Company Confidential
20
Raveon Technologies Corp.
Result: Data will be received. 1236 ANDed with FFF0 is 1230. 1234 ANDed with FFF0
is 1230. The results of the ANDing match, and thus the data will be received.
Example 4 (able to receive from a group, xx34 where xx is any two digits)
Sending Destination Address =
2234
Receivin modem’s Unit Address =
1234
Receiving
modem’s
Address Mask =
00FF
Result: Data will be received. 2234 AND 00FF equals 0034. 1234 AND 00FF equals
0034, therefore they match. The results of the ANDing match, and thus the data will be
received.
9.11 Error Correction
The
Radio Modem
has a built-in error correction mode, commonly referred to as
Automatic Repeat request (ARQ)
. It works by checking each reception for errors,
and if the data is OK, it sends a short “
ACK
” packet back over the air telling the
sending station the data was OK.
The
Radio Modem
uses a traditional Carrier Sense Multiple-Access (CSMA)
algorithm with randomized re-try time slots to determine when to re-transmit
packets that must be re-sent. The duration between re-tries increases as the
number of attempts increases.
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
Radio Modem
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.
