Rax 256p Interface Unit


In the following sections, some popular digital switching systems are described. Under early electronic switching systems, section No. 1 A ESS and No. 5 ESS explained. In popular digital switching system section, DMS 100 and EWSD system are explained. The section electronic exchanges in India overviews the Indian Telecom Organizations and Switching Systems in India.


In this section, No. 1A ESS introduced in 1976 and No. 5 ESS introduced in 1982 are described. The No. 1A ESS was the updated version of No. 1 ESS which is the first electrronic switching system developed by wester Electric and introduced to market in 1966. The switching systems developed till 1979 are analog switches and the switch No. 5 ESS developed in 1982 includes all the features of prior systems and still pupular. Thus 1A ESS and 5 ESS are described in this section.

132 Telecommunication Switching and Networks 6.6.1. No. 1 A ESS

The No. 1A ESS was developed in 1976. It replaced No. 1 ESS with advanced technology. The primary difference between No. 1 ESS and No. 1A ESS were speed and capacity. No, 1A ESS is based on the smaller magnetically latched Remreed whereas No. 1 ESS uses ferreed relay. These new network are plug compatible with the old networks so that they can be intermixed with the old design.

The original No. 1 ESS design serves maximum of 65000 lines and a maximum calling rate of 25000 calls in the busy hour. It uses maximum of 16 line switch networks. The No. 1A ESS designed with 28 line switch networks and trunk switch networks are 2048 by 2048 size. This serves a maximum of 1,30,000 lines and process 110,000 calls per busy hour.

Hardware Architecture. No. 1 A ESS consists of two subsystems. They are 1. Processor subsystems and 2. Peripheral subsystems. Each subsystems are described below :

Processor subsystem. This subsystem contains duplicated processors with call and program memories. The 1A ESS uses an improved processor which doubles the capacity of the exchange in comparison with No. 1 ESS. The 1A processor is designed as a plug in replacement for existing processors and is the control unit of the No. 1A ESS. The 1A processor is based on highspeed integrated circuit technology and achieves a speed increase by the factor of 4 to 8. Fig. 6.7 shows the processor subsystem of No. 1A ESS.

Spc Processor

To peripheral subsystem

Fig. 6.7. No. 1A ESS processor subsystem.

To peripheral subsystem

Fig. 6.7. No. 1A ESS processor subsystem.

The 1A processor has an additional set of buses. The 1A processor is most specifically used for the following :

(i) to control the 1A ESS switch

(ii) to support future switching systems

(iii) to accomodate bulk memory systems

(iv) to provide real time and continuous control through highly automatic maintenance.

In addition to address bus and data bus, 1A processor uses auxillary unit (AU) bus, Call store (CS) bus, Program store (PS) bus and peripheral unit (PU) bus.

The central control interfaces with the 1A, and performs the processing funfctions of the 1A. It also executes all maintenance routines. The program store uses the high speed semiconductor memories that stores programs, instructions and system configuration systems. The call store is used for storage of translation data and frequently changed call processing data such as status of trunks and switching network, records of network termination and maintains data related to programmed diagnostic tests. Call store also includes an emergency system recovery programs.

In auxillary file store, copies of program are held on separate discs used for program backup. In auxillary data system, magnetic tape system is used to store and retrieve data such as system reinitialization, memory dumps etc. The attached processor system uses 3B 20D computer, of which one or more are used as slave processor used for multitasking. The I/O interface is used to connect 1A to the terminals for input control messages and to receive status messages.

Peripheral subsystem. The peripheral subsystem containing the switch networks, junctors, senders and receivers. It also includes panel system and features like alarm facility, status and control. The peripheral subsystem is shown in Fig. 6.8.

Ocb 283 Exhange
SCCS-Switching control centre system Fig. 6.8. No. 1A ESS peripheral sub systems.

The switch networks are built from the small sized remreed relays. In No. 1A ESS a maximum of 28 time switch network are possible. Trunk switch networks connects 2048 inlets to 2048 outlets. The junctors are designed to provide necessary power and tone feeding conditions and necessary signalling conditions for links and trunks.

