ng about the interfaces between internal nodes in the network the routing of the VC4 in order that this extra delay does not i.e. it is only a UNI standard, and has no counterpart to the SDH significantly reduce the maximum ring size. In short, because of the NNI. The best way to view Frame Relay is as a supercharged self contained nature of the FDDI standard, together with its rather version of X.25, which is designed to handle the high speed (i.e. up narrow targeting as a high-speed LAN, as opposed to MAN or to 2Mbit/s and beyond) but bursty, LAN interconnect traffic that WAN, we expect no competition at all between SDH and FDDI. 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 Future technologies probability, in the internal nodes as well. In particular Frame Relay provides no end to end error recovery, but instead, leaves this Equipment which implements the SDH standards will be strongly function to the ISO transport layer protocols (layer 4), which are influenced by the capabilities and cost of the enabling technologies. assumed to be installed in the customer's equipment, rather than the This section reviews the impact on both equipment, and the stand- network access nodes. This makes a Frame Relay network heavily ards themselves, of some of these developments. dependent on the quality, especially the error rate, of the underlying transport network, which is where SDH comes to the rescue. The 42.8.1 Integrated circuits 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 The unrelenting trend to faster, smaller and cheaper traffic handling avoid excessive frame retransmissions from the CPE. ICs (ASICs), obviously leads to cheaper network element hardware. This symbiotic arrangement should, in theory, result in a p