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As with ISDN, communicating via xDSL requires an interface to the PC. All xDSL configurations require a modem, called an endpoint, and a NIC. Often the modem and NIC are on a single expansion card.

Frame Relay Technology

Frame relay is a WAN technology in which variable-length packets are transmitted by switching. Packet switching involves breaking messages into

chunks at the sending router. Each packet can be sent over any number of routes on its way to its destination. The packets are then reassembled in the correct order at the receiver. Because the exact path is unknown, a cloud is used when creating a diagram to illustrate how data travels throughout the service. Figure 7.2 shows a frame relay WAN connecting smaller LANs.

FIGURE 7.2 A typical frame relay configuration


Frame Relay cloud

IBM compatible    Server    IBM compatible    Server

Frame relay uses permanent virtual circuits (PVCs). PVCs allow virtual data communications circuits between sender and receiver over a packet-switched network. This ensures that all data that enters a frame relay cloud at one side comes out at the other over a similar connection.

The beauty of using a shared network is that sometimes you can get much better throughput than you are paying for. When signing up for one of these connections, you specify and pay for a Committed Information Rate (CIR) or, in other words, a minimum bandwidth. If the total traffic on the shared network is light, you may get much faster throughput without paying for it. Frame relay begins at this CIR speed and can reach as much as 1.544Mbps, the equivalent of a T1 line, which we’ll discuss next.

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