Standard Notes Tsn Unit II

8/6/2019 Standard Notes Tsn Unit II

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2 Telecommunication Switching and Networks

ECE study materials [www.pbtstudies.blogspot.com] Page 1

TIME DIVISION SWITCHING

Space and time switching:

A tandem switching centre or the route switch of a local exchange must be able to

connect any channel on one of its incoming PCM highways to any channel of anoutgoing PCM highway. The incoming and outgoing highways are spatially separate, so

the connection requires space switching.

A connection will occupy different time slots on the incoming and outgoinghighways. Thus the switching network must be able to receive PCM samples from one

time slot and re-transmit them in a different time slot. This is known as Time slot

interchange or Time switching.

Space switches:

Crosspoint matrix connects incoming and outgoing PCM highways. Differentchannels of an incoming PCM frame may need to be switched by different crosspoints in

order to reach different destinations.Crosspoint is a 2 input AND gate. One input isconnected to incoming PCM highway and another to connection store that produces a

pulse at required instants.Figure below shows space switches with k incoming, m outgoing PCM highways

carrying n channels. The connection store for each column of crosspoints is a memory

with an address location for each time slot which stores the number of the crosspoint tobe operated in that time slot. This number is written into the address by the controlling

processor in order to set up the connection.

The numbers are read out cyclically in synchronism with incoming PCM frame. Ineach time slot, the number stored at corresponding store address is read out and decoding

logic converts this into a pulse on a single lead to operate relevant crosspoint. Since a

crosspoint can make a different connection in each of n time slots, it is equivalent to ncrosspoints in a space division network.

fig. Space switch


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Time switches:

Time switch connects an incoming n-channel PCM highway to an outgoing n-channel PCM highway. Since any incoming channel can be connected to any outgoing

channel, it is equivalent to a space-division crosspoint matrix with n incoming and

outgoing trunks.

Time slot interchange is carried out by means of two stores, each having a storageaddress for every channel of the PCM frame.

Speech store consists of data of each of the incoming time slots (i.e. its speech

sample) at a corresponding address. Each address of the connection store corresponds to atime slot on outgoing highway. It contains number of time slot on incoming highway

whose sample is to be re-transmitted in that outgoing time slot. Information is read into

the speech store cyclically in synchronism with the incoming PCM system; however

random access read out is used. The connection store has cyclic read out, but writing isnon-cyclic.

To establish a connection, the number (X) of the time slot of an incoming channel is

written into the connection store at the address corresponding to the selected outgoing

channel (Y). During each cyclic scan of the speech store, the incoming PCM sample from

channel X is written into address X. During each cyclic scan of the connection store, thenumber X is read out at the beginning of time slot Y. This is decoded to select address X

of the speech store, whose contents are read out and sent over the outgoing highway.

Time switching introduces delay. If Y>X the output sample occurs later in the same

frame as the input sample. If Y


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Time Division switching networks:

The figure below shows a Space-Time-Space (S-T-S) switching network. Each ofthe m incoming PCM highways can be connected to k links by crosspoints in the A

switch, and the other ends of the links are connected to the m outgoing PCM highways by

crosspoints in the C switch. Each link contains a time switch. To make a connectionbetween time slot X of an incoming PCM highway and time slot Y of an outgoing PCM

highway, it is necessary to select a link having address X free in its speech store and

address Y in its connection store. The time switch is set to produce a shift from X to Y.

Time-Space-Time (T-S-T) switching network:Each of the m incoming and m outgoing PCM highways are connected to a time

switch. The incoming and outgoing time switches are connected by the space switch. To

make a connection between time slot X of an incoming highway and time slot Y of an

outgoing highway, it is necessary to choose a time slot Z which is free in the connectionstore of the incoming highway and the speech store of the outgoing highway. The

connection is established by setting the incoming time switch to shift from X to Z setting

the outgoing time switch to shift from Z to Y and operating the appropriate crosspoint attime Z in each frame.

Time

switch

(n x n)

Time

switch(n x n)

klinks

m outgoing

highways

m incoming

highways m x k kx m

A switch C switch

Fig. Space-time-space (STS) switching network

Timeswitch

(n x n)

Time

switch

(n x n)

Time

switch

(n x n)

Time

switch

(n x n)

B switch

m x m

moutgoing

highways

m

incoming

highways

Fig. Time-space-time switching network


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Bidirectional paths:

PCM transmission systems use four wire circuits, it is necessary to provide separatepaths for the send and receive channels. One way of doing this would be to provide a

separate switching network for each direction of transmission. However this may be

avoided by connecting the send highways of both incoming and outgoing circuits to one

side of the switch and the receive highways to the other side as shown in the figure.

