This project was written by: Levi Ehud, Hadad Eli and Epstein Amir as part of the course : Protocols and Computer Networks given by Dr.Debby Koren at the Tel-Aviv University, Israel.


I n t r o d u c t i o n

In the early 1970's there were many data communication networks(also known as Public Networks), which were owned by private companies, organizations and governments agencies.
Since those public networks were quite different internally, and the interconnection of networks was growing very fast, there was a need for a common network interface protocol.

In 1976 X.25 was recommended as the desired protocol by the International Consultative Committee for Telegraphy and Telephony (CCITT) called the International Telecommunication Union (ITU) since 1993.

X.25 is a packet switched data network protocol which defines an international recommendation for the exchange of data as well as control information between a user device (host), called Data Terminal Equipment (DTE) and a network node, called Data Circuit Terminating Equipment (DCE).

X.25 utilizes a Connection-Oriented service which insures that packets are transmitted in order.

X.25 comes with three levels based on the first three layers of the Open Systems Interconnection(OSI) seven layers architecture as defined by the International Standard Organization(ISO).

The levels are:

X.25 was originally approved in 1976 and subsequently revised in 1977, 1980, 1984, 1988 and 1992. It is currently (1996) one of the most widely used interfaces for data communication networks.

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We will examine now each level in more detail:

The Physical Level

The physical level (level 1) deals with the electrical, mechanical, procedural and functional interface between the DTE and the DCE.
The physical level is specified by the X.21, X.21-bis and the V.24 recommendation for modems and interchange circuits.

Now we will discuss in more detail the X.21 interface since it is the most commonly used one.

X.21 Digital Interface.

In 1976 CCITT recommended a digital signaling interface called X.21. The recommendation specifies how the DTE can setup and clear calls by exchanging signals with the DCE.

The physical connector has 15 pins, but not all of them are used.
The DTE uses the T and C circuits to transmit data and control information. The DCE uses the R and I circuits for data and control. The S circuit contains a signal stream emitted by the DCE to provide timing information so the DTE knows when each bit interval starts and stops.
The B circuit may also provide to group the bits into byte frames. If this option is not provided the DCE and DTE must begin every control sequence with at least two SYN characters to enable each other to deduce the implied frame boundary.

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The Link Level

The link level (also called level 2, or frame level) ensures reliable transfer of data between the DTE and the DCE, by transmitting the data as a sequence of frames (a frame is an individual data unit which contains address, control, information field etc.).

The functions performed by the link level include:

The link level uses data link control procedures which are compatible with the High Level Data Link (HDLC) standardized by ISO, and with the Advanced Data Communications Control Procedures (ADCCP) standardized by the U.S.American National Standards Institute (ANSI).

There are several protocols which can be used in the link level:

We now discuss LAPB in more detail since,as we mentioned before,it is the most commonly used.

LAPB-Link Access Protocol, Balanced.

The LAPB protocol uses the following frame structure:

There are three kinds of frames:

  1. Information: This kind of frame contains the actual information being transfered. The control field in these frames contains the frame sequence number.
  2. Supervisory: There are various types of supervisory frames.
  3. Unnumbered: This kind of frames is used only for control purposes.
  4. LAPB also provides the following commands:

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    The Packet Level

    The packet level (also called level 3 or network level) creates network data units called packets which contain control information and user data.

    The packet level provides procedures for handling the following services:

    Call Setup

    When DTE A wants to communicate with DTE B, it must set up a connection by building a CALL REQUEST packet, and passing it to it's DCE.

    DTE B gets the packet through the subnet and it's DCE. If DTE B wishes to accept the call, it sends a CALL ACCEPTED packet back.

    When DTE A receives the CALL ACCEPTED packet the Virtual Circuit is established.

    At this point the two DTEs may use a full-duplex connection to exchange data packets. When either side wants to finish the call, it sends a CLEAR REQUEST packet to the other side, which then sends a CLEAR CONFIRMATION packet back as an acknowledgment.

    The DTE determines the circuit number on outgoing calls and the DCE determines the circuit number on incoming calls If both simultaneously choose the same number then Call Collision occurs. X.25 specifies that in this case, the outgoing call is put through and the incoming one is cancelled.

    Packets Format

    We will examine now the format of the packets in X.25 protocol.

    X.25 State Diagrams

    The X.25 standard contains several state diagrams to describe event sequences such as call setup and call clearing. The diagram bellow shows the subphases of call setup:

    Initially, the interface is in state P1.A CALL REQUEST or INCOMING CALL changes the state to P2 or P3, respectively. From these states the data transfer state, P4, can be reached, either directly, or via P5.
    Similar diagrams are provided for Call Clearing, Resetting, and Restarting.

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    R e f e r e n c e s

    1. Computer Networks / Andrew S.Tanenbaum
    2. Data and Computer Communication / William Stallings
    3. Telecommunication Networks: Protocols, Modeling and Analysis / Mischa Schwartz
    4. Interconnections: Bridges and Routers / Radia Perelman
    5. Computer Networks and Their Protocols / D.W. Davies, D.L.A. Barber, W.L. Prince and C.M. Solomonides
    6. Computer Networks Architecture and Protocols / Paul E.Green

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