Chapter 2 Voice Communications Concepts and Technology

Chapter 2Chapter 2

Voice Communications Voice Communications Concepts and TechnologyConcepts and Technology

ObjectivesObjectives

Investigate PSTNInvestigate PSTN Study and understand digital voice Study and understand digital voice

communication and digitizationcommunication and digitization Understand PBXsUnderstand PBXs Understand CTI and voice servicesUnderstand CTI and voice services Introduce Digital voice Introduce Digital voice

transmission servicestransmission services

GOALGOAL

Study the business behindStudy the business behind

voice communicationvoice communication

Basic ConceptsBasic Concepts

Voice consists of sound waves of Voice consists of sound waves of varying frequency and amplitude.varying frequency and amplitude.

The transmitter part of phone The transmitter part of phone handset converts voice into handset converts voice into electrical signals to be transmitted electrical signals to be transmitted onto the analog network.onto the analog network.

The receiver part of a handset works The receiver part of a handset works the opposite of the transmitter. the opposite of the transmitter.

Getting Voice Onto and Off the Network

ElectromagnetSpeaker diaphram (moveable)

Permanent magnet

Variable magnetic field

Diaphram (moveable)

Granulated carbon

4 Wires

Sound Waves

Sound Waves

Handset

Transmitter (mouthpiece)

Receiver (earpiece)

Electrical contacts

RJ-22 connector

RJ-22 connector

RJ-11 connectors

GOLDMAN & RAWLES: ADC3e FIG. 02-01

2 wires

Basic ConceptsBasic Concepts

POTS (Plain Old Telephone Service) POTS (Plain Old Telephone Service) employs analog transmissions to employs analog transmissions to deliver voice signals from source to deliver voice signals from source to destination.destination.

The bandwidth is 4000 Hz, but The bandwidth is 4000 Hz, but guardbandsguardbands limit the useable range to limit the useable range to 300-3300 Hz.300-3300 Hz.

Today, the local loop is still analog, but Today, the local loop is still analog, but high-capacity digital circuits typically high-capacity digital circuits typically link the exchanges or Central Offices.link the exchanges or Central Offices.

Voice Bandwidth

Range of human hearing

Full bandwidth of analog

circuit

Guardband

Bandwidth available for analog voice transmission

Guardband

0

20

300

3,400

4,000

20,000

Hert

z (

Hz)

(gra

ph n

ot

to s

cale

)


Voice Network ConceptsVoice Network Concepts

PSTNPSTN Network hierarchyNetwork hierarchy Signaling and dial toneSignaling and dial tone Control and management:Control and management:

Figure 1-3 Basic Telecommunications Infrastructure

Phone Local loopC.O.

P.O.P.

Phone

C.O.P.O.P.

Local loop

Inter-exchange circuit

Belongs to IXC (Inter-eXchange Carrier)

Belongs to LEC (Local-Exchange Carrier)

LATA - A

LATA - B


Representative Voice Network Hierarchy

Residential customer

Business customer

Residential customer

Business customer

Class 1: regional centers

Class 2: sectional centers

Class 3: primary centers

Class 4: toll centers

Class 5: local central office

Local loops

Tandem office

Local loops

Local Carrier's Domain of Influence, Intra-LATA

Class 1: regional centers

Class 2: sectional centers

Class 3: primary centers

Class 4: toll centers

Class 5: local central office


Signaling and dial toneSignaling and dial tone

Requested destinations are presented to the Requested destinations are presented to the CO by dialing a series of numbers.CO by dialing a series of numbers.

Numbers tell whether the call is local, intra-Numbers tell whether the call is local, intra-LATA, or inter-LATA. Subsequently, LATA, or inter-LATA. Subsequently, concerned circuits are solicited.concerned circuits are solicited.

Numbers are generated:Numbers are generated:– Rotary phones: pulsesRotary phones: pulses– Dual-tone multi-frequency tones. Dual-tone multi-frequency tones. – Tones are used to enable specialized services Tones are used to enable specialized services

from PBX’s, carriers, banks, information services, from PBX’s, carriers, banks, information services, and retail information.and retail information.

