Business Data Communications And Networking

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American National Standards Institute (ANSI)

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The principal standards-setting body in the US. ANSI is a nonprofit, nongovernmental organization supported by more than 1,000 Trade organizations, societies and companies. Member of ITU-T, CCITT, and the ISO.

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Application Layer

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The layer that provides the interface between the applications we use to communicate and the underlying network over which our messages are transmitted. Protocols are used to exchange data between programs running on the source and destination hosts.
The user's access to the network.

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Application Service Provider (ASP)

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develops an application system (e.g., reservation system, payroll system) and companies purchase the service, without ever installing the software on their own computers. Similar to a web hosting service.

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AT&T

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American Telephone and Telegraph Company
Registered in 1885 and was the Long-Distance Telephone carrier.

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Backbone Network (BN)

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A larger, central network connecting several LANs, other BNs, MANs, and WANs. Typically span from hundreds of feet to several miles and provide very high speed data transmission

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BPS

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Bits per second
the basic unit of data communication rate measurement. Usually refers to rate of information bits transmitted.

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Broadband Communicaiton

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Originally, this was analog communications, but it has become corrupted in common usage so that it now usually means high-speed communications networks - access speeds of 1 Mbps or higher.

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Cable

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traditionally copper wire, although fiber-optic cable is becoming common, that connect the computers to the routers or switches.

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CA *net

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The Canadian Network that forms part of Internet2 - Canada's high speed network.

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Circuit

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The path over which the voice, data, or image transmission travels. Can be twisted wire pairs, coaxial cables, fiber-optic cables, microwave transmissions and so forth.

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Client

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The input-output hardware device at the user's end of a communication circuit. There are 3 major categories: computers, terminals and special-purpose terminals.

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Common Carrier

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An organization in the business of providing regulated telephone, telegraph, telex and data communications services, such as AT&T, MCI, Bell-South and NYNEX - permitted to build their own local telephone facilities and offer services to customers.

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Convergence

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The integration of voice, video, and data communication, sometimes called convergence!

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Data Link Layer

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Layer 2 of the Internet Model
responsible for moving a message from one computer to the next computer in the network path from the sender to the receiver. (Same fxs as the DLL of an OSI
Controls the physical layer by deciding when to transmit messages over the media.

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Extranet

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Uses the same technologies as the Internet but instead is provided to invited users outside the organization who access it over the Internet. Can provide access to information services, inventories, and other internal organizational databases that are provided only to customers, suppliers, or those who have paid for access.

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Federal Communications Commission (FCC)

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Governmental body that has the authority to regulate interstate telephone businesses in the US (transferred from the ICC in 1934)

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File Server

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Stores data and software that can be used by computers on the network.

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GBPS

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Gigabytes per second

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Hardware Layer

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a grouping of the data link layer and the physical layer - so closely coupled that decisions in one layer impose certain requirements on other layers.

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Hub

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Network hubs act as junction boxes, permitting new computers to be connected to the network as easily as plugging a power cord into an electrical socket and provide an easy way to connect network cables. Can also act as repeaters or amplifiers.

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Information utility

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A company that provides a wide range of standardized information services.

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Institute of Electrical and Electronics Engineers (IEEE)

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a professional society in the US whose Standards Association (IEEE-SA) develops standards. Probably most known for it's standards for LANs.

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Interexchange Carrier (IXC)

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first divided part of AT&T, in 1984. that provided long distance telephone services in competition with other IXCs such as MCI and Sprint.

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International Telecommunications Union - Tele- Communications Group (ITU-T)

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the technical standards-setting organization of the United Nations International Telecommunications Union - based in Geneva.

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Internet

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The Information highway! The network of networks that spans the world, linking more than 20 million users.

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Internet Engineering Task Force (IETF)

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a large, open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. Operates through a series of working groups which are organized by topic.

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Internet Model

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a five layer Internet Model that dominates current hardware and software; evolved from the work of thousands of people who developed pieces of the Internet.
Layer 1 - The Physical Layer
Layer 2 - The Data Link Layer
Layer 3 - The Network Layer
Layer 4 - The Transport Layer
Layer 5 - Application Layer

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Intranet

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A LAN that uses the same technologies as the Internet, but is open to only to those inside the organization.

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Kbps

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Kilobits per second - equal to 1000 bps

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layers

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entire set of communications functions broken down into a series of stages (called layers) - each of which can be defined separately.

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Local Area Network (LAN)

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a group of computers located in the same general area, covers a clearly defined small area, such as one floor or work area, a single building, or a group of buildings. Often use shared circuits where all computers must take turns using the same circuit. Support high-speed data transmission compared with standard telephone circuits, commonly operating 100 million bits per second (100 Mbps)

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Local Exchange Carrier (LEC)

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the second divided part of AT&T, in 1984, that was divided into a series of seven regional Bell operating companies that provided Local telephone services to homes and businesses

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Internet Service Provider (ISP)

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Companies that offer connections to the internet, Some charge flat rates, some charge based on usage.

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Internetwork Layer

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The transport and network layers that are so closely coupled that sometimes they are called the internetwork layer.

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Mbps

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million bits per second (100 Mbps) - the size of a textbook

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Metropolitan Area Network (MAN)

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connects LANs and BNs located in different areas to each other and to WANs. Typically span between 3 and 30 miles. Some organizations develop their own MANs using technologies similar to those of BNs. Provide moderately fast transmission rates, but can prove costly to install and operate over long distances unless Org has a continuing need to transfer large amounts of data

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Monopoly

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a business - usually highly regulated - with no competition.

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Net Neutrality

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for a given type of content (e.g., email, web, video, music) all content providers are treated the same - transmitted at the same speeds.

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Network

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A series of points connected by communication circuits.

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Network Layer

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Layer that performs routing. determines the next computer the message should be sent to so it can follow the best route through the network and finds the full address for that computer if needed.

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Open Systems Interconnection Reference Model (OSI Model)

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Helped change the face of network computing, a framework of standards for computer-to-computer communications.

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Pbps

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1 petabit, (or 1000 terabits) - equates to roughly 1/10 of th total amount of digital content stored on every computer in the world

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Peer-to-Peer Network

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network designed to connect a set of similar computers that share their data and software with each other; the computers function as equals rather than relying on a central server to store the needed data and software.

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Pervasive Networking

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communication networks will one day be everywhere, virtually any device will be able to communicate with any other device in the world.

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Physical Layer

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concerned primarily with transmitting data bits (zeros or ones) over a communication circuit. This layer defines the rules by which ones and zeros are transmitted, such as voltages of electricity, number of bits sent per second, and the physical format of the cables and connectors used.

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Print server

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connected to a printer and manages all printing requests from the clients on the network.

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Protocol

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A formal set of conventions governing the format and control of inputs and outputs between two communicating devices. This includes the rules by which these two devices communicate as well as handshaking and line discipline.

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Protocol Data Unit (PDU)

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(packet) added to message by all layers except the Physical layer that contains information needed to transmit the message through the network.

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protocol stack

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The set of software required to process a set of protocols.

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Regional Bell Operating Company

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the second divided part of AT&T, in 1984, that was divided into a series of seven regional Bell operating companies that provided Local telephone services to homes and businesses

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Request for Comment (RFC)

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A proposed standard for the Internet on which anyone in the world is invited to comment.

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Router

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A device that connects two similar networks having the same network protocol. It also chooses the best route between two networks when there are multiple paths between them.

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Server

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A computer that provides a particular service to the client computers on the network. (file, print, DB...)

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Standards

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defines a set of rules, called protocols, that explain exactly how hardware and software that conform to the standard are required to operate.

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Tbps

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terabits - a trillion bits per second.

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Transport Layer

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layer that deals with the end-to-end issues, such as procedures for entering and departing from the network. Establishes, maintains and terminates logical connections for the transfer of data between the original sender and the final destination of the message. Responsible for breaking a large data transmission into smaller packets, ensuring that all packets have been received, eliminating duplicate packets and performing flow control to ensure that no computer is overwhelmed by the number of messages it receives. Can also perform error checking.

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Voice Over Internet Protocol (VOIP)

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Protocol that allows use of network connection to make and receive telephone calls.

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Web Server

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server that stores documents and graphics that can be accessed from any Web browser. Can respond to requests from computers on this network or any computer on the Internet.

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Wide Area Network (WAN)

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Connect BNs and WANs. Circuits provided by IXCs that come in all types and sizes by typically span hundreds or thousands of miles and provide data transmission rates from 64 Kbps to 10 Gbps.

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How can data communications networks affect businesses?

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A networked business allows sharing of data and reporting. Networking - if done properly - provide more efficient use of data and sharing of information in real time and allows the computers to talk via electronic e-mail

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Discuss three important applications of data communications networks in business and personal use.

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e-mail - ability to share ideas and data
videoconferencing - reduces costs by reducing need for travel
internet - provides a plethora of information at the touch of a button.

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Define Information Lag and discuss its importance.

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Information lag is the time it takes for information to be disseminated worldwide. Collapsing the information lag speeds the incorporation of new information into our daily lives. In fact, today's problem is that we cannot handle the quantities of information we receive.

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Why are network layers important?

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Communication networks are often broken into a series of layers, each of which can be defined separately, to enable vendors to develop software and hardware that can work together in the overall network.

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Describe the five layers in the Internet network model and what they do.

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The application later is the application software used by the network user
The transport layer deals with end-to-end issues, such as procedures for entering and departing from the network, by establishing, maintaining, and terminating logical connections for the transfer of data between the original sender and the final destination of the message. The Network layer takes the message generated by the application layer and if necessary, breaks it into several smaller messages. It then addresses the message and determines their route through the network, and records message accounting information before passing in to the data link layer. The data link layer formats the message to indicate where it starts and ends, decides when to transmit it over the physical media, and detects and corrects any errors that occur in transmission. The physical layer is the physical connection between the sender and receiver, including the hardware devices and physical media.

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Describe the three stages of standardization.

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The formal standardization process has three stages: specification, Identification of choices, and acceptance.
The specification stage consists of developing a nomenclature and identifying the problems to be addressed. In the identification of choices stage, those working on the standard identify the various solutions and choose the optimum solution from among the alternatives. Acceptance - the most difficult stage - consists of defining the solution and getting recognized industry leaders to agree on a single, uniform solution. ISO standards development is pursued at the national and international levels. Authorized national technical committees can be designated as Technical Advisory Groups (TAGs) to international subcommittees or workgroups.

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Describe two important data communications standards - making bodies. How do they differ?

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The International Organization for Standardization (ISO) makes the technical recommendations about data communication interfaces.
The Telecommunications group (ITU-T) is the technical standards setting organization for the United Nations International Telecommunications Union (ITU).
Postal Telephone and Telegraphs (PTTs) are telephone companies outside of the United States. ITU-T establishes recommendations for used by PTTs, other common carriers, and hardware and software vendors.

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What are three of the largest interchange carriers (IXCs) in North America?

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AT&T, MCI, Sprint

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Name two regional Bell operating companies (RBOCs). Which one(s) provide services in your area?

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There are only 3 left, AT&T, Quest, Verizon. AT&T and Verizon provide services in my area.

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Discuss three trends in communications and networking.

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Pervasive networking - will change how and where we work and with whom we do business.
Integration of voice, video and data onto the same networks will greatly simplify networks and enable anyone to access any media at any point.
The rise in these pervasive, integrated networks will mean a significant increase in the availability of information and new services.

