Impact of broadband standards

As an ISO layer 1 transport standard, intended for building the PTO's backbone transmission networks, SDH currently has no real rivals. The nearest thing to a competitor that has emerged so far is Asynchronous Transfer Mode (ATM), and its adoption was strongly favoured by several European PTOs. However, ATM, as with most of the other popular broadband standards comes under the heading of 'Fast Packet' technologies. As with all packet transfer techniques, it tends to be better at transporting bursts of traffic, rather than the largely constant traffic load presented by the PSTN. On top of this, ATM is a standard that is really aimed at the problem of rationalising the switching machines required for the complete range of telecom services, rather than producing a bedrock transport network. In theory, it is capable of doing this, but at present, commercial implementations of ATM switches are thought to be at least five years away. In the meantime, SDH equipment will be deployed to such an extent that it will be impossible to dislodge it from its position as the workhorse of all PTO transport networks. ATM switches, when they are introduced, will appear to the SDH network as yet one more client service which needs SDH transmission bandwidth allocated to it. As the operational problems of running a cell based service are progressively overcome, we can expect an increasingly proportion of the SDH transmission network capacity to be routed to these ATM switches.

Besides ATM, there are several other Fast Packet techniques that have been proposed, which are either already available, or promised soon. They are all aimed at satisfying the business communities requirements for wide area data transmission, and, in particular, the burgeoning requirement for LAN interconnect. In view of what was said in Section 42.1 about the needs of business customers driving PTOs to install SDH networks, it is essential to determine whether the introduction of these other techniques will render SDH super­fluous

Frame Relay

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.