Tone Dialing with DTMF

2ABC

1 3DEF

A

4GHI

5JKL

6MNO

B

7PRS

8TUV

9WXY

C

* # D0operator

1209 Hz 1336 Hz 1477 Hz 1633 Hz

941 Hz

852 Hz

770 Hz

697 Hz

High (column) frequencies

Lo

w (

row

) fr

eq

ue

nci

es

Two tones as designated on horizontal (row) and vertical (column) frequency axes are combined to produce

unique tones for each button on the keypad

This column is present only on specialized government phones


Voice DigitizationVoice Digitization

PrinciplePrinciple: : – To convert voice signal from analog To convert voice signal from analog

to digitalto digital– The voice conversation must be The voice conversation must be

sampled frequently enough so that sampled frequently enough so that the resulting conversations resembles the resulting conversations resembles the voice of the call initiatorthe voice of the call initiator

Codec (coder/decoder)Codec (coder/decoder): : – device used to transform analog device used to transform analog

transmissions into a stream of binary digitstransmissions into a stream of binary digits..

Voice DigitizationVoice Digitization

Pulse Amplitude Modulation: (PAM)Pulse Amplitude Modulation: (PAM)– Varies the amplitude of the electrical Varies the amplitude of the electrical

pulsespulses– Used in earlier PBX’s.Used in earlier PBX’s.

Pulse Duration Modulation: Pulse Duration Modulation: (PDM/PWM)(PDM/PWM)– Varies the duration of electrical pulsesVaries the duration of electrical pulses

Pulse Position Modulation: (PPM)Pulse Position Modulation: (PPM)– Varies the duration between electrical Varies the duration between electrical

pulsespulses

Voice Digitization:

PAM

PDM

PPM

8

7

6

5

4

3

2

1

0

Variable: Pulse amplitude

Constants: Pulse duration, pulse position

analog signal

PAM: Pulse Amplitude Modulation

Sampling rate = 8,000 times/second

1/8000 of a second

8

7

6

5

4

3

2

1

0

Variable: Pulse duration

Constants: Pulse amplitude, pulse position

analog signal

PDM: Pulse Duration Modulation

8

7

6

5

4

3

2

1

0

Variable: Pulse position

Constants: Pulse amplitude, pulse duration

analog signal

PPM: Pulse Position Modulation

74 6 5 4 4 46 65 57

74 6 5 4 4 46 65 57


Pulse Code ModulationPulse Code Modulation

The most common method used to The most common method used to digitize voice is Pulse Code digitize voice is Pulse Code Modulation (PCM).Modulation (PCM).

PCM uses:PCM uses:– 8000 samples/sec8000 samples/sec– 8 bits/sample8 bits/sample– therefore, 1 digital voice channel therefore, 1 digital voice channel

requires 64 Kbps requires 64 Kbps – known as a DS-0 circuit.known as a DS-0 circuit.

Voice Digitization: Pulse Code Modulation (PCM)

8

7

6

5

4

3

2

1

0

8 possible amplitudes in example are really 256 (28) amplitudes in PCM

analog signal

Analog Signal to be Digitized

Sampling rate = 8,000 times/second1/8000 of a second

Step 1: Sample Amplitude of Analog Signal

Step 2: Represent Measured Amplitude in Binary Notation

Step 3: Transmit Coded Digital Pulses Representing Measured Amplitude

Amplitude in example at sample position 1 (the gray shaded box) is

Power of 2

Value

Binary notation

27 26 25 24 23 22 21 20

128 64 32 16 8 4 2 1

0 0 0 0 0 1 0 0 = 4

4

8 bits = 1 byte

0 0 0 0 0 1 0 0

8 transmitted bits = 1 transmitted byte = 1 transmitted sampled amplitude


Adaptive Differential PCM Adaptive Differential PCM (ADPCM)(ADPCM)

Each voice channel uses 4 bits. Each voice channel uses 4 bits. ADPCM supports 48 simultaneous ADPCM supports 48 simultaneous

conversations over aT1 circuit.conversations over aT1 circuit. The standard for 32-Kbps is known The standard for 32-Kbps is known

G.721.G.721. The G.721 is used as a quality The G.721 is used as a quality

reference point for voice reference point for voice transmissions (Toll Quality). transmissions (Toll Quality).

