Chapter 2 Voice Communications Concepts and Technology
Objectives • Investigate PSTN • Study and understand digital voice communication and digitization • Understand PBXs • Understand CTI and voice services • Introduce Digital voice transmission services
GOAL Study the business behind voice communication
Basic Concepts • Voice consists of sound waves of varying frequency and amplitude. • The transmitter part of phone handset converts voice into electrical signals to be transmitted onto the analog network. • The receiver part of a handset works the opposite of the transmitter.
Basic Concepts • POTS (Plain Old Telephone Service) employs analog transmissions to deliver voice signals from source to destination. • The bandwidth is 4000 Hz, but guardbands limit the useable range to 300-3300 Hz. • Today, the local loop is still analog, but high-capacity digital circuits typically link the exchanges or Central Offices.
Voice Network Concepts • PSTN • Network hierarchy • Signaling and dial tone • Control and management:
Signaling and dial tone • Requested destinations are presented to the CO by dialing a series of numbers. • Numbers tell whether the call is local, intra-LATA, or inter-LATA. Subsequently, concerned circuits are solicited. • Numbers are generated: • Rotary phones: pulses • Dual-tone multi-frequency tones. • Tones are used to enable specialized services from PBX’s, carriers, banks, information services, and retail information.
Voice Digitization • Principle: • To convert voice signal from analog to digital • The voice conversation must be sampled frequently enough so that the resulting conversations resembles the voice of the call initiator • Codec (coder/decoder): • device used to transform analog transmissions into a stream of binary digits.
Voice Digitization • Pulse Amplitude Modulation: (PAM) • Varies the amplitude of the electrical pulses • Used in earlier PBX’s. • Pulse Duration Modulation: (PDM/PWM) • Varies the duration of electrical pulses • Pulse Position Modulation: (PPM) • Varies the duration between electrical pulses
Voice Digitization: PAM PDM PPM
Pulse Code Modulation • The most common method used to digitize voice is Pulse Code Modulation (PCM). • PCM uses: • 8000 samples/sec • 8 bits/sample • therefore, 1 digital voice channel requires 64 Kbps • known as a DS-0 circuit.
Adaptive Differential PCM (ADPCM) • Each voice channel uses 4 bits. • ADPCM supports 48 simultaneous conversations over aT1 circuit. • The standard for 32-Kbps is known G.721. • The G.721 is used as a quality reference point for voice transmissions (Toll Quality).
ADPCM • ADPCM is used to send sound on fiber-optic long-distance lines as well as to store sound along with text, images, and code on a CD-ROM
Voice Compression • ADPCM is also known as voice compression. • Advanced techniques employ DSP’s. • DSP’s are able to compress voice in as little as 4800 bps. • Efficiency: 13 times more than PCM. • Voice compression may be accomplished by stand alone units, or by integral modules within multiplexers.
Private Branch Exchanges • A PBX is similar in function to a public exchange. • A PBX is exclusively used by the organization and physically located on the organization’s premises. • Provides an interface between users and the shared network (PSTN). • Additional services offered by a PBX allow users to use their phones more efficiently and effectively. • Medium to large organizations can save a lot of money by using a PBX
Popular vendors of PBX equipment • Nortel, Lucent (25% of the market) • Siemens, Rolm • NEC, Mitel
PBX Architecture • PBX overall functionality and added features are controlled by software programs. • Those programs are executed by specialized computers. • Programs reside in: CPU, stored program control or common control area. • User phones are connected to line cards. • Trunk cards allow connection of the PBX to outside world.
PBX Technology Analysis • PBX features and services end to fall into three categories: • features and services: • that provide users with flexible usage of PBX resources. • that provide for data/ voice integration • that control and monitor the use of those PBX resources.
Voice Based Features and Services • Common features: • Conference calling, Call forwarding, Call transfer, Speed dialing, Redialing, Call hold. • Least Cost Routing (Selecting lowest price LD provider). • Automatic Call distribution • Call pickup • Night Mode
Data/Voice Integration Features and Services • Data is transmitted either: • through the PBX via a dedicated connection or • a hybrid voice/data phone is used to transmit both voice and data simultaneously over a single connection. • Features: • ISDN support, T-1 support, Data interfaces, PBX to host interfaces.
Control and Monitoring Features and Services • Basic: (e.g.) • Limiting access to outside lines from certain extensions. • Advanced: • Call accounting system: program run on a separate PC directly connected to the PBX. • Process within the PBX known as station message detail recording (SMDR) where an individual detail record is generated for each call. • Used for spotting abuse and allocating phone usage on a departmental basis.
