as yet one more client service which needs SDH transmission that the number of cell corruptions is kept to a minimum. Once bandwidth allocated to it. As the operational problems of running a again, there is a technical sybiosis between SMDS and SDH, and cell based service are progressively overcome, we can expect an they will only be in direct conflict if there are inconsistencies in increasingly proportion of the SDH transmission network capacity some tariff structures. to be routed to these ATM switches. Besides ATM, there are several other Fast Packet techniques that 42.7.3 Fibre Distributed Data Interface (FDDI) have been proposed, which are either already available, or promised soon. They are all aimed at satisfying the business communities FDDI is a high speed LAN that was designed for private operators requirements for wide area data transmission, and, in particular, the who were able to install their own cables. However, the extension burgeoning requirement for LAN interconnect. In view of what was of the original interface definitions to incorporate single mode, as said in Section 42.1 about the needs of business customers driving well as multimode optical fibre has increased the maximum ring PTOs to install SDH networks, it is essential to determine whether circumference to 100km. This potentially brings it into conflict with the introduction of these other techniques will render SDH super- PTO provided MANs e.g. SMDS. This is even more likely now that fluous. there are proposed mappings of the full FDDI 25Mbit/s signal into a VC4, SO that PTO transmission facilities can be used to bridge 42.7.1 Frame Relay those spans where the private operator cannot run his own fibre. Unfortunately, there are some problems, relating to the maximum The current front runner in the Fast Packet standards is Frame Relay. delay that an FDDI ring can withstand. As it is this minimum ring This is actually a network access protocol which, like X.25, says delay which limits the size of an FDDI ring, care must be taken in nothi