In an STS network, the same speech store address in the time switch may be usedfor each direction of transmission.

In a TST network, speech in the two directions must be carried through the space

switch using different time slots.

Concentrators:

A concentrator connects to a PCM highway, a number of customers line unitsgreater than the number of time slots on the highway. In a simple concentrator, the

customers CODECS are all connected to the common highway and each may use any

time slot. A codec is operated in the required time slot by means of a connection store.Since a concentrator is connected to the route switch by a PCM highway, it may be

located at a distance from the main exchange. The concentrator can be controlled by thecentral processor in the main exchange by means of signals sent over the PCM link. IfPCM link between a remote concentrator unit and the main exchange fails, customers on

the concentrator lose all service. Duplicate PCM links are provided.

The control functions of the concentrator may be enhanced to enable it to connect

calls between its own customers if PCM link fails. Facilities must be added to receive andanalyze address signals, generate tones and make cross-switch connections between

customers lines. The unit is known as a remote switching unit.

PBX (Private Business Exchange) Switches:

A large PBX may use a switching network similar to that of a public exchange.

However, a small PBX may only generate sufficient traffic for all its connections to bemade over a single highway. All its parts, i.e. those for extension lines, exchange lines

and the operators position have CODECS connected to a common highway as shown in

the figure below.

Fig.Bidirectional transmission through time-division switching network

Outgoing

trunk

Incoming

trunk


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The CODECS are operated in the required time slots by a connection store. In order

to increase the line capacity of the PBX, the number of time slots on the common

highway may be increased by using 8-bit parallel transmission instead of serialtransmission.

Digital cross-connect units:

For a telephone call, a connection is made through a digital switching network at

the start of a call and cleared down as soon as the call ends. However a similar digital

network may be used for semi-permanent connections. It is controlled manually from anoperating terminal instead of automatically by the processor of an exchange. Such a

switching network is called a digital cross-connect unit. It is sometimes called a slow

switch in contrast to a fast switch used to connect telephone calls and connections made

by a digital cross-connect unit are called nailed-up time slots.Two functions that can be performed by digital cross-connect units are

Grooming: In grooming, 64kbits/s channels on a common PCM bearer are separated for

routing to different destination.Consolidation: In consolidation, channels on several PCM bearers that are not fully

loaded are combined onto a smaller number of bearers thereby improving utilization of

PCM systems.

Grades of service of T-D switching networks:

S-T-S network:Mode 1: B1 = [1-(1-b)

2]

k

Mode 2: B2 = [B1 + c(1-B1)]n

T-S-T network:Mode 1: B1 = [1-(1-b)

2]

n

Mode 2: B2 = B1

Connection store

Processor

Line

units PCM

highway

Extension lines

Operators console

Exchange lines

Fig. Trunking of a digital PBX


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Non-blocking networks:

Time division switching networks often have large values of connectivity and aretherefore quasi non-blocking. Time division networks can also be strictly non-blocking.

A time division switching network will be non-blocking in the strict sense if its

equivalent space division network is strictly non-blocking.A three stage space division network whose primary switches have n inlets and

tertiary switches have n outlets is strictly non-blocking if there are 2n-1 secondary

switches.

STS switch is made strictly non-blocking by providing 2m-1 time shifting links.TST switch is made strictly non-blocking by providing more time slots (64 instead of 32).

Synchronization:

Frame alignment:

If all exchange clock pulse generators are in perfect synchronism, there will be time

differences between the starting instants of different PCM frames entering a digital

exchange. To solve this problem, the line terminating unit of a PCM junction stores theincoming digits in frame-alignment buffer. Digits are read into this buffer at the rate, fa,

of the incoming line beginning at the start of each frame. They are then read out at the

rate fb of the exchange clock, beginning at the start of the PCM frame of the exchange.To cater to the maximum amount of misalignment between a digital line system and

the exchange, the aligner must have a buffer capacity of at least one frame. This

introduces delay additional to that caused by time switching. A frame alignment buffercaters perfectly to a constant misalignment.

