ISDN principles. By: Ogah, Oshoriamhe Frederick Instructor: Prof. Werner Henkel Course: Wireline Communications Date: November 30, 2004

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1 ISDN principles By: Ogah, Oshoriamhe Frederick Instructor: Prof. Werner Henkel Course: Wireline Communications Date: November 30, 2004

2 Introduction ISDN = Integrated Services Digital Network ISDN is more than just 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.

3 Aim: To support a wide range of existing voice and data services using a well-defined set of connection types and interfaces between the user and the network. Main feature: Provision of services to the subscriber over a single standardized user-network interface.

4 ISDN Principles Defined by ITU-TS TS (I.120, 1994) Voice and non-voice applications using a limited set of facilities, interfaces and standards. Support for switched and non-switched applications Circuit switching Packet switching Leased lines Layered protocol architecture

5 Introduction 1: Development At first the telephone network was analogue but due to high noise levels, the connections between two exchange points was gradually digitized. (IDN-integrated digital network) The continuation of this development (digitization) lead to an overall digital network, ISDN, which is built on IDN. ISDN allows communication with older equipment and other networks (eg. PSDN, Public Switched Packet Data Network).

6 Introduction 2: Main Characteristics The most prominent ISDN characteristics are: end to end digital connection standard terminal interface integration of multiple services (voice, data, video, multimedia transmission)

7 Digital Advantages of ISDN Reliable connection Lower interfering noise Fast speed 160 kbps for BRI 2048 kbps for PRI Fast call setup Usually less than 2 seconds Reduced costs Faster downloading

8 Advantages of ISDN (2) Flexible interface Bandwidth on Demand Easily adding new channels to the bundle of channels Multiple devices Phone, fax, PC, video conferencing system, router, each with its own sub-address.

9 Access to ISDN 1: Types of ISDN Two types of ISDN have been specified: Narrowband-ISDN (N-ISDN) (< 2 Mbps) Broadband-ISDN (B-ISDN) (> 2 Mbps) The main difference between N-ISDN N and B-B ISDN is the transmission capacity and the transfer mode.

10 Access to ISDN 2: Types of ISDN N-ISDN can serve with a capacity of up to 2Mbps, while for B-ISDN B there exists specifications for 150Mbps and 600Mbps. In this presentation, when the term ISDN is used, it would usually be in reference to N-ISDN, N or else it would be explicitly stated.

11 Access to ISDN 3: 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. A possible configuration is shown below User Station Interfaces

12 Access to ISDN 4: Connection interfaces In the diagram (in the previous slide) R, S, T, U are reference points (or interfaces) which determine mechanical and electrical characteristics, and specifications of operating procedures. TE = terminal equipment; which can be telephone, fax, computer, etc. NT = network termination; which can be divided into two units NT1 and NT2. NT1 translates the signals at reference point T into signals for the network. NT2 allows the connection of multiple TE to ISDN.

13 LT Line termination NT1 & NT2 Network termination TE1 & TE2 Terminal equipment TA Terminal adapter TE1: eg.. Phones, fax, videophones etc TE2: have no ISDN user-network interface; needs a TA for connection to the ISDN. Interface between the TE2 and the TA is the R interface

14 TA: adapts a non-isdn terminal to the ISDN Important Functions: Rate adaptation: TE2s could come in less than 64 kbps. TA then adds additional bits. Multiplexing: adding bits without wasting bandwidth. NT1: provides a conversion at the physical layer between the S interface inside a customer s premises and the subscriber loop at the U interface that connects to the local exchange. It terminates physical interfaces; Carries out maintenance functions; Power feeding to the terminal.

15 NT2: has more complex functions like switching and multiplexing. Between the NT1 and NT2 is the T interface, which is basically a subset of the S-interface. S LT: provides control over the local subscriber loops during calls It also implements local services requested by users. Provides signalling to other exchanges Provides interfaces to other public networks.

16 Access to ISDN 5: Channels A channel is a connection between two users over the network with a well defined capacity of bits per seconds (bps). A number of channels have been designed for the use of ISDN. Bearer-channel (B-channel): 64 kbps Delta-channel (D-channel): 16 kbps, 64 kbps H-channel: 384 kbps, 1536 kbps, 1920 kbps The D-channels D are used for signalling, while the B-channel B is used for transmission (clear pipe for data) An H-channel H is a higher bandwidth channel for user applications that require high-speed information transfer capabilities, eg. Video conferencing.

17 Access to ISDN 6: Basic Rate Access (BRA) The Basic Rate Access is defined as an access through 2 B-B and 1 D-channel D (16 kbps) and the data transmission rate is 144 kbps ( ). The BRA supports point-to to-point and point-to to- bus connections.

18 Access to ISDN 7: Primary Rate Access (PRA) The Primary Rate Access is an access through 30 B-B and 1 D-channel D (64 kbps) For Primary Rate Access, two different standards are established. In the US, the data transmission rate is 1536 kbps, while in Europe it is 1984 kbps. These capacities can be achieved in several ways of combining different channel types.

19 S-Interface The standard ISDN interface is the S-S interface Configuration Point to point Point to multipoint (passive bus) Point to multipoint (extended bus) NT1 (star)

20 ISDN Services Two different groups of services are supported by ISDN; the bearer services and the tele-services. Services in the ISDN

21 ISDN Services: Bearer Services With ISDN s s 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 (CS) connections.

22 ISDN Services: Tele-services 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 noise ratio and the attenuation is unaffected by distance. Teletex: Text transmission over ISDN is faster than the conventional teletex service over public data networks. The connection with teletex terminals in other networks is supported. This includes circuit- switched as well as packet-switched networks.

