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The simple answer is that VARA FM doesn¡¯t do any collision avoidance.
However, when there is an active VARA FM session between two station, any additional stations attempting to connect on the same channel will not see a response from either of the connected stations. Data communications between the connected stations will probably be impaired until the interfering station exhausts its connection retries, but the session normally resumes at that time.
Also, VARA FM supports the busy-channel detection that is implemented in the Winlink Express VARA FM session¡ if there¡¯s traffic on the channel, the operator trying to connect must click the ¡°Connect Anyway¡± button.
Keep in mind that the actual best-case?throughput for a single 1200bps connection is on the order of 500bps¡ under optimal signal conditions. Typical throughput on mildly impaired radio circuits is on the order of 300bps. The per-user throughput is a fraction of that best-case value when multiple users share a channel. This difference between the 1200bps raw data rate and 300-500bps throughput is primarily due to the overhead imposed by the AX.25 protocol.
In contrast, unlicensed VARA FM NARROW throughput is 5Kbps over moderately impaired radio channels where 1200bps packet connections are not even practical. Licensed VARA FM NARROW throughput is at least twice the unlicensed value under similar conditions. Double those numbers again for licensed and unlicensed VARA FM WIDE operation.
Realistically, minimum VARA FM NARROW throughput, for unlicensed use, is 10-15 times higher than single connection 1200bps packet¡ and 40-60 times higher for licensed VARA FM WIDE operation.
I believe that, compared to 1200bps packet operation, the throughput advantage of VARA FM (WIDE or NARROW), even for unlicensed use, more than makes up the lack of a collision detection.
Mark - AD7EF
Mark Davis