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Switching Systems

pulse code modulation

stored program common control

time/digitalelectronic1976 ESS—third generation

reed switchstored program control

space/analogSemi-electronic1972 ESS—second generation

reed switchcommon control

space/analogSemi-electronic1960 ESS—first generation

X-bar switchcommon control

space/analogElectro-mechanical1918 cross-bar

stepping switch train

distributed stage-by-stage

space/analogElectro-mechanical1892 step-by-step

plug/cord/jack

humanspace/analogmanual1878 manual operator

Type of Network

Type of Control

Method of Switching

OperationSwitching System

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Basics of Switching Systems

Switching Network

Switching Network

Switching Network

Switching Network

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• In a folded network with N subscribers, there can be a maximum of N/2simultaneous calls.

• The switching network may be designed to provide N/2 switching paths, the network then is said to be non-blocking.

• Blocking Network (Switching Paths<N/2)

• The probability that a user may get blocked is known as blocking probability.

• Switching exchanges are designed to meet an estimated maximum average simultaneous traffic, usually known as busy hour traffic. (20-30% activity)

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Elements of a switching system

• Trunk Interface (Incoming and Outgoing)• Subscriber Line Interface (I and O)• Service Circuit Interface• Junctors• Subscriber and Service line scanning and

distributor units.• Control: Direct or Common Control• Operator console• Trunk circuit scanning and distributor units.

SwitchingNetwork

Trunk Interface

SubscriberLine

Interface

ServiceCircuit

Interface

Trunk Interface

Subscriber Line

Interface

ServiceCircuit

Interface

SubscriberAnd

Service Line Scanning

and Distributor

Units

Trunk Circuit Scanning

and Distributor

Units

Control

Operator Console

JUNCTORS

Incoming Outgoing

ToIncomingTrunks

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Switching Systems (Classification)

Switching Systems

Manual

Automatic

Electromechanical Electronic (SPC)

Strowger (Step-by-Step)

Crossbar

Space Division Switching

Time Division Switching

Manual Switch-Boards

• The first switching Systems utilized operators at manual switchboards.

• The operators asked the caller for the number they wanted to call and then established the connection by plugging in a cord between terminal jacks

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RST

• R – Ring

• S – Sleeve

• T – Tip

• One wire of a wire pair is commonly referred to as the tip and other is referred to as ring, even on digital wire pairs

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Manual

LB exchange CB exchange

Magneto exchange

If 200 subscribers terminate on a switch board, what will be

the maximum number of simultaneous calls

Ans. 100

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Switch boards

Single Termination Multi Termination

Cyclic Assignment

Single termination board

TransferLines

LocalConnection

Non-LocalConnection

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SubscriberLines

Multi-Termination Boards

Cyclic Assignment

300-399

700-799

0-99

400-499

100-199

500-599

200-299

600-699

300-399

700-799

0-99

400-499

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• Almon Strowger was an undertaker who created the first automatic telephone switching machine.

• He wasn't a natural scientist nor was he immediately interested by telephones. What really spurred him on was when he imagined that his undertaking business was missing out because the lady at the phone exchange was connecting callers to a rival funeral business.

• Customers didn't immediately take to the system, and Strowger soon tired of it himself, selling his stake for a modest sum to buy a hotel. His system was later sold for $2.5 million and installed in exchanges worldwide until the 1960s.

Almon Brown Strowger

(1839-1902)

operator - get off the line!

Advantages: Automatic Switching Systems

• Language independent. • Privacy • Establishment and release of calls are

faster • Time required to establish and release a

call remains more or less of the same order.

• 24×7 • Reliable

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Functions : Automatic Switching Systems

• Signaling• Control• Switching

• Electromechanical switching– Step-by-step switch (Strowger switch)

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Uniselector

Two-Motion Selector

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Two-Motion Selector

Two-Motion Selector

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• It uses ‘direct progressive control’• The control elements of the switch are

integrated into the switching matrix• A call may be blocked even if an alternate

path exists through the switch.• Alternate routing for outgoing trunks is not

possible.• Signaling schemes other than dial pulses are

not directly usable• Number translation is impossible.

Limitations of strowger systems

Crossbar switching• a crossbar switch (also known as cross-point switch,

or matrix switch) is a switch connecting multiple inputs to multiple outputs in a matrix manner.

• The cross-points of a cross bar switch are mechanical contacts with magnets to set up and hold a connection.

• The term crossbar arises form the use of horizontal and vertical bars to initially select the contacts.

