Teletraffic Engineering - in PSTN Architectures

In PSTN Architectures

The measurement of traffic in a public switched telephone network (PSTN) allows network operators to determine and maintain the quality of service (QoS) and in particular the grade of service (GoS) that they promise their subscribers. The performance of a network depends on whether all origin-destination pairs are receiving a satisfactory service.

Networks are handled as:

• loss systems where calls that cannot be handled are given equipment busy tone or
• queuing systems where calls that cannot be handled immediately are queued.

Congestion is defined as the situation when exchanges or circuit groups are inundated with calls and are unable to serve all the subscribers. Special attention must be given to ensure that such high loss situations do not arise. To help determine the probability of congestion occurring, operators should use the Erlang formulas or the Engset calculation.

Exchanges in the PSTN make use of trunking concepts to help minimize the cost of the equipment to the operator. Modern switches generally have full availability and do not make use of grading concepts.

Overflow systems make use of alternative routing circuit groups or paths to transfer excess traffic and thereby reduce the possibility of congestion.

A very important component in PSTNs is the SS7 network used to route signalling traffic. As a supporting network, it carries all the signalling messages necessary to set up, break down or provide extra services. The signalling enables the PSTN control the manner in which traffic is routed from one location to another..

Transmission and switching of calls is performed using the principle of time-division multiplexing (TDM). TDM allows multiple calls to be transmitted along the same physical path, reducing the cost of infrastructure.