The field of Telecommunication has evolved from a stage when signs and drum beats were used for long distance communication.In March 1867,Alexander Graham Bell demonstrated his telephone set and laid the foundation of telephony.In actual telecommunication network, subscribers are not directly connected to one another, infact they are connected to switching system.Connection is established between the subscribers at the switching system.The function of switching system is establishing and releasing connection.Earlier,Manual switching systems were used such as Local Battery(LB) or Central Battery(CB).In Local Battery system,Dry cells were used in subscriber sets to power the microphone.In Central Battery system,a subscriber set is energises with a powerful central battery at the exchange.But there were certain limitation of manual switching system,due to which it lost its popularity.
LIMITATIONS OF MANUAL SWITCHING SYSTEM:
- In a manual exchange,the subscriber needs to communicate with the operator and common language becomes an important factor.
- Sufficient Privacy is not there as an operator is involved in setting up and monitoring the call.
- Moreover,the operator takes a few minutes to establish and release of calls.
All these limitations led to the development of Automatic Switching System.Basically,in automatic switching system,Electromechanical (Strowger and Crossbar ) switching system came into existence.Strowger Switching System was developed by ALMON B. STROWGER in 1889.
Strowger switching Systems are constructed using Uniselector or Two motion selector.The wiper contacts of these selectors move in direct response to dialed pulses or signals from the subscriber telephone.The wiper moves forward by one contact at a time and moves by as many contacts as the number of dialed pulses received.
Crossbar switching provides a matrix consisting of n*m set of contacts and selects one of the contacts.There is a set of horizontal and vertical bars which are attached to a set of electromagnet. When an electromagnet(in horizontal direction) is energized,the bar attached to it slightly rotates in such a way that the contact points attached to the bar move closer to its facing point but do not make any contact. Now,if an electromagnet in the vertical direction is energized,the corresponding bar rotates and the contact points comes in contact and a connection is established.
ADVANTAGE OF AUTOMATIC SWITCHING SYSTEM:
- In an automatic exchange,the subscriber does not need to communicate with the operator.
- A greater degree of privacy is obtained in automatic exchange .
- Establishment and Release of calls are faster in automatic exchanges.
Then,in due course of time,Electronic Switching System (Stored Program Control) came into its existence because Electromechanical component’s operational speed is slow and lifetime is limited.
Electronic Switching System is called Stored Program Control because the control of switching functions are programmed into memory and actions are executed by the controlling processor. There are two types of SPC(stored program control):
- Centralized SPC
- Distributed SPC
Centralized SPC: It uses a single processor to perform the exchange function.At present,Centralized SPC uses dual processor for higher reliability.If two processors are used,each must carry the full traffic load if the other fails.Further,Centralizesd SPC is divided into three sub-modes.These are Standby mode,Synchronous Duplex Mode and Load Sharing Mode.
- Standby mode:In this mode,only one of the processors will be active and other will be in standby mode.If the active processor fails,then the standby processor takes over the charge.This is further divided into two sub-modes:
- Cold standby mode:In this mode,the memory of the standby processor is not updated,so there can be disruption of calls when standby processor takes over the charge.
- Hot standby mode:Here,the standby processor is regularly updated with the details of the recent calls,so there will be no disruption of calls when change over takes place.
- Synchronous Duplex Mode:In this mode,both the processor recieves the same input and operate in synchronism to produce the same result.But the distribution of signals is done by only one processor.The other processor is updated with the details of the recent calls.The result of both the processor is compared by comparator and if any discrepancy is found then the faulty processor is taken out of the service and other processor proceeds with the task
- Load Sharing Mode:The name itself,signifies that this mode shares the load.Here,instead of comparator,an Exclusion Device is used.Here,both processors works independently and thus at any point of time,they will be performing different tasks for different calls.Exclusion Device ensures that both processors do not process the same call.If one processor fails,then other processor takes over the entire load.
Distributed SPC: In this,there is no central processor for the overall function,instead it involves a number of small processors (regional processor)to perform the task.But,there is a central processor to direct the regional processor.Since regional processors take care of the tasks such as line scanning and digit reception,therefore the cental processor is relieved of some work and thus reliability is improved.The function of exchange can be decomposed either horizontally or vertically.
So,two types are there:
- Horizontal decomposition:Here,each processor only performs some functions like call processing etc.
- Vertical decomposition:Here,all functions of the switching system are divided into smaller processing functions.
Electronic Switching System can be divided into two parts:
- Space division switching:In space division switching system,each connection is made over a different path in space.Here,the paths in the circuit are seperated from each other spatially.It was originally designsed for analog network but now it is used both in analog and digital network.
- Time division switching:Here,each connection is made over the same path in space,but at different instant of time.
Now we have to discuss the Availability of the System.Switching System must be designed to obtain Long Mean Time Between Failure(MTBF).If a system fails,it has to be rectified as soon as possible.If MTBF is longer and Mean Time To Repair(MTTR) is shorter,then the portion of time for which the system provides service is greater.So,basically the” Availability” gives the probability that the system will operate correctly when required.
In this way,we see that how we have advanced in the field of Telecommunication.We started from Manual switching system where we had to suffer from different kinds of problems and now we have come into an era of Electronic Switching System(Stored Program Control).
Modern telephone exchanges uses STORED PROGRAM CONTROL.It is only due to the use of Electronic Switching System that the call establishment and call release only takes a few seconds. It is only due to these types of advancement in the field of telecommunication,we are able to talk to our well-wishers so easily.