ISDN - Presentation Notes

Name: Peter Billes

Class: 298c Advanced Networking

Instructor: Seung Bae Im

Date: Oct. 19, 1995

ISDN (Integrated Services Digital Network)

1.0 Introduction

After a period of time, where it seemed to be forgotten, ISDN has made its way in recent years. Especially in Europe and Japan ISDN is growing very rapidly due to the power (monopoly) of the PTT's (Post Telephone Telegraph). ISDN is more than a digital communication network designed for information transfer all over the world. It incorporates the provision of services, and standardization of user interfaces and data transfer modes.

1.1 Development

ISDN is often referred as the successor of the existing public networks like the telephone network, data- and text networks (Telegraphy). The telephone network was first an 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. Only the line from the user to the local exchange is analog. For the connection between two exchange points, digital lines are used (IDN - Integrated Digital Network). A continuation of this development leads to an overall digital network. ISDN is such 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.

1.2 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

    2.0 Access to ISDN

    Two types of ISDN have been specified: Narrowband-ISDN (NISDN) and 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. Because the definition of BISDN and its services are still in process, the main point of effort will be layed on NISDN. The term ISDN will be used for NISDN. If BISDN is referred, it will be explicitly named.

    2.1 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.

    2.2 Channels

    A channel is a connection between 2 users over the network, with a defined capacity of bps. A certain number of channels have been designed for the use of ISDN. B -channel 64 kbps D16 -channel 16 kbps D64 -channel 64 kbps H0 -channel 384 kbps H11 -channel 1536 kbps H12 -channel 1920 kbps The D-channels are used for Signaling and all others for data transmission. Remaining bandwidth on the D-channels can be utilized for packet orientated data transmission.

    2.3 Basic Rate Access (BRA)

    The Basic Rate Access is defined as an access through 2B and 1D(16)-channels. The data transmission rate is 144kbps (64+64+16). If the framing-, synchronization- and D channel bits are taken in consideration, the aggregate transmission rate is 192kbps. The BRA supports point to point and point to bus connections (S-bus on NT2). With the S-bus configuration 8 Terminals can be connected to NT2, but only 2 can be active at the same time. They use the same D-channel for outband signaling.

    2.4 Primary Rate Access (PRA)

    For Primary Rate Access two different Standards are established. Un US the data transmission rate is 1536kbps while in Europe 1984kbps are used (aggregate rates: 1544 and 2048). This capacities can be achieved in several ways of combining different channel types: note: here is D = D(64)

    B -channel structure: 23B+D (30B+D in Europe)

    H0 -channel structure: 4H0 (5H0+D)

    H1 -channel structure: H1 (H12+D)

    Mixed structure: 2H0+11B+D

    3.0 ISDN Services

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

    3.1 Bearer Services

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

    3.2 ISDN Teleservices for BRA

    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:
    Texttransmission 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 teletexterminals in other networks is supported. This includes circuit-switched as well as packet-switched networks.
    Telefax (telefax 4):
    ISDN supports the pixelorientated 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. This can be seen as the first step in the direction of a multimedia document transmission service in ISDN. A multimedia document designed to be transmitted over the BRA interface, could consist of text, fax, still images with high resolution, graphics and voice. Video information could be provided in future with BISDN.
    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.
    Videophony:
    Videophony 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).

    4.0 Protocols and Frame Structure

    Protocols in ISDN are based on the ISDN-PRM (ISDN-Protocol Reference Model) which is constructed following the principles of the ISO/OSI-RM (International Standardization Organization/Open Systems Interconnection Reference Model). The ISDN-PRM consists of two different planes: the user and the control plane (see figure 3).

    The coordination of these planes is made through a management function. Both the the user and control plane could incorporate a 7 layer protocol stack as OSI, but only the first 3 are defined until now. For the D-channel definitions for layer 1-3 were made. For the circuit switched service on the B-channel, only the physical layer has been specified. In this case the user is free to decide which protocol stack to use for the higher levels. For the packet mode the layers 1-3, based on X.25 have been proposed.

    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). (see figure 4)

    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.