Fibre Distributed Data Interface (FDDI)

Date: 2015-10-07; view: 460.

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.

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.