Telecommunication switching system

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1
Telephony -
Telecommunication Switching
System

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Simple telephone communication
□ One way communication (Simplex)
□ Microphones and earphones are transducer.
□ Carbon microphones –
■ Do not give high fidelity signals
■ Gives strong electrical signals.
■ Acceptable quality
Earphone
Microphone
L
V

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□ Microphone:
□ Microphone has carbon granules in a box.
□ One side fixed, other attached to diaphragm.
□ Resistance inversely proportional to density of granules.
□ Diaphragm vibrates with sound and resistance changes.
□ V applied across box.
□ ri = ro – r sin wt
□ ro = resistance without sound
□ r = max deviation in resistance.
□ ri = instantaneous resistance
□ i = V/ {ro – r sin wt}

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□ i = V/ [ro {1 – (r/ ro) sin wt}
□ i = Io( 1 – m sin wt)-1
□ i = Io( 1 + m sin wt + m2
sin2
wt + m3
sin3
wt + …)
□ m < 1.
□ i = Io( 1 + m sin wt )
□ Carbon microphone acts as amplitude modulator.
□ m should be small to avoid harmonic distortion.
□ Energizing current Io(Quiescent current) is must.

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□ Inductor :
□ Acts as high impedance element for voice.
□ Permits DC to flow from microphone and speaker.
□ Voice goes from microphone to speaker .

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□ Earphone:
□ Converts electrical to voice signal.
□ Electro magnate with magnetic diaphragm.
□ Air gap between diaphragm and poles.
□ Voice current through electro magnet exerts variable
force on diaphragm.
□ Diaphragm vibrates and produces sound.
□ Condition for faithful reproduction:
□ Diaphragm displacement in one direction only.
□ Quiescent current provides this bias.

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□ Instantaneous flux linking poles of electromagnet and
diaphragm:
□ φi = φo + φ sin wt
□ φo = Constant flux due to quiescent current
□ φi = instantaneous flux
□ φ = max amplitude of flux variation
□ Assuming
■ vibration of diaphragm has little effect on air gap
■ Reluctance of magnetic path is constant.

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□ Instantaneous Force exerted on diaphragm is
proportional to square of instantaneous flux.
□ F = K(φo + φ sin wt)2
□ φ/ φo << 1
□ Expanding and neglecting second order terms..
□ F = K φo
2
(1 + K1 Io sin wt)
□ Force exerted proportional to input voice signal.

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Half Duplex telephone communication
□ Signal travels in both directions but not simultaneously.
□ An entity either sends or receives signal.
□ Speech of A is heard by B as well as A’s own earphone.
□ Audio signal heard by self earphone is called sidetone.
□ No sidetone: User tends to shout.
□ Too much sidetone: User tends to speak in too low volume.
□ Here entire speech intensity is heard as sidetone. Not
Desirable.
Earphone
Microphone
L
V
Earphone
Microphone

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Half Duplex circuit with Sidetone
□ At Transmitter:
□ ZL: Receiver load
□ ZB: Balancing load.
□ Earphone connected through L1 L2 L3.
□ Transmitter current I2 reaches receiver.
□ L1 very slightly different from L2 .
□ Transmitter currents I1 and I2 in opposite direction.
□ Currents divide in L1 and L2 such that very small resultant
field results.
□ Very small current induces in earpiece L3.
□ Small sidetone.

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Half Duplex circuit with Sidetone
□ At Receiver:
□ Received current flows through L1 and L2 in same
direction inducing additive field.
□ Additive signal induces in L3.
□ Strong received signal in earphone.

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Local battery exchange
□ Local battery installed at each telephone set.
□ DC supplied to transmitter. Magneto is for signaling.
□ Switch brings magneto in circuit when required.
□ Ringer has high impedance, bridged across lines.
□ At “off hook”, switch closes, DC flows through Tr.
□ Sound waves striking Tr diaphragm produces pulsating current
through primary of induction coil ,inducing AC in secondary circuit.
□ Corresponding AC flows through line reproducing sound at remote
receiver.

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Local battery exchange
□ Transformer separates transmitter and receiver ckts.
□ Prevents DC of Tr to flow through receiver.
□ Transformer may step-up voltage on line.
□ Coil matches impedance of transmitter with line.
□ Even one-to-one transformer will greatly increase percentage
change in resistance improving useful AC.
□ Capacitor is connected when number of LB sets are on same
line.
□ This ‘Sure-ring-condenser’ prevents off-hook receiver from
shunting low frequency ringing current because of high
reactance.
Induction Coil/Transformer

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Central battery exchange
□ Exchange supplies power to all phones from large
rechargeable central battery bank at exchange.
□ Subscriber lines terminated on jack mounted on switchboard.
□ One jack with light indicator for every subscriber line.

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□ As subscriber lifts handset, off-hook switch is closed
causing current to flow through handset and lamp relay
coil.
□ Lamp relay operates .
□ Indicator corresponding to subscriber lights up.
□ Operator establishes connect to subscriber through
headset key and plug-ended cord pair.
□ Cord pair has two cords connected internally and
terminated with a plug each at external ends.
□ Plug mates with jack.
□ To establishing contact, cord is plugged into subscriber
jack and keys corresponding to chosen cord is thrown in
position to connect headset.

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□ On verification that called number is free, operator
sends ringing current using plug-ended cord pair.
□ Bell B with capacitor C are always connected to circuit.
□ Capacitor allows AC ringing current from exchange to
bell but prevents the loop direct current.
□ If called party busy, called party is informed.
□ If called party answers, his indicator lamp lights up.
□ Operator connects both parties by plugging in cord
pair to called party jack.
□ In manual exchange, operator enables signaling system,
performs switching, and releases connection after
conversion.

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Signaling Tones-Automatic exchange
□ Signaling functions: establishing, maintaining and
releasing telephone conversations.
□ Done using tones in automatic switching systems.
□ Subscriber related signaling functions:
1. Respond to calling subscriber to obtain identification of
called party.
2. Inform calling subscriber that call is being established.
3. Ring bell of called party.
4. Inform calling subscriber that called party is busy.
5. Inform calling subscriber that called party is
unobtainable.

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Signaling Tones
□ Dial tone: Exchange ready to accept dialed number.
□ 33 Hz or 50 Hz or 400Hz(modulated with 25 Hz or 50 Hz)
continuous tone.
□ Ringing tone:
□ Ringing tone sent to called party.
□ Indicated to calling party by two short burst tones in a set
for 0.4s each separated by 0.2s. Two sets separated by 2s.
□ Frequency is 133hz or 400Hz.Busy Tone: burst width and gap
width both are same. 0.75s or o.375s
□ Number unobtainable:
□ 400 Hz continuous tone
□ Call-in-progress:
□ Burst duration 2.5s and off period of 0.5s.
□ Frequency 400 or 800Hz.

