User's Manual

Chapter Six Troubleshooting
Man_905UD_2.0.doc Page 43
When using this feature, the current value of the received signal strength is displayed in dBm
(decibels referenced to 1 mW). This value is updated every half second. To check the radio
path between two units, select the signal option at the local unit. The remote unit may then be
set up for tone reversals (refer 1 above) and the signal level read from the computer screen.
We have selected a bit error rate (BER) of 1 in 300 as the point at which unreliable
communications occurs. The following table lists the minimum signal required for reliable
communications (BER > 1 in 300). A system with the minimum signal level will not stay
reliable under all conditions, because of changing factors such as environmental conditions.
A fade margin of at least 15 dB should be allowed for to take account of these factors. The
table also lists our recommended minimum signal level, taking this margin into account.
Radio Baud Rate Minimum signal for BER
= 1 in 300
Recommended minimum
signal
9600 -100 dBm -85 dBm
4800 -107 dBm -92 dBm
In areas experiencing radio interference or high background noise, reliable communications
may not be achievable even with this signal level. To determine if interference is occurring,
the signal option may be selected without any other 405U units active. In a normal radio
environment, a reading of at most -110 dBm should be displayed. If a reading above this
value is displayed (for example -100 dBm), then the received signal strength from the remote
should be at least 15 dB higher than the background noise for reliable communication.
Example:
AT&T8 Displayed signal level of background noise/interference
RSSI Display - press a key to exit
-88 dBm
AT&T8 Minimum signal level for reliable comms
RSSI Display - press a key to exit
-73 dBm
When using directional aerials (YAGI aerials) this feature may be used to align the aerial in
the correct direction by selecting the peak signal when moving the aerial. Setup the remote
unit to transmit tone reversals as described above, and observe the signal indication while
adjusting the orientation of the aerial. A peak in signal level indicates optimum orientation of
the aerial.