Frame Relay

Date: 2015-10-07; view: 538.

The current front runner in the Fast Packet standards is Frame Relay. This is actually a network access protocolwhich, like X.25, says nothing about the interfaces between internal nodes in the network i.e. it is only a UNI standard, and has no counterpart to the SDH NNI. The best way to view Frame Relay is as a supercharged version of X.25, which is designed to handle the high speed (i.e. up to 2Mbit/s and beyond) but bursty, LAN interconnect traffic that X.25 is far too slow for. The reason that a Frame Relay network can deliver much higher throughputs than an X.25 network is that it uses a very 'lightweight' protocol, which removes the majority of the X.25 processing burden from the network access nodes and, in all probability, in the internal nodes as well. In particular Frame Relay provides no end to end error recovery, hut instead, leaves this function to the ISO transport layer protocols (layer 4), which are assumed to be installed in the customer's equipment, rather than the network access nodes. This makes a Frame Relay network heavily dependent on the quality, especially the error rate, of the underlying transport network, which is where SDH comes to the rescue. The high quality ISO layer 1 service provided by SDH is exactly what the lightweight layer 2 protocol of Frame Relay requires if it is to avoid excessive frame retransmissions from the CPE.

This symbiotic arrangement should, in theory, result in a peaceful coexistence, however, the issue is, as ever, clouded by that of tariffing. If a 2Mbit/s circuit, delivered by SDH, is tariffed suffi­ciently attractively, then, for many private networks, it may be cheaper to run multiple, point to point 2Mbit/s links, rather than obtain the same connectivity from a Frame Relay network. How­ever, experience to date with quasi Frame Relay networks in North America indicates that this will not be the case, and that Frame Relay and SDH will not compete directly, for business traffic.