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Configuring Frame Relay Services

2-20

117376-C Rev. 00

Traffic Control

Frame relay is unreliable. It does not include error recovery mechanisms, relying

instead on the upper layer protocols to detect and retransmit lost frames.

Hub and spoke topologies often present a “big pipe/little pipe” situation. The

central or hub site has a faster link to the frame relay service because it is a point

of concentration for many remote sites: it has a “big pipe,” with, for example, a

T1/E1 connection that can transmit data at rates of 1.54 Mb/s. The remote sites

usually support a much lower line speed, 56 Kb/s to 64 Kb/s, or “little pipe.”

Figure 2-8

illustrates this concept.

Figure 2-8. Big Pipe/Little Pipe Topology

The central site router sends traffic at its available bandwidth, in this example at

T1 rates (1.536 Mb/s in the United States and Canada, 2.048Mb/s elsewhere).

Some switches recognize that the remote site is configured at a lower speed, and

begin to drop frames above the capacity of the remote site router. At the remote

site, the frame relay interface discards frames beyond its available buffer. The

assumption is that the user application detects the lost frames and either

retransmits them or uses a flow control mechanism in the protocol, such as

windowing, to throttle, which means queue, the traffic. But not all applications

have a robust mechanism to deal with lost or out-of-sequence frames.

You can use Bay Networks WAN Compression Protocol (WCP), protocol

prioritization, congestion control, and traffic shaping either singly or in

combination to help control the flow of traffic and avoid loss of data.

FR0013A

64 Kb/s

64 Kb/s C

1.536/2.048Mb/s

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Avaya Configuring Frame Relay Services Manual de usuario Pagina 50