The ISDN is an evolutionary circuit switched network based on digital telephony. It uses a common set of interface standards and allows users to send and receive information over the network.

ISDN offers end-to-end (caller to receiver) digital connectivity between Terminal Equipment (TEs), via Network Terminators (NTs) and digital exchanges, both private and public.

ISDN Services
ISDN services are the telecommunication services to which the user has access either at an ISDN interface or a terminal connected to the ISDN. The diagram below summarises the services offered:

Bearer Services
for example:
Circuit switched speech and audio
X.25 circuit & packed switched
Frame Relay
Circuit switched data

Teleservices
ISDN for example:
Facsimile - Telephony
Telex
Videotex


Supplementary Services
for example:
Calling line Identification
Call Waiting & Forwarding
Sub-addressing
Advice of Charge
Multiple Subscriber Numbering
Direct Dial In

Equipment on the ISDN

Each piece of equipment ('functional group') on the ISDN has a label related to the function it performs. The labels are:


ISDN Equipment

ISDN Interfaces
An interface ('reference point') is said to exist between each piece of equipment on the ISDN.

Notes
In a typical BRI installation NT1 and NT2 functions are combined in single NTE (Network Termination Equipment).


ISDN Standards
ISDN is subject to standardisation by the ITU-T and ETSI, which issue recommendations and specifications covering ISDN equipment and interfaces. Standards also exist for types of service, protocols and ISDN numbering.

Pulse Code Modulation
Pulse Code Modulation (PCM) is a method of converting analogue signals into digital code. An analogue waveform is sampled at regular intervals and a measurement is taken of the sample point at various levels. The number obtained is digital. The receiver recovers the original
analogue signal by reversing the conversion.

The sampling rate is 8kHz, and the sample value is translated into an 8 bit code, to conform with ITU-TG.711. Therefore each speech signal results in a continuous bit stream at a rate of 64 kbps.

The process of converting between analogue and digital is known as encoding. An encoding device when combined with a decoder is called a Codec.

Methods of Voice Encoding
The number of levels used in the sampling, and the way they are spread, affect the amount of signal noise and distortion. There are two methods of encoding within the ITU-TG.711 recommendation: A-Law and p-Law.


A-Law encoding
This is the type of voice encoding used in Europe. Sampling consists of 4096 levels, divided into 7 segments.

p-Law encoding
This is the type of encoding used in the United States, Japan and some other regions of Asia. Sampling consists of 8159 levels, divided into 8 segments.

Line Coding Techniques
To overcome the technical difficulties raised by sending digitised information down a telephone cable originally designed for 50V pulse dialling and 4kHz speech, line coding is used. There are a number of different techniques: each one involves coding digitised speech into different voltages which can then be transmitted down the telephone lines.


2B1Q line coding
The name 2B1Q comes from the method of coding 2 binary digits (2B) as one of 4 voltage levels (IQ, where Q stands for 'quaternary').

4B3T line coding
The name 4B3T comes from the method of coding 4 binary digits (4B) as a group of 3 voltage levels (3T, where T stands for 'ternary').

Basic Rate & Primary Rate ISDN
Equipment on an ISDN network may be connected either through a Basic Rate interface or a Primary Rate interface.

* Basic Rate (BRI) consists of two B channels and one D channel, known collectively as 2B+D.

* Primary Rate (PRI) consists of 30 B channels and one D channel ('30B+D') in Europe, and 23 B
channels and one D channel ('23B+D') in the US.

The B channels contain user data at rates of up to 64Kbps. The data is switched by the network in order to provide an end-to-end transmission service.

In Basic Rate the D channel carries control and signalling data at 16kbps. It may also be used for X.25, the Packet mode service.

Basic Rate access to the ISDN

The diagram below illustrates Basic Rate ISDN operation.



In this illustration, a 4 wire interface consisting of a transmit and receive pair (known as the 'S/T interface'), is connected to the NT1. The S/T interface is terminated at the NT1 and the distant end, on both the transmit and receive pairs. Connection to the interface is usually made via 8-pin plugs which conform to ISO Specification 8877. Over this interface pass the 2B+D channels.