However if the exchanges at the two ends of a line have slightly different clock

frequencies, the contents of the buffer will change until it either overflows or empties. Ifthe buffer overflows, its contents are erased so that it can start refilling. If the buffer

empties completely, the contents of the previous frame are repeated to refill it. In either

case, a complete frame is in error. This is known as frame slip. Of course, slip can also

arise from malfunctions in switching or transmission systems.

Synchronization networks:

In a synchronous digital network, just one or two atomic reference clocks control

the frequencies of the clocks of all the exchanges in the network. This is sometimes

Timing

extraction Buffer

Digital

switch block

Exchange clock

ReadWrite

Digitalsignal

with

timing

at fa

fafa


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called despotic control. For this purpose, a synchronizing network is added to PSTN in

order to link the exchange clocks to the national reference standard.The local clock in each exchange is provided by crystal oscillator whose frequency

can be adjusted by a control voltage. This control voltage is derived from incoming digit

stream on synchronizing link which is used to determine whether the exchange clock rate

should be increased or decreased or left unchanged. Adjustments are made periodically as

a single quantum increase or decrease. This ensures that exchanges maintain the samelong term average frequency, although short term deviations may occur. This is known asmesochronous working.

Synchronizing links may be unilateral or bilateral. In first case, there is a master-

slave relationship, the clock frequency of the exchange at one end of link is controlled

solely by exchange at other end. In second case, there is a mutual relationship; eachexchange influences the frequency of the other. The principles of these methods are

shown in the figure below.

Exchange A Sync link

a.

b.

c.

d.

a.Single ended unilateral system b. Single ended bilateral system

c. Double ended unilateral system d. Double ended bilateral system

Exchange B


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Telecommunication Network

A telecommunication network must have some common standards in order toobtain a better performance and are as follows:

a) A transmission plan

b) A numbering plan

c) A charging plan

d) A routing plane) A signaling plan

f) The grades of serviceg) Switching equipment performance

h) Interconnection with other networks

i) Managing networksThese above standards are dependent and are related to each other. A

telecommunication network must give a better service to customer at the prices that they

pay. Therefore networks must compromise between performance and cost.

Analog Networks:

The number of levels in the public switched telephone network depends on the costof transmission and switching. Suppose consider an area that is small and highly

populated will have short distance between its primary centres and traffic is very high

between them.

An area that is large or less populated will have larger distances and lesser trafficbetween primary centres. Therefore the cost to provide direct routers between primary

centres is much less in the former case than in the latter. As a result, the small area and

highly populated will have more primary centres directly connected and few levels. Thedifferences are as shown in below.

fig. UK analog network

Fig above has three levels of switching center. They are group switching

centres(GSC). District Switching Centres (DSC) and Main Switching Centres(MSC). The

four wire transmission is used for the trunk network. The circuits LE and GSC employedtwo-wire switching. The traffic between trunk circuit is handled by direct circuits

between GSCs.


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The North American network is shown in fig below which differs from fig.7.1 in

several respects.

Integrated Digital Networks:

During 1950s, analog networks came into existence. Then, as a result of highcapacity, the digital optical fiber transmission system were designed, which reduced the

per circuit cost of long distance transmission system. Electromechanical analog space-

division switching systems have been replaced with electronics digital time division

switching systemThe need for channeling equipment has been eliminated by the use of both the

transmission system and switching system using digital time-division multiplexing. So

transmission and switching of channels is done through PCM framer, which resulted in a

reduced cost of the equipment. Further cost reduction is obtained by replacing channel

associated signalling with common channel signalling.Integrated Digital Network (IDN) has the compatibility of both digital transmission

and switching. This cost reduction results in changes in network configurations such asmore number of direct router between trunk exchanger and these helps in a hierarchy

with fewer levels. If there is any kind of break-downs in between the network, others can

still carry the traffic. So the advantages are ability to cope with failure and overload.Another advantage of the equipment for a digital switching network is that less

space is occupied as compared to a space division switching network. As shown in fig

below, the multiplexers and remote connections replaced the small local exchanges. Sohaving few local exchanges helps in fewer and larger junctions. So cost reduction of

junction plant and tandem switching. Advantage of having fewer local exchanges reduces

the cost of operation as well as maintenance.


