Forms of Signalling

(i) Supervisory signals or line signals. These are the signals necessary to initiate a call setup and to supervise it, once it has been established. It is also referred as subscriber loop signalling. Line signals can be transmitted by the use of a single control channel in each direction line.

Forms of Signalling

(i) Supervisory signals or line signals. These are the signals necessary to initiate a call setup and to supervise it, once it has been established. It is also referred as subscriber loop signalling. Line signals can be transmitted by the use of a single control channel in each direction line.

(ii) Routing signals or register signals. Information transfer related to call setup is usually referred to as register signals. The basic information is the dialed code which indicates to the subsequent switching centres the required routing. In addition to the basic information, signals such as route  information, terminal information, register control signals, acknowledgement signals, status of called terminal etc are also involved.

(iii) Management signals or interregister signalling. These signals are used to convey information or control between exchanges. This signalling also referred as inter exchange signalling. This signalling involves remote switching of private circuits, routing plans, modification of routing plans, traffic over load, priority of the call, class of service etc. The signalling may be performed by link-by-link basis, which passes signals exchange to exchange or end-to-end signalling which is between originating and terminating exchange referred as line signalling.

In PCM systems, signalling and speech are sampled, coded and transmitted within the frame of PCM channels. Thus, with PCM, a convenient way of transmission is possible. The signalling information and speech information carried in the same time slot is referred as inslot signalling. The signalling information carried in a separate time slot is referred as outslot signalling