s02 sovpstn210 _ overview of pstn

7/31/2019 s02 Sovpstn210 _ Overview of Pstn

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CETTM MTNL

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Overview of PSTN

Module Id : SOVPSTN210


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Public Switched Telephone Network

PSTN is public network that facilitates

predominantly voice telephony

Major Components of the Public Switched

Telephone Network (PSTN):

PSTN Switch (Exchange) Transmission facilities

Local Loop

Customer Premise Equipment (CPE)


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Public Switched Network

A switched network consists of a series ofinter-linked nodes, called switches

Switches are hardware and/or softwaredevices capable of creating temporary

connections between two or more deviceslinked to the switch but not to each other


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Typical Network ?

PSTN

switch

CPE CPE

Transmission

Facilities

PSTN

Switch


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CustomerPremiseEquipment (CPE)

Customer Premise Equipment (CPE) is the termused to identify any piece of equipment supplied

by the customer to interface with the PSTN.

Examples include: POTS/ Telephone Set

FAX/Modems or Data Sets

Private Branch Exchange (PBX)

ISDN Terminals (NT-TA)


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LocalLoop

Source:www.IPTVMagazine.com
http://www.iptvmagazine.com/http://www.iptvmagazine.com/


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PSTN Switch (Exchanges)

CONTROL

NETWORK

PSTN Switch

SUBLU


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TransmissionNetwork

Transport network used to Link different wire

centers or exchange nodes

In India, Europe E1 (2 Mbps) and its higher

order multiplex used. (PDH, SDH)

In US etc. DS1 (1.544 mbps) and its higher

order multiplex used. (SONET)

Other technologies like Frame Relay, ATM

etc also used


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Local NetworkStructure

Exchange

Area 5

Exchange

Area 4

Exchange

Area 3

Exchange

Area 2Exchange

Area 1

Wire

Center

A

Wire

Center

B


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Tandem ExchangeLocal Exchange

Local Exchange

Switching

Systems

Inter-exchange

Trunks

Local Loop

Local Loop

Local

Long

Distance

NLD POP

TAX


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Typical Local Exchange Network

Tandem node

Wire

Center

A

Wire

Center

B

Wire

Center

C

Tandem Trunk

Groups (Final)

Direct Trunk Groups

(High Usage)


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Regional Center DClass 1

Regional Center C

Class 2

Regional Center B

Class 3

Toll Center A

Class 4

FINAL

Regional Center E

Sectional Center F

Primary

Center

G

FINAL

FINAL

FIN

AL

FINAL

FINAL

FINAL

Toll Center H

Toll Connecting

Local Exchange

Local Exch

Class 5 Local

Tandem

Office

Toll

Con

nectin

gTollC

onnectingTAX-Lev2

TAX-Lev1

Long Distance (Toll) NetworkStructure


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Multiple Carrier Network

NLD1

NLD2

NLD3

Region x Long DistanceCarriers Region y


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InternationalNetwork

ILD1ILD2

ILD3

Country x

InternationalCarriers Country y

INT Gateway

INT Gateway


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Network Evolution

Earlier Networks (Up to early 70s)

analog telephone network

frequency-division multiplexing

Present:

local loop analog

Network digital (based on TDM)

Network Migration

IP based switching technology

(packet switching, Soft switches, VoIP)


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Digital Switching

Methods of switching

- Circuit switching

- Packet switching

- Message switching


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In circuit Switched Network- Path or pipe

between two ends in communication is opened

and stays open for the duration of the call

Circuit Switching


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Circuit Switch

circuit switch is a device with n

inputs and m outputs that creates a

temporary connection between an

input link and an output link.


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Crossbar Switch

Crossbar switch connects n inputs to m outputs

in a grid, using electronic micro-switches(transistors/Relays) at each cross-point


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1 2

1

2

N-1

Crossbar Switch

2X2 crossbar switch

- Cross state

- Bar state

NXN crossbar switch

- Connects N inputsand N outputs

Output blocking

- Two inputs request

the same output

1

2

1

2


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Characteristics of Crossbar Switches

Crossbar switches are non-blocking

- Connection requests to distinct outputs

are never blocked

- Complexity of the switch is (n2)

