Operator's Manual

OM-E 2072-09689-00
A-1
APPENDIX A
ALE CAPABILITIES AND FEATURES
A-1. SCOPE
This Appendix provides a concise description of the ALE capabilities and features.
A-2. SCANNING
The method used to select the best channel is based on scanning: as long as it is idle, a radio operating
in the ALE mode switches frequency at a relatively rapid rate (programmable at 2 or 5 channels per
second) within the group of channels that can be used (the ALE frequency table, with up to 100
frequencies). During this process, it collects information regarding the state of each channel (busy/free
and its background noise) that can be used together with the quality information collected by sounding
when the user initiates a call.
• When a station initiates a call, it selects the best free channel on the basis of the most recent
information, switches to that channel and then transmits a call request for a time sufficient for
any station to finish scanning all the preprogrammed channels and reach the selected channel.
• Any station that should receive this call will then stop on the channel used by the initiating
station, and will use the channel for communication. The exact response to the call request
depends on the type of call (call types are described below): as a result, it is possible to program
any desired station to ignore some types of calls (in fact, any call not specifically addressed to
it).
A-3. SOUNDING
Automatic sounding is the method used by ALE for testing the quality of channels and propagation
paths under field conditions. The sounding signal is a unilateral identifying broadcast, repeated at
periodic intervals on unoccupied channels. Identification of a sounding signal sent from a station
indicates a high probability of bidirectional communication.
The length of the sounding cycles can be set to short or long cycles, depending on propagation
conditions. When propagation changes are slow, long intervals of about 1 or 2 hours may be sufficient.
If, however, the propagation changes are erratic and rapidly changing, or if it is critical to have
updated connectivity information, then shorter intervals are recommended. Each station can also
measure BER and S/N, and update the LQA table accordingly.
The maximum score for receiving an optimum sounding is 70%; the minimum score is 30%. The
score given to a station that did not send any sounding or a station whose sounding has not been
received is also 30%.
A-3.1 Sounding Cycle Time
The sounding signal is repeated at regular intervals on all the channels in the network. A complete
round of sounding messages, or sounding cycle, can be programmed to 30, 60, 90, or 120 minutes
when the automatic sounding is on. You can also manually initiate sounding when there is a specific
need for it, even if the system is set to operate in the automatic mode.
A short sounding cycle ensures a more updated LQA memory. However, frequent sounding increases
network occupancy, leaving less channel-free time for the users. Figure A-1 illustrates network
occupancy in relation to sounding cycles of 30, 60, 90 and 120 minutes. (Note that both axes have
logarithmic scales.) The chart also shows 80% and 95% occupancy in a 120-minute sounding cycle at
a very rapid scanning rate (applicable only to multiple-channel networks).