PSTN : Switching Systems

Switching Systems
Switching systems are assemblies of equipment that setup, maintain, and disconnect connections between multiple communication lines. Switching systems are often classified by the type of network they are part of (e.g., packet or circuit switched) and the methods that are used to control the switches. The term “switch” is sometimes used as a short name for switching system. Public telephone switching systems have many switches within their network. A typical switch can handle up to 10,000 communication lines each.

Early switches used mechanical levers (crossbars) to interconnect lines. Modern switches use computer systems to dynamically setup, maintain, and disconnect communication paths through one or more switches. True computer-based switching came about through the introduction of the electronic switching systems (ESS’s). ESS EOs did not require a physical connection between incoming and outgoing circuits. Paths between the circuits consisted of temporary memory locations that allowed for the temporary storage of traffic. For an ESS system, a computer controls the assignment, storage, and retrieval of memory locations so that a portion of an incoming line (time slot) could be stored in temporary memory and retrieved for insertion to an outgoing line. This is called a time slot interchange (TSI) memory matrix. The switch control system maps specific time slots on an incoming communication line (e.g., DS3) to specific time slots on an outgoing communication line.

The public telephone network switching system architecture uses a distributed switching system that has a hierarchy of switching levels. Distributed switching systems connect calls through the nearest switching system. With distributed network architecture, the call processing requirements are distributed to multiple points. Using a multilevel hierarchy structure for switching systems allows switching to occur at lower levels of switching unless the telephone call must pass between multiple switches. At that point, the call is passed up to a higher-level switch for transfer to more distant locations.

In conjunction with distributed network architecture, the ability to perform “dynamic routing” furthers the network’s resiliency to faults. Sometimes called “adaptive routing”, dynamic routing automatically re-routes communication paths or circuits as the network traffic levels (e.g., levels of congestion) change or as paths go in or out of service.

A key part of public telephone networks is system reliability. As a result, in the event of equipment failure in such a network, backup (redundant) equipment must provide for continued service. Although this increases the reliability of switching systems, it also increases the system cost (for additional backup equipment) and complexity (recovery management systems).

Public telephone switching systems use EO telephone switches to connect the telephone network to end customers. These switches serve as an end node switch that and provides local dial and access to local and long distance services. Switches that are used to interconnect switches to each other are called tandem switches.

Some systems use mini-switches called remote digital terminals that are located near the EO switch. These mini-switches act as concentrator lines of voice channels between the end customers and the EO switching system. Concentrators grouping multiple communication lines into more efficient trunked (multi-channel) lines.