Software. The development of new software tools (such as high level programming languages, use structured programmes) has increased the productivity, speed of operation and allows to carry more features.

In 1982, Wester electric developed their fully developed digital switch called No. 5 ESS and put into service. The 5 ESS is a digital SPC switching system which utilizes distributed control, a TST switching network and modular hardware and software design. No. 5 ESS is fully digital switch and did everything of No. 1A ESS and more. Each TSI's of No. 5 ESS have their own processor. This makes the 5 ESS one of the fastest switches.

No. 5 ESS supports POT and centrex services as well as the advanced services such as ISDN, STP, SCP and AIN. Though No. 5 ESS primarily used as a local central office, it can be used as an operator services switch or as low to medium traffic volume tandem.

Hardware architecture. Its architecture can be classified as quasi-distributed since it maintains control of the system via various modules. The major components are

1. Administrative module

2. Communications module

3. Switching module.

The 5 ESS switching system uses a modular software structure and is UNIX based switch. Fig. 6.9 shows the 5 ESS system Architecture.

OSS Data Links

OSS Data Links

Ocb 283 Functional Architecture

Switch module Communications module Administrative module

Fig. 6.9. No. 5 ESS hardware architecture.

Switch module Communications module Administrative module

Fig. 6.9. No. 5 ESS hardware architecture.

Administrative module (AM) : The function of the AM is to assist in call processing functions, system maintenance, software recovery, error detection and system initialization. The AM is based on a duplexed AT & T main frame computer. It contains three major parts namely, central control, Input/output processor, mainstore and automatic message accounting (AMA) units.

Central control. It uses two 3B 20S processors. Of the two, one will be in active mode and other will be in stand by mode. It interfaces with message switch of communication module and Input/Output processor. By interfacing, it performs to control video displays, printer's tape units and monitoring master control center.

Main store. It stores programs and data. The costomer's details such as telephone number, related features, billing option (like electronic clearance) etc are stored here.

Input/Output processors. It provides TTY and data link interfaces to the 3B20S processors, 5 ESS network, Master control center (MCC), and various operational support systems (OSS).

Automatic Message Accounting (AMA) units : It uses data links to transport calling information to central revenue accounting office and AMA type.

Other functions of AM include (i) routing of calls to a particular switching module and traking their availability (ii) Controls and allocates time slots for the time multiplexed switch (TMS) and (iii) Supports hard disk access and maintenance system software.

Communication Module (CM) : CM provides communication between the AM and the switching module (SM). There are two basic components of CM. They are

(i) Message switch. It provides control message transfer between the 3B20S processor and interface modules. It contains clock for synchronizing the network. (referred as network control and timing links (NCT)). Through NCT's, the message switch performs packet switching functions between CM and its SM.

(ii) Time multiplexed switch (TMS). It performs space division switching between switching module's. It provides permanent time slot paths between each SM and the message switch for control messages between the processor and SM's.

Switching modules (SM). It provides call processing and being the first stage of switching. The common components of the switch module are shown in Fig. 6.10. Port control for lines and trunk allocations, setting up and releasing calls, scanning are performed by SM.

Interface Units. The switching module's are equipped with four types of interface units.

(i) Line units (LU). It contains a solid state two stage analog concentrator that provides access to 64 output channels. It is used for terminating analog lines. The concentrator can be fully equipped to provide 8 : 1 concentrators or can be fully equipped to provide 6 : 1 or 4 : 1 concentration.

(ii) Trunk units (TU). It is used for terminating analog trunks. Each TU requires 64 time slots. Depending on the applications of lines and trunks, SM's can be configured differently.

(iii) Data line Trunk Unit (DLTU) and Data Control Line Unit (DCLU). It is used for terminating digital trunks and remote switch module's (RSM). Each fully equipped DLTU and DCLU requires. 256 time slots.

Subscriber lines (Analog)

Trunk lines (Analog)

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