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Why has the Internet model replaced the Open Systems Interconnection Reference (OSI) model?

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Because the OSI model never caught on commercially in North America - although some European networks use it.

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In the 1980s when we wrote the first edition of this book, there were many, many more protocols in common use a the data link, network, and transport layers than there are today. Why do you think the number of commonly used protocols at these layers has declined? Do you think this trend will continue? What are the implications for those who design and operate networks?

Back 71

Each network began using the same standards to allow easily exchanged messages with on another. It does no good to network, if communications and messages cannot be accessed.

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How do local area networks (LANs) differ from metropolitan area networks (MANs), wide area networks (WANs) and backbone networks (BNs)

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LAN is a network of computers located in the same general area.
MAN (metropolitan) is a network encompassing a city or county area
WAN (wide) is a wide area network spanning city, state, or national boundaries.
BN (backbone) is a large central network that connects almost everything on a single company site.

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What is the purpose of a data communications standard?

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The use of standards makes it much easier to develop software and hardware that link different networks because software and hardware can be developed one layer at a time. The software or hardware defined by the standard at one network layer can be easily updated, as long as the interface between that layer and the ones around it remains unchanged.

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Describe the seven layers in the OSI network model and what they do.

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1. Application layer - the application software used by the network user.
2. Presentation layer formats the data for presentation to the user by accommodating different interfaces on different terminals o computers so the application program need not worry about them.
3. Session layer is responsible for initiating, maintaining, and terminating each logical session between end users.
4. Transport layer deals with end-to-end issues such as procedures for entering and departing from the network, by establishing, maintaining, and terminating logical connections for the transfer of data between the original sender and the final destination of the message.
5. Network Layer takes the message generated by the application layer and if necessary, breaks it into several smaller messages. It then addresses the message(s) and determines their route through the network, and records message accounting information before passing it to the data link layer.
6. Data Link layer formats the message to indicate where it starts and ends, decides when to transmit it over the physical media, and detects and corrects any errors that occur in transmission.
7. Physical layer is the physical connection between the sender and receiver, including the hardware devices (e.g., computers, terminals, and modems) and physical media (e.g., cables, and satellites).

Front 75

Explain how a message is transmitted from one computer to another using layers.

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The application layer is the application software used by the network user. The transport layer is responsible for obtaining the address of the end user (if needed), breaking a large data transmission into smaller packets (if needed), ensuring that all the packets have been received, eliminating duplicate packets, and performing flow control to ensure that no computer is over whelmed by the number of messages it receives. The network layer takes the message generated by the application layer and if necessary, breaks it into several smaller messages. it then addresses the message(s) and determines their route through the network, and records message accounting information before passing it to the data link layer. The data link layer formats the message to indicate where it starts and ends, decides when to transmit it over the physical media, and detects and corrects any errors that occur in transmission. the physical layer is the physical connection between the sender and receiver, including the hardware devices and physical media.

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How are Internet standards developed?

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The Internet Engineering Task Force (IETF; www.ietf.org) sets the standards that govern how much of the Internet will operate. Developing a standard usually takes 1-2 years, Usually, a standard begins as a protocol developed by a vendor. When a protocol is proposed for standardization, IETF forms a working group of technical experts to study it. The working group examines the protocol to identify potential problems and possible extensions and improvements, and then issues a report to IETF. If the report is favorable, the IETF issues a Request for Comment (RFC) that describes the proposed standard and solicits comments from the entire world. Once no additional changes have been identified, it becomes a Proposed Standard. Once at least two vendors have developed software based on it, and it has proven successful in operation, the Proposed Standard is changed to a Draft Standard. This is usually the final specification, although some protocols have been elevated to Internet Standards, which usually signifies a mature standard not likely to change. There is a correlation of IETF RFCs to ISO standards.

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What are the implications for management from Chapter 1 - intro to data communications?

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1. Networks and the Internet change almost everything, the way we operate, work, live, play and learn.
2. Today's networking environment is driven by standards which means an organization can easily mix and match equipment from different vendors.
3. As the demand for network services and network capacity increases, so too will the need for storage and server space. finding efficient ways to store all the information we generate will open new market opportunities.

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Network Applications

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the software packages that run in the application layer

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Application Architecture

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the way in which the functions of the application layer software are spread among the clients and servers in the network

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What are the four general functions derived from the division of how any application program works.

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Data Storage - require data to be stored and retrieved
Data Access Logic - the processing required to access data (e.g., DB Queries)
Application Logic (business logic) can be simple or complex depending on the application
Presentation Logic the presentation to the user and the acceptance of the user's commands.

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Host-Based Architecture

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The server (or host computer) performs virtually all the work.

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Client-based Architecture

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the client computers perform most of the work

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Client-Server Architecture

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The work is shared between the servers and the clients.
This is the dominant architecture.

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Peer-to-Peer Architecture

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computers are both clients and servers and thus share the work.

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Mainframe

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a very large general purpose computer (usually costing millions of dollars) that is capable of performing an immense number of simultaneous functions, supporting an enormous number of simultaneous users and storing huge amounts of data.

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Personal Computer

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the type of computer you use, can also be used as a server.

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Cluster

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a group of computers liked together so that they act as one computer. Requests arrive at the cluster and are distri utted amoung the computers so that no one computer is overloaded. Each computer is separate so that if one fails, the cluster still works - simply bypasses it. Clusters are more complex than singe servers because work must be quickly coordinated and shared among the individual computers. Very scalable - can always add one more...

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Virtual Server

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one computer that acts as several servers. Using special software, several operating systems are installed on the same physical computer so that one physical computer appears as several different servers to the network. Can perform the same or separate functions (e.g., print server, data warehouse...) Improves efficiency - if one crashes, the others keep working.

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What are the five commonly used types of clients?

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1. Personal Computer - most common type today
2. Terminal
3. Network Computer
4. Transaction terminal
5. Handheld Computer

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Adaptive differential pulse code modulation (ADPCM)

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the alternative used by IM and many other applications that provide voice services over lower-speed digital circuits. Works in much the same way as PCM, it samples incomoing voice signal 8,000 times per second and calculates the same 7-bit amplitude value as PCM. However, instead of transmitting the 8-bit value, it transmits the difference between the 8-bit value in the last time interval and the current 8-bit value (how the amplitude has changed from one bit to another),

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American Standard Code for Information Exchange (ASCII)

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An eight-level code for data transfer adopted by the ANSI to achieve compatibility among data devices

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Amplitude

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One of the characteristics of a sound wave, it is the height of the sound wave and is measured in decibals.

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Amplitude Modulation (AM)

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the amplitude or height of the wave is changed. The highest amplitude symbol represents a binary 1 and the lowest amplitude symbol represents a binary 0.

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Amplitude Shift Keying (ASK)

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Same as Amplitude Modulation: the amplitude or height of the wave is changed. The highest amplitude symbol represents a binary 1 and the lowest amplitude symbol represents a binary 0.

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Analog Transmission

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occurs when the signal sent over the transmission media continuously varies from one state to another in a wavy-like pattern much like the human voice.

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Bandwidth

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The difference between the highest and the lowest frequencies in a band or set of frequencies. The maximum symbol rate for analog transmission is usually the same as the bandwidth as measured in Hz.

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Bandwidth on Demand Interoperability Networking Group (BONDING)

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Standard that splits outgoing messages from one client or host across several low--speed telephone lines and combines incoming messages from several telephone lines into one circuit so that the client or host 'thinks' it has a faster circuit. The most common use is room-to-room videoconferencing.

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Baud Rate

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rate of a unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes.

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Bipolar

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a type of signaling in data transmission where the ones and zeros vary from a plus voltage to a minus voltage (like an AC current). Types of bipolar signaling: NRZ (non-return to Zero), RZ (return to zero)

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Bit Rate

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Number of bits per second data is transmitted.

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Bits per Second (BPS)

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the basic unit of data communication rate measurement. Usually refers to rate of information bits transmitted.

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Carrier Wave

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An analog signal at some fixed amplitude and frequency that then is combined with an information-bearing signal to produce an intelligent output signal suitable for transmission of meaningful information,

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Channel

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1)A path for transmission of electromagnetic signals. (synonym for line or link)
2) A data communications path. Circuits may be divided into sub-circuits.

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Circuit

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two meanings:
1. physical circuit - the actual wire used to connect two devices.
2. logical circuit - the transmission characteristics of the connection e.g., T1

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Circuit configuration

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The basic physical layout of the circuit. There are two fundamental layouts:
Point-to-point
multipoint

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coaxial cable

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a type of guided media; Cable that has a copper core (the inner conductor) with an outer cylindrical shell for insulation. The outer shield, just under the shell, is the second conductor.

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codec

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device that translates analog voice data into digital form for transmission over digital computer circuits - one at sender's end, one at receiver's end.

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coding scheme

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the language that computers use to represent data

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customer premises equipment (CPE)

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Equipment that provides the interface between the customer's centrex system and the telephone network. It physically resides at the customer's site rather than the telephone company's end office.

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Cycles per second

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Same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second. (Same as Hertz!)

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data compression

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Reduces the amount of data transmitted; V.44, (the ISO standard) for data compression, uses Lempel-Ziv encoding. As a message is being transmitted, Lempel-Ziv encoding builds a dictionary of two-, three-, and four-character combinations that occur in the message. Anytime the same character pattern reoccurs in the message,

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data rate

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(or bit rate) calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate. The data capacity of a circuit is the fastest fate ant which you can send your data over the circuit in terms of the number of bits per second.

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digital subscriber line (DSL)

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A data link layer technology that provides high-speed ("broadband") communication over traditional telephone lines. A DSL modem is used to provide three channels; a traditional voice channel, an upstream channel for communicating from the client to the ISP.

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digital transmission

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The transmission of binary electrical or light pulses in that it only has two possible states, a 1 or an 0. Digital signals are usually sent over wire of no more than a few thousand feet in length.

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fiber-optic cable

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technology that uses high-speed streams of light pulses from lasers or LEDs that carry information inside hair-thin strands of glass called optical fibers.

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frequency

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the rate at which a current alternates, measured in Hertz, kilohertz, megtahertz.... hertz and cycles per second are synonymous

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frequency division multiplexing (FDM)

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dividing the circuit "horizontally" so than many signals can travel a single communication circuit simultaneously.

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`frequency modulation (FM)

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a form of modulation in which the frequency of the carrier is varied in accordance with the instantaneous value of the modulating signal.

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frequency shift keying (FSK)

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a modulation technique whereby 0 and 1 are represented by a different frequency and the amplitude does not vary.

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full-duplex transmission

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the capability of transmitting in both directions at the same time.

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guardband

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a small bandwidth of frequency that separates two choice-grade circuits. Also the frequencies between subcircuits in FDM systems that guard against subcircuit interference.

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guided media

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those in which the message flows through a physical media such as a twisted pair wire, coaxial cable, or fiber-optic cable; the media "guides" the signal.

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half-duplex transmission

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two-way transmission is possible, but you can only transmit in one direction at a time.(similar to a walkie-talkie link)
**computers use control signals to negotiate which will send sand which will receive data at a given time.

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Hertz (Hz)

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Same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second. (Same as Hertz!)

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Inverse multiplexing (IMUX)

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combines several low-speed circuits to make them appear as one high-speed circuit to the user. One of the most common uses of IMUX is to provide T1 circuits for WANs.

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ISO 8859

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a commonly used coding scheme - standardized by the ISO. It is an 8-bit code that includes the ASCII codes plus non-English letters used by many European languages.

