User's Manual

SATELLAR Radio Unit User manual v.0.7
25
An example of how to use priority settings in a simple network is shown in the figure below.
A
B
C
Priority TX
Priority RX
Priority RX
No radio coverage
between B and C
Station A has a radio link to stations B and C and sends control commands to those. Stations B
and C respond by sending either status information or acknowledgement messages to control
commands from A and they cannot hear each others’ radio transmissions. Control commands
from station A are of high priority, so station A needs to start sending despite it has a reception
going on from either of the stations B or C. Therefore station A is set to priority TX while the
others are set to priority RX.
Priority settings only help if the radio coverage is as described in the figure above, i.e. if stations
B and C cannot hear each others’ transmissions. Consider a situation where station B is sending
to A and A then needs to send a high priority message to station C while it still has reception
ongoing from B. Due to priority setting to TX it is possible but if stations B and C are within each
others’ radio coverage the two simultaneous messages from A and B collide at C and therefore
the message from A is probably not received correctly there. This kind of situation cannot be
solved with priority settings but needs a more complicated handshaking procedure that is
explained in chapter 6.3.3. Priority settings help in getting the important messages fast through
but must be used carefully keeping in mind that the stations set to priority RX may not be within
each others’ radio coverage.
6.3.2 TX delay
TX delay can be used in a situation where a certain master station sends queries as broadcast
messages to many sub-stations. To prevent replies from the sub-stations to collide at the master
station different TX delay values can be set to each of the sub-stations. This means that a sub-
station does not reply to the query until the TX delay period has been expired. TX delay is fixed,
i.e. the maximum length of the reply message must be approximately known at the network
configuration phase in order to really avoid collisions at the master station. TX delay can be
considered as a primitive time-slot mechanism.