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fig. Existing analog network

fig. Restructured analog network

The IDN of British telecom is shown in fig below, DCCE (Digital Cell CenterExchanges) serves local areas. DCCE is accessible by customers via remote concentrators

unit (RCU).

fig. British telecom integrated digital network


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fig. Component networks of an IDNAs shown in fig above, the data for common channel signalling between digital

exchanges form a separate signalling network. This signalling network uses channels in

the basic transmission network as very similar to PSTN. Another advantage of digitaltransmission link is to provide telephone circuits that an analog network cannot provide a

satisfactory overall loudness rating for every possible connections. But digital

transmission links have 0 attenuation. So this will apply to any connection setup in anIDN.

Integrated Service Digital Network (ISDN):ISDN is a set of digital transmission protocols defined by ITU. ISDN is a modified

version of Integrated Digital Network (IDN) that provides an end-to-end digital

connectivity to support a wide range of services, including voice and non-voice services,to which users have access by a limited set of standard multi-purpose user network

interfaces. Digital networks are important for two basic reasons, high quality service andspeed.

One of the advantages of ISDN is its compatibility with much of the existinginternational telecommunication infrastructure.

ISDN Services :

ISDN must have the ability to

Handle voice, audio, interactive data, fax, video

Capability to transport continuous traffic and bursty traffic

Fast call establishment

Allocate bandwidth on demandTo handle wide range of transmission speeds

Low bit error rates

Communication securityTypes of ISDN channels

Bearer B, channels for user voice and data

B channels for circuit switched connection just like a regular telephoneconnection

Integrated

digitalnetwork


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B channels 64 kbps

Data D channels carry setup, signalling and tear down information

D-channel use packet-switched connections

D-channel 16 kbps / 64 kbps

Two forms of ISDN access are as follows:

Basic Rate Interface (BRI)Two 64 kbps B channel + one 16 kbps D-channel => 2 B + D

Primary Rate Interface (PRI)-23B + D channel for T1-30 B + D channel for E1

BRI is used in residential services.

Data Rate for BRI in 2 x 64 + 16 + (overhead for synchronization and framing)=144 + (overhead for synchronization and framing)

=192 kbps

PRI is used for connections between a PBX and a

CO. Data rate for PRI = 1.544 Mbps for T1= 2.048 Mbps for E1

ISDN Architecture:

Reference points: Conceptual points used to separate groups of functions.

Functional group: A group of functions that might be found in a typical device.

Local exchange (LE): The ISDN central office.

Terminal equipment functional group, TE1: Includes telephones, fax machines,

computers.

Terminal adapter functional group, TA: Provides the functions needed to attach a non-ISDN device to ISDN.

Network termination functional group, NT1: Connects between the signals/ wiring

from ISDN device and the signalling/ electrical standards adhered to by the local office.

NT2: A digital PBX or a LAN responsible for switching and multiplexing ISDNservices. When NT1 and NT2 are used. The S and T interfaces are separated by the NT2

which is able to reallocate channels in the form of BRI or PRI connections.


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ISDN Call Control Signaling :

ACM: Address Complete MessageANM: Answer Message

IAM: Initial Address Message

REL: Release

RLC: Release Complete

The calling party starts the sequence by sending a SETUP message to the network. In the

SETUP message, the users terminal includes the information that the network needs inorder to establish a call. Examples of the information are the desired bearer service

capability, the identity of the called party, the B-channel used for call etc. Bearer servicecapability is the essence of ISDN. Bearer capability can be offered in circuit, frame or

packet mode.

Cellular Radio NetworksThe cellular radio networks offers the mobile and portable telephone stations the

same service provided fixed stations over conventional wired loops. It has the capacity toserve tens of thousands subscriber in a major metropolitan areas. The principle is

illustrated in fig below, where the entire geographical area is divided into cells. A call is

the basic geographical unit of a cellular network. Each cell uses one of the availablefrequencies. All cells have the same number of radio frequencies.

Each cell consists of a Radio Base Station (RBS), Mobile Stations (MS), A mobile

switching center (MSC). The Radio Base Stations (RBSS) in a group of cells areconnected to a mobile switching center (MSC).

The MSCs are lined by fixed circuits and interfaced to the PSTN.

