
APPENDIX
Protocol Data Mode
2.10
•
19.01.2007
101
5.1.7 LAP
Procedures
This section describes the mechanisms behind and the operational procedures of the LAP
protocol. The purpose is to provide additional illustration to the detailed protocol
implementation as described in section 5.1.8.
5.1.7.1 States
The LAP state machine is made-up by the following states:
State
Description
Idle
A LAP link has been initiated (i.e. a LAP instance exists) but
there has been no request for data transmission from the
higher layer
WaitEstablish
The higher layer has requested the first data transmission.
The LAP instance establishes a connection to its peer.
Active
The LAP connection has been established. Depending on the
conditions described below, data may be transmitted over the
LAP link.
5.1.7.2 Conditions
While in active state, the LAP operation is determined by conditions (flags).
The
receiver
is attributed by the following conditions:
Self Busy:
The condition is set or cleared locally by the higher layer (flow control). If the
condition is set, the receiver will discard any I-frames from its peer. If the condition is cleared
the receiver is ready to receive I-frames from its peer.
Reject Exception:
The condition is set, when the receiver receives an I-frame with an
unexpected sequence number (N(S) sequence error). It is cleared, when the receiver
receives an I-frame with correct sequence number. All I-frames with wrong sequence
numbers are discarded by the receiver.
Ack Pending:
This condition indicates that an acknowledgement is pending, i.e. the receiver
has received successfully at least one I-frame which it has not yet acknowledged. The
condition is set, when the receiver receives a valid I-frame. It is cleared when all received I-
frames have been acknowledged.
The
sender
is attributed by the following conditions:
Peer Busy:
This condition indicates that the peer is busy. It is set when a RNR-frame is
received. It is cleared when either a RR- or a REJ-frame is received.
Timer Recovery:
This condition is set in case of timeout, while the sender is waiting for an
acknowledgement of a previously transmitted I-frame. While in Timer Recovery condition the
sender requests an acknowledgement by polling the receiver. The condition is cleared, when
a response on the polling command has been received.