Chapter 6 - System Operation and Control
37
stopped transmitting for the receiver to become active, data will be lost. The delay between trans-
mitter keyup and the beginning of data transmission is controlled by the command
TXDelay
.
AX.25 protocol provides for retransmitting packets if no acknowledgment is heard from the
connected Station within a certain period of time. A packet may not be acknowledged due to chan-
nel noise or "collision" with another packet transmission. 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 retransmits the packet is set by the command
FRack
(frame acknowledge time). The maximum number of retransmissions 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 retransmission.
The frame-acknowledge time is automatically corrected for the additional time required for digi-
peating. An extra time delay is added for each transmission which must be made after origination
of the packet in order to deliver the packet and receive the acknowledgment. The time interval be-
fore the unit retransmits an unacknowledged packet is therefore
RETRY INTERVAL = FRACK × (2 × n + 1) seconds
where n is the number of calls in the digipeat field of the address.
The AX.25 protocol specifies that acknowledgments of digipeated packets be made from end to
end. This means that intermediate digipeaters do not acknowledge the packets they digipeat.
When the destination station receives the packet, it generates an acknowledgment which is sent
through the reverse route used by the original packet. If there are several intermediate relays, the
chance of either the original packet or the acknowledgment being lost increases drastically. To help
alleviate this problem, an automatic wait time can be imposed on any station not transmitting a
digipeated packet. Any station which is ready to transmit a packet immediately after the carrier
drops is required to wait for this time interval unless it will be transmitting one or more digipeated
packets. This means that the chance of a collision involving a digipeated packet is reduced, since
once a transmission begins, other stations will wait for a clear channel. The digipeat wait time is
set by the command
DWait
, which specifies 10 ms intervals, In order to avoid unnecessary packet
retries with the associated channel load, the KAM implements a collision-avoidance strategy which
applies to all packets except those which are being relayed. On the second and subsequent trans-
mission of a particular packet, the KAM waits an additional random time after detecting a clear
channel before beginning transmission. This strategy is based on the assumption that packets
which were not acknowledged suffered collisions with transmissions from other stations. If the ran-
dom wait is used, repeated collisions of transmissions by the same two stations can be prevented,
since eventually they will wait different time periods. The random time is a multiple (0–15) of the
TXdelay
time. This is because
TXdelay
represents the interval during which a transmitter may have
been keyed but cannot yet be detected by other stations. The interval between the unit detecting
carrier-drop and beginning to transmit is
Wait time = DWait × 10 milliseconds
for the first transmission of a packet. For subsequent transmissions of the same packet the inter-
val is:
Wait time = (DWait + r × TXDelay) × 10 milliseconds
where r is a random number from 0 to 15.
AX.25 protocol allows multiple packets to be transmitted before waiting for an acknowledg-
ment. This permits more efficient channel use when large amounts of data are being transferred.
The maximum number of packets which the KAM will send before waiting for acknowledgment is
specified by
MAXframe
. Of course, the unit will not wait until
MAXframe
packets have been en-
tered before transmitting. This parameter is only used to limit the transmission if more than one
