Integrated Services Digital Network

Integrated Services Digital Network (ISDN)

Overview

WAN technology

Simple concept: digital "bit pipe"

Single communication interface for all services

Based on telephone infrastructure

International effort driven by ITU-T

Ultimate goal: set of end-to-end integrated digital services

consider obstacles in achieving goal (co-exist with analog service)

Service Categories

Bearer Services : information delivery -- bottom 3 OSI layers
(end-to-end services not dependent on delivery method: circuit, packet, etc)

Teleservices : end-user applications such as

Teleconferencing - video plus audio
Teletex (text-based messaging)
Telefax (fax services)
Telemetry (low bandwidth services such as alarms, utility meter reading)

Supplementary Services : mainly voice services built on the other two

Message handling
call forwarding
instant call setup
call waiting

Interface between customer and bit pipe

NT: Network Termination

NT1: a low volume box

customer side connects up to 8 ISDN devices
carrier side connects to bit pipe
box handles addressing, multiplexing and contention for bit pipe
analog devices require adapter (A/D, D/A)

NT2: high volume box

Typically used to implement Private Branch eXchange or PBX)
Customer ISDN input (from devices/adapters)
Carrier ISDN output to NT1 (not to bit pipe)
Switching unit
Control unit (computer to control switching)
Communication within PBX doesn't reach bit pipe

TE1 : Terminal Equipment 1 : ISDN device (DTE)

TE2 : Terminal Equipment 2 : non-ISDN device, requires TA

TA : Terminal Adapter

Reference Points

Standard reference point exists between each link in ISDN chain.

Longest chain is: TE2 - TA - NT2 - NT1 - bit pipe

Chain w/ reference points in parens: TE2 (R) TA (S) NT2 (T) NT1 (U) pipe

S-reference also applies between TE1 and NT2, and between TE1 and NT1.

Each reference point has standards and protocols.

Other internal reference points exist

Channels

ISDN defines pipes of varying capacity, known as channels

Specific channel configurations available under different pricing schedules.

Channel types:

A - 4 khz analog telephone

B - 64Kbps digital PCM

C - 8 or 16 Kbps digital

D - 16 or 64 Kbps digital, for control purposes

E - 64Kbps digital, internal ISDN signals

H - high speed digital (H0=6B=384Kbps, H11=24B=1536, H12=30B=1920)

Standard Channel Configurations

BRI = 2B + D

PRI = 23B + D (30B in Europe)

BRI = Basic Rate Interface, provides 2 "B" channels of 64Kbps plus 1 "D" channel of 16Kbps. There is also 48Kbps BRI overhead. Adapters are available to convert to single 144Kbps channel.

PRI = Primary Rate Interface, provides 23 "B" channels of 64Kbps plus 1 "D" channel of 64Kbps. There is also 8Kbps PRI overhead. 23 chosen so a frame will fit T1 line (30 fits CCITT 2.048Mbps)

Signaling

Digital signaling uses Pulse Code Modulation (PCM).

PCM: For analog data (e.g. voice), original analog signal is sampled every 125 microseconds, at 8 bits per sample.

Relate PCM to capacity of "B" channel. What does it tell you about the use of "Kbps" here?

Existing copper cabling can be used in most configurations, especially in local loop

Broadband ISDN (B-ISDN)

Much higher throughput than ISDN

Requires fiber cabling throughout

Service access rates are:

155.520Mbps full-duplex
620.080Mbps full-duplex
155 one way, 620 other way

B-ISDN data carried by ATM cells for fast switching

ATM cells carried on SONET fiber network for high speed xmit

Many other aspects (such as reference points) similar to ISDN

Related Home Pages: notes | CSC 465 | Peter Sanderson | Computer Science | SMSU

Last reviewed: 29 April 1998

Peter Sanderson ( pete@csc.smsu.edu )