digital telephony ppt

04/11/23 A Seminar on Digital Telephony 1

Digital Telephony

Presentation By:

Mr. Devidas Chikhale

Assist. Prof. EXTC. Dept.

Contents of presentation Merits & Demerits Overview of telephone networks Call flow in landline telephone Transmission Signaling Switching ISDN VOIP

The Term Digital Telephony:

refers to the use of digital technology

in the message path of voice

communication networks.

The term digital :

refers to a method of encoding the

digital signal.


Why Digital?

• To improve quality

• Add new features

• Reduce cost of conventional voice services


Advantages• Ease of Multiplexing• Ease of signaling• Use of Modern Technology• Integration of Transmission and Switching• Signal regeneration• Performance Monitoriability• Accomodation of other Services• Operability at low S/N or Interference ratio• Ease of encryption

Ease of multiplexing

•First inter office T carrier systems

• TDM & FDM equipments

• Analog TDM

• Requirement of TDM transmission

Ease of signaling

• Control information transmission

• Case in analog

• Use of DSL

• Insertion & extraction independent of

transmission medium

• Design of signaling equipments

• Upgradiation of digital systems

Use of Modern Technologies• Multiplexer & switching matrix

• LSI telecommunication functions

• Completely nonblocking switches

• Analog IC’s

• Single pin

• Transmission systems

• Switch on chip

• Fiber optic transmission

Integration of Transmission & switching

•Independent Organisations

• Digital switching

• Not only eliminates equip. but greately

improves voice quality.

•Removal of ADC & DAC, low error rate,

voice quality by encoding & decoding .

• long dist. Voice quality identical to short

dist.

•Echo’s are eliminated

•All trunks are implimented as subchannels .

Signal Regeneration

• Reduction of probability of error

• Certain imperfections

• Ability to localize signal degradation's

• End to end quality

• ADC: loss of signal fidelity

• Tradeoff bet. Transmission quality & bw.

Performance Monitorability

•Source independent signal structure

•Analog systems

•FDM pilot signal

• Addition of parity, CRC check

• Monitoring certain redundancies in signal

w/f itself.

Accommodation of other services

• Phase distortion

• compensation for analog transmission

impairments

• Common transmission formats

eg. Television signals

Operability at low signal to noise /Interference ratios

•Analog speech quality

• End to end S/N

•Virtually error free performance

• Ability to reject crosstalk

• Speech pauses

Ease of Encryption• Sensitive conversations

• Analog voice

• More security to digital signals


Disadvantages• Increased Bandwidth

• Need for time synchronization

• Incompatibilities with analog facilities

• Topologically restricted multiplexing

• Need for extension bridges


Telephony Background

• Overview of telephone Networks

– Analog N/w Hierarchy

– Bell system Hierarchy


Three level system HierarchyFirst Level Switch

First Level

Switch

Second

Level Switch

Third level switch

Alternate Routing in North American N/w

1

4

2

33

5

1

2

3

4

5High usuage trunks

121

21

32

1

1


Mesh –connected network

Nodes


Exchange area network

EO

EO

EO

Toll network

EO

EO

Toll offices

Tandom office

Tandomoffice

End offices

Telephone Elements

11 2 3

4 5 6

7 8 9

. 0.

Electronic AmplifierAnd Ringtone

TX( Microphone)

RX ( Earphone)Touch Tone Keypad & Frequency Generator

Twisted pair

Copper wire

Wall Socket

Call FlowCalling Subscriber Switching Center Called Subscriber

(Exchange)

Off-hook signal(1) Identification ofcalling

Dial Tone(2) Subscriber(2)

Storage Allocation (3)

Address digit (4) Selection of out going Ringing Current(7)

circuit & digit analysis Answer Off-hook (5) signal(8)

Switch path setup (6)

Ring Tone (7)

Both way Disconnect ringing Both way conversation conversation tone & current

Forward Clear Supervision(9) Backward Clear on- on-hook hook

Disconnect equip.

(10)

Transmission Systems Open Wire Paired Cable(UTP)Coaxial CableOptical fiber CableMicrowave RadioSatellites

Open Wire

•Telephone poles with cross arms.

•Glass insulators to support uninsulated wire pairs.•Low attenuation (few 100 db/mile at voice frequency.)

