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Dar es Salaam institute of Technology (DIT)

ETU 07420

Switching Systems

Ally, J

jumannea@gmail.com

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Telecommunication Switching Systems

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Introduction Telecommunication system is an important and integral part of

modern society. In addition to public switched telephone network (PSTN), it plays

vital role in radio and television networks, internet and Asynchronous transfer mode (ATM) networks.

The switching system provides various services to the subscribers. The switching system is a collection of switching elements arranged

and controlled in such a way as to setup a communication path between any two distant points.

The process of transferring message from one place to another (or line to line) is called switching related to outside the switching plant or systems.

There are three types of switching namely a circuit switching, message switching and packet switching.

In telecommunication switching, the circuit switching and message switching are used.

The switching technique used in computer communication network or data transfer is packet switching.

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Classes of Switching System There are three classes of switching system based on the division of

information in space, time and frequency. They are Space division switch, Time division switch and Frequency

division switch. The space division switch provides fixed path for the entire duration of

a call. Simplicity, unlimited bandwidth, cross talk limitations are the advantages of space division switches. But these space switches are slow to operate, bulky, and involves large amount of wiring.

In time division switching, all inlets and outlet one connected to a common switch mechanism. The switch is fast and compact. Thus time division switches have more practical value only when the signal is already in digital form.

In frequency division switching, the incoming signal is modulated onto a different carrier frequency. Switching is achieved if each outlet is provided with a demodulator which can have its carrier frequency changed. Other than radio communication, until recently, there was no practical applications with this switching. Frequency division switching is now finding applications in demand assigned satellite communication links.

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Requirements of Switching System High availability. The telephone system must be very reliable. System reliability can

be expressed mathematically as the ratio of uptime to sum of the uptime and down time.

High speed. The switching speed should be high enough to make use of the switching system efficiently. The speed of switching depends on how quickly the control signals are transmitted.

Low down time. The down time is the total time the switching system is not operating satisfactorily. The down time is low enough to have high availability. The unavailability of switching system may be due to failure of equipments, troubles in transmission media, human errors in switching etc.

Good facilities. A switching system must have various facilities to serve the subscriber. For example wake up calls, address identification on phone number or phone number identification on address, recording facilities, quick service for the emergency numbers, good accessibility etc. Also it should have good servicing facilities in case of repair of equipments, skilled technicians, standby systems, etc. Good facilities is possible any switching system whether it is at rural or town or in cities, if that exchange is not overloaded.

High security. To ensure satisfied or correct operation (i.e. providing path and supervising the entire calls to pass necessary control signals) provision should be provided in the switching system. Duplicated common control circuits, registers, processors and standby systems are used provide high security.

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Functions of Switching System The switching office performs the following basic functions irrespective of the system

whether it is a manual or electromechanical or electronic switching system. Identity. The local switching center must react to a calling signal from calling

subscriber and must be able to receive information to identify the required destination terminal seize.

Addressing. The switching system must be able to identify the called subscriber from the input information. .

Finding and pathsetup. Once the calling subscriber destination is identified and the called subscriber is available, an accept signal is passed to the switching system and calling subscriber. Based on the availability, suitable path will be selected.

Busy testing. If number dialled by the calling subscriber is wrong or the called subscriber is busy (not attending the phone) or the terminal may be free (lifting the phone) but no response (not willing to talk or children handling), a switching system has to pass a corresponding voice message or busy tone after waiting for some time (status).

Supervision. Once the path is setup between calling and called subscriber, it should be supervised in order to detect answer and clear down conditions and recording billing information.

Clear down. When the established call is completed, the path setup should be disconnected.

Billing. A switching system should have a mechanism to meter to count the number of units made during the conversation. The cumulative number of units made for a particular duration by the calling subscriber is calculated. This information and if any should be sent to the called subscriber.

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Classification of Switching System In early days, the human exchange provided switching facilities.

In manual exchanges, a human operator and the elements like switches, plugs and sacks were used to connect two subscribers.

Around 1890’s many electromechanical switching devices were introduced.

Till 1940, different electromechanical switching system were invented like strowger and cross bar switching system.

The later invention of electronic switching system (ESS) which uses stored program control (SPC) and computer controlled switching systems are presently dominating the worldwide exchanges.

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Stored Program Control (SPC) Exchange In 1965, Bell system installed the first computer

controlled switching system which uses a stored program digital computer for its control functions.

The SPC concepts permits the features like abbreviated dialing, call forwarding, call waiting etc.

The SPC provides significant advantages to end users.

The SPC enables easier number changes, automated call tracing message unit accounting (for billing) etc.

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Basic of SPC In Stored Program Control (SPC), a program or a set of

instructions are stored in its memory and executed automatically one by one by the processor.

Carrying out the exchange control functions through programs stored in the memory of a computer led to the name stored program control.

A computer can be programmed to test the conditions of the inputs and last states and decide on new outputs and states.

The decisions are expressed as programs which can be rewritten to modify or extend the functions of control system.

All switching systems manufactured for use as public switching systems now use computers and software programming to control the switching of calls.

