Table of Contents
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Chapter 6
HDLC Data Link Layer
Timeout Functions
Protocol Environment Definition
6 22
Mode Setting Contention
A mode-setting contention situation exists when a node issues a
mode-setting command and, before receiving an appropriate response
(UA or DM), receives a mode-setting command from the other mode.
Contention situations fall into one of the two following categories:
When the send and receive mode-setting commands are the same, each
node shall send a UA response at the earliest respond opportunity.
Each node shall enter the indicated mode immediately without waiting
for the response.
When the send and receive mode-setting commands are different, each
node shall enter the ADM and issue a DM response at the earliest
respond opportunity.
Timeout functions are used to detect that a required or expected
acknowledging action or response to a previously transmitted frame has
not been received. Expiration of the timeout function shall initiate
appropriate action (error recovery or reissuance of the P bit).
Each node provides a timeout function to determine that it has not
received a response frame with the F bit set to 1 in response to a
command frame it had issued with the P bit set to 1.
Also, a node with no P bit outstanding, but which has transmitted one or
more frames for which it expects responses, must start a timeout function
to determine that it has not received an expected response frame.
In the network layer, the message source must provide the transmitter
with the messages it sends. Also in the network layer, the message sink
must tell the receiver what to do with the messages it receives.
Upon request from the transmitter, the message source supplies one
network packet at a time. The transmitter must notify the source about the
success or failure of the transfer to the receiver at the other node before
supplying the next packet. When the message source is empty, the
transmitter waits in an inactive state until a message is available.
Whenever the receiver has received a data frame successfully, it attempts
to give the network packet portion (link-level data) to the message sink.
If the message sink is full, it must notify the receiver.
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Troubleshooting