ADPCMADPCM

ADPCM is used to send sound on ADPCM is used to send sound on fiber-optic long-distance lines as fiber-optic long-distance lines as well as to store sound along with well as to store sound along with text, images, and code on a CD-text, images, and code on a CD-ROMROM

Voice CompressionVoice Compression

ADPCM is also known as voice ADPCM is also known as voice compression.compression.

Advanced techniques employ DSP’s.Advanced techniques employ DSP’s. DSP’s are able to compress voice in DSP’s are able to compress voice in

as little as 4800 bps. as little as 4800 bps. Efficiency: 13 times more than PCM.Efficiency: 13 times more than PCM. Voice compression may be Voice compression may be

accomplished by stand alone units, or accomplished by stand alone units, or by integral modules within by integral modules within multiplexers.multiplexers.

Private Branch ExchangesPrivate Branch Exchanges

A PBX is similar in function to a public exchange.A PBX is similar in function to a public exchange. A PBX is exclusively used by the organization and A PBX is exclusively used by the organization and

physically located on the organization’s physically located on the organization’s premises.premises.

Provides an interface between users and the Provides an interface between users and the shared network (PSTN). shared network (PSTN).

Additional services offered by a PBX allow users Additional services offered by a PBX allow users to use their phones more efficiently and to use their phones more efficiently and effectively.effectively.

Medium to large organizations can save a lot of Medium to large organizations can save a lot of money by using a PBXmoney by using a PBX

Popular vendors of PBX Popular vendors of PBX equipmentequipment

Nortel, Lucent (25% of the market)Nortel, Lucent (25% of the market) Siemens, RolmSiemens, Rolm NEC, MitelNEC, Mitel

PBX ArchitecturePBX Architecture

PBX overall functionality and added PBX overall functionality and added features are controlled by software features are controlled by software programs.programs.

Those programs are executed by Those programs are executed by specialized computers.specialized computers.

Programs reside in: CPU, stored program Programs reside in: CPU, stored program control or common control area.control or common control area.

User phones are connected to User phones are connected to line cardsline cards.. Trunk cardsTrunk cards allow connection of the PBX allow connection of the PBX

to outside world.to outside world.

PBX Physical Architecture

CPU or common control

Switching matrix

Sta

tion

card

Trun

k ca

rd

Sta

tion

card

Sta

tion

card

Sta

tion

card

Sta

tion

card

Sta

tion

card

Sta

tion

card

Sta

tion

card

Users and phones

(stations)

PBX

Outside trunks


PBX Technology AnalysisPBX Technology Analysis

PBX features and services end to PBX features and services end to fall into three categories:fall into three categories:

features and services:features and services:1.1. that provide users with flexible usage that provide users with flexible usage

of PBX resources.of PBX resources.

2.2. that provide for data/ voice integrationthat provide for data/ voice integration

3.3. that control and monitor the use of that control and monitor the use of those PBX resources.those PBX resources.

Voice Based Features and Voice Based Features and ServicesServices

Common features:Common features:– Conference calling, Call forwarding, Conference calling, Call forwarding,

Call transfer, Speed dialing, Redialing, Call transfer, Speed dialing, Redialing, Call hold.Call hold.

Least Cost Routing (Least Cost Routing (Selecting lowest Selecting lowest

price LD providerprice LD provider).). Automatic Call distributionAutomatic Call distribution Call pickupCall pickup Night ModeNight Mode

Data/Voice Integration Features Data/Voice Integration Features and Servicesand Services

Data is transmitted either:Data is transmitted either:– through the PBX via a dedicated through the PBX via a dedicated

connection orconnection or– a hybrid voice/data phone is used to a hybrid voice/data phone is used to

transmit both voice and data transmit both voice and data simultaneously over a single connection.simultaneously over a single connection.

Features:Features:– ISDN support, T-1 support, Data ISDN support, T-1 support, Data

interfaces, PBX to host interfaces.interfaces, PBX to host interfaces.

