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Transcript of 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 Division Switching
Time Slot Interchange
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Time Slot Interchange
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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Output Associated Control
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
Output Associated Control
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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
Output Associated Control
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.
CETTM MTNLCall Processing
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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
CETTM MTNLCall Processing
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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)
CETTM MTNLCall Processing
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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
CETTM MTNLSignalling
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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
CETTM MTNLSignalling
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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 Division Switching
Time Slot Interchange
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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THIS SLIDE IS KEPT INTENTIONALLY BLANK
CETTM MTNL
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