23 ISDN Services: Tele-services for BRA (2) Telefax: ISDN supports the pixel orientated transmission of documents. Mixed service: The mixed service allows a combination of the teletex and telefax service for the transmission of a 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. 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.

24 ISDN Services: Tele-services for BRA (3) Still image transfer: This service offers the possibility of transmitting freeze frames over ISDN. 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.

25 ISDN Services: Tele-services for BRA (4) 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 B can not be used any more. An emergency call is made over the D-channel which then is fed 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 B are busy. The recipient is sent a message from his mailbox over the D-Channel. D He receives the notification even if his B-channels B are used at that time.

26 ISDN Services: Tele-services for BRA (5) Videotex: Videotex is a retrieval service which enables the subscriber to view text and graphics based images. Other services: Some other services are: Tele-writing for placing short messages (written with an electronic pen on note pad) in a mailbox. Tele-action is used for controlling installations like heating, gas, water. Distribution services allow data transmission in an unidirectional way.

27 Protocols and Frame Structure Protocols in ISDN are based on the ISDN-PRM (ISDN-Protocol Reference Model) which is constructed following the principles of o 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 ISDN Protocol Reference Model

28 Protocols and Frame Structure (2) The coordination of these two planes is made through a management function. Both the user and control plane could incorporate a 7 layer protocol stack as OSI, but only the first 3 are as yet defined. For the D-channel D definitions for layer 1-31 were made. For the circuit switched service on the B-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, 1 based on X.25 have been proposed.

29 Frame structure for the BRA Protocols and Frame Structure (3) 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 B and the D-channel D over one wire pair in each direction.

30 Protocols and Frame Structure (4) The D-echoD echo-channel channel is the reversed D-channel D and is used by the TEs to check if the D channel is free. Therefore the NT transmits all D-channel D 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. B- and D-D channels are multiplexed over the same physical interface

31 Physical Layer Frame Format Format differs depending on direction Frame length 48 bits Mechanism to avoid collisions Multiplexing of B1, B2 and D-channel D data

32 S/T Interface The interface between TE and NT - called the S/T-interface - transfers information on four wires, two in each direction. The line coding method used in the S/T-interface is called pseudo-ternary alternate mark inversion (pseudo-ternary AMI). The following rules apply to this coding variant: Pseudo-ternary rules: A binary "one" is always represented by no voltage, that is, 0. A binary "zero" is represented by voltage, -11 or +1. AMI rules: Voltage -11 and +1 is to be sent alternately. The result is a code form with no DC component as seen below Pseudo Ternary AMI code

33 The bit rate in the S/T-interface is 192 kbit/s in each direction. The information is transferred in 48-bit frames. The capacity available for information is restricted to 144 kbit/s because 12 of the 48 bits are used for frame synchronisation, terminal access, DC voltage equalisation, and other functions. Shown below is how 48 bits are used. Up to eight terminals may want to use a B-channel B at the same time, so a specific access procedure has been prescribed for this channel. The procedure ( (collision collision detection ) prevents multiple terminals from sending simultaneously. Frame structure in the S/T interface

34 U Interface The U-interface U between LT and NT. The information transfer on the U-interface U makes use of existing two-wire wire connections in the access network. The bit rate is 160 kbit/s,, which includes 2B+D, frame synchronisation, and so on. Since no standard for transmission on the U-interface U has been specified, several methods can be used. Two possible methods are echo cancelling and time compression multiplexing (TCM). Echo cancellation is characterised by: simultaneous transmission in both directions (full duplex); elimination of echo; a bit rate of 160 kbit/s. The TCM method, which is less common, is characterised by: half duplex, that is, alternate, high-speed transfer of "bursts"; and a bit rate of 360 kbit/s.

35 Echo cancelling and line code Due to imperfections in hybrids, the send signal from the user's transmitter will generate an echo signal that might disturb the received signal. The purpose of the echo canceller is to neutralise this disturbance. The principle applied is that of eliminating the t echo by means of a compensating signal. Since the echo signal looks different depending on the cable used, it must be possible to control the echo cancellation function to permit adaptation to the connection involved. Echo cancellation is more effective than TCM but is also more complicated because it needs intelligence in the form of microprocessors in the terminating equipment.

36 Principle of Echo cancelling

37 The line code used (2B1Q) is a four-level bipolar code where each signal element corresponds to two transmitted bits. The transfer rate is 160 kbit/s. Line code 2B1Q

38 The frame structure of 2B1Q code is shown below. The frame contains a total of 240 bits. Eighteen of these are used for synchronisation, and six for operation and maintenance. The remaining bits form twelve 18-bit words for the B and D-channels. D Frame structure of the 2B1Q code

39 The line code used (4B3T) is a block code that converts blocks of four consecutive data bits into one of 27 possible groupings of three baud symbols, where each symbol can have one of three levels, +, - or 0. It is a three-level line code is less complex (compared to 2B1Q) and easier to implement. This line code is popular in Europe (especially Germany).

40 References The ISDN Subscriber Loop; Nick Burd c/c-4-1.shtml

TELECOMMUNICATION SYSTEMS

TELECOMMUNICATION SYSTEMS By Syed Bakhtawar Shah Abid Lecturer in Computer Science 1 INTEGRATED SERVICES DIGITAL NETWORKS (ISDN) The original concept of ISDN dates back to the early 1970s. Its design,