User`s guide

On-Air Data Speed
In a network of multiple radio modem point-to-point links, where network usage is at
moderate levels, a situation occurs which results in each pair competing for air-time to
transmit its data. The RMX232 enables user selectable throughput rates for the radio
interface ranging from 600bps to 9600 bps (2400 bps for RMX232-151/173), thus
improving network access..
The on-air data speed varies the amount of airtime a radio modem uses to send data
thus directly affecting the throughput. Operating at the maximum rate of 9600bps a
radio modem pair, when streaming data, uses approximately 98% of available airtime.
This level of usage is fine for a single isolated point-to-point link, however it reduces
the ability of other radio modem pairs (within radio range) to operate reliably, due to an
increase in potential data collisions and data retries.
Reducing the on-air data speed gives a greater opportunity for other point-to-point
links to compete for air-time to send data, thus reducing the potential for data
collisions and retries. It does however reduce the end-to-end through put over each
point-to-point link. This is to be expected as radio is a shared medium.
Selecting the right on-air data speed is dependant on a number of factors. Some
factors to consider include the amount of data each point-to-point link in the network is
expected to transfer, the rate of transfer that must be sustained over each point-to-
point link, and the total number of point-to-point links competing for air-time within
close proximity.
It is strongly recommended that before deploying a network of multiple point-to-point
links, the expected system should be bench tested. This will allow fine tuning of the
on-air data speed and other settings such as packet retires. It will also aid in
diagnosing addressing issues etc.
Setting different on-air data speeds for point-to-point link pairs within a network
enables further optimising of network airtime usage. This can be used to give a high
priority to a link that requires a sustainable throughput of say 4800bps while other
slower devices with periodic rates averaging below 600bps would be given acceptable
access to network airtime.
Link A
•
Required 4800bps
•
airspd
setting 4800
or 9600.
Link B
•
Approx 600bps
•
airspd
setting 600
Link C
•
Approx 600pbs
•
ai
r
spd
sett
in
g
600
A
C
C
A
B
B
Example network of point-to-point links operating within close
radio proximity using different airspeed settings
User’s Guide Modem Operation Explained 4-4