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Lempel-Ziv Encoding

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The ISO standard for data compression uses Lempel-Ziv encoding - as a message is being transmitted, Lempel-Ziv encoding builds a dictionary of two-, three-, and four-character combinations that occur in the message. Anytime the same character pattern reoccurs int he message, the index to the dictionary entry is transmitted rather than sending the actual data. Data is reduced approx 6:1.

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Local Loop

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(the last mile) the wires that run from your home or business to the telephone switch that connects your local loop into the telephone network.

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Logical Circuit

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the transmission characteristics of the connection between devices (e.g., T1)

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Manchester Encoding

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a special type of bipolar signaling (used in Ethernet) in which the signal is changed from high to low or from low to high in the middle of the signal. A change from high to low is used to represent a 0, whereas the opposite (a change from low to high) is used to represent a 1.

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Microwave Transmission

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an extremely high-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points.

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Modem

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device that translates the computer's digital data into analog data that can be transmitted through the voice communication circuits, second modem at receiving end translates the analog transmission back into digital data for use by the receiver's computer.

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Multipoint Circuit

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(also called a shared circuit) many computers are connected on the same circuit - each must share the circuit with others.

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Multiplexing

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means to break one high-speed physical communicaiton circuit into several lower-speed logical circuits so that many different devices can simultaneously use it but still think that they have their own separate circuits. (the multiplexer is "transparent")

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Parallel Transmission

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the way the internal transfer of binary data takes place inside a computer. if the internal structure of the computer is 8 bits, then all 8 bits of the data element are transferred between main memory and the central processing unit simultaneously on 8 separate connections.

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Phase

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the third characteristic of a sound wave, refers to the direction in which the wave begins. Measured in the number of degrees. A wave that starts up and to the right, is defined as O degrees phase wave. Waves can also start down and to the right (a 180 degree phase wave)

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Phase Modulation (PM)

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(also called phase shift keying (PSK)) the most difficult to understand. Phase refers to the direction in which the wave begins. with phase modulation, one phase symbol is defined to be a 0 and the other phase symbol is defined to be a 1.

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Phase Shift Keying (PSK)

Back 138

(also called Phase Modulation (PM)) the most difficult to understand. Phase refers to the direction in which the wave begins. with phase modulation, one phase symbol is defined to be a 0 and the other phase symbol is defined to be a 1.

Front 139

Physical Circuit

Back 139

The actual wire used to connect two devices

Front 140

Plain Old Telephone Service (POTS)

Back 140

enables voice communication between anyu two telephones within its network. The telephone converts the sound waves produced by the human voice at the sending end into electrical signals for the telephone network. These electrical signals travel through the network until they reach the other telephone and are converted back into sound waves.

Front 141

Point-to-Point Circuit

Back 141

It goes from one point to another. Sometimes called dedicated circuits. Each computer has its own circuit running from itself to the other computers

Front 142

Polarity

Back 142

the plus (+) or minus (-) measurements of current

Front 143

Pulse Code Modulation (PCM)

Back 143

representation of a speech signal by sampling at a regular rate and converting each sample to a binary number. In PCM the information signals are sampled at regular intervals and a series of pulses in coded form are transmitted, representing the amplitude of the information signal at that time.

Front 144

Quadrature Amplitude Modulation (QAM)

Back 144

involves splitting the symbol into eight different phases (3 bits) and two different amplitudes *(1 bit), for a total of 16 different possible values. One symbol in Q!AM can represent 4 bits, while 256-QAM sends 8 bits per symbol.

Front 145

Quantizing Error

Back 145

Analog voice data that has been translated into a series of binary digits to allow transmission over a digital circuit. the digitized signal is only a rough approximation of the original signal. The original signal had a smooth flow but the digitized signal has jagged steps. The difference between the two signals is called quantizing error.

Front 146

Radio Transmission

Back 146

Wireless media that uses the same basic principles as standard radio transmission. Each device or computer on the network has a radio receiver/ transmitter that uses a specific frequency range that does not interfere with commercial radio stations.. the transmitters are very low power , designed to transmit a signal only a short distance.

Front 147

Retrain Time

Back 147

(same as turnaround time) the time required to reverse the direction of transmission from send to receive or vice versa on a half-duplex circuit.

Front 148

Satellite Transmission

Back 148

similar to microwave transmission except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space.

Front 149

Serial Transmission

Back 149

a stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion

Front 150

Simplex Transmission

Back 150

a one-way transmission such as radio or TV

Front 151

Statistical Time Division Multiplexing (STDM)

Back 151

The exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than does FDM or TDM. Not all computers will be transmitting continuously at their maximum transmission speed. Selection of transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed. Provides more efficient use of the circuit and saves money.

Front 152

Switch

Back 152

Switches connect more that two LAN segments that use the same data link and network protocol.. They may connect the same or different types of cable.

Front 153

Symbol Rate

Back 153

the speed in baud is the number of symbols per second. If each signal represents only one bit, symbol rate is the same as bits per second. When each signal contains more than one bit, symbol rate does not equal bits per second.

Front 154

Time Division Multiplexing (TDM)

Back 154

a device that combines data traffic from several low-speed communication circuits onto a single high-speed circuit. In TDM, separate time segments are assigned to each terminal during these time segments, data may be sent without conflicting with data sent from another terminal.

Front 155

Turnaround time

Back 155

(same as retrain time) the time required to reverse the direction of transmission from send to receive or vice versa on a half-duplex circuit.

Front 156

Twisted Pair Cable

Back 156

A pair of wires used in standard telephone wiring, they are twisted to reduce interference caused by the other twisted pairs in the same cable bundle. Twisted-pair wires go from homes and offices to the telephone company end office.

Front 157

Unicode

Back 157

commonly used coding scheme. UTF-8 is an 8-bit version which is very similar to ASCII. UTF-16 which uses 16 bits per character (i.e., two bytes, called a "word"), is used by Windows. By using more bits, UTF-16 can represent many more characters beyond the unusual English or Latin

Front 158

Unipolar

Back 158

a form of digital transmission in which the voltage changes between 0 volts to represent a binary 0 and some positive value (e.g., +15 bolts) to represent a binary 1.

Front 159

V.44

Back 159

The ISO standard for data compression (uses Lempel-Ziv encoding)

Front 160

Wavelength division multiplexing (WDM)

Back 160

a version of FDM used in fiber-optic cables; works by using lasers to transmit different frequencies of light (i.e,, colors) through the same fiber-optic cable. Each logical circuit is assigned a different frequency and the devices attached to the circuit don't know they are multiplexed over the same physical circuit.

Front 161

Wireless media

Back 161

Media in which the message is broadcast through the air, such as microwave or satellite.

Front 162

How does a multipoint circuit differ from a point-to-point circuit?

Back 162

a Multi-point circuit is a shared circuit, many computers are connected on the same circuit. Disadvantage: Only one computer can use the circuit at a time. Advantage: reduce the amount of cable required and typically use the available communication circuit more efficiently.

Front 163

Describe the three types of data flows.

Back 163

Simplex Transmission - permits only a one way transmission - TV, radio
Half-duplex Transmission - two-way transmission, but you can transmit in only one direction at a time - walkie-talkie, computers - use control signal to negotiate which will send and which will receive data.
Full-Duplex transmission - can transmit in both directions simultaneously with no turn-around-time.

Front 164

Describe three types of guided media.

Back 164

Twisted Pair wire: insulated pairs of wires that can be packed quite close together. Usually twisted to minimize the electromagnetic interference between one pair and any other pair in the bundle. Telephone wire - 2 pairs, Lan Cable - 4 pairs.
Coaxial cable: has a copper core (inner conductor) with an outer cylindrical shell for insulation. the outer shield, just under the shell, is the second conductor.
Fiber-optic cable: uses high-speed streams of light pulses from lasers or LEDs that carry information inside hair-thin strands of glass called optical fibers. (The media guides the transmission)

Front 165

Describe four types of wireless media.

?? can only find 3

Back 165

Radio: uses a specific frequency range that does not interfere with commercial radio stations
Microwave: an extremely gigh-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points - typically used for long distance or voice transmission.
Satellite Transmission - similar to microwave except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space.

Front 166

How does analog data differ from digital data?

Back 166

Digital data is represented in binary form - 1 (off) or 0 (on). Analog data is electrical signals shaped like the sound waves they transfer and can take on any value in a wide range of possibilities.

Front 167

Clearly explain the differences among analog data, analog transmission, digital data, and digital transmission.

Back 167

Data can be transmitted through a circuit in the same form they are produced. Most computers, for example, transmit their data through digital circuits to printers and other attached devices. Likewise, analog voice data can be transmitted through telephone networks in analog form. In general, networks designed primarily to transmit digital computer data tend to use digital transmission, and networks designed primarily to transmit analog voice data tend to use analog transmission (at least for some parts of the transmission).

Front 168

Explain why most telephone company circuits are now digital.

Back 168

Transmission is now in digital form because the cheapest and highest-quality media are digital - so its driven primarily by business reasons.

Front 169

What is coding?

Back 169

a transformation or representation of information in a different form according to some set of pre-established conventions.

Front 170

Briefly describe three important coding schemes.

Back 170

ASCII - US of Am Standard Code for Information Interchange (USASCII) - the most popular code for data communications and is the standard coe on most microcomputers.
ISO 8859 - an 8-bit code that includes the ASCII codes plus non-English letters used by many European languages.
Unicode - UTF-8 is an 8-bit version very similar to ASCII. UTF-16 uses 16bits per character(i.e., two bytes, called a 'word'), is used by Windows and can represent many more characters beyond the usual English or Latin characters such as Cyrillic or Chinese.

Front 171

How are data transmitted in parallel?

Back 171

Parallel transmission is the way the internal transfer of binary data takes place inside a computer. If the internal structure of the computer is 8-bit, then all 8 bits of the data element are transferred between main memory and the central processing unit simultaneously on 8 separate connections.Each physical wire is used to send 1 bit of the 8-bit character.

Front 172

What feature distinguishes serial mode from parallel mode?

Back 172

There is only one physical wire inside the bundle and all data must e transmitted over that one physical wire - transfer is sequential, bit by bit.

Front 173

How does bipolar signaling differ from unipolar signaling? Why is Manchester encoding more popular than either?

Back 173

In Unipolar signaling, the voltage is always positive or negative (like a DC current). In bipolar signaling the ones and zeros vary from a plus voltage to a minus voltage (like an AC current). Manchester coding is less susceptible to having errors go undetected, because if there is no transition in mid-signal the receiver knows that an error must have occurred.

Front 174

What are three important characteristics of a sound wave?

Back 174

amplitude, frequency and phase

Front 175

What is bandwidth? What is the bandwidth in a traditional North American telephone circuit?

Back 175

The difference between the highest and the lowest frrequencies in a band or set of frequencies. the bandwidth in a traditional N. Am. telephone circuit is 4,000 Hz.

Front 176

Describe how data could be transmitted using amplitude modulation.

Back 176

With amplitude modulation (AM), the amplitude or height of the wave is changed. One amplitude is defined to be zero, and another amplitude is defined to be a one.

Front 177

Describe how data could be transmitted using frequency modulation.

Back 177

Frequency modulation (FM) (also called frequency shift keying (FSK)), is a modulation technique whereby each 0 or 1 is represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does not vary. One frequency (i.e., a certain number of waves per second) is defined to be a one, and a different frequency (a different number of waves per second) is defined to be a one.

