Stored Program Control exchange
Encyclopedia
Stored Program Control exchange (SPC) is the technical name used for telephone exchanges controlled by a computer program stored in the memory of the system. Early exchanges such as Strowger
Strowger switch
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger...

, panel
Panel switch
The panel switching system was an early type of automatic telephone exchange, first put into urban service by the Bell System in the 1920s and removed during the 1970s...

, rotary, and crossbar
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....

 switches were electromechanical and had no software control. SPC was introduced on a small scale in so called electronic switching system
Electronic switching system
In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...

s in the 1960s (the 101ESS PBX was a minor Bell System example) and on a large scale in the 1970s (1ESS switch
1ESS switch
The Number One Electronic Switching System, the first large-scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...

 from Bell System, AXE telephone exchange
AXE telephone exchange
The AXE telephone exchange is a product line of circuit switched digital telephone exchanges manufactured by Ericsson, a Swedish telecom company. It was developed in 1974 by Ellemtel, a research and development subsidiary of Ericsson and Televerket.. The first system was deployed in 1976...

 from Ericsson
Ericsson
Ericsson , one of Sweden's largest companies, is a provider of telecommunication and data communication systems, and related services, covering a range of technologies, including especially mobile networks...

). SPC allowed more sophisticated calling features. As SPC exchanges evolved, reliability and versatility increased. In the 1980s SPC completely took over the industry, making the term redundant except for historical interest.

Introduction

SPC allowed more sophisticated Calling features. As SPC exchanges evolved, reliability and versatility increased. In stored program control, a program or a set of instructions to the computer is stored in its memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...

 and the instructions are executed automatically one by one by the processor. An immediate consequence of program control is full scale automation of exchange functions and introduction of a variety of new functions to users.

Introducing a computer to carry out control functions of a telephone exchange is not as simple as using computer for commercial data processing. A telephone exchange must run without interruption 24 hrs a day, 365 days a year, 30–40 years. This means the exchange must be tolerant to faults
Fault-tolerant design
In engineering, fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level , rather than failing completely, when some part of the system fails...

. Attempts to introduce electronics and computers in the control sub system of an exchange were encouraging enough to spur the development of full- fledged electronic system, in which switching network is also electronic, and the world’s first electronic switching system, known as No.1 ESS, was commissioned by AT&T at Succasunna, New Jersey, in May 1965. SPC systems grew rapidly. In the 1980s SPC completely took over the industry, hence the term lost all except historical interest. Today SPC is a standard feature in all electronic exchanges. However the attempts to replace the space division electromechanical switching matrices by semiconductor cross print were not immediately successful, particularly in large exchanges. As a result many space division electromechanical 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 one is fully electronic.

Types of SPC

There are basically two approaches to organizing stored program control: centralized and distributed. Early electronic switching system
Electronic switching system
In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...

s (ESS) developed during the period 1970-75 almost invariably used centralized control. Although many present day exchange design continue to use centralized SPC, with advent of low cost powerful microprocessors and VLSI chips such as programmable logic array
Programmable logic array
A programmable logic array is a kind of programmable logic device used to implement combinational logic circuits. The PLA has a set of programmable AND gate planes, which link to a set of programmable OR gate planes, which can then be conditionally complemented to produce an output...

 (PLA) and programmable logic controllers (PLC), distributed SPC became widespread by the early 21st century.

Centralized SPC

In centralized control, all the control equipment is replaced by single processor which must quite powerful. It must be able to process 10 to 100 calls per second
Calls per second
Calls Per Second or CPS refers to how many telephone calls can be handled in a second.CPS is one measure of the performance of Switching systems....

, depending on the load to the system. A typical control configuration is shown in the figure.

This configuration may use more than one processor. The exchange resources and memory module containing the programs for carrying out various control functions may be shared by processor, or each has dedicated access path to exchange resources.

Two processor-configurations are commonplace. Dual processor architecture may be configured to operate in one of these modes:
  1. Standby mode
  2. Synchronous duplex mode
  3. Load sharing mode

Standby mode

Standby mode of operation is the simplest of dual processor configuration operations. Normally one processor is on standby, both hardware and software wise. 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 or trunks are busy or free etc.

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
1ESS switch
The Number One Electronic Switching System, the first large-scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, was introduced in Succasunna, New Jersey, in May 1965. The switching fabric was composed of reed matrixes controlled by wire spring relays...

was a prominent example.

Synchronous Duplex Mode

In Synchronous Duplex Mode of operation hardware coupling is provided between two processors which execute same set of instructions and compare the results continuously. If mismatch occurs then the faulty processor is identified and taken out of service within a few milliseconds. When system is operating normally, the two processors have same data in memories at all times and simultaneously receive information from exchange environment. One of the processor actually controls the exchange, but other is synchronized with the former but does not participate in the exchange control. If a fault is detected by the comparator the processors are decoupled and a check-out program is run independently to find faulty processor. This process runs without disturbing the call processing which is suspended temporarily. When one processor is taken out then the other processor operates independently. When the faulty processor is repaired and brought in service then memory contents of the active processor are copied into its memory and the two are synchronized and comparator is enabled.

It is possible that a comparator fault occurs only due to transient failure which is not shown even when check out program is run. In such case three possibilities exists:

Continue with both processors

Take out the active processor and continue with the other.

Continue with active processor but remove other processor from service.

Scheme 1 is based on the assumption that the fault is transient one and may not appear again. In scheme 2 and 3 the processor taken out is subjected to extensive testing to identify a marginal failure in these cases.

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.

Distributed Stored program control

As compare to Centralized SPC, it gives
  • Better availability
  • Reliability
  1. 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.
  1. Horizontal Decomposition

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