Standard Notes Tsn Unit II
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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.