Bandwidth; Repeaters And Repeater Subgroups - Mantracourt T24-ACMi-SA User Manual

Bandwidth

Each radio channel (1-15) has a finite ability to carry information. When modules do not need to communicate
with each other they can be configured on separate radio channels and do not affect each other.
However, when multiple modules are on the same radio channel, even if they use different group keys, they are
all contributing to filling the available bandwidth.
Each message transmitted takes up around 3 milliseconds so if everything worked perfectly and all modules
transmitted at just the right time and with no gaps between then there could only ever be 300 messages per
second being transmitted on any one radio channel.
In reality there are factors that reduce this capacity.
Each module uses a technique to detect whether anyone else is transmitting before it transmits itself and this
takes a finite time. There can also be interference from other sources that can delay module transmissions.
Because of the transmission rate flexibility of the T24 modules there could be a few modules transmitting
messages at fast rates or many modules transmitting messages at slow rates or any combination of these.
Practically there is a limit of around 200 messages per second available per radio channel.
It should be noted that as the number of Transmitter modules increases there is more chance of message
collisions and so more messages are lost (remember that the Transmitter modules are sending their messages
out at regular intervals) thus reducing the average number of messages per second arriving per module.
So, for example, 2 modules may transmit at 100 times per second or 100 modules at a rate of 1 per second.

Repeaters and Repeater Subgroups

Repeaters are able to retransmit messages so that the repeated signal is stronger than the original and so can
increase the range of systems or can bypass obstacles.
The repeater must be configured to operate on the same radio channel and use the same group keys as those
modules it is repeating.
Because the radio traffic is effectively doubled by a repeater there is a mechanism to reduce unnecessary
repetition of messages.
Sometimes a repeater will still see messages from modules that do not need to be repeated (Thus filling up
available bandwidth) so both repeaters and all other T24 modules have a setting called the repeater subgroup.
By default all subgroup settings are set to zero. A repeater will repeat a message from all modules whose
subgroup is either zero or matches its own subgroup. If a repeater subgroup is zero it will repeat messages from
all modules.
This is a simple way to break down modules into smaller groups and control what messages get repeated.
Changing the repeater subgroup is not normally necessary unless the bandwidth is very full due to either many
Transmitter modules being present or very fast transmissions from modules.
Mantracourt Electronics Limited
24
T24 Telemetry User Manual