Basic Principles of Electronic Exchanges:
The prime purpose of an exchange is to provide a temporary path for simultaneous. bi-directional transmission of speech between
(i) Subscriber lines connected to the same exchange (local switching)
(ii) Subscriber lines and trunks to other exchange(outgoing trunk call)
(iii) Subscriber lines and trunks from other exchanges(incoming trunk calls) and
(iv) Pairs of trunks towards different exchanges (transit switching)
These are also called the switching functions of exchange and are implemented through the equipment called the switching network. An exchange, which can set up just the first three types of connections., is called a Subscriber or Local Exchange. If an exchange can set up only the fourth type of connection, it is called a Transit or Tandem Exchange. The other distinguished functions of the exchange are
i) Exchange of information with the external environment (Subscriber lines or other exchanges) i.e. signaling.
ii) Processing the signaling information and controlling the operation of signaling network, i.e. control, and
iii) Charging and billing
All these functions can be provided more efficiently using the computer-controlled electronic exchange, than by the conventional electromechanical exchanges.
This handout describes the basic principals of SPC exchanges and explains how the exchange functions are achieved.
An electromechanical switching, the various functions of the exchange are achieved by the operation and release of relays and switch (rotary or crossbar) contacts, under the direction of a Control Sub-System. These contracts are hard-wired in a predetermined way. The exchange dependent data, such as the subscriber’s class of service, translation, and routing, combination signaling characteristics, are achieved by hand ware and logic, by an of relay sets, a grouping of the same type of lines, strapping on Main or Intermediate Distribution Frame or translation fields, etc. When the data is to be modified, for the introduction of a new service, or change in services already available to a subscriber, the hardware change ranging from inconvenient to near impossible, are involved.
In an SPC exchange, a processor similar to a general-purpose computer is used to control the functions of the exchange. All the control functions, represented by a series of various instructions, are stored in the memory. Therefore the processor memories hold all exchange-dependent data. such as subscriber data, translation tables, routing and charging information and call records. For each call processing step. e.g. for taking a decision according to a class of service, the stored data is referred to, Hence, this concept of switching. The memories are modifiable and the control program can always be rewritten if the behavior or the use of the system is to be modified. This imparts enormous flexibility in the overall working of the exchange.
Digital computers have the capability of handling many tens of thousands of instructions every second, Hence, in addition to controlling the switching functions, the same processor can handle other functions also. The immediate effect of holding both the control programme and the exchange data, in easily alterable memories, is that the administration can become much more responsive to subscriber requirements. both in terms of introducing new services and modifying general services, or in responding to the demands of individual subscribers. For example, to restore service on payment of an overdue bill or to permit change from a dial instrument to a multi-frequency sender, simply the appropriate entries in the subscriber data-file are to be amended. This can be done by typing- in simple instructions from a teletypewriter or visual display unit. The ability of the administration to respond rapidly and effectively to subscriber requirements is likely to become increasingly important in the future.
The modifications and changes in services that were previously impossible be achieved very simply in SPC exchange, by modifying the stored data suitably. In some cases, subscribers can also be given the facility to modify their own data entries for supplementary services, such as on-demand call transfer, shortcode, (abbreviated ) dialing, etc.
The use of a central processor, also makes possible the connection of local and remote terminals to carry out a man-machine dialogue with each exchange. Thus, the maintenance and administrative operations of all the SPC exchanges in a network can be performed from a single centralized place. The processor sends the information on the performance of the network, such as, traffic flow, billing information, faults, to the center, which carries out remedial measures with the help of commands. Similarly, other modifications in services can also be carried out from the remote center. This allows better control of the overall performance of the network.
As the processor is capable of performing operations at a very high speed, it has got sufficient time to run routine test programmes to detect faults, automatically. Hence, there is no need to carry out time-consuming manual routine tests.
