RV-M7 Technical Manual
42
There can be an issue with regard to store-and-forward repeating and busy channels, particularly on
polled systems. Raveon’s M7 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.
M7 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 M7 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 M7 is configured as a repeater, and is also used to send and receive data, it will not repeat any
transmission that it originated. M7 check 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 M7 modem in a network that uses repeaters.
Summary of Contents for RV-M7
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