Control and Monitoring Features Control and Monitoring Features and Servicesand Services

Basic: (e.g.)Basic: (e.g.)– Limiting access to outside lines from certain Limiting access to outside lines from certain

extensions.extensions. Advanced:Advanced:

– Call accounting system: program run on a Call accounting system: program run on a separate PC directly connected to the PBX.separate PC directly connected to the PBX.

– Process within the PBX known as station Process within the PBX known as station message detail recording (SMDR) where an message detail recording (SMDR) where an individual detail record is generated for each individual detail record is generated for each call.call.

– Used for spotting abuse and allocating phone Used for spotting abuse and allocating phone usage on a departmental basis.usage on a departmental basis.

Call Accounting Systems Installation

CPU or common control

Switching matrix

Sta

tion c

ard

Tru

nk c

ard

Sta

tion c

ard

Sta

tion c

ard

SM

DR

outp

ut

Users and phones

(stations)

PBX

Outside trunk

PC-based, call accounting system

Report printer

Usually an RS-232 connection

Call records are either saved or discarded based on call

filtering settings


Auxiliary Voice Related ServicesAuxiliary Voice Related Services

Auxiliary add-on device that provides Auxiliary add-on device that provides the following services:the following services:– Automated attendantAutomated attendant– Voice mailVoice mail– Voice response units (VRU): Interactive Voice response units (VRU): Interactive

voice response (IVR).voice response (IVR).– Voice processor: e.g. speech Voice processor: e.g. speech

recognitionrecognition– Voice server: a LAN based server that Voice server: a LAN based server that

stores, and delivers digitized voice stores, and delivers digitized voice messages. Used with voice mail system. messages. Used with voice mail system.

– Music / ads on holdMusic / ads on hold

PBX Architecture TrendsPBX Architecture Trends

PBX user demandsPBX user demands::– Better connectivity between phones and Better connectivity between phones and

desktop PCs.desktop PCs.– Better connectivity between PBXs and Better connectivity between PBXs and

LANsLANs– More More open PBXopen PBX architecture for easier architecture for easier

access to PBX features and services from access to PBX features and services from a variety of computing platforms.a variety of computing platforms.

– Better Better integration of PBX management integration of PBX management programsprograms with enterprise network with enterprise network management packages.management packages.

Open PBX ArchitectureOpen PBX Architecture

Logical separation between the call Logical separation between the call processing functionality and the underlying processing functionality and the underlying switching fabric.switching fabric.

Allow to introduce newer and faster Allow to introduce newer and faster switching technology without redesigning switching technology without redesigning entire PBX.entire PBX.

Support of industry standard API: TAPI and Support of industry standard API: TAPI and TSAPI for telephony applications.TSAPI for telephony applications.

New services can be designed and New services can be designed and interfaced to a variety of PBXs and interfaced to a variety of PBXs and computers.computers.

Open PBX Architecture

Management Applications

Call Center Applications

Telephony Enabled

Applications

Modular Applications

Call Processing Software

Operating System

Switching Fabric

Interface Modules

Wide Area Local

TAPI or

TSAPI

System Calls

SO

FT

WA

RE

HA

RD

WA

RE

cellular phone

desktop PC

ethernet (LAN)

phone

PSTN (analog)

cellular

ISDN

T-1


PBX TrendsPBX Trends

Architectural TrendsArchitectural Trends Multi-vendor Interoperability standards:Multi-vendor Interoperability standards:

– International standard: Q.Sig (allows International standard: Q.Sig (allows interoperability between PBX’s and ISDN).interoperability between PBX’s and ISDN).

PBX integration with wireless phones PBX integration with wireless phones support:support:

Cordless Telephone generation 2 (CT2).Cordless Telephone generation 2 (CT2). Common air Interface (CAI) global standard Common air Interface (CAI) global standard

for low power wireless transmission.for low power wireless transmission.

Mini PBX’s for the Small Office Home Mini PBX’s for the Small Office Home Office Market (SOHO).Office Market (SOHO).