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STROWGER SWITCHING
□ Disadvantages:
□ Dependence on moving parts and contacts.
□ Moving parts and contacts subject to wear and tear.
□ Selector switches require regular maintenance.
□ Must be located at easily and speedily accessible
locations.
□ Problems in achieving above led to Crossbar
switching.

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CROSSBAR SWITCHING 
Principles of Common Control
□ Directorless system: Example
□
E
J
D
F
B
HC
G
A
I

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□ A to F – Two routes possible
■ Route 1 – A-B-C-J-F
■ Route 2 – A-I-H-G-F
□ All outlets are numbered to identify the paths.
□ From EX OUTLET To EX
□ A 01 B
□ A 02 I
□ B 04 C
□ C 03 J
□ I 05 H
□ H 01 G
□ G 02 F
□ J 01 F

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□ Phone number of F for A to call can be at least 4
types. e. g.
□ 02-05-01-02 A-I-H-G-F
□ 01-04-03-01 A-B-C-J-F
□ DIFFICULTIES:
□ ID no. of subscriber is route dependent.
□ User must know the topology and outlet number.
□ Number and its size for a subscriber vary depending
on exchange from which call originated.

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REMEDY: DIRECTOR SYSTEM
□ Routing done by exchange.
□ Uniform numbering scheme.
□ Number has two parts-
■ Exchange identifier
■ Subscriber line identifier.
□ Exchange must receive and store the digits dialed.
□ Translate exchange identifier into routing digits.
□ Transmit routing and subscriber line identifier digits
to the switching network.

M
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director free to process another call.
□ Not involved in maintaining the circuit for
conversation.
□ Call processing takes place independent of switching
network.
□ User assigned a logical number independent of
physical number used for establishing call
□ Logical address translated to actual physical address
for connection establishment by address translation
mechanism.
Advantage of director-
Features of Common control system

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Control functions in Switching system
□ Four broad categories.
□ Event monitoring
□ Call processing
□ Charging
□ Operation and maintenance

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network
Line unit
Register
finder
Digital receiver
And storage
register
Initial
translator
Final
translator
Register
sender
Charging
circuit
Maintenance
circuits
Operation
control
Event
monitor
Common control
subsystem
Called
subscriber
Calling
subscriber
Call processing
subsystem
Common control switching system

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Control subsystem- function I
□ Event Monitoring
□ Events occurring outside exchange are monitored by
control subsystem
□ Where-at line units, trunk junctures and inter
exchange signaling receiver/sender units.
□ Events-
□ Call request, call release signals at line units.
□ Occurrence of events signalled by relays.

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Control subsystem
□ Off-hook-
□ Event sensed,
□ calling location determined,
□ free register seized
□ Identity of caller is used to determine line
category (pulse/tone), class of service.
□ Appropriate dial tone sent to caller.
□ Waits for dialled number.
□ Initial digits received and sent to initial
translator to identify exchange.

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Off-hook-contd.
□ Remaining digits received.
□ Initial translator determines route for call through
network.
□ Puts through call depending on class of service as----.
□ Call barring – STD, ISD
□ Call priority – during network overload only priority
call subscribers are put through.
□ Call charging – Various schemes available.
□ No dialing calls – hot-line connections.
□ Origin based routing -Emergency call routed to
nearest emergency call center.
□ Faulty line – alternate route chosen

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Off-hook-contd.
□ Initial translator also called office code translator
or decoder marker.
□ ‘marker’ because desired terminals were ‘marked’ by
applying electrical signals.
□ Out-of-exchange calls- IT generates routing digits,
passes to register sender.
□ Added to subscriber identification digits and sent to
external exchange.
□ Within-exchange calls – final translator converts
subscriber identification digits to equipment number
called.
□ All above can be done by single translator also.

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Control subsystem- function II
□ Controlling operations of switching network
□ Marks switching elements to be connected by
binary data, defining the path.
□ Commands actual connection of the path.
□ Path finding done
□ At Common control unit – map-in-memory
□ Or at switching network – map-in-network.

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Control subsystem- function II
□ map-in-memory –control unit supplies complete
data defining the path.
□ Done in Stored program control.
□ map-in-network – Actual path determined by
switching network.
□ Control unit only marks inlet-outlet to be
connected.
□ More common in crossbar exchanges.

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Control subsystem- function III
□ Administration of telephone exchange-
□ Putting new subscriber lines and trunks into
service.
□ Modifying subscriber service entitlements
□ Charging routing plans based on N/W status.

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Control subsystem- function IV
□ Maintenance of telephone exchange-
□ Supervision of proper functioning of the
exchange equipment, subscriber lines and trunks.
□ Performs tests and measurements of different
line parameters.
□ Aids Fault tracing without elaborate testing.

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TOUCH-TONE DIAL TELEPHONE
□ Disadvantages of rotary dial telephone:
□ Takes 12 seconds to dial a 7 digit number.
□ Faster dialing rate not available.
□ Step-by-step switching of strowger exchange can not
respond to more than 10-12 pulses/s.
□ Exchange tied-up for duration of call.
□ Pulse dialing limited to signaling between subscriber
and exchange.
□ No end-to-end (subscriber-subscriber) signaling
possible.
□ Limited to 10 distinct signals.

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□ Advantages of Touch Tone telephone:
□ Faster dialing rate feasible.
□ Common equipment not tied-up for the duration
of the call.
□ End to end signaling feasible using voice
frequency bands.
□ Higher number of signaling capability.
□ More convenient method of signaling, using push
button keyboard.

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4
2 3
*
5 6
7 8 9
1
0 #
697
941
852
770
1209 1336 1477

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□ Touching a button generates a tone.
□ Each tone is a combination of 2 frequencies.
□ Called Lower band and upper band frequencies.
□ PROBLEMS:
□ Speech signals may be mistaken for touch tone
signals – talk-off.
□ unwanted control actions may occur.
□ Speech signals may interfere with touch tone
signaling attempted together.

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TOUCH-TONE DIAL TELEPHONE –Design considerations
□ Protection against talk-off
□ Choice of codes
□ Band separation
□ Choice of frequencies
□ Choice of power levels
□ Signaling durations
□ Human factors and mechanical aspects

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TOUCH-TONE DIAL TELEPHONE –Design considerations
□ Choice of codes:
□ Imitation of code signals by speech and music should
be difficult.
□ Single frequency structures are prone to easy
imitation as occurring in speech and music.
□ Multi frequency code required.
□ Done by selecting N frequencies
□ Tested for presence/absence.
□ 2N
combinations using N frequencies .
□ Avoid single frequency combinations.

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TOUCH-TONE DIAL TELEPHONE –Design considerations- Choice of codes
□ Number of frequencies to be transmitted
simultaneously should be small to save BW.
□ Advantageous to keep fixed number of frequencies to
be transmitted simultaneously.
□ Hence P-out-of-N code.
□ P frequencies at a time, out of N.
□ Old multi-frequency key pulsing (MFKP) with 2/6
code gave talk-off less than 1/5000.
□ Inadequate for subscriber level signaling .

