What are the Electronic Exchanges? What are the advantages and types of Electronic Exchanges?
To overcome the limitations of manual switching; automatic exchanges, having Electro-mechanical components, were developed. Strowger exchange, the first automatic exchange having a direct control feature, appeared in 1892 in La Porte (Indiana). Though it improved upon the performance of a manual exchange it still had a number of disadvantages, viz., a large number of mechanical parts, limited availability, inflexibility, bulky in size, etc.
As a result of further research and development, Crossbar exchanges, having an indirect control system, appeared in 1926 in Sundsvall, Sweden. The Crossbar exchange improved upon many shortcomings of the Strowger system. However, much more improvement was expected and the revolutionary change in field of electronics provided it. A large number of moving parts in Register, marker, Translator, etc., were replaced en-block by a single computer. This made the exchange smaller in size, volume, and weight, faster and reliable, highly flexible, noise-free, easily manageable with no preventive maintenance, etc.
1.1 The first electronic exchange employing Space-Division switching (Analog switching) was commissioned in 1965 at Succasunna, New Jersey. This exchange used one physical path for one call and, hence, full availability could still not be achieved. Further research resulted in the development of Time-Division switching (Digital Switching) which enabled sharing a single path by several calls, thus providing full availability. The first digital exchange was commissioned in 1970 in Brittany, France.
This handout reviews the evolution of the electronic exchanges, lists the chronological developments in this field and briefly describes the facilities provided to subscribers, administration and maintenance personnel.
Table 1 Chronological Development of Electronic Exchanges.
|1965||No.1 ESS||Local||Bell Labs, USA|
|1972||D 10||Local and Transit||NEC. Japan.|
|1974||No. 1 ESS Centrex||Local and Transit||Bell Labs. USA|
|1975||Proteo||Local & Transit||Proteo, Italy|
|1976||AXE||Local||PTT & LM Ericsson, Sweden|
|1976||No.4 ESS||Transit||Bell Labs, USA|
|1978||AXE||Local||LM Eiricsson, Sweden.|
Advantages of Electronic Exchange Over Electro-mechanical Exchanges
|Electromechanical Exchanges –||Electronic Exchanges|
|Category, Analysis, Routing, translation, etc;, done by relays.
Any changes in facilities require the addition of hardware and/or a large amount of wiring change. Flexibility limited.
Testing is done manually externally and is time-consuming. No logic analysis carried out.
Partial full-availability, hence blocking.
limited facilities to the subscribers.
Slow in speed. Dialing speed is max. 11 Ips and switching speed is in l milliseconds.
The switch room occupies a large volume.
Lot of switching noise.
Long installation and testing time.
Large maintenance effort and preventive
|Translation, speech path Sub’s Facilities, etc., managed by MAP and other DATA.
Changes can be carried out by simple commands. A few changes can be made by Subs himself. Hence, highly flexible.
Testing carried out periodically automatically and analysis printed out.
Full availability, hence no blocking. A large number of different types of services possible very easily.
Very fast. Dialing speed up to 11 digits /sec possible. Switching is achieved in a few microseconds.
The much lesser volume required floor space of the switch room reduced to about one-sixth.
Short installation and testing period.
Remedial maintenance is very easy due to plug-in type circuit boards. Preventive maintenance not required.
1.3 Influence of Electronics in Exchange Design.
When electronic devices were introduced in the switching systems, a new concept of switching evolved as a consequence of their extremely high operating speed compared to their former counterparts, i.e., the Electro-mechanical systems, Relays, the logic elements in the electromechanical systems, have operate and release times which are roughly equal to the duration of telephone signals to maintain required accuracy. However, to achieve the requisite simultaneous call processing capacity, it became essential for such a system to have a number of such electrical control units (Called registers in a Cross-bar Exchange), in parallel, each handling one call at a time. In other words, it was necessary to have an individual control system to process each call.
Electronic logic components, on the other hand, can operate a thousand or ten thousand times during a telephone signal. This led to a concept of using a single electronic control device to simultaneously process a number of calls on a time-sharing basis. Though such centralization of control is definitely more economical it has the disadvantage of making the switching system more vulnerable to total system failure. This can, however, be overcome by having a standby control device.
Another major consequence of using electronics in control subsystems of a telephone exchange was to make it technically and economically feasible to realize powerful processing units employing a complex sequence of instructions. Part of the control equipment capacity could then be employed for functions other than call processing, viz., exchange operation and maintenance. It resulted in greatly improved system reliability without excessively increasing system cost. This development led to a form of centralized control in which the same processor handled all the functions, i.e., call processing, operation and maintenance functions of the entire exchange.
In the earlier versions of electronic control equipment, the control system was of a very large size, fixed cost unit. It lacked modularity. It was economically competitive for very large capacity exchanges. Initially, small capacity processors were costlier due to high cost per bit of memory and logic gates. Therefore, for small exchanges, the processor cost per line was too high. However, with the progressive development of the small size low-cost processor using a microprocessor, it became possible to employ electronic controls for all capacities. In addition control equipment could also be made modular aiding the future expansion.