Front 178

Describe how data could be transmitted using phase modulation.

Back 178

Phase modulation (PM) (also called phase shift keying (PSK)), is the most difficult to understand. Phase refers to the direction in which the wave begins. Until now, the waves we have shown start by moving up and to the right (this is called a 0º phase wave). Waves can also start down and to the right. This is called a phase of 180º. With phase modulation, one phase is defined to be a zero and the other phase is defined to be a one.

Front 179

Describe how data could be transmitted using a combination of modulation techniques.

Back 179

It is possible to use amplitude modulation, frequency modulation, and phase modulation techniques on the same circuit. For example, we could combine amplitude modulation with four defined amplitudes (capable of sending two bits) with frequency modulation with four defined frequencies (capable of sending two bits) to enable us to send four bits on the same symbol.

Front 180

Is the bit rate the same as the symbol rate? Explain.

Back 180

they are not the same. The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. For example, if we use amplitude modulation with two amplitudes, we send one bit on one symbol. Here the bit rate equals the symbol rate. However, if we use QAM, we can send four bits on every symbol; the bit rate would be four times the symbol rate.

Front 181

What is a modem?

Back 181

Modem is an acronym for MOdulator/DEModulator. A modem takes the digital electrical pulses received from a computer, terminal, or microcomputer and converts them into a continuous analog signal that is needed for transmission over an analog voice grade circuit. Modems are either internal (i.e., inside the computer) or external (i.e., connected to the computer by a cable).

Front 182

What is quadrature amplitude modulation (QAM)?

Back 182

QAM is a popular technique that involves splitting the symbol into eight different phases (three bits) and two different amplitudes (one bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent four bits.s the modulation scheme used in cable plants.
Basically, QAM is how the modem encodes digital information to be sent over the RF interface of the modem.

Front 183

What is 64-QAM?

Back 183

QAM both an analog and a digital modulation scheme. A very common modulation scheme used in the downstream channels for cable modem or digital cable plants.

Front 184

What factors affect transmission speed?

Back 184

Factors affecting the transmission speed are transmission capacity, specialization spectrum of information transmission and the transmission reliability and anti-interference capability of the channel.

Front 185

What is oversampling?

Back 185

Oversampling is the process of increasing the sampling frequency by generating new digital samples based on the values of known samples.

Front 186

Why are data compression so useful?

Back 186

it helps reduce resources usage, such as data storage space or transmission capacity.

Front 187

What data compression standard uses Lempel-Ziv encoding? Describe how it works.

Back 187

V.44 - the ISO Standard, compression rate averages 6:1

Front 188

Explain how pulse code modulation (PCM) works.

Back 188

a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code.

Front 189

What is quantizing error?

Back 189

Inaccuracies in the digital representation of an analog signal. These errors occur because of limitations in the resolution of the digitizing process.

Front 190

What is the term used to describe the placing of two or more signals on a single circuit?

Back 190

Quadrature amplitude modulation is one form of doing this.

Front 191

What is the purpose of multiplexing?

Back 191

The subdivision of a transmission circuit into two or more separate circuits. This can be achieved by splitting the frequency range of the circuit into narrow frequency bands (frequency division multiplexing) or by assigning a given circuit successively to several different users at different times (time division multiplexing).

Front 192

How does DSL (Digital subscriber line) work?

Back 192

A data link layer technology that provides high-speed communication over traditional telephone lines. A DSL modem is used to provide three channels: a traditional voice channel, an upstream channel for communicating from the client to the ISP, and a downstream channel for communicating from the ISP to the client.

Front 193

Of the different types of multiplexing what distinguishes
a. frequency division multiplexing (FDM)?
b. time division multiplexing (TDM)?
c. statistical time division multiplexing (STDM)?
d. wavelength division multiplexing (WDM)?

Back 193

a. frequency division multiplexing (FDM)?
The voice grade link is divided into sub circuits, each covering a different frequency range in such a manner that each sub circuit can be employed as though it were an individual circuit.
b. time division multiplexing (TDM)?
Separate time segments are assigned to each terminal. During these time segments, data may be sent without conflicting with data sent from another terminal.
c. statistical time division multiplexing (STDM)?
A time division multiplexer that dynamically allocates communication circuit time to each of the various attached terminals, according to whether a terminal is active or inactive at a particular moment. Buffering and queuing functions also are included.
d. wavelength division multiplexing (WDM)?
A version of FDM used in fiber-optic cables. When fiber-optic cables were developed, the devices attached to them were designed to use only one color of light generated by a laser or LED. WDM works by using lasers to transmit different frequencies of light through the same fiber-optic cables. As with FDM, each logical circuit is assigned a different frequency, and the devices attached to the circuit don-t “know” they are multiplexed over the same physical circuit.

Front 194

What is the function of inverse multiplexing (IMUX)

Back 194

Hardware that takes one high-speed transmission and divides it among the several transmission circuits.

Front 195

If you were buying a multiplexer, why would you choose either TDM or FDM? Why?

Back 195

I would choose an TDM because of the ability not to conflict with the data sent from another terminal.

Front 196

Some experts argue that modems may soon become obsolete. Do you agree? Why or why not?

Back 196

I agree. Modems translates digital into analog, which is a slowly fading technology in the digital world. There will always be applications for analog technology, but not with digital communication.

Front 197

What is the maximum capacity of an analog circuit with a bandwidth of 4,000 Hz using QAM?

Back 197

16,000 bps

Front 198

What is the maximum data rate of an analog circuit with a 10 MHz bandwidth using 64-QAM and V.44?

Back 198

60Mbps

Front 199

What is the capacity of a digital circuit with a symbol rate of MHz using Manchester encoding?

Back 199

10Mbps

Front 200

What is the symbol rate of a digital circuit providing 100 Mbps if it uses bipolar NRz signaling?

Back 200

100,000 baud

Front 201

What is VoIP?

Back 201

a relatively new standard that uses digital telephones with built-in codecs to convert analog voice data into digital data. Because the codec is built into the telephone, the telephone transmits digital data and therefore can be connected directly into a local area network.

Front 202

What is the physical layer?

Back 202

the network hardware including servers, clients, and circuits.

Front 203

What is the Data Link Layer (layer 2) responsible for?

Back 203

it is responsible for moving a message from on computer or network device to the next computer or network device in the overall path from sender or receiver. It controls the way messages are sent on the physical media.

Front 204

Data Link Protocol

Back 204

determines who can transmit at what time, where a message begins and ends, and how a receiver recognizes and corrects a transmission error.

Front 205

What is the architecture of the data link layer

Back 205

it performs two main functions and is often subdivided into two sublayers:
the first layer is the LLC -Logical Link Control sublayer.
The second layer is the Media Access Control (MAC) sublayer

Front 206

LLC - Logical Link Control sublayer

Back 206

the data link layer's connection to the network layer above it. The LLC sublayer software is responsible for communicating with the network layer software (IP) and for taking the network layer PDU (usually an IP packet) to the network layer software.

Front 207

MAC - Media Access Control sublayer

Back 207

the software at the sending computer controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit. At the sending computer, the MAC sublayer software takes the data link layer PDU from the LLC sublayer, converts it into a stream of bits, and controls when the physical layer actually transmits the bits over the circuit. At the receiving computer the MAC sublayer receives a stream of bits from the physical layer and translates it into a coherent PDU. ensures that no errors have occurred in transmission, and passes the data link layer PDU to the LLC sublayer.

Front 208

Media Access Control

Back 208

refers to the need to control when computers transmit and becomes important when several computers share the same communication circuit (e.g., point-to-point configuration with a half-duplex configuration that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit.

Front 209

Two fundamental approaches to MAC (Media Access Control)

Back 209

Contention: Computers wait until the circuit is free and then transmit whenever they have data to send. (commonly used in Ethernet LANs)
Controlled Access: the wireless access point controls the circuit and determines which clients can transmit at what time.

Front 210

Two commony used Controlled Access Techniques

Back 210

Access Requests and Polling

Front 211

Access Requests:

Back 211

client computers that want to transmit send a request to transmit to the device that is controlling the circuit. The controlling device grants permission for one computer at a time to transmit. (use a contention technique to send an access request.

Front 212

Polling:

Back 212

the process of sending a signal to a client computer that gives it permission to transmit. The clients store all messages that need to be transmitted. Periodically, the controlling device polls the client to see if it has data to send,.

Front 213

List the types of Polling

Back 213

Roll-Call Polling - controller works consecutively through a list of clients; can be modified to select clients in priority so some get polled more often than others. May lock up system if a client is temporarily out of svc.
Hub Polling (token passing): one device starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so non, until it reaches the 1st computer which restarts the process.

Front 214

Two categories of Errors

Back 214

Human Errors - such as a mistake in typing a number (usually controlled through the application program.
Network Errors: those that occur during transmission , are controlled by the network hardware and software.

Front 215

Two categories of network errors:

Back 215

Corrupted data: data that have been changed
Lost data: data that was not received.

Front 216

Burst Error

Back 216

more than 1 data bit is changed by the error-causing condition. errors are not uniformly distributed in time.
(The rule rather than the exception)

Front 217

Sources of Errors

Back 217

Line Noise: undesirable electrical signals
White Noise (Gaussian Noise): caused by the thermal agitation of electrons and therefore is inescapable.
Impulse Noise: Primary source of errors in data communication - a click or crackling noise
Cross-talk: Occurs when one circuit picks up signals in another.
Echoes: the result of poor connections that cause the signal to reflect back to the transmitting equipment.
Attenuation: the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer.
Intermodulation noise: a special type of cross-talk - signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal.

Front 218

What is the only way to do error-detection performance?

Back 218

to send extra data with each message; the data are added by the DLL layer of the sender on the basis of some mathematical calculations performed on the message. the receiver performs the same mathematical calculations on the message it receives and matches its results against he error-detection data that were transmitted with the message. If the two match, the message is assumed to be correct, if not, an error has occurred.

Front 219

Parity Checking

Back 219

one of the oldest and simplest error-detection methods. The value of the additional parity bit is based on the number of 1's in each byte transmitted. Only a 50% rate of error detection.

Front 220

Checksum Technique

Back 220

a checksum (typically one byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum. values are compared and if equal - no error. 95% rate of error detection.

Front 221

Cyclical Redundancy Check

Back 221

One of the most popular error-checking schemes. It adds 8, 16, 24, or 32 bits to the message. With CRC, a message is treated as one long binary number, P. Before transmission, the DLL (or hardware device) divides P by a fixed binary number, G, resulting in a whole number, Q! and a remainder, R/G. So, P/G = Q + R/G. The remainder R is appended to the message before transmission. 100% error detection rate.

Front 222

What does the data link layer do?

Back 222

It is responsible for moving a message from one computer or network device to the next computer or network device in the overall path from sender or receiver. (the data link protocol determines who can transmit at what time, where a message begins and ends, and how a receiver recognizes and corrects a transmission error.)

Front 223

What is media access control, and why is it important?

Back 223

The MAC sublayer software at the sending computer controls how and when the physical layer converts bits into the physical symbols that are sent down the circuit. At the receiving computer, the MAC sublayer software takes the data link layer PDU from the LLC sublayer, converts it into a stream of bits, and controls when the physical layer actually transmits the bits over the circuit.

Front 224

Under what condition is media access control unimportant?