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□ Hence
□ P is 2 and N s 7 or 8 depending on requirement.
□ Frequencies divided into 2 bands.
□ One from lower and one from upper band chosen.
□ Speech contains closely spaced frequencies.
□ Codes can not be confused with speech.
□ Band separation reduces this probability.

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□ Number of valid combinations = N1 X N2
□ N1 and N2 are number of frequencies in lower and
upper band.
□ With 7 frequencies ( 4:3) 12 distinct signals by push
buttons.
□ With 8 frequencies ( 4:4) 16 distinct signals by push
buttons. –Special applications only.
□ Hence Called DTMF
□ Dual Tone Multi-frequency Frequency.

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TOUCH-TONE DIAL TELEPHONE –Design considerations- Band separation
□ Advantages of band separations:
□ At receivers, band separations can be done first
to ease frequency determination.
□ Each frequency component can be amplitude
regulated separately.
□ Speech interference can be reduced by using
extreme filters for each frequency.

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Receiver
Band
Separation
filter
LA
LB
S4
S3
S2
S1
D4
D3
D2
D1
LBF1
LBF3
LBF2
LBF4
S8
S7
S6
S5
D8
D7
D6
D5
HBF1
HBF3
HBF2
HBF4
L – Limiter
S- selector circuit
D – detector

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Receiver
□ After band filter, only one valid frequency each
side.
□ If mixed, limiter receives one strong valid
frequency and other invalid weak frequencies.
□ Limiter peaks strong signal and further
attenuates weak signal.
□ If both signals have same strength, limiter o/p is
much below full o/p and neither signal dominates.

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Choice of Frequency
□ Choice of frequency for touch tone signaling
depends on-
□ Attenuation characteristics
□ Delay distortion characteristics
□ In band 300hz to 3400Hz.
□ Required-
□ A flat amplitude response with very low
attenuation.
□ A uniform delay response with low relative delay
values.

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Choice of Frequency
1 2 3 4
2
1
4
3
Delay (ms)
f (KHz)
1 2 3 4
2
1
4
3
Attenuation (dB)
f (KHz)
Best choice- 700 Hz to 2200 Hz
Actual range – 700 Hz to 1700 Hz

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Choice of Frequency
□ Actual range – 700 Hz to 1700 Hz
□ Spacing depends on detection accuracy.
□ Minimum spacing chosen more than 4%.
□ 1:2 or 2:3 such harmonic relationship are to be
avoided-
□ between two adjacent frequencies of same band.
□ between pairs of frequencies in different bands.
□ Improves talk-off performance.
□ Chosen frequencies almost remove talk-off.

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Signal power
□ Only two frequencies.
□ Hence signal power can be as large as possible.
□ 1dB above 1mW nominal value.
□ Attenuation increases with frequency.
□ Worst case attenuation in 697-1633 can be 4dB.
□ Hence upper band frequencies powers are 3dB
higher than lower band frequencies.
□ Nominal values for output power are –
□ Lower band power = -3.5dBm
□ Higher band power = - 0.5dBm

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Signaling Duration
□ The probability of talk-off can be reduced if
check for presence of a frequency is done for a
longer time.
□ This requires subscriber to keep button pressed
for long time than normal.
□ But with efficient circuit designs, lower
durations can be tested.
□ Fast dialer pauses for 200ms between digits.
□ In normal practice tone duration 160ms and inter
digit gap 350ms followed.

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DIVA – an advantage
□ Data-in-voice-answer is a major advantage of end-
to-end signaling using touch tone dialing.
□ Examples-
□ Fault lodging in telephone services where operator
sends voice message and user sends digits
corresponding to answers.
□ Airline and railways services where user dials digits
to opt for various services (information, reservation)
in response to operator’s voice message.
□ Best example of dialing and voice conversation
together.

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STORED PROGRAM CONTROL
□ Program or set of instructions to the computer are
stored in its memory.
□ Instructions executed automatically one by one by
the processor.
□ Programs are for Telephone exchange switching
control functions.
□ Hence called SPC.

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FEATURES
□ Full scale automation of exchange functions.
□ Common channel signaling
□ Centralized maintenance
□ Automatic fault diagnosis.
□ Interactive human machine interface.
!
□ REQUIREMENTS FROM COMPUTER-
□ Telephone exchange must operate without
interruption 24 hrs a day, 365 days a year. for
years to come.
□ And hence the computers.

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Centralised SPC
□ Control equipments must be replaced by a single
powerful computer.
□ Must be capable of processing more than 100 calls
per second along with other tasks.
□ May use more than one processor for redundancy.
□ Each processor has access to all exchange
resources and function programs.
□ Each processor capable of executing all control
functions.

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Centralised SPC – no redundancy
▪
Signal
Distributor
Scanners
Processors
Maintenance
Console
Memory
Secondary
Storage: Call
Recording, Program
Storage etc
To lines From lines

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Centralised SPC – With redundancy
!
!
!
!
!
!
□ Redundancy at the level of processors, exchange
resources and function programs .
R1 R2
Rt
PpP2P1
FtF2F1
Resources
Processors
Function
programs

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□ Practically, resources and memory modules are
shared by processors.
□ Each processors may have dedicated path to
exchange resources.
□ Each processors may have its own copy of
programs and data in dedicated memory
modules.
□ Two Processor configuration is most common.

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Modes of Dual Processor Architecture
□ Standby mode
□ Synchronous mode
□ Load sharing mode

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Standby mode 
Exchange
Environment
P1 P2
Secondary Storage

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Standby Mode
□Simplest
□H/W and S/W of one processor are active.
□Other is standby.
□Standby processor is brought to line only when active
processor fails.
□Standby processor should be able to reconstitute the
state of exchange system during takeover.
■Which subscriber or trunk are busy or free.
■Which paths connected through the network.

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Standby Mode
□ Small exchanges:
■By scanning all status signals during takeover.
■Only the calls being established at time of failure are
disturbed.
□Large exchanges:
■Not possible to scan all status signals within reasonable
time.
■Active processor copies the status of the system
periodically into secondary storage.
■Most recent updates are taken by standby at takeover.
■All calls which changed status after last updates are
disturbed.

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Synchronous duplex mode
Exchange
Environment
P1 P2
M1 M2
C

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□ Both processors execute same instructions.
□ Results compared continuously.
□ During fault, comparator results mismatch.
□ Each processor have same data in its memory.
□ Each receive same information from exchange.
□ One processor actually controls .
□ Other synchronises but does not participate.

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□ During fault:
□ P1 & P2 decoupled
□ Run checkout program in each machine.
□ Call processing suspended temporarily without
disturbing the current call.
□ Good processor takes control.
□ Once repaired, other processor copies contents of
active processor in its memory.
□ Comparator is enabled.

