In addition to supporting the basic
communication functions, the master terminal is required to provide the
following channel management functions. In this section, parameters labelled
“user alterable” are required to be readily adjustable to permit user tuning of
the communication facilities.
The system supplier is expected to provide
default values for these parameters to support initial field operation.
Message Priorities
The capability to manage the transmissions
on each channel based on relative priorities assigned by non protocol processes
to waiting messages should be supported. The capability to assign different
priorities to the various message functional types (e.g., status versus analog
data acquisition) should be supported.
A priority order based on remote terminal
address should be supported for party line remotes. The assignment of
priorities should be user alterable.
Message Queuing
The capability to manage a queue of
messages waiting for access to each channel should be supported. The size of
each queue shall be user alterable. The capability to establish alarm
thresholds indicating imminent queue overflow and excessive queuing time for
each queue should be supported. The values of the alarm thresholds should be
user alterable.
Message Retransmission
When any master terminal message requiring
a response is transmitted, a “No-Response” timer should be started and
subsequently reset on receipt of the response. If the user-alterable time-out value
is exceeded, the previous master terminal message may be retransmitted.
The capability for user-alterable
assignment of the permitted maximum number of retries for each message
functional type should be supported. Some message types may not require any
retries.
Communication Performance
The capability to alarm channels as
inoperative or marginal should be supported. The criteria for declaring a
channel inoperative or marginal should be user-alterable and be based on
measured channel quality parameters and/or error statistics.
As a minimum, an error count should be
accumulated for a user-defined message count to alarm a channel as inoperative
or marginal when the error count exceeds a useralterable limit. Channel quality
parameters may include signal-to-noise ratio, carrier level, etc., which are
monitored by communication equipment.
Rejection of messages by remote or master
terminals may be due to conditions such as a low quality channel, marginal
modems, or terminal equipment failure.
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