The impact of electronics on exchanges is not static and it is still changing as a function of advances in electronic technology.
- Phased Developments
Many electronic switching systems, including the recent ones, had an electromechanical switching network and used miniature electromagnetic relays injunction and subscriber line equipment None-the-less the trend is towards all electronic equipment for both public and private switching and the switching network has already been made fully electronic with the advent of digital switching.
However, very recently, several countries have developed or specified stored-program equipment for upgrading electromechanical exchanges. This typically involves replacing the registers and translators of crossbar exchanges by processor-based facilities. These allow the exchange subscribers to benefit from new services like abbreviated dialing call forwarding automatic alarm call, and detailed billing. They, very significantly, enhance exchange administration and maintenance capabilities for day-to-day operations, such as modifying a subscriber’s class of service, changing the way traffic is routed, collecting traffic and load data, call charging, etc.
1.5 Facilities provided by Electronic Exchanges.
Facilities offered by electronic exchanges can be categorized in three arts.
(i) Facilities to the Subscribers.
(ii) Facilities to the Administration.
(iii) Facilities to the Maintenance Personnel.
- Facilities to the Subscribers.
MFC Push-button Dialing.
All subscribers in an electronic exchange can use push-button telephones, which use Dual Tone Multifrequency, for sending the dialed digits. Sending of eleven digits per second is possible, thus increasing the dialing speed.
Priority Subscriber Lines
Priority Subscriber lines may be provided in electronic exchanges. These subscribers are attended to, according to their priority level, by the central processor, even during heavy congestion or emergency.
Toll (Outgoing Call) Restriction
The facility of toll restriction or blocking of subscriber line for specific types of outgoing traffic, viz., long-distance STD calls, can be availed of by all subscribers. This can be easily achieved by keying-in certain service codes.
Incoming calls to a subscriber can be automatically forwarded during his absence, to a customer service position or a recorded announcement. The customer service position answers the calls and forwards any message meant for the subscriber.
Most subscribers very often call only a limited group of telephone numbers. By dialing only prefix digit followed by two selection digits, subscribers can call up to 100 predetermined subscribers connected to any automatic exchange. This shortens the process of dialing all the digits.
The subscriber having the call forwarding facility can keep his telephone in the transfer condition in case he wishes his incoming calls to be transferred to another telephone number during his absence.
Do Not Disturb
This service enables the subscriber to free himself from attending to his incoming calls. In such a case, the incoming calls are routed to an operator position or a talking machine. This position or machine informs the caller that called subscriber is temporarily inaccessible.
Subscribers can set up connections to more than one subscriber and conduct telephone conferences under the provision of this facility.
Camp On Busy
Incoming call to a busy subscriber can be “Camped on” until the called subscriber gets free. This avoids wastage of time in redialing a busy telephone number.
The ‘Call Waiting’ service notifies the already busy subscriber of a third party calling him. He is fed with a special tone during his conversation. It is purely his choice either to ignore the third party or to interrupt the existing connection and have a conversation with the third party while holding the first party on the line.
Instead of camp on busy, a call can automatically be repeated. The calling party can replace his handset after receiving a busy tone. A Periodic check is carried out on the called party’s status. When the idle status is ascertained, the connection is set up and ringing current fed to both the parties.
This system permits consultation and the transfer of a call to other subscribers. Consultation can be initiated by means of a special signal from the subscriber telephone and by dialing the directory number of the desired subscriber without disconnecting the previous connection.
The priority of calls to Emergency Positions
Emergency calls such as ambulance, fire, etc., are processed in priority to other calls.
Subscriber charge Indicator
By placing a charge indicator at the subscriber’s premises the charges of each call made can be ascertained by him.
Call Charge printout or immediate Billing
The subscriber can request automatic post call charge notification in the printout form for individual calls or for all calls. The information containing called number, date and time, and the charges can be had on a Tele-type-write.
Malicious Call Identification
Malicious Call Identification is done immediately and the information is Obtained in the printout form either automatically or by dialing an identification code.
Interception or Announcement.
In the following conditions, an announcement is automatically conveyed to calling subscribers.
- Change of a particular number of transferred subscribers.
- Dialing of unallocated code.
- Dialing of an unobtainable number.
- Route congested or out of order.
- Subscriber’s line temporarily out of order.
- Suspension of service due to non-payment.
Connection Without Dialing.
This allows the subscribers to have a specific connection set up, after lifting the handset, Without dialing. If the subscriber wishes to dial another number, then he has to start dialing within a specified time period, say 10 seconds, after lifting the handset.
Automatic Wake Up.
Automatic wake up service or morning alarm is possible, without any human intervention.
Hot Line or Private Wire.
Hotline service enables the subscriber to talk to a specific subscriber by only lifting the handset. This service cannot be used. along with normal dialing facility. The switching starts as soon as the receiver is lifted.