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Load Sharing mode
Exchange
Environment
P1 P2
M1 M2
ED
Exclusion Device

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Load Sharing mode
□ Both processors are active simultaneously.
□ Both share the load and resources dynamically.
□ Both processors have access to entire exchange.
□ Incoming call is assigned randomly
□ or in a predetermined order to one of the
processors.
□ Assigned processor handles the call through
completion.
□ Both have separate memories for storing temporary
call data .

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Load Sharing mode
□ Both are in mutual coordination through inter
processor link.
□ If information exchange fails, healthy processor
takes over.
□ Exclusion devise prevents both to be active
together.
▪ Current calls are transferred.
□ Calls being established are lost.

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Load Sharing mode
□ Traffic sharing depends on the conditions of the
processors and their requirements.
□ During testing on one, other can take more traffic.
□ Gives much better performance during traffic
overloads.
□ It’s a step towards distribution control.

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Availability of the single processor system 
□ Main purpose of redundant configuration is to
increase availability.
□ Availability of single processor:
□ A = MTBF / (MTBF + MTTR)
□ MTBF = mean time between failure
□ MTTR = mean time to repair
□ Unavailability U = 1-A
□ = 1- {MTBF / (MTBF + MTTR)}
□ MTTR / (MTBF + MTTR)
□ If MTBF>>MTTR
□ U = MTTR / MTBF

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Availability of the Dual processor system 
□A dual processor is said to have failed only when both
the processors fail.
□System is totally unavailable.
□Condition – One processor has failed.
■ Other also fails before first is repaired.
□Conditional probability that second fails during MTTR
of first.
□MTBF of dual processor can be given in terms of
MTBF and MTTR of single processors as-
□ MTBFD = (MTBF)2
/ 2MTTR -using conditional
probability.

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Availability of the Dual processor system 
□ AD = MTBFD/ (MTBFD+ MTTR)
□ AD = (MTBF)2
/ [(MTBF)2
+ 2(MTTR)2
]
□ UD = 1- AD
□ = 2(MTTR)2
/ [(MTBF)2
+ 2(MTTR)2
]
□ If MTBF>>MTTR
□ UD = 2(MTTR)2
/ (MTBF)2

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Assignment
□ Given that MTBF = 2000 hours and MTTR = 4 hours,
calculate the unavailability for single and dual
processor system.
!
□ U = 4/2000 = 2 X 10-3
□ 525 hours in 30 years.
!
□ UD = 2 X 16/2000 = 8 X 10-6
□ 2.1 hours in 30 years

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Functions of control subsystem
□ Event monitoring
□ Call processing
□ Charging
□ Operation and maintenance
□ Grouped under 3 levels
Call processing
Event monitoring
and distribution
O & M and charging
Real time
constraint
increases
Level 1
Level 2
Level 3

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□ Event monitoring has highest priority, O&M and
then charging the least.
□ Real time constraint asks for priority interrupts.
□ If an EVENT occurs during O&M, it will be
interrupted and event will be handled.
□ Then O&M will be resumed.
□ Nesting interrupt to suspend low level functions and
take up higher level functions.

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□ Interrupt processing
Level n process
Suspend level n
Take up level n + x
Suspend level n + x
Take up level n + x + y
Level n + x + y complete
Resume level n + x
Level n + x complete
Resume level n

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□ When an interrupt occurs, program execution is
shifted to an appropriate service routine address in
memory through branch operation.
□ Non-vectored interrupt:
□ Branch address fixed.
□ Interrupt service routine scans interrupt signals
and decides on appropriate routine to service.

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□ Vectored interrupt:
□ Branch address not fixed.
□ Branch address supplied to processor by
interrupting source.
□ Set of address called interrupt vector.
□ Faster as can be addressed directly, without full
scanning

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DISTRIBUTED SPC
□ Control functions shared between many processors
within the exchange.
□ Low cost microprocessors offer better availability
and reliability than centralised SPC.
□ Exchange control functions decomposed
horizontally or vertically.

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DISTRIBUTED SPC – Vertical decomposition 
□ Exchange divided into blocks.
□ Each block assigned to a processor.
□ Performs all control functions related to that block
of equipments.
□ Total control system consists of several control
units coupled together.
□ Processor in each block may be duplicated for
redundancy.
□ Operates in any of three dual processor modes as
explained earlier.
□ Modular so that more can be added when exchange
is expanded.

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DISTRIBUTED SPC – Horizontal decomposition 
□ Each processor performs one or some of exchange
control functions.
□ Chain of processors for 3 functions.
□ Entire chain may duplicate for redundancy.
Call processing
Event monitoring
and distribution
O & M and charging
Real time
constraint
increases
Level 1
Level 2
Level 3

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DISTRIBUTED SPC – Horizontal decomposition 
□ Entire chain may duplicate for redundancy.
Exchange environment
EM & DP EM & DP
CP CP
O & MP O & MP
Level 3
Level 2
Level 1
EM & DP-Event monitoring and distribution

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DISTRIBUTED SPC – Level 3 processing
□ Handles scanning, distribution and marking
functions.
□ Operations simple, specialised and well defined.
□ Sets or senses binary conditions in F/F or registers.
□ Achieves control by sensing or altering binary
conditions using CONTROL WORD
□ Hard wired or micro programmed device.
□ Compare micro programmed control to Hard wired
control.

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□ Set of control words stored in memory and read
one by one.
□ Horizontal control - One bit per every control
signal.
□ Flexible and fast.
□ Expensive as large width - depends on number of
signals.
□ Vertical control – Each signals binary encoded as a
word.
□ Time too large as at a time only one signal.
□ Mid approach chosen.
□ Control word contains group of encoded words.

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□Processors for call processing.
□Called switching processors.
□Instructions designed to allow data to be packed more
tightly in memory without increasing access time.
□Processor designed to ensure over 99.9% availability,
fault tolerance and security of operation.
□I/O data transfer order of 100 kilobytes per s.
□I/O technique:
■Program controlled data transfer .
■Direct memory access.

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□ Traffic handling capacity of control equipment
limited by capacity of switching processor.
□ Load on switching processor measured by occupancy
t.
□ Occupancy: Fraction of unit time for which
processor is occupied.
□ t = a + bN
□ a = fixed overhead depending on exchange capacity
and configuration
□ b = average time to process one call.
□ N = number of calls per unit time.

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□ a depends on scanning workload which depends on
number of subscriber lines, trunks and service circuits
in exchange.)
■a estimated by knowing total lines, instructions
required to scan one line and average execution time per
instruction.
□ b depends on type of call process.
■Incoming call process time less than outgoing or transit
calls etc..
■Results of party busy or no answer etc also affect.
■ Type of subscribers (DTMF/rotary dial) also affect as
grouping PBX lines change.

