Chapter 4Voice Communications

• Public Switched Telephone Network or Plain Old Telephone Service (POTS)

• Private Telephone Systems

• Computer Telephone Integration

• Wireless Communication

• Voice Transmission alternatives

Public Switched Telephone Network

• Central office

• Lines

• Voice digitization

• Numbering

• Services

Telephone Set

• Transmitter– Analog signal

• Receiver– Sidetone

• Switchhook– On-hook and off-hook

• Dialing– Rotary dial

– Dual-tone-multifrequency (DTMF) or touch tone

• Ringing– Call setup time

Central Office• Equipment

– Manual switching

– Electromechanical switching

– Electronic switching• Reliable

• One total failure in 40 years

• Quite & efficient

• Less Labor costs with higher skill levels

• Functions– AT & T dynamic non hierarchical routing (DNHR)

• direct, redundancy, and alternate routing

• Maximum 4 toll office

Central Office Hierarchy

• Class 5: local central office

• Tandem office

• Class 4: toll center

• Class 3: primary center

• Class 2: sectional center

• Class 1 regional center

Inter Office Signaling• Special network between central offices to set up

telephone calls• Common channel interoffice signaling system

(CCIS) by CCITT• Signaling system No.7 (SS7) By CCITT

– Advanced intelligent network (AIN)

– Software-defined network (SDN)

– Calling number identification

Lines

• Local loop (pair of copper wires: tip and ring)

• Drop wire

• Distribution cable

• Feeder cable

• Trunk– Copper wire

– Coaxial cable

– Microwave radio

– Fiber optic cable

• Internet line usage problem

Voice Digitization• Analog signals to digital signals (8000 samples/

sec)• Equipment

– coder/decoder or codec (signal conversion)

– Channel (signal conversion & loading into T-1)

• Methods– Pulse amplitude modulation (PAM)

– Pulse duration modulation (PDM) or pulse width modulaiton (PWM)

– Pulse position modulation (PPM)

– Pulse code modulation (PCM): most used with 256 possible amplitudes (2 to 8)

– Adaptive differential pulse code modulation (ADPCM)

Pulse Code Modulation

• Sample amplitude of analog signal (4)• Present measured amplitude in binary

notation– 7 5 6 4 3 2 1 0– 128 64 32 16 8 4 2 1– 0 0 0 0 0 1 0 0

• Transmit coded digital pulses representing measured amplitude (00000100)

Voice Digitization Bandwidth

• DS-0 with 64,000 bits/sec

• T-1 with 24 Ds-0

• Adaptive differential pulse code modulation (ADPCM)– Need half of bandwidth for PCM– Used for voice compression with less quality

Telephone Numbering

• 9 geographic zones in the entire world

• Composition – Access code, carrier’s code,– Zone and country code– Area/city code– Exchange code– Subscriber code

Telephone Services - I

• Types of calls– Local calling– Long distance calls– International calls– Operator services

Telephone Services - II

• Calling card calls

• Discounted calls

• 800 or 888 service– Geographic: interstate, intrastate, international

(Universal International Freephone Numbering -UIFN)

– Call direction: in, out, or both

• 900 service

Telephone Services - III

• Software defined network (SDN)• Foreign exchange (FX) lines• Integrated Services Digital Network (ISDN)• Selection criteria for telephone service

– Time

– Duration

– Number

– Location Usage pattern

– Usage pattern (busy hour)

– 1-5% (Blocking) grade service level

Private Telephone Systems

• Key systems: 3 to 50 telephones

• Private branch exchange (PBX) / private automatic branch exchange (PABX) / computer branch exchange (CBX): 50 to 10,000 or more

• Centrex

PBX System Feature• Automatic Call Distribution (ACD)

• Data communication

• Direct inward dialing (DID)

• Hunt group (alternative trunks)

• Least cost routing

• Paging

• Pick-up group

• Prioritization

• Night mode

• Station message detail recording/call detail recording (SMDR)/(CDR)

• Voice messaging

PBX Station Feature• Automatic redial• Automatic reminder• Call forward• Call transfer• Call waiting• Camp on (call back)• Conference calls• Distinctive ringing• Do not disturb• Speed dialing

PBX Data/Voice Integration

• ISDN support

• T-1 support

• Data Interfaces

• PBX-to-host interfaces

PBX Auxiliary Service

• Automated attendant

• Voice mail

• Voice response units (VRUs) or interactive voice response

• Voice processor same as VRU

• Voice server

• Music/ads on hold

PBX Security

• Block remote access

• Change access code and password

• Block outgoing calls during off-hours

Selection Criteria of PBX

• Maintenance

• Training

• Other support

• Price

• System feature

• Station feature

• Security feature

PBX Trends

• Multivendor interoperability standards

• PBX integration with wireless phones

• Mini-PBXs for home and small business

Centrex Service

• Regulated by public utilities commission

• PBX equipment in central office

Selection CriteriaBetween PBX and Centrex

• Regulation

• Maintenance

• Reliability

• Growth

• Control

• Capital investment

Computer Telephone Integration (CTI)

• Architectures

• Functions

• Application standard

CTI Architectures

• PBX-to-host interfaces

• Desktop to CTI

• Client/server CTI

CTI Functions

• Call control (on- line directory, conference calls)• Automated attendant• Automated call distribution• Audiotex• Fax-on-demand• Interactive voice response (IVR)• Outbound dialing• Unified messaging or universal in-box for voice

mail, e-mail, faxes and paper messages

CTI LAN-based Application Standards & Tools

• Standards– TSAPI (telephony services application program

interface) by AT&T and Novell– TAPI (telephony application program interface)

by Intel and Microsoft

• Development tools– Visual Basic or Visual C++– Graphical code generator, debugging tool,

interfaces to other applications (fax, sound)

Wireless Communication - I

• Cordless telephone

• Cellular telephone service– Transmitting voice & data– Security

• Eavesdropping

• Spread spectrum

• Accident

Wireless Communication - II

• Personal communication service– personal communication network (PCN) /

personal handyphone system (PHS) / personal communication service (PCS)

– Communication levels• Macrocells - car

• Microcells -walking

• Picells - building

• Marine and aeronautical telephone services– Radio communication

Voice Transmission Alternatives

• Internet - IP

• Frame relay

• Asynchronous transfer mode

• Others

Internet Telephone Calls

• Facilities– Hardware– Software

• Benefits

• Problems

• Impacts (industry & regulation)

• Future

Frame Relay

• Problems– Variable-length frames– Delay

• Frame relay access device (FRAD)– Voice prioritization– Data frame size limitation– Separate voice and data queues

Asynchronous Transfer Mode

• Fixed-length frames (53 bytes cells)

• Constant bit rate (not efficient)

• Optimal methods– Voice compression– Silence compression– Variable bit rate (max: peak voice bit rate, min:

guaranteed voice bit rate)

Others

• Voice/data multiplexers via T-1

• Voice/data modem– Analog simultaneous voice and data: use

switches– Digital simultaneous voice and data: digitize

voice using analog line

• ISDN BRI (basic rate interface) with two 64-kbps channels

Assignment

• Review chapters 1-4

• Read chapter 5

• Prepare exam 1