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Basic Control Structure of SPC The SPC uses processors designed to meet the various requirements

of the exchange. More than one processors are used for the reliability. Also the SPC system uses distributed software and hardware

architectures. To carry over the maintenance functions of the switching system, a

separate processor is used.

There are two types in SPC exchanges, namely centralized SPC and distributed SPC.

How to organise stored program control (SPC) There are two approaches to organizing stored program control

(SPC): Centralized: In this control, all the control equipment is replaced by

a single processor which must be quite powerful. Distributed: In this control, the control functions are shared by

many processors within the exchange itself. Typically Centralized SPC Organization

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Centralized SPC Early electronic switching systems are centralized SPC exchanges and

used a single processor to perform the exchange functions. Presently centralized exchanges uses dual processor for high reliability.

All the control equipments are replaced by the processors. A dual processor architecture may be configured to operate in standby

mode, synchronous duplex mode, and Load sharing mode. Standby mode: In this mode, one processor is active and the other is on

standby, both hardware and software wise. The standby processor brought online when active processor fails.

Synchronous duplex mode: In this mode, hardware coupling is provided between the two processors which execute the same set of instructions and compare the results continuously. If a mismatch occurs, the faculty processor is identified and taken out of service immediately.

Load sharing mode: In load sharing operation, an incoming call is assigned randomly or in a predetermined order to one of the processors which then handles the call right through completion. Thus both the processors are active simultaneously and share the load and the resources dynamically.

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Availability

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Availability (2)Dual Processor. A dual processor system is said to have failed only when both processor fails and the total system is unavailable. The MTBF of dual processor is given by

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ExampleGiven that MTBF = 2000 hrs and MTTR = 4 hrs. Calculate theunavailability for single and dual processor systems for 10 years and 30 years.

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Distributed SPC The introduction of distributed SPC enabled customers to be provided

with a wider range of services than those available with centralized and electromechanical switching system.

The distributed SPC offers better availability and reliability than the centralized SPC.

Vertical decomposition, the exchange environment is divided into several blocks and each block is assigned to a processor that performs all control functions related to that block of equipments.

Horizontal decomposition, each processor performs one or some of the exchange control functions.

Advantages of SPC in Telephone Switching Easy to control

Easy to maintain

Flexible

Wide range of services can be provided to customers.

Increase level of automotive in switching

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Switching Techniques Switches are hardware and/or software devices used to connect

two or more users temporarily. Message switching, circuit switching and packet switching are the

most important switching methods. The terminals of the message switching systems are usually

teleprinters. This switching is also called store and forward switching.

The circuit switching sets up connection between the telephone, telex networks etc, which interchange information directly. Thus circuit switching is also referred as lost call system.

The modified form of message switching is called packet switching.

Packet switching system carries data from a terminal or computer as a short packets of information to the required destination.

This system is midway between message switching and circuit switching.

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Message Switching In message switching, there is no direct link between the sender and

the receiver. A message delivered to the destination is rerouted along any path

before it reaches the destination. It was common in 1960’s and 1970’s. In message switching no complete connection is required. Thus the

each message includes a header contains the destination address, routing information and priority information (for special cases).

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Circuit Switching Circuit switching creates a direct physical connection between

two devices such as phones or computers. In order to setup a direct connection over many links it is

necessary that each link to be simultaneously free.

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Packet Switching The first packet switching system Arpanet, was developed

by the U.S. Defence Advanced Research Projects Agency (DARPA) in 1969.

The data stream originating at the source is divided into packets of fixed or variable size.

Each packet contains a portion of the user’s data plus some control information.

Routing control decides how the network will handle the stream of packets as it attempts to route them through the network and deliver them to the intended destination.

The routing decision is determined in one of two ways. They are Datagram Virtual circuit

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Datagram In datagram, each packet within a stream is independently routed. A routing table stored in the router (switch) specifies the outgoing link

for each destination. The table may be static or it may be periodically updated.

In the second case, the routing depends on the router’s estimate of the shortest path to the destination.

Therefore each packet must contain bits denoting the source and destination.

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Virtual Circuit In virtual circuit, a fixed route is selected before any data is transmitted

in a call setup phase similar to circuit switched network. All packets belonging to the same data stream follow this fixed route

called a virtual circuit. Packet must now contain a virtual circuit identifier. Once the virtual circuit is established, the message is transmitted in

packets.

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Circuit Switch and Packet Switch ComparisonCircuit Switching Packet Switching

Dedicated transmission path. No dedicated transmission path.

Transmission of data. Transmission of packets.

Operate in real time. Near real time.

Message not stored. Message held for short time.

Path established for entire message. Route established for each packet.

Call setup delay. Packet transmission delay.

Busy signal if called party busy. No busy signal.

Blocking may occur. Blocking cannot occur.

User responsible for message-loss protection.

Network may be responsible for each packet but not for entire message.

No speed or code conversion. Speed and code conversion.

Fixed bandwidth transmission. Dynamic use of bandwidth.

No overload bits after initial setupdelay.

Overload bits in each packet.

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Thanks!

Technology changes but communication lasts.