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DISTRIBUTED SPC – Level 1 processing (O&M)
□ Administer the exchange H/W and S/W.
□ Add, modify and delete information in translation
table.
□ Change subscriber class of service.
□ Put a new trunk or line into operation.
□ Supervise operation of the exchange.
□ Monitor traffic.
□ Detect and locate fault and errors.
□ Run diagnostic and test programs.
□ Man-machine interaction.
!
▪

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DISTRIBUTED SPC – Level 1 processing (O&M)
□ Less subject to real time constraint.
□ Less need for concurrent processing.
□ Single O&M computer caters to many exchanges.
□ Helps diagnosis of many from one location.
Operator Maintenance Personal
Exchange PExchange 2Exchange 1
O & M
Computer

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SINGLE STAGE NETWORKS
□ No of cross points will be 10 x 10 = 100.
□ Fully connected so no blocking.
□ Used for 10-25% time on average.
□ Remains idle. Waste of infrastructure.
10 inputs
10 outputs

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TWO – STAGE NETWORKS
□ N X N two stage network with K simultaneous
connections-
!
!
!
□ Full connectivity to K simultaneous calls.
□ Blocking after K.
□ Each stage has NK switching elements.
□ Assuming 0nly 10% connectivity K can be N/16.safe
□ Switching elements each stage = N2
/16
□ Total switching elements = N2
/8.
□ If N = 1024, switching elements = ?
N X K K X NN N

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□ For large N, N X K is unrealizable.
□ Remedy: Using smaller size switching matrices.

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p 1 s
p 2 s
p r-1 s
p r s
r 1 q
r s q
M inlets N outlets

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□ M = pr. ( p inlets per p blocks)
□ N = qs.
□ Full availability: Atleast one outlet from each
block in 1st stage must be connected to inlet of
every block in 2nd
stage.
□ No. of Switching elements = S =
□ S = psr + qrs
□ S = Ms + Nr
□ No of simultaneous calls – switching capacity SC
=
□ SC = sr
□ Condition:

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□ These N/W are blocking.
□Under 2 conditions:
■If calls are uniformly distributed, (rs + 1)th
call arrives.
■Calls are not uniformly distributed.
□Probability that given inlet in Ith
block is active = α .
□Probability that given outlet in Ith
block is active = β .
□ β is
■inversely proportional to number of outlets in each
block.
■Directly proportional to number of inlets in each block.
□ β = p α/s
□Probability that another inlet becomes active and asks

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□ Blocking means –
□ All outlets are already active, and no free outlets.
□ The probability that an already active outlet is sought =
□ = probability that the particular outlet is active AND
□ other outlets are not sought.
□ PB = p α/s[1-{(p-1) α/(s-1)}]
□ If p = M/r
!
□ PB = {M α(s-1) – ((M/r) –1) α} / {rs(s-1)}

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THREE – STAGE NETWORKS
r 1 q
r s q
p 1 s
p 2 s
p r-1 s
p r s
N
inlets
N
outlets
s 1 p
s 2 p
s r-1 p
s r p
p x s s x p
r x r

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THREE – STAGE NETWORKS
□ N inlets = r blocks of p inlets each.
□ Same for p outlets.
□ Stage 1 --– p x s.
□ Stage 2 --– r x r.
□ Stage 3 --– s x p.
□ No of switching elements = S =
□ rps + sr2
+ srp
□ = 2Ns + sr2
.
□ = s(2N + r2
).

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TIME DIVISION SPACE SWITCHING
1
2
N-1
N
1
2
N-1
N
BUS
k – to 2k
decoder
Modulo – N
counter
Cyclic control
Clock
SWITCHING STRUCTURE

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□
!
!
!
!
!
!
!
□ N X 1 and 1 X N switching matrix for 1st
and 2nd
stage.
□ 1 interconnecting link.
□ Speech in PAM analog - analog time division
1
N
1
N
Two – stage equivalent circuit

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□ Inlet-outlet pair connected to bus through control mechanism.
□ Number of simultaneous conversations SC = 125/ts
□ ts is time in µs to set up a connection.
□ Inlet-outlet selection dynamic.
□ Simplest is cyclic. ( i connected to i.)
□ Hence no switching.
□ Hence lacks full availability.
□ Inlet or the outlet control can be memory based to achieve
switching as…

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Input controlled TIME DIVISION SPACE SWITCHING
□ 1
2
N-1
N
1
2
N-1
N
BUS
k – to 2k
decoder
Modulo – N
counter-
cum-MAR
Cyclic control
Address
decoder-
cum-MDR
7
4
1
5

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□ Sequence required is stored serially in memory
address register at outlet side.
□ Input serially.
□ 7-4-1-5 stored in locations 1, 2, 3 and 4.
□ Inlet 1 connected to outlet 7……
□ Full availability.
□ Called inlet or input controlled as outlet is chosen
depending on inlet being scanned.
□ Control memory has N words for N inlets.
□ Width of log2N bits. (Stored in binary.)
□ Cyclic control means all subscriber scanned whether
active or not.

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□ Decoder o/p enables proper outlet to be connected
to bus.
□Sample signal is passed from inlet to outlet.
□Any inlet I can be connected to any outlet k.
□Full availability.
□If inlet inactive-
■Memory location has null value.
■Address decoder does not enable any outlet
line.
□Bus–single switching element–time shared by N
connections.
□All can be active simultaneously.
□Physical connection established between inlet and

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Output controlled TIME DIVISION SPACE SWITCHING
1
2
N-1
N
1
2
N-1
N
BUS
Decoder
7
4
1
5
Modulo-N
counter
Decoder
Cyclic control
CLK

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□ Sequence required is stored serially in memory address
register at inlet side.
□ Output serially.
□ 7-4-1-5 stored in locations 1, 2, 3 and 4.
□ Outlet 1 connected to inlet 7……
□ Full availability.
□ Called outlet or output controlled as inlet is chosen depending
on outlet being scanned.
□ For active outlet i, inlet address stored in location i.
□

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□ SC = N = 125/( ti + tm + td + tt)
□ ti = Time to increment the modulo-N counter.
□ tm = Time to read the control memory
□ td = Time to decode address and select inlet or outlet.
□ tt = Time to transfer sample value from inlet to outlet.
□ All times in µs.
□ Clock rate 8 X N KHz

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Some more on TIME DIVISION SPACE SWITCHING
□ For two direction data transfer-Two independent buses.
□ Simultaneous data transfer on two buses.
□ Or single bus with time sharing two directional traffic.
□ All lines scanned irrespective of active or inactive.
□ Waste as only 20% are active.
□ Hence control on memory on both sides more useful.
□ Hence memory-controlled time division space switching.