In order to make a call , the mobile user access the cellular network through a base

station. For a call to a mobile user, initially the directory number of the user must beknown. Therefore for the network, it is necessary to determine in which cell the user is

located. The network must keep track of the user during the period of conversation,

because the user move form one cell to another cell. This process of moving from one

cell to another cell is called handoff. The mobile telephone continuously monitors acontrol channel and so receives the information that identifier the area. If the received


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signal strength falls below threshold, the mobile telephone automatically switches to

another control channel.Each mobile telephone has a home switching center. This contains a home location

register, which stores customers data, including the directory number, equipment serial

number and class of service. There are number of cellular radio standards in use. These

include

1) The advanced mobile phone system2) The N order mobile Telephone Service (NMT)3) Total Access Communication System (TACS)4) The Nippon Automatic Mobile Telephone System (NAMTS)

Intelligent NetworkFor many interconnected exchanges in a network, it is costly and the time

consuming to make software upgrades and hardware changes. Consequently, these

changes are made very infrequently and it is very difficult to introduce the new services.A possible solution to this problem is to separate the software that controls the basic

function of exchange from the software required for providing more complex services.

These include free phone services, calling card service, and value added services. Themore complex services can be controlled by a centralized processor called a service

control point (SCP). A telecommunication network that has been enhanced in this way is

called intelligent network (IN). The exchange that makes the required connection is

called a service switching point (SSP).

The architecture of intelligent network is shown in fig above. The SCP is a

centralized processor and its software is organized in three levels.

a) Node software : This software provides common facilities such as signaling,database access, transmission and alarm.

b) Services logic program (SLP): These are program that control various services.c) The services logic execution environment (SLEE): This program hosts various

SLPs and network with the basic call control and switching of SSP.

There is a Service Management System which is connected to all the SCPs by datalinks. This allows the addition of new customers, updates of data and data reloads.


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At present IN use a centralized structure as shown in fig.7.12. In future IN may use

a decentralized structure. The decentralized IN may give faster response and reduce loadon the network and it is cheaper and widely used services. However, centralized SCP

control is better to enable services to be provided.

Private Networks

Private networks are secured leased lines that were access protected by aunique user name and a password , private network encrypt data transmitted from anaccess point before it bits the network and decrypt all the received traffic before it gets

the user. This basic functionality not the medium the PN spans, is the true definition of a

PN.Fig below. shows the overall topology of a private network.

The private network data pathA typical end to end PN data could contain:

Several machines not under control of the corporation

A security gateway

An internal segment

An external segment

Private Data NetworksPublic data networks consists of two parts:

1)The data network identification code (DNIC) of 4 digits2)The network terminal number (NTN) of 10 digits.

DNIC consists of a data country code DCC of 3 digit followed by a network digit.

By this means a single country can have upto ten different data networks. A

country can have more than one DDC. The format user for the ten digits NTNcan be determined by the network from another country.


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CONTROL OF SWITCHING SYSTEMS

Switching systems were fast evolving from being manually controlled to being

controlled by relays and electronically. The change from manual to Strowger systembrought about a change from centralized to distributed control. Also it became possible to

perform a number of functions using the same hardware by using different programs.

This revolutionary technique is known as the stored program technique.

Call processing functions:For any call to be made, a sequence of operations takes place in which the calling

and the called customers lines and the connections to them change from one state to

another. The various states for a simple call between two customers whose lines

terminate in the same exchange is described below.

Idle state: Initially, the customers hand-set is in the on-hook condition. The line is said

to be idle (state 0). The exchange meanwhile is continuously monitoring the line to detecta change in state.

Call request signal: When the customer lifts the handset, current flows into the line and

a signal is sent to the exchange. This is also known as seize signal.

Calling line identification: The exchange now detects the line in which the callingcondition originated. For this equipment number (EN) to directory number (DN)

translation occurs.

Determination of originating class of service: The originating class of service (COS)

corresponding to the range of services available to the calling customer. In an SPC

exchange, the customers COS is stored as data.

Identification of calling party: If the originating COS indicates a multi-party line, it is

necessary to ensure that the correct party is billed for the call.Connection to the calling line: The exchange now makes a connection to the callingline.

Proceed to send signal: The exchange now sends a signal (dial tone) to the calling party

indicating to him to send the identity of the number he wants to call. The exchange nowwaits for this information (state 1).

Address signal: The calling customer now dials the number of the person he wants tocontact in his hand-set. This is the address signal to the exchange.

Selection of outgoing line termination: The exchange now determines the requiredoutgoing line termination from the address signal it just received. For this DN-EN

translation is done.

Determining the terminating class of service: Just as in the case of the caller, the COSof the called line must also be checked.