Mini PBX’s for the Small Office Home Mini PBX’s for the Small Office Home Office MarketOffice Market

Mini PBX’s known as multifunction Mini PBX’s known as multifunction telephony boards or PC expansion telephony boards or PC expansion boards:boards:

Multiple workers share a small number Multiple workers share a small number of phone lines with features such as:of phone lines with features such as:

Auto attendant softwareAuto attendant software Integration with cell phones, pagers, and Integration with cell phones, pagers, and

voice mail systems.voice mail systems. ISDN SupportISDN Support

Computer Telephony Computer Telephony Integration (C T I)Integration (C T I)

CTI attempts to integrate the phone CTI attempts to integrate the phone and the computer to increase and the computer to increase productivity.productivity.

Examples of the integration:Examples of the integration:– Call control: allows users to control their Call control: allows users to control their

telephone functions through their telephone functions through their computer.computer.

– Fax on demand: users can dial-in using a Fax on demand: users can dial-in using a telephone and request fax documents.telephone and request fax documents.

C T I C T I

– Unified messaging: Unified messaging: keeps voice-mail, e-mail, and faxes together in keeps voice-mail, e-mail, and faxes together in

one place (universal in-box).one place (universal in-box).

– Audiotex:Audiotex: Deliver audio information to users based on Deliver audio information to users based on

responses on the touch-tone keypad to responses on the touch-tone keypad to prerecorded questions (hotline questions).prerecorded questions (hotline questions).

– Interactive Voice Response (IVR):Interactive Voice Response (IVR): Support online transaction processing. Used in Support online transaction processing. Used in

banks to allow users to transfer funds between banks to allow users to transfer funds between accounts.accounts.

C T I ArchitectureC T I Architecture 1 - PBX to host interfaces1 - PBX to host interfaces

In PBX-to-host interface CTI was In PBX-to-host interface CTI was achieved by linking mainframes to PBXs achieved by linking mainframes to PBXs via PBX-to-host-interface.via PBX-to-host-interface.

Applications were compatible with Applications were compatible with computer and PBX.computer and PBX.

Systems linked to an automatic call Systems linked to an automatic call distribution unit (ACD)distribution unit (ACD)

All phones are controlled by CTI All phones are controlled by CTI application running on mainframe application running on mainframe computer.computer.

Expensive systems.Expensive systems.

C T I ArchitectureC T I Architecture

2 - Desktop C T I2 - Desktop C T I

First party call controlFirst party call control Less expensive alternative to PBX-host Less expensive alternative to PBX-host

architecture.architecture. PC’s are equipped with telephony boards PC’s are equipped with telephony boards

and associated control software.and associated control software. Each PC controls only the telephone to Each PC controls only the telephone to

which it is attached.which it is attached. No overall automatic call distribution No overall automatic call distribution

across multiple agents and their phones.across multiple agents and their phones. No sharing of call related data among No sharing of call related data among

the desktop CTI PC’s.the desktop CTI PC’s.

C T I ArchitectureC T I Architecture

3 - Client/Server C T I3 - Client/Server C T I

CTI server computer interfaces to the PBX CTI server computer interfaces to the PBX or ACD to provide overall system or ACD to provide overall system management. management.

Individual client based CTI applications Individual client based CTI applications execute on multiple client PCs. execute on multiple client PCs.

Multiple CTI applications on multiple client Multiple CTI applications on multiple client PCs can share the information supplied by PCs can share the information supplied by the single CTI Server.the single CTI Server.

Offers overall shared control of the PBX-Offers overall shared control of the PBX-to-host CTI architecture at a cost closer to to-host CTI architecture at a cost closer to that of the desktop architecture.that of the desktop architecture.

CTI

Architectures

CTI client applications



desktop phone service



data server

CTI server

CTI Server applications

customer information

PBX ACD

Client/Server CTI

PC running 3270

emulationPC running

3270 emulationPC running

3270 emulationdesktop phone service



mainframe computer PBX ACD

PBX-to-Host Interfaces

CTI applications

proprietary PBX-to-Host CTI interface

CTI card

CTI card

PBX

PSTN

-or-

-or-

-or-

Desktop CTI

desktop CTI application

desktop CTI application


Figure 2-20 Technology Required to Develop and Implement CTI Applications

FAX Software

CTI Application Development Tool

Programming Language- Visual Basic 3.x - Visual Basic 4.0 - Visual C++

DEVELOPMENT WORKSTATION

FAX API

CTI API

Development

Phone Network

customer phonenetwork

interface card

network interface

card

voice board

CTI/IVR application

customer information

Data Server CTI server

DTMF tones

Digital Signal Processor

(DSP) voice queries

analog voice lines-or- ISDN

-or- T-1/E-1

Implementation

voice card specific drivers

Interactive Voice Response (IVR) Application


Alternatives to PSTNAlternatives to PSTN

Voice over the InternetVoice over the Internet– using a sound card, microphone, and using a sound card, microphone, and