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Generalised TIME DIVISION SPACE SWITCHING
1
2
N-1
N
1
2
N-1
N
BUS
Decoder
7
4
1
5
Modulo-SC
counter
Decoder
CLK
MDR
MAR
Read/Write
Data input

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□ Control memory word has two address.
□ Inlet and out let address.
□ Word width is 2[log2N] bits.
□ Operation:
□ Inlet k and outlet j addresses entered into free
location of control memory via data input.
□ The Location then marked busy.
□ Modulo – SC counter updated at clock rate.
□ Control memory word read out one by one.
□ Addresses are used to connect respective inlet and
outlet.
□ Sample transferred from inlet to outlet.
□ Clock updates counter.

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□ Busy / free information stored in bit vector.
□ 1 bit per location.
□ Bit set – busy.
□ SC = 125/ts
□ Clock rate = 8 SC kHz.
□ Ts = ti + tm + td + tt
□ If is tm dominant, control memory busy throughout 125 µs.
□ One write cycle reserved for input purpose in every 125 µs.

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TIME DIVISION TIME SWITCHING
□ Memory block in place of bus.
□ PCM samples.
□ Serial data taken in and out.
□ But parallel data written and read out of memory.
□ Serial/ parallel converter at inlet and vice versa.
□ MDR is a single register.
□ Gating mechanism to connect inlet and outlet.
□ No physical connection between inlet and outlet.
□ Information not transferred in real time.
□ Data first stored in memory, then transferred to
outlet.
□ Hence called TIME DIVISION TIME SWITCHING.

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▪In
g
at
e
O
ut
g
at
e
D
at
a
o
ut
D
at
a
inS/P
Data
memory N
words of 8
bits each
P/S
1
N N
1
MAR
MDR
Control memory N
words of log2N bits
each
Modulo-N
counter
M
A
R
Data
in
MDR

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□Equivalent circuit-
!
!
!
!
□Types:
□Sequential write/random read
□Random write/sequential read
□Random input/random output
□Inlets and outlets and control memory scanned
sequentially.
□Data memory read/written sequentially/random.
□Three forms can operate in any of two modes:
N X 1 1 X NDelay
NN
11

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TIME DIVISION TIME SWITCHING – Phased operation
□ Two phases.
□ Sequential write/random read – Phase one –
□ Inlets scanned sequentially 1, 2, …N.
□ Data stored in Data memory sequentially 1, 2, …N.
□ Control memory stores inlet addresses as required by outlets.
□ Inlet numbers 5, 7, 2, … for outlets 1, 2, 3, …
□ Phase Two –
□ Outlets scanned sequentially 1, 2, 3, ….
□ Data read from data memory randomly 5, 7, 2,….
□ Data reading controlled by control memory.

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□ First phase - one memory write per inlet (total N)
□ Second phase– one control memory read + one data memory
read per outlet.
□ Total time taken = in µs
□ ts = Ntd + N(td + tc)
□ td= read/write time for data memory
□ tc= read/write time for control memory
□ If td = tc = tm ,
□ ts = 3N tm
□ Number of subscribers = N = 125/3 tm
▪

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□ Number of subscribers can be increased
□ By overlapping read cycle of data memory and
control memory.
!
!
!
!
!
!
□ Last cycle of phase 1, memory write coincides with
□ -first location of control memory read having inlet
address.
□ Gives out data 1 and reads next control
Phase 1
DM write
Phase 1
DM read
a1 a2 aN
N21
CM read

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TIME DIVISION TIME SWITCHING – slotted operation
□
!
!
!
!
!
□ Sub periods i = 125/N µs.
□ Operations in each sub periods:
□ 1. Read inlet i and store data in data memory
location i.
□ 2. Read location i of control memory and read
address say j.
125µs
N21
DM write
CM read
DM read

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TIME MULTIPLEXED SPACE SWITCHING
□ Time division switches means:
□ An inlet or an outlet corresponds to single
subscriber
□ with one sample speech appearing every 125 µs.
□ Used in local exchanges.
□ Time multiplexed switches means:
□ Used in transit exchanges.
□ Inlet and outlets are trunks carrying TDM data.
□

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□ 12M
12M
12M
12M
1
2
N-1
N
1
2
N-1
N
Decoder Cyclic
Control
MAR
!
CM
!
MN words

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□ N incoming trunks and N outgoing trunks.
□ Each carry a TDM stream of M samples per frame.
□ Frame time 125 µs.
□ One frame time – MN samples.
□ One time slot = 125 µs
□ One time slot – N samples are switched.
□ Output controlled switch - Output Cyclically
scanned.
□ Corresponding to each outlet, M locations in control
memory.
□ M blocks of N words each.
□ Two dimensional location address .

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□ Block address i corresponds to time slot i.
□ Word address j corresponds to outlet j.
□ First N locations corresponds to first time slot.
□ And so on.
□ If inlet address k is present in location (i,j)- ( output controlled)
□ Means inlet k is connected to outlet j during time slot i.
□ Number of trunks supported =
□ N = 125/Mts
□ ts= is switching time including memory access time per inlet-
outlet pair.
□ Physical connection provided between inlet and outlet.

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□ Cost of switches = No of switches + no of memory
words
□ = 2N + MN
□ Cost of equivalent single stage switch = (MN)2
.
!
!
!
▪ ASSIGNMENT:
▪ Calculate number of trunks that can be supported
on a time multiplexed space switch, given that
▪ a) 32 channels are multiplexed in each stream.
▪ b) Control memory access time is 100 ns.

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SOLUTION
□
□ M = 32
□ ts = 100 + 100 = 200 ns
!
□ N = 125/M ts = 20

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TIME MULTIPLEXED TIME SWITCHING
□ Time switch does not give physical connection.
□ Data stored and then transferred during another
slot.
□ Delay.
□ Employs TIME SLOT INTERCHANGER.
!
!
□

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Time Slot Interchanger
□ Let one incoming trunk and one outgoing trunk.
□ M channels multiplexed in 125 microseconds.
□ Sequential write / random read
□ Time slot duration tTS= 125/M
□ MtTS= 125
▪

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▪
12M
1
2
3
4
M
5
6
DM
38
76
27
13
51
19
26
1
2
3
4
M
5
6
CM
1
4
27
7
Time slot
counter
CTS
1 2 3 M
38 42 51 19
O/P slot number
Control data
memory location
frame
frame

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□Clock runs at time slot rate.
□Time slot counter incremented by one at end of each
slot.
□Counter contents provide
■locations addresses for data memory .
■locations addresses for control memory .
□Data memory and control memory access
simultaneously at beginning of time slot.
□Content of CM used as address of data memory.
□Respective data read to output trunk.

