1ESS Switch - Switching Fabric

Switching Fabric

The voice switching fabric plan approximately followed that of the earlier 5XB switch in being bidirectional and in using the call-back principle. The largest full access matrix switches in the system, however, were 8x8 rather than 10x10 or 20x16. Thus they required eight stages rather than four to achieve large enough junctor groups in a large office. Crosspoints being more expensive in the new system but switches cheaper, system cost was minimized with fewer crosspoints organized into more switches. The fabric was divided into Line Networks and Trunk Networks of four stages, and partially folded to allow connecting line-to-line or trunk-to-trunk without exceeding eight stages of switching.

Although the following discussion is technically detailed, a brief review of the principles can aid in understanding. Suppose we have the need to connect 1000 input customers to 1000 output customers. A full connection would require a matrix of 1000x1000 or 1 million physical switches for full interconnection possibility. When one considers that a large telephone system can have many more than 1000 input customers and 1000 output customers, the hardware to establish a full interconnection can grow rapidly and exceed practical implementations. There is a reasonable compromise first theorized by Agner Krarup Erlang which is based upon the concept that not all calls need to be connected at the same time. From statistical theory, it is possible to design hardware that can connect "most of the calls" (in the sense of a high percentage) and block others as exceeding the design capacity. These are commonly referred to as "blocking switches" and are the most common in modern telephone exchanges. They are generally implemented as smaller switch fabrics in cascade. In many, a randomizer is used to select the start of a path through the multistage fabric so that the statistical properties predicted by the theory can be gained.