Point-to-Point and Point-to-Multipoint links
The layer 2 of a S/T interface can operate in either Point-to-Point (PP) or Point-to-Multipoint (PMP) mode.

. On a PP link, only one TE may be connected at the end of up to 1km of cable.

. On a PMP link, up to 8 terminals can be connected in parallel along the bus. The length of the bus is limited to about 200m, depending on the cable.

Basic Rate power
Power is provided across the network so that, in the event of a local mains power failure, a basic telephone service is still provided.


The 0S1 7-Layer Model
The Open Systems Interconnection (0S1) is an agreed, international standard governing the way systems communicate. The standard model has seven layers, as follows:

Layer 1 

The physical layer. Transmits bits between the terminal and the network. It defines connectors, line coding, transmission rates and anything else concerning the transfer of
bits.

Layer 2 

The data link layer. Provides link level control. Error detection and correction are
handled by assembling the bits into frames. All Layer 2 formats derive from a standard
known as High Level Data Link Control (HDLC).

Layer 3 

The network layer. Routes messages to their destination.

Layer 4 

The transport layer. The end-to-end layer which sets up and maintains connections.

Layer 5 

The session layer. Handles the co-ordination between processes.

Layer 6 

The presentation layer provides data formatting and code conversion.

Layer 7 

The application layer This is the task to be performed, e.g. remote login and file transfer.


Layer 1 (BRI) 
Layer 1, at the S/T reference point, transfers information bits in frames between the terminals and the NT1.

Each layer 1 frame is 48 bits long and lasts for 250 microseconds-its structure depends on the direction of transmission. It contains two 8 bit groups of channel Bi, two 8 bit groups of channel B2, 4 bits of channel D and 12 bits used for other functions.

D Channel Contention
If two TEs attempt to make a call simultaneously, a procedure known as D Channel Contention Resolution prevents a collision.

Activation and deactivation
TEs and NTs can be deactivated in order to reduce power consumption, then reactivated to normal power. Both the TE and NT can receive activation messages, but only an NT can instruct a TE to deactivate.


'Info' signals
Activation and Deactivation messages are transmitted using 'Info' signals.

Info 0 

idle state-neither the TEs or the NT are operating.

Info 1 

TE to NT-requests activation.

Info 2 

NT to TE-requests activation/responds to Info 1.

Info 3 

TE to NT-contains operational data.

Info 4 

NT to TE-contains operational data.


Layer 2
Layer 2 provides a secure, error-free connection for Layer 3 call control information, by organising the Layer 3 octets into error checked frames. Layer 2 procedures, based on HDLC, are known as Link Access Procedure for a D Channel (LAPD).

A Layer 2 frame is structured as follows:

Flags Indicate the start and end of the frame.

Address A unique value identifying the message type and the chan nel to which it refers.
Contains a SAPI (Service Access Point Indentifier) to identify the service that the frame is intended for, a TEl (Terminal Endpoint Indentifier) to identify the terminal, and a Command/Response bit.


Control Carries information that identifies the frame.

Information Carries the TEl management protocol or layer 3 message.

FC Sequence Used for error detection.

Layer 3

Layer 3 routes messages to their destination. The diagram below shows the structure of the Layer 3 signalling messages.

Protocol Discriminator Identifies the protocol.

Call ReferenceValue Identifies the call with which a message is to be associated.

Message Type Describes the intention of the message: CONNECT, SETUP, etc.

Information Elements A number of these may be included. The number and content depend on the message type.

Protocols
A protocol is an agreed set of rules for carrying out a particular function such as the exchange of information.

ETSI
The ITU Q921 and Q931 recommendations are the basis of what is commonly known as the ETSI protocol. Q931 defines the call control messages and Q921 defines the frames that carry the messages. ETSI is the standard ISDN protocol used within Europe.

VN4
VN4 is the national ISDN protocol for France and it is also used in private networks urilising equipment of French origin. VN4 is similar in many respects to the ETSI protocol, the main differences being the construction of layer 3 messages and information elements, new message types and different teleservices.

I TR6
The national protocol for the ISDN environment in Germany. Also often used in private networks utilising equipment of German origin.

TPH
The national protocol for the ISDN environment in Australia. Also often used in private networks utilising equipment of Australian origin.

Reproduced by kind permission Trend Communications