speakersspeakers– can call others using the same can call others using the same

productproduct– gateways are being established to gateways are being established to

allow Internet voice callers to reach allow Internet voice callers to reach regular telephone users as well.regular telephone users as well.

Voice over the InternetVoice over the Internet

Underlying transport protocols (IP and Underlying transport protocols (IP and UDP) deliver voice conversation.UDP) deliver voice conversation.

IP = internet protocolIP = internet protocol UDP = User datagram protocolUDP = User datagram protocol IP and UDP can be used in any of the IP and UDP can be used in any of the

following:following:– Modem based point-to-point connectionsModem based point-to-point connections– Local area networksLocal area networks– Private Internets (Intranets) Private Internets (Intranets)

VOIP Transmission Technology and Topologies

router router

LAN hubs

INTERNET/INTRANET

LAN attached PCs with required Client technology.

IP protocols REQUIRED

LAN hub

LOCAL AREA NETWORK

LAN attached PCs with required Client technology.

IP protocols REQUIRED

PSTN

POINT-TO-POINT/MODEM-TO-MODEM

modemmodem

analog dial-up lines

PC with required Client technology

PC with required Client technology

Internet Access

REQUIRED CLIENT TECHNOLOGY

Client workstation

Internal

External-or-

Modem

ONLY required for dial-up connections.

ONLY required for

Internet-based voice transmission

LAN connection

ONLY required for LAN-based

voice transmission

Voice/sound technology

sound card speakers microphone IP-based Voice Client

software

an

d /

or

an

d /

or

an

d

an

d

PO

SS

IBL

E IM

PL

EM

EN

TA

TIO

N T

OP

OL

OG

IES

Internet -or-

Intranet


Alternatives to PSTN Alternatives to PSTN (cont’d)(cont’d)

Example of products:Example of products:– CoolTalkCoolTalk– Internet PhoneInternet Phone– Intel Internet PhoneIntel Internet Phone– Microsoft NetMeetingMicrosoft NetMeeting– WebtalkWebtalk– TeleVoxTeleVox

Voice over Frame relayVoice over Frame relay

Variable-length delay introduced by Variable-length delay introduced by variable-length frames is unacceptable variable-length frames is unacceptable for voice.for voice.

Frame relay access device (FRAD) Frame relay access device (FRAD) accommodates both voice and data:accommodates both voice and data:– Voice prioritization: priority given to voice Voice prioritization: priority given to voice – Data frame size limitation: to limit delaysData frame size limitation: to limit delays– Separate voice and data queues. Separate voice and data queues.

Voice over Frame relayVoice over Frame relay Voice conversations require 4 – 16 Kbps Voice conversations require 4 – 16 Kbps

of bandwidth.of bandwidth. Permanent Virtual Circuit (PVC): Permanent Virtual Circuit (PVC):

maintained end to end connection maintained end to end connection throughout the voice conversation.throughout the voice conversation.

Voice conversation can take place Voice conversation can take place between locations directly connected to a between locations directly connected to a frame relay network.frame relay network.

No current standards defined between No current standards defined between frame- relay networks and the voice frame- relay networks and the voice based PSTN.based PSTN.

Voice Transmission over a Frame Relay Network

FR FRvoice

data

voice

data

voice and data

voice and data

Local Area Network

Telephone service Telephone service

Local Area Network

PBX PBX

PSTN

NO voice interoperability between Frame Relay and

PSTN networks

FRAD prioritizes voice

traffic

FRAD prioritizes voice

traffic

Frame Relay Network


Voice over ATMVoice over ATM

ATM is a switched-based WAN service ATM is a switched-based WAN service using fixed-length frames (using fixed-length frames (cellscells).\).\

Cells assures fixed length processing by Cells assures fixed length processing by ATM switches enabling predictable delay ATM switches enabling predictable delay and delivery time.and delivery time.

