Chapter 4 Voice 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 – – – Transmit coded digital pulses representing measured amplitude ( )

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 Criteria Between 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, , 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