•Used for long, rural customer loops

•Needs large amount of copper

(25 times more than cable).

Unshilded Twisted Pair(UTP)

For making trunks in early days. To connect terminal equipments to the exchange. Installed with two pairs to the home with extra pair making it possible to add another line.

Coaxial Cable

Copper wire

Shield( metallic, foil or braid)

Outer Sheath Insulation

For High bandwidth and Video Services before OFC

Coaxial cable• To satisfy long haul requirement of the toll

n/w.

• To combat attenuation: repeater amp. at

5.5 miles.

• Capacity of open wire/cable: 12 voice

circuits.

•Capacity increase by : larger diameter

Decreasing distance between repeaters.

• Improving noise figure, linearity & bw of

repeater amplifier.

Optic Fibre Cable

CoverCladding( Refractive coating)

Core

Higher bw, lower maintenance, lower attenuation,

Lower cost.

Optical Fiber Cable Fibers assembled into stranded or ribben

cable. Multifibre ribbon with min. 12 fibers. Loose or Tight buffering cable Transfer information as light of impulses

along a strand of glass. Repeaters at 100km Loss is calculated in dB/KM

Microwave Radio• Requires line of sight bet. antennas installed.

Parabolic antennas

Switch store

Pylon

40KM

A Satellite Link

Satellite

Earth Station

Thin route & data applicationsInherent propagation delay (250 msec)

Signaling

The signaling function of a telephone

network refer: to the means for

transferring network related control

information between Switching

nodes,various terminals, and users of the

network. Categorised as 1) Supervisory signals

2) Information bearing signals

Types of signaling

Signaling

Pulse

Signaling

DTMFSignaling

In ChannelSignaling

In Band Out of Band

Channel

Associated Signaling

(CAS)

Common Channel signaling (CCS)

SS7Signaling

Dc Pulse Signaling

Make(Closed)

Break(Open) <>< >

Digit length

Inter digit time

Pulse length

MakeBreak

Pulse Dialing

DTMF Signaling

DTMF Phone

Button digit or designation

Low band High band

1 697 1209

2 697 1336

3 697 1477

4 770 1209

5 770 1336

A [FO(flash override)

697 1633

B[ F( Flash) 770 1633

C[ I ( Immediate) 852 1633

D [ p( Priority)] 941 1633

In Channel Signaling

o Also called per trunk signaling.

o Uses the same transmission facilities or channel as for voice.

o Inband: Signaling information in the same band of freq’s used by voice.

o It can be used on any transmission

medium

o Eliminates mutual interference. Eg. 2600Hz

In Channel Signaling Out of band signaling:- Uses same facilities as the voice band but

different portion of the freq band. eg. Dc signaling on most customer loops. It depends on the transmission medium. Implimented above the cuttoff freq. of

voice separation filters. But <4khz. 3825 Hz for the purpose.

Channel Associated Signaling

T1 channel carries sequence of frames.

eg. 8*24+1 = 193bits

Framing bit


Frame: 24 channels, 8data bits+ 1frame bit

Total : 193 bits/frame T1 frames are sent at 8khz. The first vertical row of 8 bits sent first.

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

- c - s - c - s - c - s - c - s - c - s - c - s


Super Frame:- 12 frames make super frame Framing bits of even numbered frames as

signaling bit. Framing bits line up in particular

pattern100011011100. Which decides

which 8 bits belong to which pattern.This

discerning called frame synchronization.It looks specially at every 6th frame for

CAS signaling. These bits are called A/Bbits


Extended Super frame:- Twice the size than super frame. Provide continuous error checking

& separate data channel. Every fourth bit provides framing

synchronization. 4 signaling bits/channel. provides 16 signaling states in the 193rd

bit to ensure adequate synchronization,

supervisory control and maintenance capabilities.

COMMON Channel Signaling

It refers to, instead of sending signaling

information over same facilities that carry

the message traffic ( voice signals), uses

dedicated data link between stored

program elements (computers) of

switching systems. Data link sends messages to identify

specific trunks & events related to it.

