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55 Cards in this Set
- Front
- Back
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Which of the following best describes the main function of OSI layer 1 protocols?
a. Framing b. Delivery of bits from one device to another c. Addressing d. Local Management Interface (LMI) e. DLCI |
b. Delivery of bits from one device to another
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Which of the following typically connects to a four-wire line provided by a telco?
a. Router serial interface b. CSU/DSU c. Transceiver d. Switch serial interface |
b. CSU/DSU
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Which of the following typically connects to a V.35 or RS-232 end of a cable when cabling a leased line?
a. Router serial interface b. CSU/DSU c. Transceiver d. Switch serial interface |
b. CSU/DSU
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On a point to point WAN link using a leased line between two routers located hundreds of miles apart, what devices are considered to be the DTE devices?
a. Routers b. CSU/DSU c. The central office equipment d. A chip on the processor of each router e. None of these answers are correct. |
a. Routers
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Which of the following functions of OSI Layer 2 is specified by the protocol standard for PPP, but is implemented with a Cisco proprietary header field for HDLC?
a. Framing b. Arbitration c. Addressing d. Error detection e. Identifying the type of protocol that is inside the frame |
e. Identifying the type of protocol that is inside the frame.
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Router1 has three point to point serial links, one link each to three remote routers. which of the following is true about the required HDLC addressing at Router1?
a. Router 1 must use HDLC addresses 1,2, and 3 b. Router 1 must use any three unique addresses between 1 and 1023 c. Router1 must use any three unique addresses between 16 and 1000 d. Router1 must use three sequential unique addresses between 1 and 1023 e. None of these answers are correct |
E. Although HDLC has an address field, its value is immaterial on a point to point link, as there is only one intended recipient, the device on the other end of the circuit.
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What is the name of the Frame Relay field used to identify Frame Relay virtual circuits?
a. Data-link connection identifier b. Data-link circuit identifier c. Data-link connection indicator d. Data-link circuit indicator e. None of these answers are correct. |
A. Data-link circuit identifier
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Which of the following is true about Frame Relay virtual circuits (VCs)?
a. Each VC requires a separate access link b. Multiple VCs can share the same access link. c. All VCs sharing the same access link must connect to the same router on the other side of the VC d. All VCs on the same access link must use the same DLCI |
B one of the main advantages of Frame Relay is that a router can use a single access link to support multiple VCs, with each VC allowing the router to send data to a different remote router. To identify each VC, the router must use a different DLCI, because the DLCI identifies the VC.
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How are long links created
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The cabling is owned and managed by a company that has the right to run cables under streets. Companies used these leased line.
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What is a telco?
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It is the local telephone company. It is a government controlled monopoly. They are also refereed as (PTT)public telephone and telegraph companies, and also more commonly called service providers.
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How long can an individual trunk run for?
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Ethernet does not support trunks to run for 1000 miles. Even if you had a trunk that ran 1000 miles you are not allowed to bury the cable over 1000 miles.
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The difference between LANs and WANs?
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LANs tend to reside in a single building and among buildings in a campus using optical cabling approved for Ethernet. WAN connections typically run longer distances across town or between cities.
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How does a Teleco work?
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WAN leased lines acts like the telco gave you two twisted pairs of wires between the two sites on each end of the line.
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What do routers connect to (point to point leased line)?
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Routers, typically connect to an external channel service unit/data service unit. (CSU/DSU). The router connects to the CSU/DSU with a short cable (<50ft).
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What does the channel service unit/data service unit connect to (CSU/DSU)?
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It is connected to a line from the building to the street. It is usually a four wire cable the telco plugs into your CSU/DSU. (Point to point leased line)
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What is on the other side of the CSU/DSU? (point to point leased Line)
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Usually the central office (CO) and it connects to a device called a WAN switch.
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Who owns the CSU/DSU?
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The telco customer
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Who owns the gear inside the Central office and wiring coming to the CO?
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The telco
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What is a Demarc?
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A demarc is short for demarcation point, it refers to the point at which the telco's responsibility is on one side and the customer's on the other side. It is the concept of responsibility of the telco and customer.
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In the US where is the Demarc loacated?
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The demarc is typically located where the telco physically terminates the set of two twisted pairs inside the customer building. The customer will designate a room and the lines from the telco.
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What is CPE?
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Customer premises equipment refers to devices that are at the customer site from the telco perspective. For instance, both CSU/DSU and the router are CPE devices.
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Can the telco own other equipment?
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Yes, They can actually own the router, wires, CSU/DSU. They can demarc at different parts of the line. CPE still refers to the equipment at the telco customer's location.
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What supports synchronous communication?
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For point to point serial links or frame relays links in this chapter, the router uses a synchronous communication.
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What type of connector is used between a router and CSU/DSU?
