APPENDIX
Protocol Data Mode
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5.1.5 LAP
Protocol
Overview
LAP (link access protocol) is a widely spread protocol for safe data transmission. It provides
end-to-end error correction and flow control for a data link. Each data link requires its own
instance of LAP.
There exist multiple variants of LAP which differ in some details of their frame structures and
their ways to establish a connection. E.g. the LAP variant LAP-D is used in the ISDN D-
channel. ITU recommendation Q.921 includes a detailed description.
The LAP variant used in LU3 data transmission is more precisely referred to as LAP-U. It is
described in EN 300 651. The firmware uses LAP-U over the air interface.
The LAP protocol used in protocol mode is very similar to LAP-U. The only differences result
from embedding the LAP packet into a HDLC frame. This implies using the HDLC ADDR
field for transport of LAP information elements and using the HDLC FCS, whereas LAP-U
uses a different type of checksum.
LAP is a peer-to-peer protocol. The peers correspond by exchanging LAP frames. These
frames may have variable length.
The LAP protocol is full duplex. The two directions are independent, therefor we only regard
one direction for this description. Due to this approach we distinguish in this text between
sender and receiver. The sender transmits information to the receiver. The receiver returns
acknowledgements to the sender. Each LAP instance comprises a sender and a receiver.
LAP support multiple frame operation. The sender may send k I-frames, before it needs an
acknowledgement from the receiver. The firmware uses k=4.
LAP provides an efficient flow control mechanism using supervisory frames.