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If received data has an error in it or if the receiving modem does not get the
data due to interference, the receiving modem does not send the ACK back,
and the sending station will automatically re-send the data. There are two
aspects to configuring ARQ:
1)
Enable the ARQ mode in the modem to allow it to transmit ACKs
(ATAK 1 command). This command enables the modem to
transmit ACK packets.
2)
Configure the number of retries the modem should attempt if an
ACK packet is not received back when it sends data (ATRB xx
command). For example, if you set ATRB to 5 with the ATRB 5
command, the modem will wait for an ACK whenever it sends data.
If it receives and ACK back from the modem that it sent data to, it
will do nothing more. But if it does not receive an ACK, it will
resend the same data, trying up to 5 more times.
The factory default condition is not to send or require ACK packets, so if you
wish to use this mode, program the FireLine to transmit ACKs with the ATAK
1 command. This will cause the modem to send an ACK anytime it receives
data from another modem. Note: If the destination address was a broadcast
(FFFF, FFxx, or xxFF), it will not wait for an ACK.
The number of times it retries to send data that does not get through (does
not get an ACK) is up to the user, but a number of 5 is usually a good
compromise. If after 5 times, the data does not get through, then there
probably is something seriously wrong with the channel or system.
The retried-transmissions are randomly spaced at intervals between
approximately 200mS and 400mS, increasing by 50mS for each attempted
transmission. Once the modem has sent its data the number of times the
user specified in the ATRB xx command, the data is discarded, and the
modem will continue to operate as normal.
It is very important that if one modem is configured to send ACK packets
(ATAK 1) that all other modems communicating with it are also set to expect
ACK packets using the ATRB xx command.
To disable ARQ (the default condition of the modem), disable ARQ with the
ATAK 0 and set retries to zero with the ATRB 0 command.
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