M7 GX Technical Manual
36
Raveon Technologies Corp.
To delete an entry in the table so it has no effect on the operation, set the fields to
0. For example, to disable entry 1, use the
ATX1 0 0 0 0
command.
There can be an issue with regard to store-and-forward repeating and busy
channels
, particularly on polled systems. Raveon’s M6 wireless modem has a
number of provisions in it to make store-and-forward repeating work smoothly.
For example, in the diagram above, assume A is the master station, and C is a
remote station being polled. When the store-and-forward repeater B sees a packet
it should repeat, immediately upon reception of the packet, it keys its transmitter
and repeats the packet. The scenario that can cause problems is if the end
receiving station C actually heard the original transmission from A. In a polled
scenario, the end station C will typically then respond to the poll, and want to
transmit. Station C’s transmission can happen at the same time as the repeater B
is trying to repeat the original transmission.
This contention can be reduced/eliminated in the following ways:
1. Turn busy-channel lock-out on (
ATBC 1
) on all modems. This stops them from transmitting on
a busy channel (stops them from transmitting when the repeater is transmitting).
2. Set the serial port baud-rate on the end-stations to be fairly slow (
ATBD x
). Thus, when they
receive a poll request, there is a delay as they send data in/out of their serial ports, and during
this delay, the repeater can do its thing.
3. Increase the serial port time-out value from 20mS to say 250mS (
ATR3 250
). Then, when the
polled station responds, there is a 250mS delay before the end station’s data gets sent out over
the air. This gives a little gap for the repeater to use for repeating messages.
4. Any combination of 1-3.
M6 radio modems will not repeat or receive duplicate versions of the same data
packet. If two repeaters are used in the same system, each will repeat a
transmission only one time, even if they are within communication range of each
other. A repeater will not repeat a transmission if it was the originator of the
transmission. If another M6 in the system has the same ID as the repeater, the
repeater will not repeat data from that particular unit.
The repeater’s ID must be
unique in the system.
If a M6 is configured as a repeater, and is also used to send and receive data, it
will not repeat any transmission that it originated. M6 checks the ID of the station
that originated the transmission to determine if the message should be repeated. If
the transmission was originated by a station with the same Unit ID as the Unit ID in
the receiving station, the data will not be repeated. This is why it is important to
have a different ID for each M6 modem in a network that uses repeaters.
7.6. Data Communication Security
The M6 has a number of data encryption modes to secure the transmissions and
reception of data. Encryption is turned off on a modem by setting
ATDEN 0
and
setting the
KEYPHRASE 0
.
For legacy support, the KEYPHRASE can be set to an encryption key and then
GPS position transmissions will be encrypted. The serial data transmissions will
