Supervision of acknowledgements on the Abis, by the BSC and by the BTS.
When BTS sends a message on Abis to the BSC, the BSC should reply with an acknowledgment.
The BTS waits T200 ms (def T200 = 300ms).
If ack is received before T200, then fine. Life goes on.
If T200 expires (no ack received), then BTS sends the initial message one more time.
It will then wait T200, for the ACK.
Still no ack ? Then the process is repeated N200 times (N200 = 12, by default).
So 12 attempts to send 1 message, still no ack received… then the Abis link is assumed to be dead, and all communications are lost in the BTS. TRX, Abis and BSC boards are reset (depends on vendor behaviour).