Westermo RM-505U-2-E - Manual (Page 14)

505U Radio Telemetry Module
Page 14 © September 2002
2.4 How to Design a Remote Monitoring System
2.4.1 Achieving reliable radio transmission
A system can theoretically have an unlimited number of 505U modules, but in practice, the number
is limited by the amount of radio traffic on one frequency in the system. When a radio channel
becomes unreliable because of radio traffic, then a second radio channel must be used to increase
the size of the system. This limit is not a function of the number of modules, but the number of radio
messages.
A system comprises 505U and 105U modules - each can transmit input signals. The 105U can
"hear" other radio messages, and will hold off transmitting a message until the radio channel is clear.
The 505U module cannot, and there is a possibility that an individual transmission will clash with
another transmission, and both transmissions will be corrupted. This possibility increases as the
density of transmissions increases. Configuring the re-transmit feature (transmission of each message
several times) will increase the chance of each message being received successfully, but will increase
the overall density of radio traffic.
For large systems, a compromise is required between the number of re-transmissions, and the
update times for each input. High priority inputs should have shorter update times than lower
priority inputs.
The peak transmission density should be calculated for large systems. These values are calculated
by determining the number of transmissions from inputs changing value and the number of update
transmissions per hour.
The probability of success for an individual message depends on the transmission density and the
number of re-transmissions for each message. This is shown in the following graph:
Probability of successful transmission
0 %
10 %
2 0 %
3 0 %
4 0 %
5 0 %
6 0 %
7 0 %
8 0 %
9 0 %
100%
0 1000 2000 3000
M e s s a g e s P e r H o u r
1
2
3
4
5
Transmissions
p e r m e s s a g e
This assumes that the radio path is reliable and that there is no other radio users on this radio
channel. If intermediate repeaters are used, then each repeated message should be counted as
another message.