SupportDocumentationRoutersAccess RoutersAR100&200Configuration & CommissioningConfiguration Guide

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010 CLI-based Configuration Guide - WAN

ISDN Layer 2 Protocols

ISDN Layer 2 Protocols

LAPD

ISDN Layer 2 protocols comply with the Q.921 standard, which defines Link Access Procedures on the D-channel (LAPD). LAPD has the following functions:
  • Transmits signaling packets over ISDN D channels.
  • Allocates and manages terminal endpoint identifiers (TEIs).
  • Controls the packet sequence.
  • Detects and rectifies faults.
  • Controls traffic to prevent links from being overloaded.

LAPD Frame Format

Two frame formats are available: format A for frames that have no Information field and format B for frames containing the Information field. Figure 10-3 shows the LAPD frame format.
Figure 10-3  LAPD frame format

The meanings of the fields are as follows:
  • Flag field: is 1 byte in length and has a fixed value of 0x7E. A frame starts and ends with the Flag field.
  • Address field: is 2 bytes in length. Figure 10-4 shows the Address field format.
    Figure 10-4  Address field format

    • SAPI: is short for service access point identifier and specifies the message type. The value 63 indicates a Layer 2 TEI management message, and the value 0 indicates a Layer 3 call management message.
    • C/R: is the command/response field bit. The C/R bit identifies a frame as a command or a response. The user side sends commands with the C/R bit set to 0, and responses with the C/R bit set to 1. The network side sends commands with the C/R bit set to 1, and responses with the C/R bit set to 0.
    • EA1: is the first address extension and has a value of 0.
    • EA2: is the second address extension and has a value of 1.
    • TEI: is short for terminal endpoint identifier and has the following values:
      • Values 0 to 63 are selected by users. The device does not support user-defined TEI values.
      • Values 64 to 126 are requested by users and allocated by the network device.
      • The value 127 is used to address packets to an unidentified device.
  • Control field: is 1 byte or 2 bytes in length according to the frame type. This field identifies the frame type:
    • I: indicates information frames, which transmit valid information or data.
    • S: indicates supervisory frames, which are used for error control and flow control. Supervisory frames help to detect error data so that error data can be retransmitted.
    • U: indicates unnumbered frames, which are used to establish, tear down, and control links.

    For more information about these frames, see the LAPD protocol.

  • Information: consists of an integer number of bytes and transmits TEI management messages or Layer 3 call management messages.
  • FCS: is short for frame check sequence. The FCS field is 2 bytes in length and ensures the validity of received frames.