ISDN

DEFINITION

Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires that gives good quality and high speeds. ISDN is a fixed-wire end-to-end digital telecommunications network.

HISTORY AND DEVELOPMENT

ISDN is often referred as the successor of the existing public networks like the telephone network, data- and text networks (Telegraphy). Since the inception of telephony, the telephone user has been connected to the telephone office by a pair of copper wires in the local loop (analog network). In the early days of telephony, switching was made by the telephone operator, by plugging both ends of a wire in the right holes of a connection board. In those times the phones had no dialing mechanism. The user had to pick up the phone and tell the operator which connection he desires. After the invention of the dialing disk, switching could be automated, but fast connection set up was possible only after a partially digitization of the telephone net. Today the biggest part of the Telephone system is digital. The last mile of the network has remained analog even as the backbone of the network was converted to all-digital transmission and switching. In mid 1970’s it became apparent that the demand for data services would necessitate making the telephone network entirely digital as well bas capable of providing access to a wide range of services including voice and data. In the early 1980’s the CCITT developed the

Integrated Services Digital Network (ISDN) standards for providing end-to-end digital connectivity. ISDN is a network providing moreover services available - in the past - only in other networks and/or with other interfaces, eg. Teletex, Telefax. ISDN is built on top of IDN. It allows communication, not only with older equipment, for instance an analog telephone connected analog to the local exchange, but also with other networks, eg. PSPDN (Public Switched Packet Data Network). Thus ISDN can be seen as an evolution rather than a revolution.

Main characteristics

The probably most important keywords concerning ISDN are:  end to end digital connection

 integration of multiple services (voice-, data-, video-, multimedia transmission)

 standard terminal interface

Figure: Integrated Services Digital Network

Access to ISDN

Two types of ISDN have been specified:

1. Narrowband-ISDN (NISDN) and 2. Broadband-ISDN (BISDN).

The main difference between NISDN and BISDN is the transmission capacity and the used transfer mode. NISDN can serve with a capacity of up to 2Mbps, while for BISDN exist specifications for 150Mbps and 600Mbps.

Reference Points (connection interfaces)

The entry level interface to ISDN is the Basic Rate Interface (BRI), a 144 kbit/s service

delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into

two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or

Data channel).BRI is sometimes referred to as 2B+D

The Interface specifies the following network interfaces:

The U interface is a two-wire interface between the exchange and the Network

Terminating Unit which is usually the demarcation point in non-North American networks.

The T interface is a serial interface between a computing device and a Terminal Adapter,

which is the digital equivalent of a modem.

The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T

reference points are commonly implemented as a single interface labeled 'S/T' on an NT1

The R interface defines the point between a non-ISDN device and a terminal adapter

(TA) which provides translation to and from such a device.

Primary Rate Interface

The other ISDN service available is the Primary Rate Interface (PRI) which is carried over an E1

(2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of

64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered

on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23

'B' channels and 1 'D' channel for signaling (Japan uses a circuit called a J1, which is similar to a

T1).

In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is

sometimes called "23B+D + n*24B". D-channel backup allows you to have a second D channel

in case the primary fails. One popular use of NFAS is on a T3.

PRI-ISDN is popular throughout the world, especially for connection of PSTN circuits to PBXs.

Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth

BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits

serving PBXs.

Data Channel

The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz

with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more

than digital channels. Each one of these channels is known as a DS0.Most B channels can carry a

64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was

more of a problem in the past, and is not commonly encountered nowadays.

Signaling Channel

The signaling channel (D) uses Q.931 for signaling with the other side of the link.

Reference Points (connection interfaces)

User terminals can be attached in different ways to the ISDN network, depending on the terminal capabilities and configuration needs of the user station. The CCITT (International Telegraph and Telephone Consultative Committee) Recommendations contain a specification of the subscriber station and its interface to the network. Possible configurations are shown in figure 1.

R, S, T, U are reference points (or interfaces) which determine mechanical and electrical characteristics, and specifications of operating procedures. TE is the terminal equipment. It can be a telephone, telefax, computer etc. NT is the network termination, and can be divided into the two units NT1 and NT2. NT1 translates the signals at ref. point T into Signals for the network. NT2 allows the connection of multiple TE to ISDN. Up to 8 TE's can be connected with Basic

Rate Access to the so called S-bus. If only this passive bus (S-bus) is attached to NT2, the NT2 doesn't need to provide any function and can be called "zero NT2". In other configurations NT2 can act as a private branch exchange, concentrating the traffic of several TE's including the passive bus. NT2 could also handle the internal traffic between TE's. Older Terminal equipments which do not meet the specification of the S reference point can be attached through a Terminal Adaptor. The responsibility of the network provider can end at the ref. points S, T or U, depending on the national regulations.