In an SPC exchange, all control equipment can be replaced by a single processor. The processor must, therefore, be quite powerful, typically, it must process hundreds of calls per second, in addition to performing other administrative and maintenance tasks. However, totally centralized control has drawbacks. The software for such a central processor will be voluminous, complex, and difficult to develop reliably. Moreover, it is not a good arrangement from the point of view of system security, as the entire system will collapse with the failure of the processor. These difficulties can be overcome by decentralizing control. Some routine functions, such as scanning, signal distributing, marking, which are independent of call processing, can be delegated to auxiliary or peripheral processors. These peripheral units, each with a specialized function, are often themselves controlled by small stored programmes processors, thus reducing the size and complexity at the central control level. Since they have to handle only one function, their programmes are less voluminous and far less subjected to change than those at central. Therefore, the associated programme memory need not be modifiable (generally, semiconductors ROM’s are used).
Despite the many difference between the electronic switching systems, and
all over the world there is a general similarity between most of the systems in terms of their functional subdivisions. In it’s simplest form. an SPC exchange consists of five main sub-systems, as shown in fig.
- Terminal equipment provides on an individual basis for each subscriber line and for interexchange trunk.
- The switching network may be space- division or time-division, uni-directional or bi-directional.
- Switching processor, consisting mainly of processors and memories.
- Switching peripherals ( Scanner, Distributor, and Marker ), are Interface Circuits between control system terminal equipment and switching network.
- Signaling interfaces depending on the type of signaling used, and
- Data Processing Peripherals ( Tele – typewriters, Printers, etc. ) for man-machine dialogue for operation and maintenance of the exchange.
In this equipment, line, trunk, and service circuits are terminated, for detection, signaling, speech transmission, and supervision of calls. The Line Circuits carry out the traditional functions of supervising and providing battery feed to each subscriber line. The Trunk Circuits are used on outgoing, incoming and transit call for battery feed and supervision. Service Circuits perform specific functions, like, transmission and reception of decadic dial pulses or MF signals, which may be economically handled by a specialized common pool of circuits. In contrast to electromechanical circuits, the Trunk and Service circuits in SPC exchanges, are considerably simpler because functions, like counting, pulsing, timing charging, etc. are delegated to stored programme.
In an electronic exchange, the switching network is one of the largest sub-systems in terms of the size of the equipment. Its main functions are,
- Switching, i.e., setting up a temporary connection between two or more exchange terminations, and
- Transmission of speech and signals between these terminations, with reliable accuracy.
There are two types of electronic switching systems. viz. Space division and Time Division.
2.1 Space Division Switching System.
In a space Division Switching system, a continuous physical path is set up between input and output terminations. This path is separate for each connection and is held for the entire duration of the call. Path for different connections is independent of each other. Once a continuous path has been established., Signals are interchanged between the two terminations. Such a switching network can employ either metallic or electronic cross-points. Previously, usage of metallic cross-points, viz., reed relay, mini-cross bar derivative switches, etc.were favored. They have the advantage of compatibility with the existing line and trunk signaling conditions in the network.
2.2 Time Division Switching System.
In Time Division Switching, a number of calls share the same path on a time-division sharing basis. The path is not separate for each connection, rather, it is shared sequentially for a fraction of time by different calls. This process is repeated periodically at a suitable high rate. The repetition rate is 8 Khz, i.e. once every 125 microseconds for transmitting speech on the telephone network, without any appreciable distortion. These samples are time-multiplexed with staggered samples of other speech channels, to enable sharing of one path by many calls. The Time Division Switching was initially accomplished by Pulse Amplitude Modulation (PAM) Switching. However, it still could not overcome the performance limitations of signal distortion noise, cross-talk, etc. With the advent of Pulse Code Modulation (PCM), the PAM signals were converted into a digital format overcoming the limitations of analog and PAM signals. PCM signals are suitable for both transmission and switching. The PCM switching is popularly called Digital Switching.
Compatibility with Existing Network
In this area, the application of electronic techniques has encountered the greatest difficulty. To appreciate the reasons, let us consider the basic requirements of a conventional switching network.
- High OFF resistance and low ON resistance.
- Sufficient power handling capacity for transmitting ringing current, battery feed, etc…, on subscriber lines.
