Kantronics KAM 98, Руководство пользователя

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Selective Monitoring
With MONITOR ON, the BUDLIST command can help in setting up your
monitoring so you will see only those packets you want to see. You can list up to
10 calls in BUDLIST.
Timing
Dwait vs. Persistence and Slottime
When the TNC acts as a digipeater, packets received that need to be relayed are
retransmitted as soon as the frequency is clear. Other TNCs on frequency
(should) hold off or wait their turn, using a PERSISTENCE/SLOTTIME algo-
rithm or DWAIT method to determine when to transmit. This practice has been
adopted to reduce collisions with packets being digipeated, since these packets
must (by the AX.25 protocol) be acknowledged end-to-end. Kantronics’ current
TNCs (e.g. KPC-3, KPC-3 Plus, KPC-9612, KPC-9612 Plus, among others), use
the PERSIST/SLOTTIME algorithm to gain channel access. The default value
for DWAIT is set to zero. If the DWAIT method is used, the TNC sending
non-digipeat packets will wait for DWAIT (times 10 milliseconds) time of clear
channel before beginning to key-up the radio to transmit.
The algorithm used with the PERSIST and SLOTTIME parameters helps
avoid collisions by randomizing the wait time before transmitting. The more
random the timing the less chance of two TNCs transmitting at the same time
and colliding.
Once the TNC detects a clear frequency it will wait SLOTTIME (times 10 milli-
seconds). Then, if the channel is still clear, it will generate a random number. If
this number is smaller than the setting of PERSIST the TNC will transmit. If it is
larger it will wait another SLOTTIME and then generate another random number
and again decide whether to transmit or not. When using PERSIST and
SLOTTIME you should set DWAIT to 0, since both will be used if specified.
As an example, let’s assume that PERSIST is set to 63, and SLOTTIME is set to
10. This value of SLOTTIME results in a random number being generated every
100 milliseconds. When the TNC sees that the channel is clear, it waits 100 ms,
then generates a random number between 0 and 255 (inclusive). If, in our exam-
ple, the number was 83, then the TNC would not start the key-up of the transmit-
Overview of Packet Communication Packet Modes of Operation
KAM’98 v 8.3 151 User’s Guide