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Operation is a Time Slot
▪
t
t t
tTS
Read input data;
Write into DM;
Read CM.
Read DM;
Write data
to output

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□ I/P data available to read at beginning of time slot.
□ Data ready for writing on O/P at end of time slot.
□ Storage action.
□ Hence delay of minimum one time slot even if no
time slot interchange.
□ Output delayed by tTS microsecond.
IS1 IS2
OS1 OS2
0 tTs 2tTs

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□ Delay depends on to which output slot, input slot is
switched.
□ Previous cycle, all DM is filled/ written in.
□ In current cycle, CM is read for DM address.
□ CM1 =1, contents of DM1 switched to O/P1.
□ Current contents can be switched only in this case.
□ Delay tTS microseconds.
□ CM2=7, contents of DM7 switched to O/P 2.
□ Delay = [(M-7)+2+1] tTS = (M-4) tTS microseconds.
□ CM3=4, contents of DM4 switched to O/P 3.
□ Delay = [(M-4)+3+1] tTS = M tTS microseconds.

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□ We have MtTS= 125
□ Two sequential memory access per time slot.
□ tTS = 2 tm
□ 2 tm M = 125
□ No switching elements.
□ Cost equal to number on memory elements.
□ M locations in each of CM and DM.
□ C = 2M units.

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Assignment
□ Calculate the maximum access time that can be
permitted for the data and control memories in a
TSI switch with a single input and output trunk
multiplexing 2500 channels. Also estimate cost of
the switch and compare it with single stage space
division switch.
□ 2 tm M = 125
□ tm = (125 X 103
)/(2500 X2) = 25ns
□ C = 2 X 2500 = 5000 units.

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Traffic Engineering
□ Provides basis for design and analysis of
telecommunication networks.
□ Blocking probability is major issue for design.
□ Blocking probability depends on time for which
following are busy –
□ Subscriber
□ Digit receiver
□ Inter stage switching links
□ Call processors
□ Trunk between exchanges

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Traffic Engineering
□ Traffic pattern on the network varies throughout
the day.
□ Traffic engineering provides a scientific basis to
design cost effective network taking all above into
account.
□ It helps to determine ability of network to carry a
given traffic at a particular loss probability.
□ Provides a means to determine quantum of common
equipment required to provide a particular level of
service for a given traffic pattern and volume.

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Traffic load and parameters
□ Typical traffic load of a day
Hour of the day
Number oh calls
In the hour
1 2413

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□ Traffic pattern varies for domestic and official
areas.
□ Varies for working and non-working days.
□ Busy hour- 1 hour interval lying in time interval
concerned in which traffic is highest (Max call
attempts).
□ Peak busy hour- The busy hour each day.
□ Time consistent busy hour- particular 1 hour period
which is peak busy hour each day over the days
under consideration.

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□ CCR Call Completion Rate – ratio of number of
successful calls to number of call attempts.
□ Used in dimensioning the network capacity.
□ Designed to provide overall CCR of 0.70.
□ CCR=0.75 considered excellent.
□ Higher CCR is not cost effective.

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□ BHCA Busy Hour Call Attempts – Number of call
attempts during busy hour.
□ It is an important parameter in deciding the
processing capacity of common control or stored
program control in an exchange.
□ Busy hour calling Rate – average number of calls
originated by a subscriber during the busy hour.
□ It is useful in sizing the exchange to handle peak
traffic.
□ Rural area – 0.2 typical
□ Business area – 3 typical

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□ Example: An exchange serves 2000
subscribers. If the average BHCA is 10000 and
CCR is 60%, calculate the busy hour calling rate.
□ Only 60% of total attempts are successful.
□ Average busy hour calls = 10000 X 0.6 = 6000
□ Busy hour calling rate = 6000/2000
□ = 3 calls per subscriber.

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□ Day-to-busy hour traffic ratio – ratio of busy
hour calling rate to average calling rate for the
day.
□ Gives how much of day’s total traffic is carried
in busy hour.
□ Business area - 20
□ Rural area - 6-7

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□ Traffic intensity Ao – Ratio of period for which a
server is occupied to total period of observation.
□ Server includes all common equipments irrespective
of locations.
□ This gives traffic on the network in terms of the
occupancy of the servers in the network.
□ Generally period of observation is 1 hour.
□ Ao is dimensionless.
□ Called erlang (E) in honour of scientist.
□ 1 erlang of traffic – servers occupied for entire
period of observation.

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□ A group of 10 servers, each is occupied for 30
minutes in an observation interval of 2 hours.
Calculate the traffic carried by the group.
□ Traffic carried per server = 30/120
□ = 0.25E
□ Total traffic carried by the group = 10 X 0.25
□ = 2.5E
□ Erlang measure indicateds average number of
servers occupied .
□ Useful in driving average number of calls put
through during period of observation

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□ A group of 20 servers carry a traffic of 10 erlang.
If the average duration of the call is 3 minutes,
calculate number of calls put through by a single
server and the group as a whole in one hour period.
□ Traffic per server = 10/20 = 0.5 E
□ Server busy for 0.5 of total period.
□ Hence a server busy for 0.5 * 60 = 30 minutes
□ Total number of calls/server = 30/3 = 10 calls.
□ Total number of calls by group = 10*20 calls.
□ =200 calls

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□ Traffic intensity also measured in CCS
□ Centum Call Second represents call time product.
□ Valid only in telephone circuits.
□ 1 CCS can be 1 call for 100s duration or 100 call
for 1s duration or any other.
□ Total duration same = 100s.
□ Some times CM or CS are used to measure TI.
□ 1E = 36CCS = 3600 CS = 60 CM
□ 1E means busy full duration of 60 CM.
□ 100CS = 1CCS

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□ A subscriber makes 3 phone calls 3m, 4m and 2m
duration in a 1-hour period. Calculate subscriber
traffic in erlang, CCS and CM
□ Subscriber traffic in erlang =
□ =busy period/total period = 9/60 = 0.15E
□ Traffic in CCS = 36*0.15 = 5.4 CCS
□ Traffic in CM = 60* 0.15 = 9
□ Or Traffic in CM = 3+4+2 = 9

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□ Traffic intensity is a call-time product.
□ Parameters –
□ Average call arrival rate C
□ Average holding time per call th
□ Load offered to network = A = Cth

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□ Assignment: Over a 20 minute observation interval,
40 subscribers initiate calls. Total duration of the
calls is 4800s. Calculate the load offered to the
network by the subscribers and average subscriber
traffic.
□ Average call arrival rate = 40/20 = 2 calls/m
□ Average holding time
□ = 4800/40 =120s = 2m/call
□ Offered load = 2*2 = 4E
□ Average subscriber traffic = 4/40 = 0.1E

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Action during overload
□ Two options.
□ Loss system-Overload traffic may be rejected.
□ Delay system – Held in queue until NW facilities
are made available again.
□ Conventional automatic exchanges are based on
loss system.
□ User has to retry.

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Grade of service and blocking probability
□ In loss system, traffic carried by NW is lower
than actual traffic offered to NW.
□ Overload traffic is rejected.
□ Grade of service –GOS
□ Is an index of quality of service.
□ Is amount of traffic rejected by network.
□ Is ratio of lost traffic to offered traffic.