- Good frequency response (300-3400 Khz )
- Bi-directional path (preferable)
- C. signaling path to work with existing junction equipment (preferable)
- Easy to control.
- Low power consumption, and
- Immunity to extraneous noise, voltage surges.
The present-day electronic devices cannot meet all these requirements adequately. It is seen that requirement iii,v, vi and vii only, can easily be met by electronic devices. These considerations show that substitutions of the analog mode of electromechanical switching network by fully electronic equipment are not, straightway practical. The main virtue of the existing electromechanical devices is their immunity to extraneous noise voltage surge, etc., which are frequently experienced in a telephone network. Moreover, metal contact switches offer little restriction on the voltages and currents to be carried. In the existing network and subscriber handsets, typically, 80-volt peak to peak ringing current is required to be transmitted on the line. This is difficult, if not impractical, for electronic switches to handle. Therefore, to avail of the advantages of the electronic exchanges, either of the two following alternatives may be adopted.
- Deploy a new range of peripherals/equipment, suited to the characteristics of the electronic switching devices, on one hand, and requirements of telephone network on the other hand. i.e. employ Time Division Switching systems, or
- Continue to use metal contact switches, while other sub-systems may be changed to electronic. i.e. semi-electronic type of exchanges rather than fully electronic exchanges, to employ Space Division Switching Systems.
The switching processor is a special purpose real-time computer, designed and optimized for dedicated applications of processing telephone calls. It has to perform certain real-time functions (which have to be performed at the time of occurrence and cannot be deferred), such as, reception of dialed digits, and sending of digits in case of transit exchange. The block schematic of a switching processor, consisting of a central control programme store is shown in fig.2
Central Control (CC) is a high-speed data processing unit, which controls the operation of the switching network. In the Programme store, sets of instructions. called programmes are stored. The programmes are interpreted and executed by the central control. Data Store provides for the temporary storage of transient data, required in processing telephone calls, such as digits dialed by the subscriber, busy/idle states of lines and trunks, etc. The translation Store contains information regarding lines. e.g. category of calling and called line. routing code, charging information, etc. Data Stores is temporary memory, whereas Translation and Programme Stores are of a semi-permanent type. The information in the Semi-permanent memories does not change during the processing of the call, but the information in Data Store changes continuously with origination and termination of each call.
4 Switching Peripheral Equipment
The time intervals, in which the processor operates, is in the order of microseconds, while the components in the telephone switching section operate in milliseconds ( if the switching network is of the analog type). The equipment, known as the switching peripheral, is the interface between these two types of equipment working at different speeds. The interface equipment acts as speed buffer, as well as, enables the conversion of digital logic signals from the processor to the appropriate electrical signals to operate relays and cross-points, etc. Scanners, Signal distributors and Marker fall under this category of devices.
Its purpose is to detect and inform CC of all significant events/signals on subscriber lines and trunks. connected to the exchange. These signals may either be continuous or discrete. The equipment at which the events/signals must be detected is equally diverse.
- Terminal equipment for subscriber lines and inter-exchange trunks and.
- Common equipment such as DTMF (Dual – Tone Multi-Frequency) or MFC digit receivers and inter-exchange signaling senders/receivers connected to the lines and trunks.
In view of this wide diversity in the types of lines. trunks and signaling, the scanning rate, i.e. the frequency at which scan points are read, depends upon the maximum rate at which events/signals may occur. For example, on a subscriber line, with decadic pules signaling with 1:2 make -break ratio, the necessary precision, required for pulse detection, is of the order of ten milliseconds, while other continuous signals ( clear, off-hook, etc.) on the same line are usually several hundred mili-seconds long and the same high precision is not required. To detect new calls, while complying with the dial tone connection specifications, each line must be scanned about every 300 milliseconds. It means that in a 40,000 line exchange ( normal size electronic exchange ) 5000 orders are to be issued every 300 milliseconds, assuming that eight lines are scanned simultaneously.
