When a radio channel is allocated to a mobile, the BTS reports the signal quality seen by the mobile by transmitting periodically 08.58 measurement results.
If using mainBcch, when a mobile initiates a location update but doesn’t get an accept/reject from the network, after 10 seconds we see that the RXLEV-FULL-up and RXLEV-SUB-up contained in the 08.58 measurement results drop to 0, indicating that the mobile is not transmitting any 04.08 measurement reports anymore.
In other words, the mobile is not even listening on that radio channel after these 10 seconds. The mobile didn’t release sapi 0 as we would expect.
This behaviour has been seen consistently with different mobile manufacturers.
Similarly, consistently if we use bcchCombined, we see that the mobile gives up only after 20 seconds and that it cleanly releases SAPI 0 when it does so.
Maybe this behaviour is linked to the BTS model?