Switching
Roger L. Freeman · 2002
Abstract This chapter deals with the switching function of the PSTN. It is the general policy of telephone companies/administrations and the ITU not to dictate switch design to manufacturers. The ITU and Bellcore (now Telcordia) are, however, concerned with the digital network and its performance. A switch is an integral part of this network. For example, a switch provides master clock to outgoing trunks connected to it. A switch can degrade error performance. It adds to the total jitter and wander accumulation across the network. Queueing and processing delays can add to total end‐to‐end delay. Careful switch design can keep these impairments within reasonable bounds. Many of these reasonable bounds are established in the ITU Q.500 recommendations and by Bellcore. Switch interfacing is very important to the network designer. A local serving switch usually encounters analog subscriber loops (lines) on one side and digital trunks (junctions) on the other. Concentrators may or may not be used on the line side. This switch compresses and expands. In other words there will be more lines than trunks, with a ratio as high as 12 to 1 or as low as 1.5 to 1 depending on whether the traffic sources are predominantly residential or commercial/industrial. A switch will be designed for a grade‐of‐service 0.01, often to 0.005 or better. We would expect to encounter CCITT Signaling System No. 7 on the trunk side, and one of several types of subscriber loop signaling on the line side. The line side may utilize a DSL (digital subscriber line) which would appear as a multiplex configuration, often related to DS1 or E1. If a switch interfaces with ISDN basic rate, digital subscriber signaling will probably be used. There is a North American version and a CCITT version. Tandem and transit exchanges do not provide compression and expansion. The number of trunk inlets will equal the number of trunk outlets. Tandem switches may provide a CAMA (centralized automatic message accounting) function. Modern tandem/transit exchanges use CCITT Signaling System No. 7. Digital switch design is almost universally such that with small changes in peripheral equipment, the switch will serve either in an E1 or DS1 environment. The switch will consist of a switching fabric, peripheral equipment, and a processor/computer based control function. The switching fabric will often consist of time and space switching stages.