COMMON Channel Signaling

Matrix Matrix

CPU CPU

Voice& Signaling

Voice& Signaling

Voice only

Control

Per channel signaling

Per channel signaling

ControlData Link

COMMON Channel SignalingAdvantages:- Only one set of signaling facilities for each

associated trunk groups. A single dedicated control channel. Dedicated channel inaccessible to users Simple procedure for transferring

information directly bet. Switching

processors. connections involving multiple switching

offices can be setup more rapidly.

COMMON Channel Signaling Disadvantages:- Control information relayed from one

node to other in store & forward fashion. High degree of reliability is required

in terms of physical facilities & error

control for data link.eg. Disconnect signal No automatic test of voice circuits as when

voice channel is used to transfer control

information.All trunks in a group do not terminate at

the same switch in some instances.

SS7 ( Signaling System 7)

SS7 Network Architecture

It consist of three signaling

components

I) Signal switching point or Service

switching point(SSP) ii) Signal control point or service

control point(SCP) III) Signal Transfer point or Service Transfer

point.( STP)


Signal /service switching point:-(SSP)

o Local exchange that carries telephone

conversations.

o Each SSP has numeric point code to

identify source & destination.

o Uses routing table to select path for

message.

o Combination of voice switch & SS7 switch.

o Became mixture of circuit related & packet

related messages.


SS7 Protocol suite:- Layer Protocols

Application

Transport

Network

Datalink

Physical

ISUP, TCAP, etc.

SCCP

Message Transfer Part (MTP3) Level3



TCAP: Transaction capabilities application part

SCCP: Signaling connection control part


The Application layer (level 5)

ISUP: ISDN user part to setup telephone

calls correctly bet. Diff. Switches.

It is part of ITU-T recommendations

or ANSI.

Transport layer:(level 4):-Responds to request from application layer typ. Processors in host computer.

The signaling connection control part(sccp) routing protocol provides end to end routing for TCAP messages to their proper database.


TCAP( Transacation capabilities application part):-

i) Deployment of advanced intelligent

network services by supporting non

circuit related information exchange bet.

Signaling points using SCCP

connectionless services.

ii) Supports remote control.

iii) TCAP messages are contained within

SCCP portion of a message signal unit.

SS7 Network ArchitectureMSU:- Message Signal Unit

i) Provides structure to carry the

information in the ss7 n/w.

ii) Information can be circuit based or non

circuit based,ie routing info. N/w

management data.

iii) Originate from MTP3or MTP user.

iv) Serves as payload carrier for high level

messages as SCCP, ISUP, TCAP

v) Helping transfer user messages bet. two peer

level 4 protocols in signaling points.


SCCP:- ( Signaling connection control part)

i) Provides connection oriented and

connectionless n/w services above n/w

layer.

ii) Provides subsystems numbers to allow

messages to be addressed to specific

applications or subsystems at signalling

points.

iii) Serves as transport layer for TCAP based

such as free phone,calling card,wireless roaming


iv) Provides the means by which STP can perform global title translation(GTT).

For MTP users, SCCP messages are composed of three parts.

a) A mandatory fixed part

b) A mandatory variable part

c) optional part


MTP ( Message Transfer Part):- Designated as joint transport system. Meet the requirment of diff. Users.Network Layer(MTP3) provides functional & procedural means

of transferring signaling messages. Responsible for network management. Routers send data through the extended

n/w making internet possible.

SS7 Network ArchitectureMTP2:-Message Transfer part2

functional & procedural means for data

transfer bet. N/w entities. Detects & correct errors that occur in

physical layer. Error messages can be retransmitted

using signal units or packets.

MTP1:-Message Transfer part1

Uses a timeslot in T/E carrier.

SS7 Network ArchitectureSignal Control Point:- (SCP) Act as gateway bet. Core SS7& world of

intelligent peripheral (IP). IP helps the SCP decide how to deliver

enhanced services to the subscriber, voice

customization, recognition,multimedia

person to person messaging. It heralds approach of next generation

networks which blend PSTN & PSDN.


Signal Transfer point:- (STP)

Most versatile, packet switches, acts like

router in SS7 n/w.

Major component in the n/w.

Acts like a firewall, screening messages to

& from , & with, other n/w’s.

Routes messages from & to outgoing

signaling links over SS7 n/w.

Switching

Refers to the setup & release connection

between transmission channels on an

“ as needed basis”. Switching categories for voice circuits.

