CS457/557 - Computer Networks: Chapter 4 - Voice Communications and Voice/Data Integration

Introduction

Voice Transmission Basic Concepts

Getting Voice Onto/Off of the Network

uses transmitters and receivers to convert sound waves to electrical signals and back

Voice Bandwidth

range of hearing - 20-20,000 Hz
bandwidth of POTS - 300-3300 Hz

Voice Digitization

requires 8000 samples/sec using a codec (coder/decoder)
channel bank - bank of codecs arranged in a modular chassis to digitize analog voice conversations and load onto a shared high-capacity circuit (e.g. T1)

PAM(pulse amplitude modulation)
PDM(pulse duration modulation) or pulse width modulation
PPM(pulse position modulation)
PCM(pulse code modulation) requires a DS-0 (64K) circuit

8000 samples/sec * 8 bits/sample = 64,000 bits/sec (DS-0 circuit)
24 DS-0s = 1.536 Mbps + 8K of framing bits = 1.544 Mbps (T-1)
ADPCM(adaptive differential pulse code modulation) requires 32K (=> 48 simultaneous conversations over a T-1)

Voice Compression

often uses DSPs(digital signal processors) which take the digitized PCM code and further compress it
CODEC(Coder/Decoder) - samples analog transmission and transforms into digital
also used to multiplex several digital conversations onto a single channel

PBX Basic Concepts

PBX - private branch exchange = used for circuit switching

Basic PBX Architecture

requires:

PBX CPU
stored program control (common control area)
station cards - slide-in modules for terminating phone conneCITons
trunk cards - used to increase capacity of conneCITons to the outside network

PBX Installation

station lines are "punched-down" on a Telco (66M) punch-down block and then connected to the station card with a 25-pair Telco cable

tie lines allow for a virtual PBX, i.e. conneCITng a remote site directly to the PBX

PBX Benefits of Digitization

allows for the same switching and trnasmission equipment to handle voice, fax, and data
various types of information can be multiplexed or interleaved over the same transmission circuit
over long distances , digital information is more error-free

PBX Features and Services

Voice-based PBX features and services
Feature/Service	Description
Least Cost Routing (LCR)	PBX chooses the most economical path for any given call
Automatic Call Distribution (ACD)	Incoming calls are routed directly to certain extensions without going through central switchboard
Call Pickup	Allows user to pickup/anser another user's phone without forwarding
Paging	Ability to use page speakers
Direct Inward Dialing	Allows calls to pass central switchboard
Hunting	Allows incoming calls to get through on alternate trunks
Prioritization	Individual extensions are given priority
Night Mode	can still make calls after switchboard is closed

Computer Intergrated Telephony CIT

can use automatic number identification (ANI) or calling line identification display(CLID) to display information on a screen for the call receiver
SMDR (station message detail recording) - call accounting system

Voice Processing

Auxilliary voice-related services/devices
Service/Device	Description
Automated attendant	recorded message system that works with touch-tone (requests extension)
Voice mail	some can be forwarded, copied, annoted, saved and recalled
Voice Response Units(VRU)	uses synthesized voice to offer menu choices to be chosen by touch-tome phone
Voice Processor	uses speech recognition
Voice server	stores, processes and delivers digitized voice messages
Music / ads on hold

CIT Application Development and Implementation

TSAPI (Telephony Services Application Program Interface) - developed by Novell & AT&T
TAPI (Telephony API) - developed by Intel and Microsoft

PBX Trends

* Multivendor Interoperability Standards
* PBX Integration with Wireless Phones

Voice Network Concepts

Central Office (CO)
Local Access Transport Area (LATA)
Point of Presence (POP)

Signalling the Central Office: Dialing

DTMF - dual-tone multi-frequency: touch-tone dialing

PSTN (public switched telephone network) Hierarchy

Local calls - handled by local office
intra-LATA handled by local carrier
inter-LATA handled by long-distance carrier; routed via a POP (class 4 toll center) and then routed through class 3 primary center then class 2 seCITonal centers and then class 1 regional centers

Network Interoffice Signalling Standards

use standardized out-of-band signalling (common channel interoffice signalling) to include intelligent services
Signalling System 7 - CCITT standard for out-of-band signalling (suite of protocols that control the structure and transmission of information via out-of-band signalling between CO switches

SS7 protocols

OSI Model

SS7

Application

Operations Maintance Application Part (O&MAP)

TransaCITon Capabilities Application Part (TCAP)

Presentation

Session

ISDN User Part

Transport

Network

Signalling ConneCITon Control Part (SCCP)

Datalink

Message Transfer Part (MTP)

Physical

PCS: Personal Communications Services

Personal Phone Numbers

Personal Communications System Challenges
Challenge	Explanation/Implication
Spectrum Allocation	worldwide allocation required
Operational Standards	worldwide standards required
Societal and User Issues	Who pays for calls? Should location be broadcast? Privacy issues? Potential misuse of PCS services?
Operations, Administration, and Management	How will various network segments and industries transparently interoperate on a technical and business basis?
Who is in Charge?

Potential Benefits of PCS

Wireless Centrex
Two-way calling
Paging
Voice Mail
Handoff and Roaming

The Wireless Challenge: Maximize Calls on Minimum Bandwidth

TDMA time division multiple access
FDMA frequency division multiple access
CDMA code division multiple access

Wide Area Network Options for Data/Voice Transmission

Network options for data/voice transmission
Network Option	Examples	Characteristics
Public	Carrier services--SMDS, frame relay	Central control. Maintained by TELCO staff. Highly redundant network
Hybrid	Combination of some public network carrier services and some leased lines	Voice is often transmitted via public network, while leased lines are used for data
Virtual Private	Software defined network, Centrex	Offers closed groups on a public network. Offers control and features of private network without capital investment and operations personnel
Private	Leased lines	Offers user control over guaranteed bandwidth facilities on a 24-hour-a-day basis for a fixed monetary fee
Bypass	User-owned and controlled or third party VSAT Microwave	May bypass local TELCO and bring voice and data directly to POP of long distance carrier

VSAT:Very Small Aperature Terminal Satellite Transmission

data must travel 22,000 miles to orbiting satellite and back
rain fade
requires an inhouse specialist
does VSAT support multiple transport protocols

Microwave

requires line-of-sight (5-30 miles) towers

Public Network Services

800 Service
Business Network Services: Flexible Billing Formats
Information Services: Network Service of the Future?
value-added information services - AOL, CompuServe, Prodigy

Wide Area Data/Voice Integration Does Not Imply ISDN

End Devices Connected	WAN Service Employed	Integration Device	Integration Elements
Voice: PBXs or channel bank Data:mainframe to terminal traffic or LAN traffic	Frame relay accessed via T-1	Frame relay switch	Data; Voice
Voice: PBXs Data:mainframe to terminal traffic or LAN traffic	DDS lines to 56K or DS1 64K digital leased line	Remote bridges	Data; Voice; Fax
Voice: PBXData: mainframe to terminal traffic	T-1, fraCITonal T-1, DDS or analog leased lines	Multiplexor	Data; Voice; Fax
Voice: single phone line Data: PC	Switched analog dial-up line	DOV (data over voice) unit and software	Data; Voice; Fax
Voice: PBX Data: LAN Traffic	Analog or digital leased lines to DS-0 (64 K)	PC expansion card	Data; Voice; Fax
Voice: Data: PC	voice over the Internet	sound card; microphone; speakers; software	Data; Voice; Fax

[e-mail your answers to summers_wayne@ColumbusState.edu before 4 p.m. on Thurs., Sept. 24, 1998]Review Questions: 8, 34

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