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APPENDIX A: RF Topics
This section discusses some topics to ensure the best RF coverage range.
RF Channel Selection
The 802.15.4 wireless standard uses 16 RF channels numbered 11 through 26.
Each channel has a bandwidth of 2MHz and channels are separated by 5MHz. When a
ZW-CM is powered on it automatically searches for a Receiver with the same Network
ID by scanning each RF channel. The Receiver dictates the RF channel the ZW-CM will
use. You may choose to manually select your RF Channel at the Receiver. See you
Receiver User Manual for more information.
You may want to manually select an RF channel if you are running multiple
receivers in one area. While multiple Receivers can coexist on the same RF channel,
selecting different channels is recommended. Using separate RF channels will reduce
potential interference between systems. This is particularly true for large deployments.
Wi-Fi networks may also create interference with the ZW-CM. Wi-Fi networks
operate on fixed frequencies with channels occupying 20MHz or 40MHz of bandwidth.
While there are many Wi-Fi Channels there are only a few non-overlapping channels
that are generally used.
Figure A.1
shows the 802.15.4 channels mapped against the
occupied bandwidth of the most commonly used Wi-Fi Channels in North America and
Europe. If you know what WiFi channels are currently in use, select an 802.15.4
channel that does not overlap with it to reduce interference.
Signal Strength Indicator
When the ZW-CM transmits, it reports its Received Signal Strength to the Receiver.
This reading can be viewed on your Receiver and indicates how well the ZW-CM can
hear the Receiver. The received signal strength is important because each transmission
sent by the ZW-CM must be confirmed by the Receiver. If the ZW-CM does not receive
a confirmation it will retry sending the data until successful.
The ZW-CM has a receive sensitivity of -96dBm or better. It is recommended that
the received signal be at least -86dBm on average to maintain reliable communications.
In most cases this means that the Signal Strength reading should be 15% to 20%.
Figure A.2
shows the rough mapping from Received Power to Signal Strength. Receive
power is measured in dBm which is a logarithmic term. For every 10dBm increase in
receive power the actual power is increased by 10 times.
If the wireless link is weak the ZW-CM will need to re-transmit often which
lowers its battery life. Use the Diagnostic page on your Receiver to view the
Signal Strength of the ZW-CM and reposition it if necessary.
Figure A.1 – Global Frequency Chart