Testing called line termination: The exchange first tests the called line before making a

connection to it because it may be busy or out of service.

Status signal: A status signal, called a call progress signal is now sent back to inform thecaller regarding the progress of the call. If the busy tone or number unobtainable tone is

sent, then the caller replaces the hand-set and connection is released. Idle (0) state is

resumed.


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Connection to called line: If the called line is obtainable and free, the exchange now

makes a connection to it.

Alerting called customer: The exchange now sends an alerting signal to the called line,

i.e. the phone at the called line end now rings indicating the called party to answer the

call. At the same time, it sends a ring-back tone to the caller. The exchange now waits for

an answer (state 2).

Answer signal: When the called customer answers by lifting the hand-set, the line islooped and current flows. This provides an answer signal to the exchange and thus it

ceases to send the ringing tone back to the caller and the called line. If the customer doesnot reply, the calling line hangs up and idle (0) state is resumed.

Completion of the connection: On receiving the answer signal from the called customer,

the exchange completes the connection between the called and the calling parties.

Conversational state: Since connection is complete, they can converse as long as they

want (state 3). The exchange supervises the connection to detect the end of the call.

Clear signal: When each customer replaces the hand-set, line current ceases and

provides a clear signal to the exchange.

Release of connection: The exchange then clears down the connection and hence idle (0)

state is resumed.Since the calling party is billed, the connection is released after the calling party

hangs up.Various other situations resulting in problems can occur in case one of them hangs

up and the other does not. These problems are rectified using certain time based circuits.

Signal exchanges

Signals sent in the direction away from the caller (towards the called line) are calledforward signals. Signals sent towards the caller (and away from the called line) are called

backward signals. Also each signal should produce a response in the opposite direction,

thus verifying correct operation as follows:

1. The call request signal is answered by the proceed to send signal.2. The address signal is answered by the call status signal.3. The answer signal is a response to the alerting signal4. The caller responds to the answer signal by commencing the conversation.5. The backward clear signal is a response to the forward clear signal (or vice versa)

For a call over a junction between two exchanges, the actions between the

customers calling signal and connection to an outgoing line occur at the originatingexchange. The exchange then sends a seize signal to the terminating exchange. After the

originating exchange has sent the address information to the terminating exchange, the

actions from receipt of address information to alerting the called customer take place at

the terminating exchange. When the answer signal is got from the called customer,

connection is made and after the conversation has ended, then connections are released.


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Called

terminal

Calling

terminal

Switching

system

Alert

Answer

Forward clear

Backward clear

Forward clear

Backwardclear

Call request (seize)

Proceed to send

Address

Status

Answer

Fig. Signal exchange diagram for a local call

Idle

Call request (seize)

Connect to calling terminal

Proceed to send

Address signal

Connect to called terminal

Status signal

Alert called terminal

Answer

Connection

Clear signal

Disconnect

Time not to scale

E

v

en

t

Fig. Timing of signals exchanged for a local call

Idle


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State transition diagrams

The sequence of operations discussed previously can be represented by a graphicalmethod known as state transition diagram (STD). The various symbols used are as

follows:

State boxes: They are labeled with a number and a title. Address information canalso be included.

Event boxes: Rectangular box. If action is that of sending a signal, thenprotruding arrow is used.

Decision boxes: Diamond shaped boxes used in computing flowcharts.

Below is an example of a STD for a local call:


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Common control

A common control performs a specific call-processing function. Thus, the control ofthe switching system employs functional division. A common control is hence brought

into play only when required and released when not.

Switching networks are generally lost call systems. When common controls arebusy, calls offered to them are not normally lost but are delayed. The traffic performance

of common controls therefore can be discussed using queuing theory.

Common control units have been designed using relays, electronic digital signals

and stored program control (SPC). An SPC common control enables the same logiccircuits to perform different tasks under the control of different programs. A separate

small network using switches similar to those in the main switching network can be used

to common control the trunks as required.

Control of the exchange is distributed among a number of processes, each

performing a specific function. These communicate with each other and with customersconnection by means of temporary connections made through the main switching

network, as required.

When it is necessary to exchange signals between numbers of functional units butthe signal exchanges need only take place one at a time, the units can be connected by a

common bus. The bus can be used both in serial as well as parallel modes.