For large n, the number of crosspoints

becomes very large

Multistage networks

- Reduce the number of crosspoints


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Multistage Switch

Multistage switch combines crossbar switches in several

stages

Design of a multistage switch depends on the number of

stages and the number of switches required (or desired)

in each stage


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Switching Path


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Multiplexing

Frequency Division Multiplexing Time Division Multiplexing

Code Division Multiplexing

Space Division Multiplexing Polarisation Division Multiplexing


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Frequency division multiplexing

frequency

time

f1

f2

f3

f4

f5

frequency

time

Time division multiplexing

FDM v/s TDM


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Time-division multiplexing (TDM)

Method of putting multiple data streams in a singlesignal by separating the signal into many segments,each having a very short duration

Each individual data stream is reassembled at thereceiving end based on the timing

Multiplexer combines signals at the source (transmitting)

end of a communications link. (It accepts the input fromeach individual end user, breaks each signal intosegments, and assigns the segments to the compositesignal in a rotating, repeating sequence. The compositesignal thus contains data from multiple senders.)

At the receiving end, the individual signals are separatedout by demultiplexer, and routed to the proper end users


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The data are organized in frames where each frame

contains a cycle of time slots A sequence of slots dedicated to one source is a channel

Data from different sources is inserted into slots or channelsin some sequence

Synchronous TDM slots are filled from a predetermined

sequence of sources. If there is no data to transmit anidle signal is sent

Statistical (Asynchronous)TDM fills slots as data isavailable. There is not preset sequence. Therefore, datamust be associated with the source by address. Noempty or idle slots are sent if any source has dataready to transmit. Idle is sent only if all channels have nodata to transmit

Time division multiplexing (TDM)


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Telephone switch digitizes voice call (80008-bit samples per second)

Switching method is TDM

Switch multiplexes calls, 8-bit slot every

125 s

E1 (PCM) F


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E1 (PCM) Frame structure

Voice information PCM coded 8bits per sampleSample rate 8khz (125 s)

Channel capacity 64kbps


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TDM Switching

Each subscriber occupies a time slot duringconversation

To connect any two subscribers, it is

necessary to interconnect the time-slots

which maybe on same or different PCMhighways

The interconnection of timeslots is called

switching


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TDM switching

Switching Types

- Time division switching

- Space division switching

- Hybrid

Ti Di i i S it hi


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Time Division Switching

In Time switching TSx of I/C PCM HWY1 are

transferred to TSy of O/G PCM HWY2

DIGITAL SWITCH

HW 1

HW 2

HW 3

HW 4

I/P O/P

TS6

TS18

Ti Di i i S it hi


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Time Slot Interchange


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TSI consists of random

access memory (RAM) withseveral memory locations. The number of locations is

the same as the number ofinputs.

The size of each location is

the same as the size of asingle time slot.

The RAM fills up withincoming data from timeslots in the order received.

Slots are then sent out in anorder based on thedecisions of a control unit.

Time slot Interchange Technique


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1

2

24

1

2

24

24 23 12

2 241 23

Time-slot Interchange Technique

Time division switch

Slots are written and readfrom memory

Each slot requires one

write and one read

operationThe memory cycle time,

the size of the sample,

and input rate determine

the size of the switch

Digital Time Switch


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Digital Time Switch

Consists of SPEECH BUFFER and

CONTROL(address) memory

Number of storage locations are equal to

number of input time slots. For e.g. for 32 TS I/P

PCM, speech memory will have 32 locations

Writing/reading of samples in speech memory is

controlled by control memory

Di it l Ti S it h


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Digital Time Switch

Speech Memory

It stores the content of the time slotsNumber of location equal to number of time slots

Control Memory

Provide Read/Write addressing for reading speech

memoryIt has one location per input timeslot

Each location contain address of one of thespeech memory locations where the speech sampleis either written or read

Addresses are written in the control memory bycentral control of the exchange

Time Switch


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Time Switch

SPEECH X

XCONTROL

UNIT

SPEECHBUFFER

MEMORY

CONTROL

MEMORY

CONTROLLED WRITE

SEQUENTIAL READ

X

0

n

0

n

y

SEQUENTIAL WRITE

CONTROLLED READ

Time switch operating mode


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Time switch operating mode

Output Associated ControlSequential written but controlled reading

Input Associated controlControlled writing sequential reading

Output Associated Control


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Time switch operation.

