Link Access Procedure/Protocol (LAPF) as defined in the ITU Q.922, an enhanced LAPD (Q.921) with congestion control capabilities, is for Frame Mode Services in the Frame Relay network. The core functions of LAPF are:
- Frame delimiting, alignment, and flag transparency
- Virtual circuit multiplexing and de-multiplexing
- Octet alignment: Integer number of octets before zero-bit insertion
- Checking minimum and maximum frame sizes
- Error detection, Sequence and non-duplication
- Congestion control
The purpose of LAPF is to convey data link service data units between DL-service users in the User Plane for frame mode bearer services across the ISDN user-network interface on B-, D- or H-channels. Frame mode bearer connections are established using either procedures specified in Recommendation Q.933 or (for permanent virtual circuits) by subscription. LAPF uses a physical layer service, and allows for statistical multiplexing of one or more frame mode bearer connections over a single ISDN B-, D- or H-channel by use of LAPF and compatible HDLC procedures.
LADF is used in the Frame Relay network for end-to-end signaling.
Protocol Structure
The LAPF is the same as the LAPD with the following address field:
| 6 | 7 | 8 | 12 | 13 | 14 | 15 | 16 |
| DLCI | C/R | E | DLCI | FECN | BECN | DE | EA |
- DLCI C Datalink Connection Identifier field represents the address of the frame and corresponds to a PVC.
- C/R - Designates whether the frame is a command or response.
- EA - Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses.
- FECN - Forward Explicit Congestion Notification (see ECN below).
- BECN - Backward Explicit Congestion Notification (see ECN below).
- DE - Discard Eligibility.
LAPF control field format:
| Control field bits (Modulo 128) | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| I Format | N(S) | 0 | ||||||
| N(R) | P/F | |||||||
| S Format | X | X | X | X | su | su | 0 | 1 |
| N(R) | P/F | |||||||
| U Format | M | M | M | P/F | M | M | 1 | 1 |
- N(S) - Transmitter send sequence number.
- N(R) - Transmitter receive sequence number.
- P/F - Poll bit when used as a command, final bit when used as a response.
- X - Reserved and set to 0.
- Su - Supervisory function bit.
- M - Modifier function bit.
Related protocols: LAPD, ISDN, X.25, Frame Relay
Sponsor Source: LAPF is defined by ITU-T (http://www.itu.org) Q.922: ISDN Data Link Layer Specification for Frame Mode Bearer Services
Reference: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm: Understand Frame Relay
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