Back 224

It is unimportant with point-to-=point full-duplex configurations, because there are only two computers on the circuit and full duplex permits either computer to transmit at any time!

Front 225

Compare and contrast roll-call polling, hub polling (or token passing) and contention.

Back 225

Contention - computers wait until the circuit is free - no other computers are transmitting - and then transmit whenever they have data to send.
Polling - the clients store all messages that need to be transmitted, periodically, the controlling device polls the client to see if it has data to send. If it does, then it sends the data. If it doesn't, it responds negatively and the controller goes on the the next client.
Roll-call polling - the controller works consecutively through a list of clients, first polling client 1, then client 2, and so on until all are polled. Roll-call polling can be modified to select clients in priority so that some get polled more often than others.

Front 226

Which is better, controlled access or contention? explain.

Back 226

Contention approaches work better than controlled approaches for small networks that have low usage because there is little chance for collision. For large networks with high usage, controlled access works better Many computers want to transmit and the probability of a collision using contention is high. Collisions are very costly in terms of throughput because they waste circuit capacity during the collision and require both computers to retransmit later. Controlled access prevents collisions and makes more efficient use of the circuit, and although response time does increase, it does no more gradually.

Front 227

Define two fundamental types of errors.

Back 227

There are human errors and network errors. Human errors, such as a mistake in typing a number, usually are controlled through the application program. Network errors, are those that occur during transmission, are controlled by the network hardware and software. There are two categories of network errors one is corrupted data (data that have been changed) and lost data.

Front 228

Errors normally appear in ____________, which is when more than 1 data bit is changed by the error-causing condition.

Back 228

bursts

Front 229

Is there any difference in the error rates of lower-speed lines and higher-speed lines?

Back 229

The difference in the error rates of lower speed lines when compared to higher speed lines is slower speed lines have a lower error rate than high speed lines because higher transmission speeds are more error prone.

Front 230

Briefly define noise.

Back 230

Noise is the unwanted change in waveform that occurs between two points in a transmission circuit. It is also undesirable electrical signals (for fiber optic cable, it is undesirable light). Noise is introduced by equipment or natural disturbances, and it degrades the performance of a communication circuit. Noise manifests itself as extra bits, missing bits, or bits that have been flipped.

Front 231

Describe four types of noise. Which is likely to pose the greatest problem to network managers?

Back 231

White noise is caused by the thermal agitation of electrons and therefore is inescapable. Impulse noise is heard as a click or a crackling noise and can last as long as 1/100 of a second. Cross-talk occurs when one circuit picks up signals in another. Echo noise are caused by poor connections that cause the signal to reflect back to the transmitting equipment. The type of noise that poses the greatest problem to network managers is impulse noise which is the primary source of errors in data communications. Impulse noise may not really affect voice communications, but can obliterate a group of data, causing a burst error.

Front 232

How do amplifiers differ from repeaters?

Back 232

Amplifier and repeater are both devices used to boost the strength of a
signal. An amplifier takes the incoming signal, increases its strength, and retransmits it on the next section of the circuit. These are typically used on analog circuits such as telephone Company’s voice circuit. Repeaters are commonly used on digital circuits. A repeater receives the incoming signal, translates it into a digital message, and retransmits the message. Because the message is re-created at each repeater, noise and distortion from the previous circuit are not amplified.

Front 233

What are three ways of reducing errors and the types of noise they affect?

Back 233

Three ways of reducing errors are shielding, moving cables, and changing multiplexing techniques. Shielding protects wires by covering them with an insulating coating. Moving cables away from sources of noise (especially power sources). Changing multiplexing techniques from FDM to TDM or changing the frequencies or size of the guardbands in FDM can help. These affect impulse noise, cross-talk, and intermodulation noise. Impulse noise (sometimes called spikes) is the primary source of errors in data communication. Impulse noise is heard as a click or a crackling noise and can last as long as 1/100 of a second. Cross-talk occurs when one circuit picks up signals in another. Intermodulation noise is when signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal.

Front 234

Describe three approaches to detecting errors, including how they work, the probability of detecting an error, and any other benefits or limitations.

Back 234

parity checking, checksum, and cyclic redundancy checking.
1. Parity is one of the oldest and simplest errors –detection methods. It is an addition of non-information bits to a message to detect any changes in the original bit structure from the time it leaves the sending device to the time it received.
2. With the technique checksum, 1 byte is added to end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum calculate the checksum. The receiver calculates its own checksum in the same way and compares it with the transmitted checksum. If the two values are equal, the message is presumed to contain no errors. Use of checksum detects close to 95 percent of the errors for multiple-bit burst errors.
3. Cyclic Redundancy Check is one of the most popular error-checking schemes. It adds 8, 16, 24, or 32 bits to the message.

Front 235

Briefly describe how even parity and odd parity work.

Back 235

Even parity is when the seven bits of an ASCII character have an even (2,4, or 6) number of 1s, and therefore a 0 is placed in the eight parity position. Odd parity is when the seven bits of an ASCII character have an odd (1, 3, 5, or 7) number of 1s, and therefore a 1 is placed in the eighth parity position.

Front 236

Briefly describe how checksum works.

Back 236

With the technique checksum, 1 byte is added to end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum calculate the checksum. The receiver calculates its own checksum in the same way and compares it with the transmitted checksum. If the two values are equal, the message is presumed to contain no errors. Use of checksum detects close to 95 percent of the errors for multiple-bit burst errors.

Front 237

How does CRC work?

Back 237

Cyclic Redundancy Check (CRC) adds 8, 16, 24 or 32 bits to the message. With CRC, a message is treated as one binary number; P. Before transmission, the data link layer (or hardware device) divided P by affixed binary number, G, resulting in a whole number, Q, and a remainder, R/G. So, P/G = Q+R/G. For example if P = 58 and G= 8, then Q = 7 and R = 2. G is chosen so that the remainder R will be either 8 bits, 16 bits, 24 bits, or 32 bits.
The remainder, R, is appended to the message as the error-checking character before transmission. The receiving hardware divides the received message by the same G, which generates an R. The receiving hardware checks to ascertain whether the received R agrees with the locally generated R. If it does not, the message is assumed to be in error.

Front 238

How does forward error correction work? How is it different from other error-correction methods?

Back 238

Forward error correction uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the receiving end without retransmission of the original message. Forward error correction is different because it corrects the error at the receiving end without retransmission.

Front 239

Under what circumstances is forward error correction desirable?

Back 239

Forward error correction is desirable when sending satellite transmission because error rates fluctuate depending on the condition of equipment , sunspots, weather. Weather conditions make it impossible to transmit without any errors, so forward error correction is desirable.

Front 240

Compare and contrast stop-and-wait ARQ and continuous ARQ.

Back 240

Stop and Wait ARQ the sender stops and waits for a response from the receiver after packet. After receiving the packet the receiver sends an acknowledgement (ACK) if the packet did not contain any errors and a negative acknowledgement (NAK) if the packet contained errors. Continuous ARQ the sender does not wait for an acknowledgement after sending a message it immediately send the next one. The sender examines the stream of returning acknowledgements, if it does receive an NAK the sender will retransmit the needed message. The packets being retransmitted must contain errors ( called Link Access Protocol for Modems [LAP-M]) or maybe the first packet with an error and all the others followed it (called Go Back N ARQ).

Front 241

Which is the simplest (least sophisticated) protocol described in this chapter?

Back 241

I think that point to point protocol is the simplest and least sophisticated protocol described in this chapter. Point to point is byte oriented protocol that is designed to transfer data over a point to point telephone line but provides an address so that it can be used on multipoint circuits.

Front 242

Describe the frame layouts for SDLC

Back 242

SDLC is a mainframe protocol developed by IBM in 1972 that is still in use today. Each begins and ends with a special bit pattern 01111110, known as the flag. The address field id's the destination. The lengths usually 8 bits but can be set at 16 bits(all comps on network must have the same) Control field indicates the kind of frame transmitted. An information frame is used for the transfer and reception of messages, frame numbering of contiguous frames and the like.
A supervisory frame is used to transmit acknowledgements (ACK/Naks), Message field is of variable lengths and is the user's message. Frame check sequence field is a 32-bit CRC code.

Front 243

Describe the frame layouts for Ethernet

Back 243

The frame starts with a 7-byte preamble which is a repeating pattern of ones and zeros (10101010). This is followed by a start of frame delimiter, which marks
the start of the frame. The destination address specifies the receiver, whereas the source address specifies the sender. The length indicates the length in 8-bit bytes of the message portion of the frame. The VLAN tag field is an optional 4-byte address field used by virtual LANs (VLANs),

Front 244

Describe the frame layouts for PPP

Back 244

similar to an SDLC or HDLC frame. The frame starts with a flag, and has a one-byte address (which is not used on point-to-point circuits). The control field is typically not used. The protocol field indicates what type of data packet the frame contains (e.g., an IP packet). The data field is variable in length and may be up to 1,500 bytes. The frame check sequence is usually a CRC-16, but can be a CRC-32. The frame ends with a flag.

Front 245

What is transmission efficiency?

Back 245

Transmission efficiency is the total number of information bits divided by the total bits in transmission.

Front 246

How do information bits differ from overhead bits?

Back 246

Information bits are used to convey the user’s meaning. Overhead bits are used for error checking and marking the start and end of characters and packets.

Front 247

Are stop bits necessary in asynchronous transmission? explain by using a diagram

Back 247

Stop bits are indeed necessary in asynchronous transmission. Stop bits inform the receiver that the character has been received and resets it for recognition of the next start bit. It is typically represented with a 1. It is the last bit on the diagram. The diagram starts with the start bit at 0, then the ASCII data, then the parity bit, then lastly the stop bit.

Front 248

During the 1990s, there was intense competition between two technologies(10-Mbps Ethernet and 16-Mbps token ring) for the LAN market. Ethernet was promoted by a consortium of vendors, whereas token ring was primarily an IBM product, even through it was standardized. Ethernet won, and no one talks about token ring anymore. Token ring used a hub-polling-based approach. Outline a number of reasons why Ethernet might have won. Hint: the reasons were both technical and business.

Back 248

One of the reasons for Ethernet success was that it was more universal, while Token Ring was mainly IBM used. Ethernet was a more simplified version, therefore easier to use and cheaper to mass produce. It has no transparency problems and is basically easier for the consumer to understand.

Front 249

Under what conditions does a data link layer protocol need an address?

Back 249

Under Point-to-Point protocol; It is designed to transfer data over a point-to-point circuit but provides an address so that it can be used on multipoint circuits. Its often used in WANs.

Front 250

Are large frame sizes better than small frame sizes? Explain.

Back 250

Small frames are more efficient for error prone circuits because they are less likely to contain errors and if the co, they cost less in terms of circuit capacity to retransmit if there is an error. Large frames are better suited to error-free circuits but their drawback is they they are ore likely to be affected by errors and thus require more retransmission.

Front 251

Show how the word "HI" would be sent using asynchronous transmission using even parity (make assumptions about the bit patterns needed.) Show how it would be sent using Ethernet.

Back 251

The word would look like this in binary
01001000 01001001
Even parity would set the first bit of each "letter" to either 1, if the number of 1's is odd Or there aren't any). So with even parity, you would have

01001000 11001001

Parity doesn't directly relate to ethernet, but to the protocol transmitting over ethernet, so you may deal with odd parity

Front 252

What does the transport layer do?