ISDN ServicesTwo different groups of services are supported by ISDN. The bearer services and the teleservices (see figure 2).

Bearer ServicesWith ISDNs bearer services it is possible to transfer data between two subscribers. The network is acting in this case as a bit pipe. The bearer service can be divided into packet switching (PS), with both connection orientated (CO) and connectionless (CL) modes, and circuit switched connections.

ISDN Teleservices for Basic Rate Access Telephony:This service enables subscribers to make phone calls using the ISDN. It is a considerable improvement to the (partially) analog telephone-system, as it offers a better signal-noise ratio and the attenuation is unaffected by distance. In the future it would also be possible to provide a greater voice bandwidth, e.g. 7kHz (now 3.1kHz) and stereo sound, an important service attribute for audio conferencing.

Teletex:

Text transmission over ISDN is faster than the conventional teletex service over public data networks. It is possible to transmit a page (legal size or OSI A4) in less than 1 second. The connection with teletex terminals in other networks is supported. This includes circuit-switched as well as packet-switched networks.

Telefax (telefax 4):

ISDN supports the pixel orientated transmission of documents. CCITT has made recommendations for telefax4 and group 4 facsimile machines with a resolution of 300dpi and optionally 400dpi or 1200dpi. One page of legal size (or OSI A4) paper can be transmitted in 15 seconds when using the 400dpi resolution.

Mixed service:

The mixed service allows a combination of the teletex and telefax service for the transmission of one document. Thus it is possible to send a letter where the text is sent character coded and a picture and/or signature is sent pixel coded.

Still image transfer:

This service offers the possibility of transmitting TV freeze frames over ISDN. A still image sequence can be compressed and send with an update rate between 1 and 10 seconds, depending on the contents of the image.

Video phony:

Video phony is used to transmit moving pictures from person to person or person to group. Because of the relatively low transmission rate (64kbps or 2x64kbps) the quality of the video frames are inferior to TV frames (480x240 Pixel) and must be transmitted at a very high compression.

Alarm Services:

It is possible to make emergency calls over ISDN, even if the power supply on the user side breaks down and the B-channels can not be used any more. An emergency call is made over the D-channel which then is feeded by the provider.

Messaging:

This service provides a mailbox function for the user. Text and voice mail can be deposited in the mailbox, if the user is not available or both of his B-channels are busy. The recipient is sent a message from his mailbox over the d-Channel. He receives the notification even if his B-channels are used at that time.

Videotex:

Videotex is a retrieval service which enables the subscriber to view text and graphics based images.

Other services:

Some other services are: Telewriting for placing short messages (written with an electronic pen on note pad) in a mailbox. Teleaction is used for controlling installations like heating, gas, water. Distribution services allow data transmission in an unidirectional way - but this service is much more interesting with the higher transmission rates of BISDN (video and TV transmission).

The different protocols:

LAP-D (US) Euro-ISDN DSS1 (Europe) 1TR6 (Germany)

Layer 3

Speed protocols

HDLC (worldwide) X75 (Europe) V110 (CCITT) V120 (CCITT, Europe)

Link protocols

PPP Multilink-PPP SLIP HDLC

Bonding

Bonding is an ISDN protocol that allows for up to 31 B-channels to be 'bonded' to one huge data channel to support speeds over 64k. The incoming high-speed data stream is split into various 64k channels, transported over the ISDN, and then the separate channels are combined again. The problem that has to be overcome is synchronization, since the different 64k pipes are not all travelling the same route.

ISDN Frame Structure

The size of the frames in ISDN is 48 bits and each frame is sent in 250 micro seconds (=192kbps). Time division multiplex is used to transmit the two B-channels and the D-channel over one wire pair in each direction (TE->NT and TE<-NT).

The D-echo-channel is the reversed D-channel and is used by the TEs to check if the D channel is free. Therefore the NT transmits all D-channel bits back to the TE (echo). As transmission code the inverted AMI code is used. 0 is represented by a pulse and 1 by no pulse.

REPORT BY

ROLL NUMBER NAME

21 DILIP RAI

22 SANIL RANA

23 PRASHANT RATHOD

24 MEENU RATHORE

25 FORAM RAVAL