01

02

03

04

31

Speech Sample

O/G PCM

TS 6

I/C PCM

TS 4

Time slotcounter

Four

Control/connection/address

memory

Read Address

Buffer / Speech Memory

6



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Sequential written but controlled reading

In this mode 32 samples of incoming PCMare written cyclically in the speech memorylocations in the order of time-slots of I/CPCM

Each location of control memory is rigidlyassociated with the corresponding time slot ofO/G PCM and contains the address of thetime slot of incoming PCM to be connected.

Example:- Output TS6 has got control word 4 in

its control location, hence TS6 will readout thespeech buffer location 4


Time Switch Characteristics


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Time Switch Characteristics

There is no blocking in a time switch

In a time switch ,there are as many memory

locations as in the control and speech

memories

Corresponding to free incoming and outgoingtime-slots, there is always a free path

available to interconnect them

Space Switching


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TS 10

TS 10

Space Switching

Interchange of Highway

Time slot remains same, call switched to anotherhighway or PCM

In Space switching TSi of an I/C HWY1 is

switched to same time-slot, TSi of an O/G HWY2


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TST Switch


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TST Switch

TST Switch


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TST Switch

There are two time stages separated by a

space switch

The time switch is a time slot changer

Space switches have n x n cross points. It is

a highway changer

Possible to connect any TS in I/C PCM to any

TS in O/G PCM


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Telephone Network


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p

Circuit switching

64 Kbps circuitestablished between

the source and

destination switch

Only delay is thepropagation delay

No queueing delay

Statistical Multiplexing


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Fe

wer

Trunks

Many

Lines

p g

Many input lines aremultiplexed into a few trunks

Exploit the fact that not allusers are making calls all thetime

When a new call arrives and allthe trunks are busy, the call isblocked (lost)

Key design issueGiven a demand, determine

the number of trunks thatwill maintain the blocking

probability below a certainvalue

CETTM MTNLBlocking Model


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N(t)

1

2

3

4

5

6

7

all trunks busy

trunk

#

g

An new arriving call is

assigned a free trunk

A call uses the trunk for

a call holding time

When all the trunks arebusy, a new incoming

call is blocked (all

circuits busy message)

CETTM MTNLBlocking Probability


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g y

State of the system: number of active

calls Call arrival rate = l calls/second

(Possion Process)

Call holding time = 1/m second

(negative exponential) Offered load a = l/m Erlangs

Blocking probability =

=

=c

k

k

c

b

ka

caP

0

!

!

CETTM MTNLPSTN Switch (Exchange) Function


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PSTN Switch provide a mechanism for

connecting four types of calls

Call to a subscriber

- Originating call

- Terminating call Call From/to Trunk

- Incoming call

- Outgoing call

( g )

CETTM MTNLGeneral Architecture of Exchange


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g

Subscriber

Access

SwitchingNetwork

From another Telephone exchange

Trunks

Access

To another

subscriber or

Telephone

Exchange

N/W controller

Control Unit(Call Processing and

Control ProcessingPrograms)

CETTM MTNL

PSTN switch (Exchange)


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Subscriber interfacing equipment provides

access to the customer

Trunk interfacing equipment provide access for

incoming calls from other exchanges

Switching Network provides path for throughconnection

Control unit, does the call processing control

and charging function

CETTM MTNLPSTN Numbering Scheme


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ITU standard E.164 Numbering format- E.164 uses a

country code, area code and phone numberInternational public telecommunication number for

geographic areas (maximum 15 digits)

Generally international numbers are limited to 12

digits and National Numbers to 10 digits- Country code

- National Destination code (optional)

- Subscriber Number

In India PSTN follows- SDCA code+SubscriberNumber (10 digit)

CETTM MTNLCall Dialling


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CETTM MTNLCall Processing


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When subscriber lifts handset off-hook is

detected as a call event in the exchange. Path is established from subscriber line unit to

signaling/tone unit providing dial tone to the

subscriber

Exchange receives the dialed digits.

For receiving the tone dialing digits there are

frequency receivers in the exchanges.