In scanning, electronic gates, forming the equivalent of a rotating switch, connect a

common control to each trunk in turn. Since, at any time only one trunk can communicatewith the common control, there can be no contention.

However scanning presents the following requirements which may conflict,

In every cycle, the scanner must connect the common control to each trunk for asufficiently long period to exchange the required signals.

The period of a complete scanning cycle must be sufficiently short for thecommon control to detect every change that occurs in the states of the trunks

being scanned.


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In case of conflict between the requirements, then start-stop scanning is used.

Reliability, availability and security

Switching systems must be reliable. The strowger system was very fault tolerant

because of its distributed nature in common control and particularly centralized control,makes the operation of an exchange dependent on a small number of equipments. The

ideal time interval between failures is fixed as a standard known as Mean Time BetweenFailures (MTBF). Once failure of equipment occurs, fault must be diagnosed andrectified. The longer the MTBF and the shorter the Mean Time To Repair (MTTR), then

greater is the proportion of time for which an equipment provides service. This

proportion is called the availability of the equipment.

Availability = MTBF/ (MTBF + MTTR)This gives the probability that the equipment will operate correctly. Now

unavailability is defined as 1availability = MTTR/(MTBF + MTTR)

Security

The failure of an entire exchange is a very serious matter. So availability must be as

close to unity as possible.If common control is used, an exchange that had the minimum possible

configuration of equipment would be unlikely to obtain adequate availability, even if

constructed with the most reliable components.The security measures used are as follows:

Line circuits :noneSwitching network :none or partial duplication)

Common controls :1 in n sparing

Central processors :replicationExplanation:

Since a terminating unit for a customers line contains relatively fewcomponents, it should suffer faults less frequently than the line itself.

Therefore no additional measures are needed.

I/O

Control bus

CPU

1

CPU

2

Read/

Write

memory

Backing

store

MMI

Fig. Functional units interconnected by a bus


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For space division switching network, since a choice of many paths is therefor a connection, a fault in one does not affect the entire system greatly. This

is not so in the case of a time division network, less equipment is used, thissaving in the cost can be used to provide more reliable equipment.

For common control equipments, such as registers, 1 in n sparing is used.Here when only n equipments are actually needed for functioning, n+1

equipments are provided. This way, when one fails other is used. Before onemore fails, the previous one can be repaired.

If a single processor is handling all the calls, then its failure would result inmass inconvenience. Therefore two or more processors are provided. Also,

when one is being upgraded, the others handle the traffic without anydeterioration in the grade of service.

Stored program control

Processor architectureIn order to obtain adequate security, a switching system with a minimum of two

processors are required. They can be configured in the following ways:

Worker and Standby

Load sharing

Synchronous operationIn a system with a worker and a standby, either a cold or a hot standby may be used.

In cold standby, the spare processors memory is not updated resulting in disruption

during change-over. Whereas in hot standby, there is no disruption because the spare

processors memory is constantly updated.In load sharing, as the name itself suggests, both the processors perform different

tasks for different calls. This causes problems during change-over. Also, contention must

be prevented. Additional processors can be provided as the traffic grows.In synchronous dual operation, both processors receive identical inputs and operate

in synchronism to produce the same output at the same time. Comparisons of outputs

result in fault detection. A test program is used to detect the faulty one. This cannot beused in case of a software fault.

In large exchanges, to handle more traffic, a multi-computer or a multi-processor is

used. The load in a multi-computer is usually divided between them on a geographicalbasis, each handling processing for a different part of the exchange. In multi-processor

system, the processors share programs and data stores; each one controls any part of the

exchange, using any program.A software fault is rectified using roll-back, i.e. re-configuration.

Distributed processing:

The reduction in the cost of processing brought about by the microprocessor has

enabled the control of switching systems to be de-centralized. Here, routine tasks,associated with parts of the system or particular functions are delegated to separate small

processors called regional processors. Since a connection involves a number of different


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functions and passes through different parts of the system, a central processor is still

required to direct the regional processors and also to perform complex functions.In early SPC systems, fully centralized systems placed a restriction on the amount

of directly addressable memory that was available. With distributed microprocessors,

each one with its own extensive RAM, limitations are removed. Since regional processors

are used, the complexity is reduced and reliability and maintainability are improved. As a

result of data being stored at the regional processors, no processor requires access to datathat is beyond its direct addressing capability. However copies of all software are needed

as back-up.

Documents

Standard Notes Tsn Unit II