Wan

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Circuit-switched

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A circuit-switched network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate.

PSTN and ISDN are two types of circuit-switching technology that may be used to implement a WAN in an enterprise setting.

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Packet-switched

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In contrast to circuit switching, packet switching splits traffic data into packets that are routed over a shared network. Packet-switching networks do not require a circuit to be established, and they allow many pairs of nodes to communicate over the same channel.

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Connectionless versus Connection-oriented

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Connectionless systems, such as the Internet, carry full addressing information in each packet. Each switch must evaluate the address to determine where to send the packet.

Connection-oriented systems predetermine the route for a packet, and each packet only has to carry an identifier. In the case of Frame Relay, these are called Data Link Connection Identifiers (DLCIs). The switch determines the onward route by looking up the identifier in tables held in memory. The set of entries in the tables identifies a particular route or circuit through the system. If this circuit is only physically in existence while a packet is traveling through it, it is called a virtual circuit (VC).

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Permanent Virtual Circuit

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(PVC)-A permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with establishing and terminating VCs, but they increase costs because of constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.

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Switched Virtual Circuit

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(SVC)-A VC that is dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases: circuit establishment, data transfer, and circuit termination.

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SVC is appropriate in which situations?

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SVCs are used in situations in which data transmission between devices is intermittent, largely to save costs. SVCs release the circuit when transmission is complete, which results in less expensive connection charges than those incurred by PVCs, which maintain constant virtual circuit availability.

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Examples of packet- or cell-switched connections

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Examples of packet- or cell-switched connections include:

X.25
Frame Relay
ATM

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Time-division multiplexing

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(TDM) gives each conversation a share of the connection in turn. TDM assures that a fixed capacity connection is made available to the subscriber.

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Private WAN connections

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Private WAN Connection Options

Private WAN connections include both dedicated and switched communication link options.

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Dedicated communication links
(also called leased lines)

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When permanent dedicated connections are required, point-to-point lines are used with various capacities that are limited only by the underlying physical facilities and the willingness of users to pay for these dedicated lines. A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are also called leased lines. (E1/T1)

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Switched communication links

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Switched communication links can be either
1) circuit switched: (PSTN) and ISDN.
<or>
2) packet switched: Frame Relay, ATM, X.25, and Metro Ethernet

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Public WAN Connection Options

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broadband services such as DSL, cable modem, and broadband wireless, and combined with VPN technology to provide privacy across the Internet.

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Requirements for leased lines

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A router serial port is required for each leased line connection. A CSU/DSU and the actual circuit from the service provider are also required.
Leased lines are generally priced based on the bandwidth required and the distance between the two connected points.

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Integrated Services Digital Network

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(ISDN) is a circuit-switching technology that enables the local loop of a PSTN to carry digital signals, resulting in higher capacity switched connections. ISDN changes the internal connections of the PSTN from carrying analog signals to time-division multiplexed (TDM) digital signals.

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Two types of ISDN interfaces

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1) Basic Rate Interface (BRI)-ISDN is intended for the home and small enterprise and provides two 64 kb/s B channels and a 16 kb/s D channel.

2) Primary Rate Interface (PRI)-ISDN is also available for larger installations. PRI delivers 23 B channels with 64 kb/s and one D channel with 64 kb/s in North America, for a total bit rate of up to 1.544 Mb/s. This includes some additional overhead for synchronization.

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X.25

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X.25 networks are now in dramatic decline being replaced by newer Layer 2 technologies such as Frame Relay, ATM, and ADSL. However, they are still in use in many portions of the developing world, where there is limited access to newer technologies.

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Difference between X.25 and Frame Relay

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Although the network layout appears similar to X.25, Frame Relay differs from X.25 in several ways. Most importantly, it is a much simpler protocol that works at the Data Link layer rather than the Network layer. Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter. Frame Relay offers data rates up to 4 Mb/s, with some providers offering even higher rates.

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ATM

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It is built on a cell-based architecture rather than on a frame-based architecture. ATM cells are always a fixed length of 53 bytes. The ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload. Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. Video and voice traffic do not have to wait for a larger data packet to be transmitted.

The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25. Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload. When the cell is carrying segmented Network layer packets, the overhead is higher because the ATM switch must be able to reassemble the packets at the destination. A typical ATM line needs almost 20 percent greater bandwidth than Frame Relay to carry the same volume of Network layer data.

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DSL

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DSL technology is an always-on connection technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, and provides IP services to subscribers. A DSL modem converts an Ethernet signal from the user device to a DSL signal, which is transmitted to the central office.

Multiple DSL subscriber lines are multiplexed into a single, high-capacity link using a DSL access multiplexer (DSLAM) at the provider location. DSLAMs incorporate TDM technology to aggregate many subscriber lines into a single medium, generally a T3 (DS3) connection. Current DSL technologies use sophisticated coding and modulation techniques to achieve data rates of up to 8.192 Mb/s.

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Cable Modem

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Network access is available from some cable television networks. Coaxial cable is widely used in urban areas to distribute television signals. Cable modems provide an always-on connection and a simple installation. A subscriber connects a computer or LAN router to the cable modem, which translates the digital signals into the broadband frequencies used for transmitting on a cable television network. The local cable TV office, which is called the cable headend, contains the computer system and databases needed to provide Internet access. The most important component located at the headend is the cable modem termination system (CMTS), which sends and receives digital cable modem signals on a cable network and is necessary for providing Internet services to cable subscribers.

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WiMAX

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Worldwide Interoperability for Microwave Access (WiMAX) is a new technology that is just beginning to come into use. It is described in the IEEE standard 802.16. WiMAX provides high-speed broadband service with wireless access and provides broad coverage like a cell phone network rather than through small WiFi hotspots. WiMAX operates in a similar way to WiFi, but at higher speeds, over greater distances, and for a greater number of users. It uses a network of WiMAX towers that are similar to cell phone towers.

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VPN

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A VPN is an encrypted connection between private networks over a public network such as the Internet. Instead of using a dedicated Layer 2 connection such as a leased line, a VPN uses virtual connections called VPN tunnels, which are routed through the Internet from the private network of the company to the remote site or employee host.