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Called

Subscriber

Call

Processor

Calling

Subscriber

goes off hook

Tone/GeneratorFrequency Receiver

Unit

Out going

Trunks

Speech pathSignaling path

Subscriber

ConnectionUnit

Subscriber

Connection

Unit

Switching

Unit

PCMs



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For every call processing event, call treatment

programs become active

Dedicated subsystems of a telephone exchange

do the call treatment to find out

- Type of call- Path to next node

- Charging

CETTM MTNL

Call Processing


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Depending on the dialed digits and the customer

profile the exchange software decides where toroute the call. Dilled digits are mapped to

exchange equipment No.

The control unit of the exchange

(distributed/centralized) controls the callestablishment and facilitates finding of the

switched path via the switching network of the

exchange

CETTM MTNL

Call Processing


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If the call is for a subscriber, path is found

towards the corresponding subscriber lineunit

If its a call for other exchange the call path

is found to a trunk

The control unit also records the call eventswhich facilitates call charging and generation

of call data records (CDRs)



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For Inter exchange calls the dialed digits and

call control information needs to be sent toother exchange

Signaling mechanism is used to direct and

control the setup and disconnection of inter

exchange calls. Two types of signaling normally used

- Channel Associated Signaling (CAS)

- Common Channel Signaling (CCS)

CETTM MTNLSignalling


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In Channel Associated Signaling (CAS) also

call in-band signaling, the signaling takesplace on the same TS or channel of thespeech path

In Common Channel Signaling (CCS) alsocalled out- of-band signaling, a separate

dedicated channel is used to send and receivesignaling information for a group of trunks.Signaling is facilitated by means of labeledmessages

CCS No-7 Signaling is most commonly used

signaling mechanism in PSTN network



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Originating

Subs

Terminating

Exchange

Terminating

Subs

Originating

Exchange

Off-hook

Dial Tone

Dialed Digits

JXn Seizure

Acknowledgment

Dialed Digits

Ring Back Ringing

Answer Off-hook

Disconnect

Idle1

2

3



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Terminating

Exchange

Originating

Subs

Terminating

Subs

Originating

Exchange

Off-hook

Dial Tone

Dialed Digits

JXn Seizure Rq

Acknowledgment

Dialed Digits

Ringing

Answer Off-hook

Disconnect

Idle1

2

4

3

5 6

6

Ring Back


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Signalling


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Originating

Subs

Terminating

Subscribert

Terminating

Subs

Originating

Exchange

Off-hook

Dial Tone

Dialed Digits

JXn Seizure Rq

Acknowledgment

Dialed Digits

Ringing

Answer Off-hook

Disconnect

Idle1

2

4

3

5

7

6

6

8

9

10 10

Audible Ring

8

CETTM MTNL

Common Channel Signalling


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CCS Signalling: telephony signaling carried overseparate network from telephone calls (data)

- allows for signaling between any switches (notjust directly-connected )

- allows for signaling during call (not just before/after)

- security: in-band tone signaling helps phonephreaks; out of band signaling more secure SS7 network: packet-switched but calls circuit-

switched Redundancy (for reliability) in signaling network

links, elements

CETTM MTNL


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Public Switched Telephone Network

Public Switched Network Typical Network ?

Customer Premise Equipment (CPE)

Local Loop

PSTN Switch (Exchanges)

Transmission Network

Local Network Structure

Typical Local Exchange Network

Multiple Carrier Network

CETTM MTNL


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International Network

Network Evolution Digital Switching

Circuit Switch

Crossbar Switch

Characteristics of Crossbar Switches

Multistage Switch

CETTM MTNL

S it hi P th


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Switching Path

Multiplexing

Time-division multiplexing (TDM)

E1 (PCM) Frame structure

TDM Switching



Time-slot Interchange Technique

Digital Time switch

Common Channel Signalling

CETTM MTNL


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Time Switch

Time switch operating mode Output associated control

Output associated control

Time switch Characteristics

Space switching

Time Switch V/s Space Switch

Hybrid Switches

TST Switch

CETTM MTNL

Combination switches


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Combination switches

Telephone Network

Statistical Multiplexing

Blocking Model

Blocking Probability

PSTN switch (Exchange)

PSTN Numbering Scheme

Call Dialling

Call Processing

Signalling

CETTM MTNL


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THIS SLIDE IS KEPT INTENTIONALLY BLANK

CETTM MTNL


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CETTM MTNL


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s02 sovpstn210 _ overview of pstn

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