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A 60 pin D shell connector Connects the router to the CSU/DSU.
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What is the typical connector interface between the CSU/DSU and the telco?
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It Is the Rj-48 connector that has the same size and shape as the RJ 45 connector used for Ethernet cables.
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The interface between the CSU/DSU and the telco can be internal to the router?
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Yes, It does not need a cable connecting it to an external CSU/DSU. This case the serial cables are not needed. The RJ-48 port in the router serial interface card.
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The network contacts the telco and orders a circuit. What things are needed to be defined by the network engineer?
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The engineer must define the speed in kilobits per second. The engineer burchases two CSU/DSUs and installs one at each site and connects cables from each router to respective CSU/DSU cables.
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An order service is ordered and the service provider runs at a predefined speed that the engineer ordered. What does the Engineer must do?
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The engineer must Configure the CSU/DSU on each end of the link to match the defined speed by the service provider.
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What is synchronization?
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It is to synchronize their clocks so that they run at exactly the same speed a process called synchronization.
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How do synchronous circuits work?
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They impose time ordering at the link's sending and receiving ends. Essentially, all devices agree to try to run at the exact same speed.
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How does synchronization work with a leased line?
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It occurs by having one CSU/DSU (slave) adjust its clock to match the clock rate of the other CSU/DSU (master). Networking devices synchronize their clocks several times per second.
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What is the clocking hierarchy for a leased line?
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The clocking hierarchy for a leased line is Telco> CSU/DSU > Router.
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What is the device that provides clocking called and what is the device receiving called?
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The CSU/DSU is considered to be the data communications equipment DCE. The device receiving is typically a router, the router is referred to as data terminal equipment. DTE.
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DS0 speed
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bit rate is 64kbps
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DS1
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or T1 is 1.544Mbps (24 DSOs, plus 8 kbs overhead)
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DS3
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T3 44.736 Mbps (28 DS1s, Plus management overhead)
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E1
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2.048 Mbps (32 DS0s)
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E3
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34.368 Mbps (16 E1s, plus management overhead)
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J1
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Y1 2.048 Mbps (32DS0; Japanese standard)
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What are the two most popular protocols used on point-to-point links?
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High-level Data Link Control (HDLC) and Point-to-Point Protocol (PPP)
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What are some attributes of High-Level Data Link Control (HDLC)?
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HDLC needs to determine if the data passed the link without errors. It discards the frame if errors occurred. It needs to identify the type of packet inside the HDLC frame so receiving device knows the packet type.
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What does a proprietary HDLC frame contain?
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What is the difference between standard HDLC and Proprietary HDLC?
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Standard HDLC contains 1 byte in the flag, address control. 4 bytes in the FCS and variable for the data.
A proprietary HDLC may contain 1 byte in the Flag, Address, Control. 2bytes in the type. 4 bytes in the FCS and variable in the data portion. The main difference is that there is a type portion in the frame and reserves 2 bytes. |
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Does HDLC preform error detection?
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Yes, It uses an Frame check sequence in the HDLC trailer. (FCS) If it has errors in it it discards the frame.
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Why was PPP created?
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The Internet Engineering Task Force saw the need for another data link layer protocol between routers over a point-to point link. Thus PPP was born.
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What is the differences and similarities between HDLC and PPP?
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The framing looks identical to proprietary HDLC framing. PPP discards errored frames that do not pass the FCS. PPP also uses a 2 byte type field and is standard for PPP. Any vendor can conform to the PPP standard and communicate with other vender products. When connecting to a different vendor router though point to point serial link PPP is the data link protocol of choice.
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Synchronous
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A device tries to use the same speed as another device on the other end of a serial link. The device can notice slight variations in the speed on each end and adjust its speed accordingly.
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Clock source
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The device to which the other devices on the link adjust their speed when using synchronous links
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CSU/DSU
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Channel service unit/data service unit. Used on digital links as an interface to the telephone company. Routers use a short cable from a serial interfaces to a CSU/DSU, Which is attached to the telco with similar configuration at the other router on the end of the link.
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Telco
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Telephone company
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Four-wire circuit
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A line from the telco with four wires, composed of two twisted-pair wires. Each pair is used to send in one direction. A four-wire circuit allows full-duplex communication.
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T1
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A line from the telco that allows transmission of data at 1.544 Mbps.
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E1
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Similar to T1, but used in Europe. It uses a rate of 2.048 Mbps 32 64-kbps channels.
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What is a packet switching service?
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A packet-switching service, is similar to a leased line. A company can connect a large number of routers to the packet-switching service. Using a single serial link from each router into the packet-switching service. When connected it acts like a hub or switch that sends data directly to each other.
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What are the two packet switching service that are popular today?
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Frame Relay and Asynchronous Transfer Mode (ATM). With frame relay being more common.
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