Back 252

The transport layer links the application software in the application layer with the network and is responsible for the end-to-end delivery of the message. The transport layer accepts outgoing messages from the application layer (e.g., Web, email, and so on, as described in Chapter 2) and segments them for transmission.

Front 253

What does the network layer do?

Back 253

The network layer takes the messages from the transport layer and routes them through the network by selecting the best path from computer to computer through the network (and adds an IP packet).
The network and transport layers also accept incoming messages from the data link layer and organize them into coherent messages that are passed to the application layer.

Front 254

What are the parts of TCP/IP and what do they do?
Who is the primary user of TCP/IP?

Back 254

The parts of TCP/IPTCP/IP allows reasonably efficient and error-free transmission. Because it performs error checking, it can send large files across sometimes unreliable networks with great assurance that the data will arrive un-corrupted. TCP/IP is compatible with a variety of data link protocols, which is one reason for its popularity.

Front 255

Compare and contrast the three types of addresses used in a network.

Back 255

The three types of addresses are:
With TCP/IP, each application layer software package has a unique port address.
When an application layer program generates an outgoing message, it tells the TCP software its own port
address (i.e., the source port address) and the port address at the destination computer (i.e., the destination port address).

Front 256

How is TCP differenct from UDP?

Back 256

TCP is different from UDP in that Typically, UDP is used when the sender needs to send a single small packet to the receiver (e.g., for a DNS request). When there is only one small packet to be sent, the transport layer doesn’t need to worry about segmenting the outgoing messages or reassembling
them upon receipt, so transmission can be faster. UDP does not check for lost messages,

Front 257

How does TCP establish a session?

Back 257

When the sending computer wants to send a message to the receiver, it usually starts by establishing a session with that computer. Connection-oriented messaging sets up a TCP connection (also called a session) between the sender and receiver. To establish a connection, the transport layer on both the sender and the receiver must send a SYN (synchronize) and receive a ACK (acknowledgement) segment. This process starts with the sender (usually a client) sending a SYN to the receiver (usually a server). The server responds with an ACK for the sender’s/client’s SYN and then sends its own SYN. SYN is usually a randomly generated number that identifies a packet. The last step is when the client sends an ACK for the server’s SYN. This is called the three-way handshake and this process also contains the segment size negotiation.

Front 258

What is a subnet and why do networks need them?

Back 258

subnetworks or subnets are designed on the network that subdivide the network into logical pieces.it is customary to use the first 3 bytes of the IP address to indicate different subnets, it is not required. Any portion of the IP address can be designated as a
subnet by using a subnet mask.

Front 259

What is a subnet mask?

Back 259

Every computer in a TCP/IP network is given a subnet mask to enable it to determine which computers are on the same subnet (i.e., LAN) that it is on and which computers are outside of its subnet. For example, a network could be configured so that the first two bytes indicated a subnet (e.g., 128.184.x.x), so all computers would be given a subnet mask giving the first two bytes as the subnet indicator. Subnet masks tell computers what part of an Internet Protocol (IP) address is to be used to determine whether a destination is on the same subnet or on a different subnet. A subnet mask is a 4-byte binary number that has the same format as an IP address and is not routable on the network.

Front 260

How does dynamic addressing work?

Back 260

Dynamic Addressing is when a server is designated to supply a network layer address to a computer each time the computer connects to the network. This is commonly done for client computers but usually not done for servers.

Front 261

What benefits and problems does dynamic addressing provide?

Back 261

A benefit is the assignment of IP address configurations is automated and controlled from a central position. Another benefit is It drastically reduces the time to set up client computers and eliminates the likelihood of configuration errors. A problem would occur when users reconfigure their own IP addresses and they should be discouraged, or preferably prevented, from doing so. Another problem would be if you run out of numbers for the range of your default router addresses.

Front 262

What is address resolution?

Back 262

To send a message, the sender must be able to translate the application layer address (or server name) of the destination into a network layer address and in turn translate that into a data link layer address.

Front 263

How does TCP/IP perform address resolution from URLs into network layer addresses?

Back 263

When a computer does not have the IP address of the URL logged in it's local IP look-up table, is uses one or more DNS servers to resolve the address. Once the address associated with the URL is resolved the message is then recorded to the senders look-up table and the message is forwarded to the intended receiver

Front 264

How does TCP/IP perform address resolution
from IP addresses into data link layer addresses?

Back 264

To send a message to a computer in its network, a computer must know the correct data link layer address. In this case, the TCP/IP software sends a broadcast message to all computers in its subnet. A broadcast message, as the name suggests, is received and processed by all computers in the same LAN (which is usually designed to match the IP subnet). The message is a specially formatted TCP-level request using Address Resolution Protocol (ARP) that says “Whoever is IP address xxx.xxx.xxx.xxx, please send me your data link layer address.” The TCP software in the computer with that IP address then responds with its data link layer address. The sender transmits its message using that data link layer address. The sender also stores the data link layer address in its address table for future use.

Front 265

What is routing?

Back 265

Routing is the process of determining the route or path through the network that a message will travel from the sending computer to the receiving computer. In some networks (e.g., the Internet), there are many possible routes from one computer to another. In other networks (e.g., internal company networks), there may only be one logical route from
one computer to another.

Front 266

How does decentralized routing differ from centralized routing?

Back 266

With centralized routing, all routing decisions are made by one central computer or router. Centralized routing is commonly used in host-based networks and in this case, routing decisions are rather simple. Decentralized routing. Decentralized routing means that all computers or routers in the network make their own routing decisions following a formal routing protocol.

Front 267

What are the differences between connectionless and connection-oriented messaging?

Back 267

When connectionless messaging is desired, the TCP segment is replaced with a User Datagram Protocol (UDP) packet. When connection-oriented messaging is desired, TCP is used.

Front 268

What is a session?

Back 268

A session can be thought of as a conversation between two computers. When the sending computer wants to send a message to the receiver, it usually starts by establishing a session with that computer. The sender transmits the segments in sequence until the conversation is done, and then the sender ends the session. This approach to session management is called connection-oriented messaging.

Front 269

What is QoS routing and why is it useful?

Back 269

QoS routing is a special type of connection-oriented messaging in which different connections are assigned different priorities. Different classes of service are defined, each with different priorities. For example, a packet of videoconferencing images would likely get higher priority than would an SMTP packet with an email message and thus be routed first.

Front 270

Compare and contrast unicast, broadcast,
and multicast messages.

Back 270

Unicast messages are the most common type of message in a network is the transmission between two computers. One computer sends a message to another computer (e.g., a client requesting a Web page). Broadcast message that is sent to all computers on a specific LAN or subnet. A multicast message is used to send the same message to a group of computers.

Front 271

Explain how multicasting works.

Back 271

Computers wishing to participate in a multicast send a message to the sending computer or some other computer performing routing along the way using a special type of packet called Internet Group Management Protocol (IGMP). Each multicast group is assigned a special IP address to identify the group. Any computer performing routing knows to route all multicast messages with this IP address onto the subnet that contains the requesting computer. The routing computer sets the data link layer address on multicast messages to a matching multicast data link layer address. Each requesting computer must inform its data link layer software to process incoming
messages with this multicast data link layer address. When the multicast session ends (e.g., the videoconference is over), the client computer sends another IGMP message to the organizing computer or the computer performing routing to remove it from the multicast group.

Front 272

Explain how the client computer in Figure 5.14
(128.192.98.xx) would obtain the data link
layer address of its subnet router.

Back 272

When a computer is installed on a TCP/IP network (or dials into a TCP/IP network), it knows the IP address of its subnet gateway. This information can be provided by a configuration file or via a bootp or DHCP server. However, the computer does not know the subnet gateway’s Ethernet address (data link layer address). Therefore, TCP would broadcast an ARP request to all computers on its subnet, requesting that the computer whose IP address is 128.192.98.1 to respond with its Ethernet address.

Front 273

Why does HTTP use TCP and DNS use UDP?

Back 273

HTTP uses TCP because of its error control and delivery-assurance. HTTP at the Application Layer would pass its message packet with overhead to the Transport Layer (where TCP would complete packetization) and hand it off to the Network Layer. TCP is a connection-oriented protocol, which means that TCP sets up a virtual circuit between the sender and receiver. The network layer makes one routing decision when the connection is established, and all packets follow the same route. All packets in the same message arrive at the destination in the same order in which they were sent.
DNS uses UDP because each DNS’s object is small enough to go in one datagram (no need to reorder) and there is no need for acknowledgement. UDP is a connection-less protocol and each packet is treated separately and makes its own way through the network. It is possible that different packets will take different routes through network depending upon the type of routing used and the amount of traffic. But since each DNS object would fit into one datagram, we do not have to worry for reordering problem.

Front 274

How does static routing differ from dynamic
routing? When would you use static routing?
When would you use dynamic routing?

Back 274

With static routing, routing decisions are made in a fixed manner by individual computers or routers. The routing table is developed by the network manager, and it changes only when computers are added to or removed from the network.Static routing is commonly used in networks that have few routing options that seldom change.
With dynamic routing (or adaptive routing), routing decisions are made in a decentralized manner by individual computers. This approach is used when
there are multiple routes through a network, and it is important to select the best route. Dynamic routing attempts to improve network performance by routing messages over the fastest possible route, away from busy circuits and busy computers.

Front 275

What type of routing does a TCP/IP client
use? What type of routing does a TCP/IP
gateway use? Explain.

Back 275

The TCP/IP client uses static routing because the client must always point to a single gateway router.
The TCP/IP gateway router uses dynamic routing because typically it must process multiple requests for routing beyond the single segment that it physically supports.

Front 276

What is the transmission efficiency of a 10-byte Web request sent using HTTP, TCP/IP, and Ethernet? Assume the HTTP packet has 100 bytes in addition to the 10-byte URL. Hint: Remember from Chapter 4 that efficiency = user data/total transmission size.

Back 276

User data = 100 Bytes
Total Transmission size = 181 Bytes
Efficiency = User Data / Total Transmission size
= 100 / 181
= 0.5525
Therefore, the transmission efficiency is 55.25%

Front 277

What is the transmission efficiency of a 1,000-byte file sent in response to a Web request HTTP, TCP/IP, and Ethernet? Assume the HTTP packet has 100 bytes in addition to the 1,000-byte file. Hint: Remember from Chapter 4 that efficiency = user data/total transmission size.

Back 277

100 bytes + 1000 bytes = 1100 bytes to transmit. Packets must be under 1500 bytes, so it'll fit in one packet.

Ethernet Frame:
www.inetdaemon.com/tutorials/...me_format.html
Start Frame Delimiter = 1 byte
Destination MAC = 6 bytes
Source MAC = 6 bytes
Length = 2 bytes
FCS (trailer) = 4 bytes

TCP header:
http://www.freesoft.org/CIE/Course/Section4/8.htm
Source Port = 2 Bytes
Destination Port = 2 Bytes
Sequence Number = 4 Bytes
ACK Number = 4 Bytes
Data Offset = 4 BITS
Reserved = 6 BITS
Control Bits = 6 BITS (2 bytes total for this, and the two above)
Window = 2 Bytes
Checksum = 2 Bytes
Urgent = 2 Bytes
Option = Varies, for this, 1 byte
Padding = varies, to make sure the packet header is exactly 32 bytes

IP Header = 20 Bytes
http://www.erg.abdn.ac.uk/users/gorr...ip-packet.html

19 + 32 + 20 = 71 bytes transmission overhead + 1100 data = 1171 bytes transmitted.

