MFJ-1270C/1274C MANUAL ADVANCED OPERATION
Page 65
Packet Timing
The AX.25 protocol provides for re-transmitting packets if no acknowledgment
is heard from the intended destination station within a certain period of time. A
packet might not be acknowledged due to channel noise or "collision" with an-
other packet transmission, and since there may be other stations on the channel,
the receiving station may not be able to acknowledge the received packet
immediately. The time lapse before the originating station re-transmits the
packet is set by the command
FRACK
(frame acknowledge time). The
maximum number of re-transmissions before the originating station terminates
the connection is set by the command
RETRY
. The maximum number of
transmissions of a packet is RETRY+1, since the initial transmission does not
count as a re-transmission. Setting
RETRY
to 0 specifies an infinite number of
retries.
The frame-acknowledge time is automatically corrected for the additional time
required for digipeating. The time interval before MFJ TNC re-transmits an
unacknowledged packet is
Retry interval = FRACK * (2*n + 1)
in seconds, where n is the number of digipeaters in use for this connection.
Acknowledgments of digipeated packets are made from end to end, and
digipeaters do not acknowledge the packets they relay. If there are several
intermediate relays, the chance of either the original packet or the
acknowledgment to be lost increases drastically. To reduce this problem, an
automatic wait time can be imposed on any station not transmitting a digipeated
packet. Stations waiting for a clear channel to transmit packets wait for this time
interval after the channel clears before transmitting. This wait does not apply if
the station will be transmitting one or more digipeated packets. This usually
gives the digipeater a clear chance at the channel.
The wait time is set by the command
DWAIT
, which specifies 40 ms intervals.
If no digipeating is being done by anyone in the local area, this parameter can be
set to 0, but in any event it should be set to the same value by all members of a
local packet group.
In order to avoid unnecessary packet retries, the MFJ TNC implements a
collision-avoidance strategy which applies to all packets except those being
digipeated. On the second and subsequent transmissions of a particular packet,
the MFJ TNC waits an additional random time after detecting a clear channel