Marker performs physical setup and release of paths through the switching network, under the control of CC. A path is physically operated only when it has been reserved in the central control memory. Similarly, paths are physically released before being cleared in memory, to keep the memory information updated vis-a-vis switching network, Depending upon whether it is switching is Time-division or Space division, the marker either writes information in the control memory of time and space stages. (Time Division Switching), or physical operates the cross – points (Space Division Switching)
It is a buffer between high – speed – low – power CC and relatively slow-speed-high-power signaling terminal circuits. A signal distributor operates or releases electrically latching relays in trunks and service circuits, under the direction of central control.
4.4 Bus System
Various switching peripherals are connected to the central processor by means of a common system. A bus is a group of wires on which data and commands pulses are transmitted between the various subunits of a switching processor or between switching processor and switching peripherals. The device to be activated is addressed by sending its address on the address bus. The common bus system avoids the costly mesh type of interconnection among various devices.
4.5 Line Interface Circuits
To enable an electronic exchange to function with the existing outdoor telephone network, certain interfaces are required between the network and the electronic exchange.
4.5.1 Analogue Subscriber Line Interface
The functions of a Subscriber Line Interface, for each two wirelines, are often known by the acronym: BORSHT
B : Battery feed
O : Overload protection
R : Ringing
S : Supervision of loop status
H : Hybrid
T : Connection to test equipment
All these functions cannot be performed directly by the electronic circuits and, therefore, suitable interfaces are required.
4.5.2 Transmission Interface
Transmission interface between analog trunks and digital trunks (individual or multiplexed) such as A/D and D/A converters are known as CODEC, These may be provided on a per-line and per-trunk basis or on the basis of one per 30 speech channels.
4.5.3 Signaling Interfaces
A typical telephone network may have various exchange systems (Manual, Strowger, Crossbar, electronic) each having different signaling schemes. In such an environment, an exchange must accommodate several different signaling codes.
Initially, all signaling between automatic exchanges was decadic i.e. telephone numbers were transmitted as trains of 1to 10 pulses, each train representing one digit. To increase the speed at which the calls could be set up, and to improve the reliability of signaling, compelled sequence multi-frequency signaling system was then introduced. In this system, each signal is transmitted as a combination of 2 out of a group of say 5 or 6 frequencies. In both decadic and multi-frequency methods, the signals for each call are sent over a channel directly associated with the inter-exchange speech transmission circuit used for that call. This is termed as channel associated signaling. Recently, a different technique has been developed, known as common channel signaling. In this technique, all the signaling information for a number of calls is sent over a signaling link independent of the inter-exchange speech circuits. The higher transmission rate can be utilized to enable the exchange of a much larger amount of information. This results in faster call setup, the introduction of new services, e.g.., abbreviated dialing, and more retrials ultimately accomplishing higher call completion rate, Moreover, it can provide an efficient means of collecting information and transmitting orders for network management and traffic engineering.
4.5.4 Data Processing Peripherals.
The following basic categories of Data Processing Peripherals are used in the operation and maintenance of exchange.
i) Man-machine dialogue terminals, like Tele-typewriter (TTY) and
Visual Display Units (VDU), are used to enter operator commands and to give out low-volume data concerning the operation of the switching system. These terminals may be local i.e. within a few tense of meters of the exchange or remotely located. These peripherals have been adopted in the switching Systems for their ease and flexibility of operation.
ii) Special purpose peripheral equipment is, sometimes employed for carrying out repeated functions, such as subscriber line testing, where speed is more important than flexibility.
iii) High-speed large-capacity data storage peripherals (Magnetic Tape Drives, magnetic Disc Unit) are used for loading software in the processor memory.
iv) Maintenance peripherals, such as Alarm Annunciators and Special Consoles, are used primarily to indicate that automatic maintenance procedure has failed and manual attention is necessary.
The electronic exchanges work on the principle of Stored Programme Control. All the call processing functions are performed on the basis of the pre-designed programme which is stored in the memory of the Central Processor. Though the initially designed Electronic Exchanges had a single centralized processor. the control is being decentralized, providing dedicated microprocessor-controlled subsystems for improved efficiency and security of the system. This modular architecture also aids future expansions.
Principles of Electronic Exchanges, Principles of Electronic Exchanges