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915U-2 Wireless I/O
Page 39
3.3 System Design
3.3.1 Radio Channel Capacity
Messages sent on a cable link are much faster than on a radio channel, and the
capacity of the radio channel must be considered when designing a system. This
becomes more important as the I/O size of a system increases.
The modules are designed to provide "real-time" operation or Change of State (COS).
When an input signal changes, the change message is sent to the output. The system
does not require continuous messages as in a polling system. Update messages are
intended to check the integrity of the system, not to provide fast operation. Update times
should be selected based on this principle. The default update time in the mappings is
10 minutes - we recommend that you leave these times as is unless particular inputs
are very important and deserve a smaller update time.
It is important that radio paths be reliable. For large systems, we recommend a
maximum radio channel density of 300 messages per minute, including change
messages and update messages. We suggest that you do not design the system with
more than 300 messages per minute as this does not take into account any network
communication overheads. Note that this rate assumes that all radio paths are reliable
and the network topology (mesh) is stable - poor radio paths will require retransmissions
and will reduce the channel density. If there are other users on the radio channel, then
this peak figure will also decrease.
Having remotes radios dropping in and out of communications can also increase overall
network traffic because the network would need to relearn the communication paths
each time the module comes back on line.
Dual Band Operation
The 915U-2 radio band is split into two sub-bands, 902-914 MHz (Low) and 915–928
MHz (High). In America and Canada, the 915U-2 uses both sub-bands - but in other
countries, e.g. Australia only the high band is available. In America and Canada, it is
possible to restrict the frequency hopping of the 905U to only the high or low band. If
there are many 905U systems in the same area, this technique will help to separate
systems to avoid radio interference. Note that this technique is only possible in
countries that utilize the full 902-928MHz bandwidth, i.e. America / Canada, etc.
The radio sub-band can be changed by selecting the "Hop Set" on the Radio page.
3.3.2 Radio Path Reliability
Radio paths over short distances can operate reliably with a large amount of obstruction
in the path. As the path distance increases, the amount of obstruction that can be
tolerated decreases. At the maximum reliable distance, "line-of-sight" is required for
reliable operation. The curvature of the earth becomes more of an obstacle if the path is
greater than several kilometres (or miles), and therefore needs to be allowed for. For
example, the earth curvature over 5 miles (8km) is approx 10 feet (3m), requiring
antennas to be elevated at least 13 feet (4m) to achieve "line-of-sight" even if the path is
flat.
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915U-2 Wireless I/O
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