TSSN - Elements of a Switching System


In this chapter, we will discuss the elements of a switching system. Though there are different kinds of switching systems from manual to automatic, a few basic elements play an essential role for the functioning of a switching system. Along with the switching network, there are different sub systems such as control sub system, signaling system, trunk and subscriber line interfaces, distributor units, operator console, juncture circuits, essential for the operation of the whole switching system.

Switching System

In this section, we will understand the structure of the switching system. We will also understand how the different elements work in it. The block diagram of the switching system given below show the essential elements of a switching system.


The diagram shown above contains different blocks of the switching system. The blocks are discussed below.

Switching Network

It provides the switching paths between the called subscribers and the calling subscribers.

Control Subsystem

This is the critical part of the switching system, which actively establishes the switching paths, by identifying the inlet and outlet lines and interpreting the signaling information received on these lines.

This control subsystem, controls the making and breaking of the connection by sensing the signal transfer on the lines. The control sub system sends out signaling information to the subscriber and other exchanges connected to the outgoing trunks.


The signaling formats and requirements for the subscriber, the trunks and the sub systems differ significantly. Accordingly, a switching system provides for three different forms of signaling −

  • Subscriber loop signaling
  • Interexchange signaling
  • Intraexchange or register signaling

A switching system is composed of elements that perform switching, control and signaling functions.

Trunk Interface

The Trunk lines used for connections between the switching systems, are terminated at this port. The Trunk interface is the point where the trunk lines are connected to the system.

Subscriber Line Interface

The Subscriber lines used for connections between the subscribers and the switching systems are terminated at this port. The subscriber line interface is the point where the lines from the subscribers are connected to the system.

Line Scanning Unit

The line scanning unit senses and obtains the signaling information from the respective lines. The information obtained from these lines are given to the control sub system to identify the inlets and outlets.

Distributor Units

The distributor units are used for distributing or sending out the signaling information on the respective lines. The distribution of information through the trunk lines, is done through the distribution units.

Operator Console

The operator console permits interaction with the switching system for maintenance and administrative purposes.

Service Circuit Interface

The service circuit interface provides interaction between circuits for maintenance and testing purposes.


The Junctures is a junction that provides a folded connection for the local subscribers and the service circuits. If the called subscriber and the calling subscriber both are local, then the folded connection helps in making the connection to a local call, whereas the trunk lines will not be in use.

Direct and Indirect

The switching systems are of the following two types −

  • the direct control switching system
  • the indirect control switching system

Direct Control Switching System

The Switching systems where the control sub systems form an integral part of the network are called the Direct Control Switching systems. For example, the Strowger switching system.

Indirect Control Switching System

The Switching system in which the control sub system is present outside the switching network is called the Indirect Control Switching system or the Common Control Switching system or the Register Control switching system. The examples of this system include Crossbar switching system, Electronic switching system or Stored Program Control method of switching systems.