42.7.2 Switched Multimegabit Data Service (smds)

Like Frame Relay, SMDS is another broadband wide area service aimed at achieving economies in transmission bandwidth by statis­tical multiplexing of bursty data traffic. Rather than being a standard in its own right, SMDS refers to a service which is delivered over a wide area network, which employs the DQDB standard for its access protocol. Originally it was targeted at rather higher hand-widths than Frame Relay i.e. l.5Mbit/s to 45Mbit/s, but now, it too is being repositioned to serve the LAN interconnect market. SMDS encapsulates the customer's data in trains of fixed sized cells, which are then relayed, via the SMDS switches to their destinations. Like Frame Relay, SMDS needs a high quality layer 1 service, in order that the number of cell corruptions is kept to a minimum. Once again, there is a technical sybiosis between SMDS and SDH, and they will only be in direct conflict if there are inconsistencies in some tariff structures.

42.7.3 Fibre Distributed Data Interface (fddi)

FDDI is a high speed LAN that was designed for private operators who were able to install their own cables. However, the extension of the original interface definitions to incorporate single mode, as well as multimode optical fibre has increased the maximum ring circumference to 100km. This potentially brings it into conflict with PTO provided MANs e.g. SMDS. This is even more likely now that there are proposed mappings of the full FDDI 125Mbit/s signal into a VC4, so that PTO transmission facilities can be used to bridge those spans where the private operator cannot run his own fibre. Unfortunately, there are some problems, relating to the maximum delay that an FDDI ring can withstand. As it is this minimum ring delay which limits the size of an FDDI ring, care must be taken in the routing of the VC4 in order that this extra delay does not significantly reduce the maximum ring size. In short, because of the self contained nature of the FDDI standard, together with its rather narrow targeting as a high-speed LAN, as opposed to MAN or WAN, we expect no competition at all between SDH and FDDI.

42.8 Future technologies

Equipment which implements the SDH standards will be strongly influenced by the capabilities and cost of the enabling technologies. This section reviews the impact on both equipment, and the stand­ards themselves, of some of these developments.

42.8.1 Integrated circuits

The unrelenting trend to faster, smaller and cheaper traffic handling ICs (ASICs), obviously leads to cheaper network element hardware. This, in turn, will eventually lead to the introduction of SDH equipment onto the premises of business customers. The trend to higher functional integration will probably lead to changes in PTO premises design because of increased heat dissipation in a given volume of rack space.

42.8.2 Optical interfaces

Very low cost SDH optical interfaces are expected to produce the long awaited shift in PTO station cabling from coaxial copper to optical fibres. There are several advantages to optical interconnec­tion e.g. relatively long range, no crosstalk, physically small calling volume. However, perhaps the biggest advantage is the future proof­ing which results from the fact that an optical interconnect cable can, within reasonable limits, carry any hit rate. Besides enabling the reuse of cables that would be difficult to reuse otherwise, it also reduces the problem of successive layers of interconnect cables physically preventing the withdrawal of the older, disused, cables that they are burying.