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Crossbar switching• Uses ‘common control’

• Control function implementation is separate from the switch implementation

• Logical addresses instead of physical line numbers

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• Early crossbar systems were slow in call processing as they used electro-mechanical components for common control subsystems.

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Stored Program control (SPC)

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

• SPC lead to full scale automation of exchange functions and introduction of variety of new services to the users.

SPC

Centralized Distributed

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Centralized

Processor

Signal distributor

Scanners

MaintenanceConsole

Memory Secondary Storage: Call Recording,

Program Storage etc.

• Redundancy may also be provided at the level of exchange resources and function programs.

• A dual processor architecture may be configured to operate in one of three modes:- Standby, Synchronous duplex, and Load Sharing.

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Standby Dual Processor Config.

Exchange Environment

P1 P2

Secondary Storage

Synchronous Duplex

Exchange Environment

P2

M2M1

P1 C

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Load Sharing

Exchange Environment

P2

M2M2

P1

ED

Distributed SPC

• In distributed control, control functions are shared by many processors within the exchange itself.

• Exchange functions may be decomposed either “horizontally” or “vertically” for distributed processing.

• Better reliability and availability• Owes its existence to low cost processors

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Levels of Control Functions(Horizontal decomposition)

Event Monitoring and Distribution

O & M and Charging

Call Processing

Dual Chain Distributed Control

Exchange Environment

EM & DP

O & MP

CP

EM & DP

O & MP

CP

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• 1965: Bell system installed the first electronic switching system (No. 1 ESS)

• Allowed introduction of new features such as abbreviated dialing, call forwarding, call waiting,…

• Simplified administration and maintenance tasks.

• 1976: AT&T’s No.4 ESS is a high capacity toll switch using SPC and digital electronics for its switching matrix.

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Drive Mechanism of Rotary Switch

Uniselector

2

2-motion selector

2-motion selector

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Step-by-step switching

Selector HuntersOr

Line Finders

Group SelectorStage

Final Selector

From Calling Subscriber

To CalledSubscriber

Line Equipment Part (Pre-selector

Stage)

Switching NetworkPart

1st Selector, 2nd Selector, &

so on….

Connector Part(Numerical Selector)

Selector Hunters

o Off hook --- interrupter mechanism activated ---wiper steps till find a free selector group --- first selector once located ---interrupter mechanism deactivated --- first selector sends dial tone ---accept pulses --- group selection continues till final selector.

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Line Finders

o Off-hook signal sensed by all line finders ---interrupter mechanism of line finder whose associated first selector is free, gets activated ---line finder wiper steps till it reaches the subscriber contact --- first selector sends dial tone.

Design Parameters

• Number of Subscriber lines, N• No. of Switching Elements, S• Cost of Switching System, C = S×Cs + Cc + Cch

• Switching Capacity, SC – maximum number of simultaneous calls.

• Traffic Handling Capacity, TC = SC / Theoritical Max. Load = SC / (N/2) = 2SC / N

• Equipment Utilization Factor, EUF = Number of switching elements in operation when SC is fully utilized / total number of switching elements in the system.

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• Number of Switching Stages, K• Average Switching Time per stage, Tst

• Call set up time, Ts = Tst × K + T0

• Cost capacity index, CCI = switching capacity / cost per subscriber line = N(SC) / C

• Blocking Probabilty, PB or B

Design Parameters

Design 1: 100-line exchange (uniselectors)

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Design 2: 100-line exchange (uniselectors)

• 10 uniselectors in the second stage for every one uniselector in the first stage of Design 1.

Design 3: 100-line exchange (2-motion selectors)

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Design 3: 100-line exchange with selector finders

Design 4: 100-line exchange with line finders

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1000-line exchange

Common Control Switching System

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3X3 Crossbar Switching

6X6 Crossbar Matrix

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Connection in Crossbar

• Energize horizontal bar --- energize vertical bar --- de-energize horizontal bar.

• Energize vertical bar --- energize horizontal bar --- de-energize vertical bar.

• Suitable arrangement so that Latch is maintained, even after de-energizing the concerned bar.

Diagonal Crosspoint Matrix

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Blocking Crossbar Switch

• Energize horizontal, A

• Energize vertical, P

• De-energize horizontal, A

• Energize horizontal, B

• Energize vertical P’

• De-energize, B

• Energize A & B, • Energize P, • De-energize A & B

Local non-blocking, external blocking

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Local blocking, external blocking