User Manual

Operation
RFI-9256 Radio Modem User Manual Page 24 of 96
• Both master and slave radio must have the same local address, and it must be in the range 10-99 (i.e.,
two characters).
• All other radios in the network must have local addresses in the range 1000-9999.
These rules come about due to the way Hayes dial-up works in a back-to-back repeater network. See section
4.3.3 for more information on Hayes dial-up networking.
Back-to-back repeaters should be synchronised, as discussed in the section Synchronisation on page 22, in
order to prevent them from jamming each other due to collocated antennas.
 The slave must be configured to output a synchronisation signal.
 The master must be configured to follow a repeater synchronisation signal.
This guarantees that both master and slave will transmit at the same time.
4.2.3 Performance Tuning
LATENCY REDUCTION
Some systems will require a radio link that induces very low latency, usually where the RFI-9256 is being
used to replace an existing wired system. There are a number of parameters that can be traded off in order to
reduce latency.
 Throughput. The throughput of the RFI-9256 can be reduced, with a corresponding reduction in
latency. This is achieved by reducing the frame time as the frame time has a direct impact on
latency. A smaller frame time will give lower latency. See section Latency on page 18 for a
discussion on why this is the case.
 Reliability. By reducing the maximum number of retries the latency performance of the RFI-9256
can be improved in harsh RF environments. Given that each retry will add an extra frame time to
the latency induced by the radio, calculate the maximum acceptable latency induced by retries, and
divide by the frame time to give the maximum retries that should be configured. Note that by
reducing the maximum number of retries, data may be lost so the underlying system should be
robust enough to handle this situation.
TIME BASED PACKETISATION OF DATA
Many protocols delimit packets of data by silence on the communications line for a set period of time. A
common example of such a protocol is Modbus.
Due to the TDD nature of the RFI-9256, simply passing this data into the radio can cause it to become
disrupted as there is no guarantee that timing will be maintained over the air link (see section Data Timing
on page 19). In order to correct this problem, two configuration items should be used.
 Set the RX packetisation timer to the number of milliseconds that will appear between each
packet.