Denied Incoming Call
A Subscriber may desire that no incoming call should come on a particular line. He can ask for such a facility so that he can use the line for making only outgoing calls.
A few subscribers may like to have their telephone sets locked up against any misuse. Dialing of secret code will extend such a facility to them.
Free of charge Calls
Calls free of charge are possible on certain specialized services such as booking of complaints, booking of telegrams, etc.
If so desired, the incoming subscriber is billed for all the calls made to him, instead of the calling subscriber.
- Facilities to the Administration
Reduced Switch Room Accommodation
Reduction in switch room accommodation to about 1/6th to 1/4th as compared to the Cross-bar system is possible.
Faster installation and Easy Extension
The reduced volume of equipment, plug-in assemblies for interconnecting cables, printed cards and the automatic testing of exchange equipment result in faster installation (about six months for a 10,000 line exchange) Due to the modular structure, the expansion is also easier and quicker
The switching speed is much faster as compared to the Cross-bar system, the use of the principle of full availability of trunk circuits and other equipment makes the system economically superior to electromechanical systems.
Automatic test of Subscriber line
Routine testing of subscriber lines for Insulation, capacitance, foreign potential, etc., are automatically carried out during the night. The results of the testing can be obtained in the printout form, the next day.
- Maintenance Facilities
The automatic fault processing facility is available for checking all hardware components and complete the internal working of the exchange. The changeover from a faulty sub-system to a stand-by sub-system is automatically affected without any human intervention. Only information is given out so that the maintenance staff is able to attend to the faulty sub-system.
Once a fault is reported by the system, ‘on-demand’ programs are available which help the maintenance staff to localize the fault, who can replace the defective printed card and restore the faulty sub-system. The faulty card is attended at a centralized maintenance centre specifically equipped for this purpose.
Statistical programs are available to gather information about the traffic conditions and the trunks occupancy rate to assess and plan the solutions in cases of anticipated problems. This facility helps the maintenance and administration personnel to maintain a specified level of grade of service.
In case of congestion or breakdown of a specific route, the facility of blocking such routes is available in modes, such as
(i) Blocking of a specified percentage of calls in such a route either automatically or manually.
(ii) Blocking a specific category of subscribers.
Overloading of central processor in an electronic exchange can lead to disastrous results. To prevent this, central processor occupancy is measured automatically periodically, when it exceeds a specified percentage, audio-visual alarms are activated, in addition to printing out the message. Maintenance personnel has the following options.
(i) Block some of the facilities temporarily, or
Reduce the load by blocking some of the congested routes.
1.6 Constraints of Electronic Exchanges
Though there are a number of definite advantages of Electronic exchanges, over the electromechanical exchanges, there are certain constraints, which should be considered, at the planning stage for deciding between the two systems.
Traffic Handling Capacity
Apparently, the traffic handling capacity of exchange is limited by the number of subscriber lines and trunks connected to the switching network, and the number of simultaneous paths available through the switching network. However, in electronic exchanges, the prime limitation is the number of simultaneous calls, which can be handled by the control equipment, as it has to execute a number of instructions depending on the type of the call. Therefore the extent of loading of the exchange will be guided solely by the amount of processor loading. Moreover, the facilities to the subscribers will also have to be limited accordingly.
The power supply should be highly stable for trouble-free operation as the components are sensitive to variations beyond +10%. It is almost essential to have a standby power supply arrangement.
Total Protection from Dust
All possible precautions should be observed for ensuring a dust-free environment.
Temperature and Humidity Control
Due to the presence of quiescent current in the components and because of their compactness, the heat generated per unit volume is highest in electronic exchanges. Moreover, as the component characteristics drift substantially with the temperature and humidity, the air-conditioning load is higher. Obviously, the air-conditioning system should be highly reliable and preferably there should be a stand-by arrangement. The installation is also carried out in an air-conditioned environment.
Static Electricity and Electromagnetic interference.
Due to the presence of static electricity on the body of persons handling the equipment, the stored data may get vitiated. Handling of PCB’s therefore should be done with the utmost care and should be minimized care should also be taken to protect the cards from exposure to stray electromagnetic fields.
The repair of PCB’s is extremely complicated and sophisticated equipments are required for diagnosing the faults. This results in having a costly inventory and a costly repair center. With the frequent improvement and changes in the cards, proper documentation of cards becomes essential.
The changes in the field of electronics are almost revolutionary with very fast improvements. Hence, current technology becomes obsolete at a very fast rate. The equipment becomes obsolete before it can possibly complete one-third of its life and it might be impossible to get spare parts for the entire currency of the life of the system.
After 1950, the development in the field of electronic devices induced the telephone system designers to make use of innumerable advantages offered by their inventions. Therefore, the telephone switching system with both electronic and electromechanical components was evolved. Later on, the Stored Program Control concept was evolved and adapted to the electromechanical exchanges. This developmental step opened a new era of innumerable additional facilities to the subscribers, administration and maintenance personnel.Download