Auxiliary Voice Related Services • Auxiliary add-on device that provides the following services: • Automated attendant • Voice mail • Voice response units (VRU): Interactive voice response (IVR). • Voice processor: e.g. speech recognition • Voice server: a LAN based server that stores, and delivers digitized voice messages. Used with voice mail system. • Music / ads on hold
PBX Architecture Trends • PBX user demands: • Better connectivity between phones and desktop PCs. • Better connectivity between PBXs and LANs • More open PBX architecture for easier access to PBX features and services from a variety of computing platforms. • Better integration of PBX management programs with enterprise network management packages.
Open PBX Architecture • Logical separation between the call processing functionality and the underlying switching fabric. • Allow to introduce newer and faster switching technology without redesigning entire PBX. • Support of industry standard API: TAPI and TSAPI for telephony applications. • New services can be designed and interfaced to a variety of PBXs and computers.
PBX Trends • Architectural Trends • Multi-vendor Interoperability standards: • International standard: Q.Sig (allows interoperability between PBX’s and ISDN). • PBX integration with wireless phones support: • Cordless Telephone generation 2 (CT2). • Common air Interface (CAI) global standard for low power wireless transmission. • Mini PBX’s for the Small Office Home Office Market (SOHO).
Mini PBX’s for the Small Office Home Office Market • Mini PBX’s known as multifunction telephony boards or PC expansion boards: • Multiple workers share a small number of phone lines with features such as: • Auto attendant software • Integration with cell phones, pagers, and voice mail systems. • ISDN Support
Computer Telephony Integration (C T I) • CTI attempts to integrate the phone and the computer to increase productivity. • Examples of the integration: • Call control: allows users to control their telephone functions through their computer. • Fax on demand: users can dial-in using a telephone and request fax documents.
C T I • Unified messaging: • keeps voice-mail, e-mail, and faxes together in one place (universal in-box). • Audiotex: • Deliver audio information to users based on responses on the touch-tone keypad to prerecorded questions (hotline questions). • Interactive Voice Response (IVR): • Support online transaction processing. Used in banks to allow users to transfer funds between accounts.
C T I Architecture1 - PBX to host interfaces • In PBX-to-host interface CTI was achieved by linking mainframes to PBXs via PBX-to-host-interface. • Applications were compatible with computer and PBX. • Systems linked to an automatic call distribution unit (ACD) • All phones are controlled by CTI application running on mainframe computer. • Expensive systems.
C T I Architecture2 - Desktop C T I • First party call control • Less expensive alternative to PBX-host architecture. • PC’s are equipped with telephony boards and associated control software. • Each PC controls only the telephone to which it is attached. • No overall automatic call distribution across multiple agents and their phones. • No sharing of call related data among the desktop CTI PC’s.
C T I Architecture3 - Client/Server C T I • CTI server computer interfaces to the PBX or ACD to provide overall system management. • Individual client based CTI applications execute on multiple client PCs. • Multiple CTI applications on multiple client PCs can share the information supplied by the single CTI Server. • Offers overall shared control of the PBX-to-host CTI architecture at a cost closer to that of the desktop architecture.
Figure 2-20 Technology Required to Develop and Implement CTI Applications
Alternatives to PSTN • Voice over the Internet • using a sound card, microphone, and speakers • can call others using the same product • gateways are being established to allow Internet voice callers to reach regular telephone users as well.
Voice over the Internet • Underlying transport protocols (IP and UDP) deliver voice conversation. • IP = internet protocol • UDP = User datagram protocol • IP and UDP can be used in any of the following: • Modem based point-to-point connections • Local area networks • Private Internets (Intranets)
Alternatives to PSTN (cont’d) • Example of products: • CoolTalk • Internet Phone • Intel Internet Phone • Microsoft NetMeeting • Webtalk • TeleVox
Voice over Frame relay • Variable-length delay introduced by variable-length frames is unacceptable for voice. • Frame relay access device (FRAD) accommodates both voice and data: • Voice prioritization: priority given to voice • Data frame size limitation: to limit delays • Separate voice and data queues.
Voice over Frame relay • Voice conversations require 4 – 16 Kbps of bandwidth. • Permanent Virtual Circuit (PVC): maintained end to end connection throughout the voice conversation. • Voice conversation can take place between locations directly connected to a frame relay network. • No current standards defined between frame- relay networks and the voice based PSTN.