Chapter 5

Voice Communications

Agenda

• Telephone• Central office• Public Switched Telephone Network

– Lines– Signals– Numbering– Services

• Private telephone system• Wireless communication

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 - I

• Switching equipment– Manual switching– Electromechanical switching

• Step-by-step switches• Crossbar switches

– Electronic switching• Reliable• One total failure in 40 years• Quite & efficient• Less Labor costs with higher skill levels

• Design considerations– Forecasting demand (time, duration, distance, culture)– Grade of service or blocking

Central Office - II

• End office or serving central office

• Toll office or switching office

• Central hub

Public Switched Telephone Network

• Line types– 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

Analog Signals - I

• Signal frequency– Sine wave– Cycle– Hertz (Hz): frequency per second

• Kilohertz (kHz)• Megahertz (MHz)• Gigahertz (gHz)

• Bandwidth– 0 to 4,000 Hz (between 300 and 3,000 Hz)– Guard channel or guard band (buffer)

Analog Signals - II

• Amplitude– Loudness

• Frequency– Vibration speed

• Phase– Relative position of wave measured in degree

Analog Signals - III

• Modulation & demodulation (voice &carrier wave• Three methods

• Amplitude modulation (AM)

• Frequency modulation (FM)

• Phase modulation (PM)

• Multiplexers– Base group, channel group or just group (12 - 4kHz)

– Supergroup (5)

– master group (10)

– Jumbo group

Time Assignment Speech Interpolation (TASI)

• Pack multiple voice conversations onto a single telephone line

• Eliminate 10% of quite time in conversation

Inter Office Signaling• Direct current (DC) signals

– Dial tone signal

• Tone signals– Ringing, busy, and congestion signals

• Common channel signals– Common channel interoffice signaling system

(CCIS) by ITU-T– Signaling system No.7 (SS7)– Use separate lines to set up telephone calls– Optimizing the digital networks– Offer enhanced services

Telephone Numbering

• 9 geographic zones by ITU-T

• 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 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

Internet Telephone Calls

• Voice over IP (VoIP)

• Benefits

• Problems

• Impacts

• Future

Private Telephone Systems

• Key systems: 3 to 50 telephones

• Private branch exchange (PBX) / private automatic branch exchange (PABX) / computer branch exchange (CBX): over 50

• Tie line or tie trunk for lease line

• Centrex

• Communication server

PBX System Features

• Data communication

• Direct inward dialing (DID)

• Hunt group

• Least cost routing

• Pickup group

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

PBX Station Features

• Automatic reminder• Call forward• Call transfer• Call waiting• Camp on (call back)• Distinctive ringing• Do not disturb• Speed dialing

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

Centrex Service

• Regulated by public utilities commission

• PBX equipment in central office

Selection CriteriaBetween PBX and Centrex

• Regulation

• Maintenance

• Reliability

• Growth

• Control

• Capital investment

Audio Teleconferencing & Voice Mail

• Audio Teleconferencing

• Voice mail– Automated attendant– Audiotex– Interactive voice response

Wireless Communication - I

• Cordless telephone• Cellular telephone service

– Transmitting voice & data– Security

• Eavesdropping• Spread spectrum• Accident

– Second generation systems• Time division multiple access (TDMA)• Global system for mobile communication (GSM)• Code division multiple access (CDMA)

Wireless Communication - II

• Cellular telephone service– Third generation systems

• Japan: I-mode

• European: Wireless application protocol (WAP)

• Bluetooth: 33-foot radius

• Marine and aeronautical telephone services– Radio communication

Points to Remember

• Telephone• Central office• Public Switched Telephone Network

– Lines– Signals– Numbering– Services

• Private telephone system• Wireless communication

Discussion

• Design a voice communication system for an airline