PLP of X.25 Network

Packet Layer Protocol (PLP) is basically Network Layer Protocol for X.25 protocol suite and handles the virtual circuit. It also provides multiplexing capability. This layer is also responsible for call setup and termination and for handling transmission of data packets or frames. PLP was basically designed to manage and handle exchange of data among two Data Terminal Equipment (DTE) stations across virtual circuits. It might also be used on ISDN with the help of Link Access Procedures D-channel (LAPD).

Different Modes of PLP :

  1. Call Setup Mode –
    This mode is managed by PLP among two DTE stations with the help of 14-digit X.21 addressing scheme to usually set up the virtual circuit. It is only done using SVC’s but not with PVC’s as PVCs are already connected after they are built. Each call is also independent of other calls, so that calls,

    • Call Request Packet :
      The calling DTE sends this request as it is used to establish a connection on an SVC’s. It also includes the address of the remote DTE to be contacted.



    • Call Accept Packet :
      This packet is used to accept a call.

    • Clear Request Packet :
      This packet is used to clear a call.
  2. Data Transfer Mode –
    This mode is managed by PLP among two DTE stations across an SVC’s and also across PVC’s. In this mode, padding segmentation and reassembly, error and flow control are managed and handled.

  3. Idle Mode –
    It is an intermediate mode among connection and data transfer modes. This mode is basically used when a virtual call is established but there is no transferring of data.

  4. Call Clearing Mode –
    This mode is basically used to tear down a virtual circuit and also end a call among two DTE stations. In other words, it is used to end sessions among DTE stations and to terminate virtual circuits. Call Request Packet is transmitted to remote DTE whenever DTE wants to terminate or end a call.

  5. Restarting Mode –
    This mode is used for transmission synchronization among two DTE stations. This mode is locally connected to DCE (Data Communication Equipment’s). This mode is used prior to entering data transfer mode and also to re-establish synchronization whenever it is lost.

PLP Packet Fields :

There are basically four different PLP packet fields as shown below :


  1. General Format Identifier (GFI) :
    It contains an indication of whether modulo-8 or modulo-128 sequence numbers are used or not, and also Q-bit and D-bit. This identifier is used to identify different parameters of data frames or packets whether data packets contains data or control information or not. It is also used to identify type of windowing that is performed and also to ensure that is it needed to use delivery confirmation or not.

  2. Logical Channel Identifier (LCI) :
    This identifier is composed of 4-bit LCGN (Logical Channel Group Number) along with 8-bit LCN (Logical Channel Number). It is basically used to identify local virtual circuit.

  3. Packet Type Identifier (PTI) :

    It is used to identify only one packet type from 17 different PLP packet types such as call accept, call request, clear confirmation, clear request, interrupt request, receive ready, or receive not ready, etc.

  4. User Data :
    The user data field only exists or present in data packets. It also contains encapsulated data from an upper-layer protocol like TCP/IP.

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