1000 = User Data & 1171 = Total Transmission Size
1000/1171=.82

Front 278

Describe the anatomy of a router. How does
a router differ from a computer?

Back 278

The anatomy of a router is a computer processor, RAM and flash memory, and ethernet ports. a router involves itself with network protocols (IP, IPX, etc). A router can only deal with those protocols it was designed to understand.A router creates an internetwork, where members of each network can communicate with members of the other network(s). Routers are better suited to very large and/or complex topologies (like the Internet) where data may make several hops to reach its destination, because a higher level approach is taken to understand the topology of the internetwork. Inside the router is a computer. This computer is responsible for inputting data from the network interfaces, deciding what to do with it, and outputting it to the correct network interface. This job sounds simple, but there can be a lot of administrative overhead.The router has a microprocessor just like a personal computer (although it is not normally an Intel processor).A router has memory and ROM, and sometimes, flash memory and non-volatile memory.The flash memory stores the router's operating system. The operating system is normally the only software which exists on the router. The non-volatile memory stores the router's configuration. A computer has much more software and no means to connect to the internet. Routers enable computers to communicate and they can pass information between two networks—such as between your home network and the Internet.

Front 279

Define local area network.

Back 279

a computer network that links devices within a building or group of adjacent buildings.A local area network is a group of microcomputers or other workstation devices located within a small or confined area and are connected by a common cable. A LAN can be part of a larger backbone network connected to other LANs, a host mainframe, or public networks.

Front 280

What are the distinguishing features of a LAN?

Back 280

The distinguishing features of a LAN are:
a. the small confined area in which it operates
b. very high speed transmissions
c. they operate outside the government-regulated communication environment, because their circuits do not cross public thoroughfares (roads) and, therefore, do not require licensing or regulatory approvals to operate.

Front 281

What are two reasons for developing LANs?

Back 281

There are two basic reasons for developing a LAN: information sharing and resource sharing. Information sharing refers to having users who access the same data files, exchange information via electronic mail, or search the Internet for information. The main benefit of information sharing is improved decision making, which makes it generally more important than resource sharing. Resource sharing refers to one computer sharing a hardware device (e.g., printer) or software package with other computers on the network, in order to save costs.

Front 282

What is the function of LAN metering software?

Back 282

LAN metering software prohibits using more copies of a software package than there are installed licenses. Many software packages now sell LAN versions that do this automatically, and a number of third-party packages are also available.

Front 283

Discuss why it is important for organizations to enforce policies restricting use of employee-owned hardware and software and unauthorized copies of software.

Back 283

Organizations must have policies on employee-owned hardware and software and unauthorized software because:
a. employee-owned software may contain viruses
b. employee-owned hardware and software may encourage theft of the organization's assets by thoughtless, dishonest, or disgruntled employees
c. illegal copies of software may make the organization vulnerable to legal action
d. employees should not use the organization's assets for their own benefit.

Front 284

Describe at least three types of servers.

Back 284

A LAN can have many different types of dedicated servers. Four common types are file servers, database servers, print servers, and communication servers.
1. File servers allow many users to share the same set of files on a common, shared disk drive.
2. A database server usually is more powerful than a file server. It not only provides shared access to the files on the server, but also can perform database processing on those files associated with client-server computing.
3. Print servers handle print requests on the LAN. By offloading the management of printing from the main LAN file server or database server, print servers help reduce the load on them and increase network efficiency.
4. Communications servers are dedicated to performing communication processing. There are three fundamental types: fax servers, modem servers, and access servers.

Front 285

Describe the basic components of a wired LAN.

Back 285

The first two are the client computer and the server. The other components are network interface cards (NICs), network circuits, hubs/switches/access points, and the network operating system. Most LANs are built with unshielded twisted-pair (UTP) cable,
shielded twisted-pair (STP), or fiber-optic cable. (Common cable standards are discussed on the next page. We should add that these cable standards specify the minimum quality cable required; it is possible, for example, to use category 5 UTP wire
for a 10Base-T Ethernet.)

Front 286

Describe the basic components of a
wireless LAN.

Back 286

Any client computer, laptop or wireless device with a compatible wireless client adapter allows connectivity with an access point. The client adapter is a radio transmitter with firmware that supports any of 802.11a/b/g signaling. That is needed before the device can associate and authenticate with the access point. Some access points have a no client mode that doesn’t allow any association from clients extending network distance. Client wireless software included with the adapter must be implemented with specific Windows platforms. The client adapter will be integrated with the laptop, PCMCIA slot or desktop PCI bus slot. They will support specific wireless standards, antenna characteristics, WiFi certification, WDS, network range and security.

Front 287

What types of cables are commonly
used in wired LANs?

Back 287

Most LANs are built with unshielded twisted pair (UTP) wires, shielded twisted pair (STP), coaxial cable, or fiber optic cable (although fiber optic cable is far more commonly used in backbone networks). Many LANs use a combination of shielded and unshielded twisted pair. Although initially it appeared that twisted pair would not be able to meet long-term capacity and distance requirements, today this is one of the leading LAN cabling technologies. Its low cost and the availability of shielded wiring that can handle higher transmission make it very useful.

Front 288

Compare and contrast category 5 UTP, category 5e UTP, and category 5 STP.

Back 288

Twisted pair cables are widely used in transmitting information, especially across great distances. The twist in the wire cancels out any magnetic interference that may develop in the wiring. There are two common types of twisted pair cabling, STP and UTP. The S stands for Shielded, the U stands for Unshielded, and the TP stands for twisted pair for both. STP simply has additional shielding material that is used to cancel any external interference that may be introduced at any point in the path of the cable. UTP cables have no protection against such interference and its performance is often degraded in its presence.
Category 5 UTP 100 Mbps 100Base-T Ethernet Cost:.07
Category 5 STP 100 Mbps 100Base-T Ethernet Cost:.18
Category 5e4 UTP 1 Gbps 1000Base-T Ethernet Cost.10

Category 5e is an improved version of category 5 that has better insulation and a center plastic pipe inside
the cable to keep the individual wires in place and reduce noise from cross-talk, so that it is better suited
to 1000Base-T

Front 289

What is a cable plan and why would
you want one?

Back 289

A cable plan is a plan for the network layout, including how much cable is used, where the cables are, how many and where hubs are located, how many ports are available, what local city fire codes must be followed, and what are the identification labels of the cable. Most buildings under construction today have a separate LAN cable plan as they do or telephone cables and electrical cables. The same is true for older buildings in which new LAN cabling is being installed. It is common to install 20 to 50 percent more cable than you actually need to make future expansion simple. it is critical to plan for the effective installation and use of LAN cabling. The cheapest time to install network is during the construction of the building adding cable to an existing building can cost significantly more.

Front 290

What does a NOS do? What are the major
software parts of a NOS?

Back 290

The network operating system (NOS) is the software that controls the network. Every NOS provides two sets of software: one that runs on the network server(s), and one that runs on the network client(s). The server version of the NOS provides the software that performs the functions associated with the data link, network, and application layers and usually the computer's own operating system. The client version of the NOS provides the software that performs the functions associated with the data link and the network layers, and must interact with the application software and the computer's own operating system.

Front 291

How does wired Ethernet work?

Back 291

All computers are connected to one half-duplex circuit running the length of the network that is called the bus. The top part of Figure 6.4 shows Ethernet’s logical topology. All frames from any computer flow onto the central cable (or bus) and through it to all computers on the LAN. Every computer on the bus receives all frames sent on the bus, even those intended for other computers. Before processing incoming frames, the Ethernet software on each computer checks the data link layer address and processes only those frames addressed to that computer.

Front 292

How does a logical topology differ from
a physical topology?

Back 292

A logical topology is how the network works conceptually, much like a logical data flow diagram (DFD) or logical entity relation diagram (ERD) in systems analysis and design or database design. A physical topology is how the network is physically installed, much like a physical DFD or physical ERD.

Front 293

Briefly describe how CSMA/CD works.

Back 293

Ethernet uses a contention-based media access control technique called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD, like all contention-based techniques, is very simple in concept: wait until the circuit is free and then transmit. Computers wait until no other devices are transmitting, then transmit their frames. As long as no other computer attempts to transmit at the same time, everything is fine. However, it is possible that two computers located some distance from one another can both listen to the circuit, find it empty, and begin simultaneously. This simultaneous transmission is called a collision. The two frames collide and destroy each other. The solution to this is to listen while transmitting, better known as collision detection (CD). If the NIC detects any signal other than its own, it presumes that a collision has occurred and sends a jamming signal. All computers stop transmitting and wait for the circuit to become free before trying to retransmit. The problem is that the computers that caused the collision could attempt to retransmit at the same time. To prevent this, each
computer waits a random amount of time after the colliding frame disappears before attempting to retransmit. Chances are both computers will choose a different random amount of time and one will begin to transmit before the other, thus preventing a second collision. However, if another collision occurs, the computers wait a random amount of time before trying again. This does not eliminate collisions completely, but it reduces them to manageable proportions.

Front 294

Explain the terms 100Base-T, 100Base-F,
1000Base-T, 10 GbE, and 10/100/1000 Ethernet.

Back 294

The original ethernet specification was a 10 Mbps data rate using baseband signaling on thick coaxial cable, called 10Base5 (or "Thicknet"), capable of running 500 meters between hubs. Following 10Base5 was 10Base2 or thinnet as we used to say. Thinnet or RG-58 coaxial cable, similar to what is used for cable TV was considerably cheaper and easier to work with, although it was limited to 185 meters between hubs. The 10Base-2 standard was often called "Cheapnet."

When twisted pair cabling was standardized for supporting Ethernet (app. 1988) the T replaced the 2 to represent "twisted-pair". Twisted pair is the most commonly used cable type for Ethernet. 10BaseT breaks down as 10 Mbps, baseband, and the "T" means it uses twisted pair wiring (actually unshielded twisted pair). It was the 10Base-T standard that revolutionized Ethernet, and made it the most popular type of LAN in the world.

Eventually the 10BaseT standard was improved to support Fast Ethernet or 100BaseT that breaks down as 100Mbps baseband over twisted-pair cable. This eventually was improved even further to 1000BaseT or 1 Billion BITs per second baseband. There is currently a revised standard evolving which makes Ethernet even faster. It is known as the 10GbE or 10 Billion BITs per second Ethernet. Though proven to work it has yet to reach the marketplace. But it would be astute to consider that it will be here in the near future.
Finally, 10/100Mbps Ethernet refers to the standard that can autosense which speed it needs to run at between the two speeds of 10Mbos or 100Mbps. It comes down to the type of NIC running at the individual node and the type of switch port that the node connects into. It is commonplace to run 10/100Mbps switches in LAN operating environments where there are older NICs already operating and no real business case requirements for upgrading these nodes.

Front 295

How do Ethernet switches know where to
send the frames they receive? Describe how
switches gather and use this knowledge.

Back 295

When a switch receives a frame from a computer, it looks at the address on the frame and retransmits the frame only on the circuit connected to that computer, not to all circuits as a hub would. Therefore, no computer needs to wait because another computer
is transmitting; every computer can transmit at the same time, resulting in much faster performance. The
switch uses a forwarding table that lists the Ethernet address of the computer connected to each port on
the switch. When the switch receives a frame, it compares the destination address on the frame to the addresses in its forwarding table to find the port number on which it needs to transmit the frame.

Front 296

Compare and contrast cut-through, store and
forward, and fragment-free switching.

