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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
)
GOLDMAN & RAWLES: ADC3e FIG. 02-02
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
GOLDMAN & RAWLES: ADC3e FIG. 01-03
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
GOLDMAN & RAWLES: ADC3e FIG. 02-04
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
GOLDMAN & RAWLES: ADC3e FIG. 02-05
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
GOLDMAN & RAWLES: ADC3e FIG. 02-07
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
GOLDMAN & RAWLES: ADC3e FIG. 02-08
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
GOLDMAN & RAWLES: ADC3e FIG. 02-11
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
GOLDMAN & RAWLES: ADC3e FIG. 02-14
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
GOLDMAN & RAWLES: ADC3e FIG. 02-17
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
CTI client applications
CTI client applications
desktop phone service
desktop phone service
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
desktop phone service
desktop 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
GOLDMAN & RAWLES: ADC3e FIG. 02-19
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
GOLDMAN & RAWLES: ADC3e FIG. 02-20
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
GOLDMAN & RAWLES: ADC3e FIG. 02-22
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
GOLDMAN & RAWLES: ADC3e FIG. 02-26
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
GOLDMAN & RAWLES: ADC3e FIG. 02-27
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
GOLDMAN & RAWLES: ADC3e FIG. 02-28
Recommended