Voice transmitted using Constant Bit Rate Voice transmitted using Constant Bit Rate (CBR) reservation scheme. (CBR) reservation scheme.

CBR does not make optimal use of CBR does not make optimal use of bandwidth because of moments of silence.bandwidth because of moments of silence.

Most common method: reserve a CBR of Most common method: reserve a CBR of 64Kbps for one conversation digitized via 64Kbps for one conversation digitized via PCM.PCM.

Optimizing voice over ATMOptimizing voice over ATM Voice Compression:Voice Compression:

– achieved via ITU G series of standards:achieved via ITU G series of standards: G.726: 48, 32, 24 or 16 KbpsG.726: 48, 32, 24 or 16 Kbps G.728: 16 KbpsG.728: 16 Kbps G.729: 8 KbpsG.729: 8 Kbps

Silence suppression:Silence suppression:– Cells containing silence are not allowed and Cells containing silence are not allowed and

replaced at the receiver with synthesized replaced at the receiver with synthesized background noise.background noise.

Use of VBR (Variable bit rate):Use of VBR (Variable bit rate):– Combines positive attributes of both voice Combines positive attributes of both voice

compression and VBR.compression and VBR.

Voice Transmission over an ATM Network

ATM Network

voice

data

voice

data

voice & data

Local Area Network

Telephone service Telephone service

Local Area Network

PBX PBX

-either- CBR - Constant Bit Rate

-or- VBR - Variable Bit Rate

transmission through ATM network

silence suppression/background

noise synthesis

ATM cells

silence suppression/background

noise synthesis

ATM cells

voice compression/

decompression

voice compression/

decompression

voice & data

ATMATM


Voice/Data MultiplexersVoice/Data Multiplexers

A voice/data multiplexer A voice/data multiplexer simultaneously transmits digitized simultaneously transmits digitized voice and data over a single digital voice and data over a single digital transmission service.transmission service.

Voice and data channels are Voice and data channels are assigned to separate channels. assigned to separate channels. (multiplexing).(multiplexing).

Voice/Data ModemsVoice/Data Modems

Two standards exist for the simultaneous transmission Two standards exist for the simultaneous transmission of voice and data over a single analog phone lineof voice and data over a single analog phone line. .

ASVD:ASVD:– Switches between voice and dataSwitches between voice and data– Voice takes priorityVoice takes priority– Standard known as: ITU V.61Standard known as: ITU V.61

DSVD:DSVD:– Digitizes voice and combines it with data over the Digitizes voice and combines it with data over the

single analog transmission line. single analog transmission line. – Voice compressed into 9.6 to 12 Kbps leaving 16.8 Voice compressed into 9.6 to 12 Kbps leaving 16.8

to 19.2 Kbps for data (V.34 DSVD compliant to 19.2 Kbps for data (V.34 DSVD compliant modem).modem).

– DSVD is known as ITU V.70DSVD is known as ITU V.70

Integrated Services Digital Integrated Services Digital Network (ISDN) Network (ISDN)

A newer switched digital service A newer switched digital service used for small business and used for small business and residential users.residential users.

It offers two 64 Kbps channels, one It offers two 64 Kbps channels, one for voice and the other for data.for voice and the other for data.

Simultaneous Voice/Data Transmission with DSVD and ISDN

PC

PSTN

DSVD - Digital Simultaneous Voice Data

DSVD modem (V.34)

Analog Phone

PC

DSVD modem (V.34)

Analog Phone

data and voice share a single 28.8Kbps channel (V.34)

analog analog

digitized voice/data (28.8Kbps)

PC

ISDN

ISDN - Integrated Services Digital Network

ISDN modem data/voice

Analog Phone

PC

ISDN modem data/voice

Analog Phone

data and voice on separate channels

digital digital

64Kbps voice

64Kbps data


Documents

Chapter 2 Voice Communications Concepts and Technology