A major component of a switching system or an exchange is the set of input and output circuits called inlets and outlets,respectively.The primary function of a switching system is to establish an electrical path between a given inlet-outlet pair. The hardware used for establishing such a connection is called the switching matrix or the switching network. It is important to note that the switching network is a component of the switching system and should not be confused with telecommunication network. The switching network is called a symmetric network.
The inlets/outlets may be connected to the subscriber lines. In this case ,the output lines are folded back to the input and hence the network is called a folded network. Four types of connections may be established:
1. Local call connection between two subscribers in the system.
2. Outgoing call connection between a subscriber and an outgoing trunk.
3. Incoming call connection between an incoming trunk and a local subscriber
4. Transit call connection between an incoming trunk and an outgoing trunk
In a folded network with N subscribers, there can be a maximum of N/2 simultaneous calls or information interchanges. The switching paths, in which case the network is said to be nonblocking.In a no blocking network, as long as a called subscriber is free, a calling subscriber will always be able to establish a connection to the called subscriber. In other words, a subscriber will not be denied a connection for want of switching resources.But,in general, it rarely happens that all the possible conversations take place simultaneously. It may, hence be economical to design a switching network that has as many expected. In this case, it may occasionally happen that when a subscriber requests a connection, there are no switching paths free in the network, and hence he is denied connection. In such an event, the subscriber is said to be blocked, and the switching network is called a blocking network. In a blocking network, the number for simultaneous switching paths is less that the maximum number of simultaneous conversations that can take place. The probability that a user may get blocked is called blocking probability.
All switching exchange are designed to meet an estimated maximum average simultaneous traffic, usually known as busy hour traffic. Past records of the telephone traffic indicate that even in a busy exchange, not more than 20-30 percent of the subscribers are active at the same time.Hence; switching systems are designed such that all the resources in a systems are treated as common resources and the required resources are allocated to a conversation as long as it lasts. The quantum of common resources is determined based on the estimated busy hour traffic. When the traffic exceeds the limit to which the switching system is designed, a subscriber experiences blocking. A good design generally ensures a low blocking probability.
The traffic in a switching network is measured by an internationally accepted unit of traffic intensity known as earlang (E), named after an illustrious early contributor to traffic theory. A switching resource is said to carry one erlang of traffic if it is continuously occupied throughout a given period of observation.
In a switching exchange, all the inlet/outlet connections may be used for interexchange transmission. In such a case, the exchange does not support local subscribers and is called a transit exchange. A switching network of this kind is called a no folded network. In a nonfolded network with N inlets and N outlets, N simultaneous information transfers are possible.Consequently, for a nonfolded network to be nonblocking,the network should support N simultaneous switching paths.
While the switching network provides the switching paths, it is the control subsystem of the switching system that actually establishes the path. The switching network does not distinguish between inlets/outlets that are connected to the subscribers or to the trunks. It is the job of the control subsystems to distinguish between these lines and interpret correctly the signaling information received on these lines. It senses the end of information transfer and releases connections. A connection is established, based on the signaling information received on the inlet lines. The control subsystem sends out signaling information to the subscriber and other exchanges connected to the outgoing trunks. In addition, signaling also involved between different subsystems within an exchange. The signaling formats and requirement for the subscriber, the trunks and the subsystems differ significantly.
A switching system is composed of elements that perform switching, control and signaling functions. The subscriber lines are terminated at the subscriber line interface circuits, and trunks at the trunk interface circuits. There are some service lines used for maintenance and testing purposes.Junctor circuits imply a folded connection for the local subscribers and the service circuits. It is possible that some switching systems provide an internal mechanism for local connections without using the junctor cirtuits.Line scanning units sense and obtain signaling information from the respective lines. Distributor units send out signaling information on the respective lines. Operator console permits interaction with the switching system for maintenance and administrative purposes. In some switching systems, the control subsystem may be an integral part of the switching network itself. Such systems are known as direct control switching systems. Those systems in which the control subsystem is outside the switching network are known as common control switching systems.Strowger exchanges are usually direct control systems,wheras crossbar and electronic exchanges are common control systems. All stored program control systems are common control systems. Common control is also known as indirect control or register control.
With the advent of automatic switching systems, the manual exchanges have almost gone out of use. Today, operator assistance is required on a routine basis, only to connect the incoming calls at a private automatic branch exchange (PABX) to the required extension numbers. Even this requirement will cease to exist with the large scale introduction of what is known as direct inward dialing.
When number of subscribers increases, multiple switch boards and operators are required to handle the traffic. In this case the switch boards may be of two types:
1. Single termination switching boards
2. Multitermination switchboards
The terms nonmultiple and multiple are sometimes used to denote single termination and multitermination schemes respectively.
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