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□ Offered traffic- A = Cth
□ C -- Average number of calls generated by the
user.
□ th– average holding time per call.
□ Carried traffic – actual traffic carried by NW.
□ Is average occupancy of server.
□ is period for which a server is occupied out of
total observation time.

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□ GOS = (A-Ao)/A
□ A = offered traffic
□ Ao = carried traffic
□ A - Ao= lost traffic
□ GOS as small as possible for better service.
□ Recommended value = 0.002
□ 2 calla per 1000 calls.

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Blocking probability PB- Loss system
□ Defined as probability that all servers in system
are busy.
□ Any new arrival is blocked.
□ Not same as GOS.
□ If an exchange has same number of servers and
subscribers-
□ GOS is zero.
□ Blocking probability non zero.

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Blocking probability PB
□ GOS is a measure from subscriber point of view.
□ Blocking probability is a measure from network
or switching system point of view.
□ GOS is arrived at by observing number of
rejected subscriber calls.
□ Blocking probability is arrived at by observing
the busy servers in switching system.
□ GOS called call congestion/loss probability.
□ PB called time congestion

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Blocking probability -Delayed system
□ In system, traffic carried = load offered.
□ All calls are put through the network as and when
NW facilities are available.
□ GOS always zero.
□ Delay probability- prob that a call experiences a
delay.
□ If input rate far exceeds NW capacity, undesirably
long queue and delay.
□ Unstable as never recovers.

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Flow control -Delayed system
□ If queue size more that acceptable level-
□ Made to act as loss system till queue size below
acceptable level.

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FASCIMILE
□ Means exact reproduction.
□ Of a document or a picture.
□ Band width required very small.
□ Suitable for transmission over telephone lines.
□ USES: Transmission of
□ photograph.
□ Document, weather maps etc..
□ Language texts for which tele-printer is not
available

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FASCIMILE -Sender
□ Message—
□ A single page
□ Narrow continuous tape.
□ Continuous sheet paper.
□ Scanning methods—
□ Optical scanning-light spot traverses the message.
□ More common.
□ Resistance scanning-character of message offers
varying resistance,
□ Brought into circuit using a stylus.

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FASCIMILE-Sender
□ Cylindrical Scanning

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Cylindrical scanning
□ Message fixed around drum.
□ Drum rotated about its axis and moves along axis
simultaneously.
□ Moves below a fixed scanning spot.
□ Reflected light focused on photo cell .
□ Photo cell converts light to electrical signal.
□ Solid state amplifiers amplify signal.
□ Spot made very small using mask or lenses.
□ Spot follows spiral path.

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Cylindrical scanning
□ Uncommon alternate arrangements-
□ Scan in series of closed rings.
□ Drum stationary, spot moves.
!
□ Traversing speed – 1/100 inch per second
□ Rotation speed – 60 rpm
□ 100 scanning lines on each 1 inch width of
picture.

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Tape scanning

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Tape scanning
□ Message taken directly off a printed tape.
□ Scanning beam falls from top and travels across
the rape.
□ Achieved using hexagonal prism.
□ Prism rotates and deflects beam to travel across
the tape.
□ New trace at start of each face of prism.

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Scanning spot
□ Shape of scanning spot determines wave shape
of signal output.
□ Preferred- Rectangular shape without gap or
overlap.
□ Less preferred – Trapezoidal with little overlap.
□ Average width of top and bottom widths is P.

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Scanning spot

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Facsimile Receiver- photographic reception
□ Equipment used is identical but process is
reverse.
□ Input is electrical and output optical.
□ Received electrical signal varies intensity of
light beam.
□ Light beam falls on photographic material.

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□ Small coil of fine wires suspended in strong magnetic
field.
□ Small mirror is mounted in coil.
□ Electrical signal through coil deflects the mirror as
per its strength.
□ Mirror is kept off center at no signal.
□ Small signal deflects mirror less and less light passes
through aperture.
□ Large signal deflects mirror more and larger light
passes through aperture.
□ Provides positive image on photographic plate.

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□ Alternate method.
□ Crater lamp containing neon, argon, or helium.
□ Glows when voltage is applied.
□ Intensity of light changes with voltage .
□ Signal is applied to lamp.
□ Output light is made to fall on a photographic
plate.
□ Not very efficient and responsive.

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Facsimile Receiver- Direct Recording reception
□ Highly absorbent chemically treated paper is used.
□ Electrolyte held in paper disassociate when voltage is
applied.
□ Signal voltage applied via a metal stylus.
□ Metallic salt so produced reacts with colour chemical
on paper.
□ Produces a mark on paper.
□ Intensity of mark depends on amount of
disassociation.
□ Hence depends on electrical signal.
□ Paper is damp and must be kept sealed

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Facsimile Receiver- Direct Recording reception
□ Paper is damp and must be kept sealed
□ Cheap.
□ Tonal range less.
□ Suitable for low grade applications.

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Synchronization
□ For documentary, need for synchronization is not
severe.
□ Can be achieved using synchronous motor at both
ends, operated off frequency controlled mains.
□ For picture, receiver must be synchronized with
transmitter.
□ By sending synchronizing signals at 1020Hz.
□ Sender speed bears known relation to 1020Hz
□ Receiver speed adjusted using stroboscope.

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Synchronization
□ With carrier transmission, carrier is sent along
with USB.
□ Carrier helps in recovering 1020Hz.
□ Speed of receiver adjusted with this 1020Hz.
□ If receiver has constant speed error, picture
would be distorted.
□ Phase error breaks the picture.
□ Can be avoided by sending 1020Hz pulsed
momentarily to indicate start of the transmission.
□ Pulse releases the switch holding the receiver
drum.

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□ No error
Constant speed
error
Phasing
error

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Index of cooperation
□ Height/width ratio must be same for
transmitted and received pictures.
□ Hence scanning pitch and drum diameter must be
same at both ends.

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D- sending drum Diameter  
d – receiving drum Diameter 
P – Sender scanning pitch 
p – Receiver scanning pitch 
n – number of lines scanned

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□ Width of transmitted picture – nP
□ Width of transmitted picture – np
□ Height of transmitted picture is proportional to
D.
□ Height of received picture is proportional to d
with same constant of proportionality.
□ For correct height/width ratio-
□ D/nP = d/np
□ D/P = d/p

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□ Ratio of diameter to scanning pitch should be
same at both ends.
□ Called Index of cooperation.
□ IEEE defines it as product of stroke length and
scan density.
□ For drum scanner, stroke length is ΠD
□ Scan density is lines per unit length = 1/P
□ IOC(IEEE) = ΠD /P
□ IOC(CCITT) = D/P

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□ Effect of different index of cooperation.

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□ Assignment:
□ The drum diameter of a facsimile machine is
70.4mm, and the scanning pitch is 0.2 mm per
scan. Calculate IOC
□ IOC(IEEE) =
□ 1106
□ IOC(CCITT) =
□ 352

Telecommunication switching system

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