One effect of this is that an address mask of 0000 will cause the
FireLine
modem to received any data from any unit that transmits. The Destination
Address will effectively be ignored if the mask is set to 0000.
Addressing Examples:
Example 1 (default configuration)
Sending Destination Address = 1234
Receiving Unit Address = 1234
Receiving Unit’s Address Mask = FFFF
Result: Unit will receive the data, because the addresses identically match. When the
addresses are identical, the value of the mask is not important.
Notes: This is the default configuration. All units have address 1234, and all modems will
talk to all other modems with address 1234.
Example 2 (a configuration that won’t work)
Sending to Destination Address =
1236
Receiving Unit Address =
1234
Receiving Unit’s Address Mask =
FFFF
Result: No data will be received, because the address do not match, and the address
mask of FFFF requires that all digits in the address match. .
Example 3 (able to receive a data from a group, 1230 – 123F)
Sending to Destination Address =
1236
Receiving
FireLine
Unit Address =
1234
Receiving
FireLine
Address Mask =
FFF0
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
Receiving
FireLine
’s Unit Address =
1234
Receiving
FireLine
’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.
4.6.
Error Correction
The
FireLine
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
FireLine
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.