I) Local ( line to line)

ii) Transit or (Tandem) switching

III) Call distribution

Switching Functions

Transit

concentrator

PBX

Endoffice

Tandem/Toll switchinglocal

Trunks

Transit

Transit Transit

Types of Switching

Space division switching Time division switching Time slot interchangeSpace time space Time space time

Space Division Switching

Crosspoints

N inlets

M outlets

Time division switching

A time division multiplexed frame, each

Sample 1,2,3,4 with consecutive time slot

time

Sample Frame Timeslot

3 4 1 2 3 4 1 2

Timeslot interchange switch

Input TDM Stream

Output TDM

Stream

3 4 A B C D 1 2 3 4

3 4 1 2 3 4 B D C A

A frame is a sequence of timeslot interchange switch

Time

Time

TST Switching Timeslotinterchange work by writing each

frame into a buffer memory & then reading

the information out in a different order

onto the output.

A Timeslot Interchange (TSI) switching element

InletsOutletsM

K

K

P

T R TSI

Space-Time-Space switching

M inlets

M

K

K

N

L R

L R

K

L R

NoutletsA-buses B-buses

PrimarySecondaries Teritary

Three stage space-time-space switch

STS SwitchingL:-space division matrices M:I/p’s,K:o/p’s

R:- space division matricesK: I/p’s N: o/p’s Inlets are TDM with L timeslots per frame. Outlets are TDM with R timeslots per

frame. Pri. & teritary are time division matrices. K secondaries are buffers Calling timeslot must be idle in A-bus & Called timeslot must be idle in B-bus making Kroutes spatially disjointed.

Time –Space-Time Switching

L K

L K

M

N

K

K R

K R

M inlets

Primaries Tertiaries

NoutletsA -buses B-buses

Time –Space-Time SwitchingL timeslots per frame at the inlet & R

timeslots per frame at the outlet. All frames have equal periodic timeslots. Secondary is a time division matrix

having k timeslots. Pri. & Tertiaries are buffers. K routes are not spatially disjoint bcz

each route occupies a diff.timeslot in the

same physical pair of buses Matching must occur in time

What is ISDN? INTEGRATED SERVICES DIGITAL NETWORK

•ISDN is a set of standards which define an end to end Digital Network

Features of ISDN Uses digital signal. Can transport many types of digital

traffic voice,data,video,text,graphics. Faster data rate than modem Alternate to using leased lines. Uses existing telephone wiring

ISDN Architecture

ISDN CHANNELS

• Bearer channels ( B-channels)

• Delta/Demand channels ( D channels)

• High capacity channels ( H channels)

Features of B - CHANNEL End to end four wire digital circuits. Shared n/w access for voice ,data &

leased lines. Relatively high bandwidth data channels

( 64 kbps) Lower error rates than typical voiceband

modems In service performance monitoring

Features of D-Channel Calling number identification User to user message transfer Access to packet switching network Signaling simultaneous with active

components. Far end supervision Telemetry for fire alarms,security,meter

reading.

ISDN Service

BRI (Basic Rate Interface)Connection from the ISDN office to the user location provides for

access to three channels. The channels are two 64Kb B-channels and one 16Kb D-channel

The B-channels and the D-channel provide the user with access to the circuit switched network

ISDN ServicePRI (Primary Rate Interface)

ISDN Primary Rate Interface service provides digital access via a T1 line. A T1 line provides a 1.544MBps bandwidth. This bandwidth is

divided into 24 64Kb channels. The ISDN PRI service uses 23 B channel access and uses the 24th (D) channel for signaling purposes.

Telecommuter/Remote Office

Components & ConsiderationsISDN Router

Multiple remote users at the same location

ISDN ProtocolsProtocols which start with the following letter:

E - Protocols recommend telephone network standards for ISDN

I - Protocols for Concepts, terminology and general methods

Q - Protocols, how switching and signaling should operate, call setup etc.

How VOIP Works?

Continuously sample audio

Convert each sample to digital form

Send digitized stream across Internet in packets

Convert the stream back to analog for playbackWhy VoIP?

IP telephony is economic; High costs for traditional telephone switching equipments.

How VOIP Works?

digital telephony ppt

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