TSSN - Stored Program Control


In this chapter, we will discuss the Stored Program Control works in Telecommunication Switching Systems and Networks. In order to increase the efficiency and speed of control and signaling in switching, the use of electronics was introduced. The Stored Program Control, in short SPC is the concept of electronics that ringed in a change in telecommunication. It permits the features like abbreviated dialing, call forwarding, call waiting, etc. The Stored Program Control concept is where a program or a set of instructions to the computer is stored in its memory and the instructions are executed automatically one by one by the processor.

As the exchange control functions are carried out through programs stored in the memory of a computer, it is called the Stored Program Control (SPC). The following figure shows the basic control structure of an SPC telephony exchange.

spc exchange

The processors used by SPC are designed based on the requirements of the exchange. The processors are duplicated; and, using more than one processor makes the process reliable. A separate processor is used for the maintenance of the switching system.

There are two types of SPCs −

  • Centralized SPC
  • Distributed SPC

Centralized SPC

The previous version of Centralized SPC used a single main processor to perform the exchange functions. The dual processor replaced the single main processor at a later stage of advancement. This made the process more reliable. The following figure shows the organization of a typical Centralized SPC.

to lines and from lines

A dual processor architecture may be configured to operate in three modes like −

  • Standby Mode
  • Synchronous Duplex Mode
  • Load Sharing Mode

Standby Mode

As the name implies, in the two processors present, one processor is active and the other is in the standby mode. The processor in the standby mode is used as a backup, in case the active one fails. This mode of exchange uses a secondary storage common to both the processors. The active processor copies the status of the system periodically and stores in the axis secondary storage, but the processors are not directly connected. The programs and instructions related to the control functions, routine programs and other required information are stored in the Secondary storage.

Synchronous Duplex Mode

In the Synchronous Duplex mode, two processors are connected and operated in synchronism. Two processors P1 and P2 are connected and separate memories like M1 and M2 are used. These processors are coupled to exchange the stored data. A Comparator is used in between these two processors. The Comparator helps in comparing the results.

During the normal operation, both of the processors function individually receiving all the information from the exchange and also related data from their memories. However, only one processor controls the exchange; the other one remains in synchronism with the previous one. The comparator, which compares the results of both the processors, identifies if any fault occurs and then the faulty processor among them is identified by operating them individually. The faulty processor is brought into service only after the rectification of fault and the other processor serves meanwhile.

Load Sharing Mode

Load sharing mode is where a task is shared between two processors. The Exclusion Device (ED) is used instead of the comparator in this mode. The processors call for ED to share the resources, so that both the processors do not seek the same resource at the same time.

In this mode, both the processors are simultaneously active. These processors share the resources of the exchange and load. In case one of the processor fails, the other one takes over the entire load of the exchange with the help of ED. Under normal operation, each processor handles one-half of the calls on a statistical basis. The exchange operator can however vary the processor load for maintenance purpose.

Distributed SPC

Unlike Electromechanical switches and Centralized SPC, the introduction of Distributed SPC has enabled to provide a wide range of services. This SPC has separate small processors called the Regional Processors that deal with different works, rather than just one or two processors working on the whole thing like in the centralized system. However, when these regional processors are required to perform complex tasks, the centralized SPC helps by directing them.

The Distributed SPC has more availability and reliability than Centralized SPC, because entire exchange control functions may be decomposed either horizontally or vertically for distributed processing. Such distributed control where switching equipment is divided into parts, each of which have its own processor, is indicated in the figure below.


The exchange environment in vertical decomposition is divided into several blocks and each block is assigned to a processor that performs all the control functions that are related to specific block of equipment, whereas each processor in horizontal decomposition performs one or some of the exchange control functions.