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Link Access Procedure for Frame Relay

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Link Access Procedure for Frame Relay (LAPF) is part of the Frame Relay protocol for WANs where it is used for end-to-end signaling. It is defined in ITU standard Q.922.


In Frame Relay Local Management Interface (LMI) messages are carried in a variant of LAPF frames.

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APF: Link Access Procedure/Protocol for Frame Mode Services (Q.922)

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.

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