Stored program control: Difference between revisions

Revision as of 19:45, 12 July 2015

Revision as of 19:59, 19 January 2016

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A telephone exchange must run continuously without interruption at all times, by implementing a [[fault-tolerant design]]. Early trials of electronics and computers in the control sub systems of an exchange were successful and resulted in the development of fully electronic systems, in which the switching network was also electronic. A trial system with stored program control was installed in [[Morris, Illinois]] in 1960. It used a [[flying-spot store]] with a word size of 18 bits for semi-permanent program and parameter storage, and a barrier-grid memory for random access working memory.<ref>A. E. Joel, ''An Experimental Electronic Switching System'', Bell Laboratory Record, October 1958 p.359</ref> The world’s first electronic switching system for permanent production use, the [[No.1 ESS]], was commissioned by AT&T at [[Succasunna, New Jersey]], in May 1965. By 1974, AT&T had installed 475 No. 1ESS systems. In the 1980s SPC displaced electromechanical switching in the telecommunication industry, hence the term lost all but historical interest. Today SPC is a standard feature in all electronic exchanges.

The attempts to replace the electromechanical switching matrices by semiconductor cross point switches were not immediately successful, particularly in large exchanges. As a result, many space division switching systems used electromechanical switching networks with SPC. Nonetheless, private automatic branch exchanges (PABX) and smaller exchanges do use electronic switching devices. The two types of space division electronic switching systems are using electromechanical switching network and the other using electronic switching network. The second type is fully electronic.

==Types==

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===Centralized control===

In centralized control, all control equipment is replaced a central processing unit. It must be able to process 10 to 100 [[calls per second]], depending on the load to the system.{{cncitation needed|date=July 2015}} Multiprocessor configurations are commonplace and may operate in various modes, such as in load-sharing configuration, in synchronous duplex-mode, or one processor may be in stand-by mode.

====Standby mode====

Standby mode of operation is the simplest of a dual-processor configuration. Normally one processor is in standby mode. The standby processor is brought online only when the active processor fails. An important requirement of this configuration is ability of standby processor to reconstitute the state of exchange system when it takes over the control; means to determine which of the subscriber lines or trunks are in use.

In small exchanges, this may be possible by scanning the status signals as soon as the standby processor is brought into action. In such a case only the calls which are being established at the time of failure are disturbed. In large exchanges it is not possible to scan all the status signals within a significant time. Here the active processor copies the status of system periodically into secondary storage. When switchover occurs the recent status from the secondary memory is loaded. In this case only the calls which change status between last update and failure are affected. The shared secondary storage need not to be duplicated and simple unit level redundancy would suffice. [[1ESS switch]] was a prominent example.

====Synchronous duplex mode====

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====Load-sharing mode====

In load-sharing operation, an incoming call is assigned randomly or in a predetermined order to one of the processors which then handles the call right through completion. Thus, both the processors are active simultaneously and share the load and the resources dynamically. Both the processors have access to the entire exchange environment which is sensed as well as controlled by these processors. Since the calls are handled independently by the processors, they have separate memories for storing temporary call data. Although programs and semi permanent data can be shared, they are kept in separate memories for redundancy purposes.

There is an inter processor link through which the processors exchange information needed for mutual coordination and verifying the 'state of health’ of the other. If the exchange of information fails, one of the processors which detect the same takes over the entire load including the calls that are already set up by the failing processor. However, the calls that were being established by the failing processor are usually lost. Sharing of resources calls for an exclusion mechanism so that both the processors do not seek the same resource at the same time. The mechanism may be implemented in software or hardware or both. Figure shows a hardware exclusion device which, when set by one of the processors, prohibits access to a particular resource by the other processor until it is reset by the first processor.

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Distributed SPC is both more available and more reliable than centralized SPC.<ref>[http://books.google.com/books?id=ghCiNHzWvL4C&pg=PA96&dq=4.3+Distributed+SPC&hl=en&sa=X&ei=ToAhT6fJAsHL0QHhr4zVCA&ved=0CDIQ6AEwAA#v=onepage&q=4.3%20Distributed%20SPC&f=false] T Wiswanathan</ref>

*Vertical decomposition:

*:Whole exchange is divided into several blocks and a processor is assigned to each block.This processor perform all the task related to that specific blocks.Therefore, the total control system consists of several control units coupled together.For redundancy purpose processor may be duplicated in each blocks.

*Horizontal Decomposition:

*:In this type of decomposition each processor performs only one or some exchange function.