Back 296

There are three modes in which switches can operate. The first is cut-through switching. With cut-through switching, the switch begins to transmit the incoming
packet on the proper outgoing circuit as soon as it has read the destination address in the frame. In other words, the switch begins transmitting before it has received the entire frame. Advantage: Low Latency. Disadvantage: the switch begins transmitting before it has read and processed the frame check sequence
at the end of the frame; the frame may contain an error, but the switch will not notice until after almost all of the frame has been transmitted. Cut-through switching can only be used when the incoming data circuit has the same data rate as the outgoing circuit.
store and forward switching, the switch does not begin transmitting the outgoing frame until it has received the entire incoming frame and has checked to make sure it contains no errors. Only after the switch is sure there are no errors does the switch begin transmitting the frame on the outgoing circuit.
If errors are found, the switch simply discards the frame. This mode prevents invalid frame from consuming network capacity, but provides higher latency and thus results in a slower network (unless many frames contain errors). Store and forward switching can be used regardless of whether the incoming data circuit has the same data rate as
the outgoing circuit.
fragment-free switching, lies between the extremes of
cut-through switching and store and forward switching. With fragment-free switching, the first 64 bytes of the frame are read and stored. The switch examines the first 64 bytes (which contain all the header information for the frame) and if all the header data appears correct, the switch presumes that the rest of the frame is error free and begins transmitting. Fragment-free switching is a compromise between cut through and store and forward switching because it has higher latency and better error control than cut through switching,
but lower latency and worse error control than store and forward switching. Most switches today use cut through or fragment-free switching.

Front 297

Compare and contrast the two types of
antennas.

Back 297

the two types of antennas are Omnidirectional and Directional. Most WLANs are installed using APs that have omnidirectional antennas, which means that the antenna transmits in all directions simultaneously. The other type of antenna that can be used on APs is the directional antenna, which, as the name suggests, projects a signal only in one direction. Because the signal is concentrated in a narrower, focused area, the signal is stronger and therefore will carry farther than the signal from an AP using an omnidirectional antenna.Directional antennas are most often used on the inside of an exterior wall of a building, pointing to the inside of the building. This keeps the signal inside the building (to reduce security issues) and also has the benefit of increasing the range of the AP.

Front 298

How does Wi-Fi perform media access control?

Back 298

Media access control in Wi-Fi is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which is similar to the contention-based CSMA/CD approach used by Ethernet. With CSMA/CA, computers listen before they transmit and if no one else is transmitting, they proceed with transmission. Detecting collisions is more difficult in radio transmission than in transmission over wired networks, so Wi-Fi attempts to avoid collisions to a greater extent than traditional Ethernet. CSMA/CA has two media access control approaches.
The first media access control method is the distributed coordination function (DCF) (also called physical carrier sense method because it relies on the ability of computers to physically listen before they transmit). With DCF, each frame in CSMA/CA is sent using stop-and-wait ARQ. After the sender transmits one frame, it immediately stops and waits for an ACK from the receiver before attempting to send another frame. When the receiver of a frame detects the end of the frame in a transmission, it waits a fraction of a second to make sure the sender has really stopped transmitting, and then immediately transmits an ACK (or a NAK). The original sender can then send another frame, stop and wait for an ACK, and so on.
The second media access control technique is called
the point coordination function (PCF) (also called the virtual carrier sense method). Not all manufacturers have implemented PCF in their APs. With this approach, any computer wishing to transmit first sends a request to transmit (RTS) to the AP, which may or may not be heard by all computers. The RTS requests permission to transmit and to reserve the circuit for the sole use of the requesting computer for a specified time period. If no other computer is transmitting, the AP responds with a clear to transmit (CTS), specifying the amount of time for which the circuit is reserved for the requesting computer. All computers hear the CTS and remain silent for the specified time period.

Front 299

21. How does Wi-Fi differ from shared Ethernet
in terms of topology, media access control,
and error control, Ethernet frame?

Back 299

The logical and physical topologies of Wi-Fi are the same as those of shared Ethernet. Detecting collisions is more difficult in radio transmission than in transmission over wired networks, so Wi-Fi attempts to avoid collision to a greater extent than the traditional Ethernet.

Front 300

22. Explain how CSMA/CA DCF works.

Back 300

The first media access control method is the distributed coordination function (DCF) also called physical carrier sense method because it relies on the ability of computers to physically listen before they transmit. When a node wants to send a message, first listens to make sure that the transmitting node has finished, then waits a period of time longer. Each frame is sent using stop-and –wait ARQ, by waiting, the listening node can detect that the sending node has finished and can then begin sending its transmission. ACK/NAK sent a short time after a frame is received. Message frames are sent a somewhat longer time after (ensuring that no collision will occur)

Front 301

23. Explain how CSMA/CA PCF works.

Back 301

The second media access control technique is called the point coordination function (PCF) is also called virtual carrier sense method. When hidden node problem exists, the AP is the only device guaranteed to be able to communicate with all computers on the WLAN. Therefore, the AP must manage the shared circuit using controlled-access techniques, with this approach, any computer wishing to transmit first sends a request to transmit (RTS) to the AP, which may or may not be heard by all computers. The RTS requests permission to transmit and to reserve the circuit for the sole use of the requesting computer for a specified time period. If no other computer is transmitting, the AP responds with a clear to transmit (CTS), specifying the amount of time for which the circuit is reserved for the requesting computer. All computers hear the CTS and remain silent for the specified time period.

Front 302

Explain how association works in WLAN.

Back 302

Before a computer can transmit in a WLAN is must first establish an association with a specific AP, so that the AP will accept its transmissions. Searching for an available AP is called scanning and NIC can engage in either active or passive scanning.
During active scanning, a NIC transmits a special frame called probe frame on all active channels on its frequency range. When an AP receives a probe frame, it responds with a probe response that contains all the necessary information for a NIC to associate with it. A NIC can receive several probe responses from different APs. It is up to the NIC to choose with which AP to associate with. This usually depends on the speed rather than distance from an access point. Once a NIC associates with an access point they start exchanging packets over the channel that is specified by the access point.
During passive scanning, the NIC listens on all channels for a special frame called beacon frame that is sent out by an access point. The beacon frame contains all the necessary information for a NIC to associate with it. Once a NIC detects this beacon
frame it can decide to associate with it and start communication on the frequency channel set by the access point.

Front 303

What are the best practice recommendations for wired LAN design?

Back 303

Many organizations today install switched 100Base-T over category 5e wiring for their wired LANs. It is relatively low cost and fast. Another good alternative is 10/100/1000 over cat 5e wires. It is critical to plan for the effective installation and use of LAN cabling. The cheapest point at which to install network cable is during the construction of the building; adding cable to an existing building can cost significantly more. Most buildings under construction today have a separate LAN cable plan, Each floor has a telecommunications wiring closet that contains one or more network hubs or switches. Cables are run from
each room on the floor to this wiring closet.

Front 304

What are the best practice recommendations for WLAN design?

Back 304

Selecting the best practice wireless technology is usually simple. You pick the newest one, cost permitting. Today, 802.11n is the newest standard. Designing the physical WLAN is more challenging than designing a wired LAN because the potential for radio interference means that extra care must be taken in the placement of access points. The placement of the access points needs to consider both the placement of other access points as well as the sources of interference in the building.

Front 305

What is a site survey and why is it important?

Back 305

The site survey determines the feasibility of the desired coverage, the potential sources of interference, the current locations of the wired network into which the WLAN will connect, and an estimate of the number of APs required to provide coverage.

Front 306

How do you decide how many APs are needed and where they should be placed for best performance?

Back 306

WLANs work very well when there is a clear line of sight between the AP and the wireless computer. The more walls there are between the AP and the computer, the weaker the wireless signal becomes. The type
and thickness of the wall also has an impact; traditional drywall construction provides
less interference than does concrete block construction. Wi-Fi has a long range, but real-world
tests of Wi-Fi in typical office environments have shown that data rates slow down dramatically when the distance from a laptop to the AP exceeds 50 feet. Therefore, many wireless designers use a radius of 50 feet when planning traditional office environments,
which ensures access high-quality coverage. Most designers plan wireless LANs using 50- to 75-foot squares, depending on the construction of the building: smaller squares in areas where there are more walls that can cause more interference and larger squares in areas with fewer walls.

Front 307

What is a bottleneck and how can you locate one?

Back 307

A Bottleneck is the part of the network that is restricting the data flow. The bottleneck will lie in one of two places. The first is the network server. In this case, the client computers have no difficulty sending requests to the network server, but the server lacks sufficient capacity to process all the requests it receives in a timely manner. The second location is the network circuit, connecting the LAN to the corporate BN. In this case, the server can easily process all the client requests it receives, but the circuit lacks enough capacity to transmit all the requests to the server.
The first step in improving performance, therefore, is to identify whether the bottleneck lies in the circuit or the server. To do so, you simply watch the utilization of the server during periods of poor performance.
If the server utilization is high (e.g., 60 to 100 percent), then the bottleneck is the server; it cannot process all the requests it receives in a timely manner.
If the server utilization is low during periods of poor performance (e.g., 10 to 40 percent), then the problem lies with the network circuit; the circuit cannot transmit requests to the server as quickly as necessary.
Things become more difficult if utilization is in the midrange (e.g., 40 to 60 percent). This suggests that the bottleneck may shift between the server and the circuit depending on the type of request, and it suggests that both should be upgraded to provide the best performance.

Front 308

Describe three ways to improve network performance on the server.

Back 308

1. Software: The NOS is the primary software-based approach to improving network performance. Some NOSs are faster than others, so replacing the NOS with a faster one will improve performance.
2. Each NOS provides a number of software settings to fine-tune network performance. Depending on the number, size, and type of messages and requests in your LAN, different settings can have a significant effect on performance.
3. One obvious solution if your network server is overloaded is to buy a second server (or more). Each server is then dedicated to supporting one set of application software (e.g., one handles email, another handles the financial database, and another stores customer records).
More: Sometimes, however, most of the demand on the server is produced by one application that cannot be split across several servers. In this case, the server itself must be upgraded. The first place to start is with the server’s CPU. Faster CPUs mean better performance. If you are still using an old computer as a LAN server, this may be the answer; you probably need to upgrade to the latest and greatest. Clock speed also matters: the faster, The better. Most computers today also come with CPU-cache (a very fast memory module directly connected to the CPU). Increasing the cache will increase CPU performance.

Front 309

Many of the wired and wireless LANs share the same or similar components (e.g., error control). Why?

Back 309

The past few years have seen major changes in LAN technologies (e.g., gigabit Ethernet, high-speed wireless Ethernet). As technologies have changed and costs have dropped, so too has our understanding of the best practice design for LANs. One of the key questions facing network designers is the relationship between Wi-Fi and wired Ethernet. The data rates for Wi-Fi have increased substantially with the introduction of 802.11n, so they are similar to the data rates offered by 100Base-T wired Ethernet. The key difference is that 100Base-T wired Ethernet using switches provides 100 Mbps to each user, whereas Wi-Fi shares its available capacity among every user on the same AP, so as more users connect to the APs, the network gets slower and slower.

Front 310

As WLANs become more powerful, what are the implications for networks of the future? Will wired LANS still be common or will we eliminate wired offices?

Back 310

I believe the networks of the future will be hybrids and they will run the best of both worlds -wired and wireless.
I believe the efficiency and security of the wired network will always be necessary for certain computing functions, however the convenience of wireless is also a mitigating factor.