it is the story of a MS that does a call setup…
after the sdcch phase, the BSC looks for a TCH to give to the MS : that is counted as a “TCH REQUEST”.
if the BSC finds a TCH in the cell, then it will ask the BTS to do the channel activation. It is only possible if the cell is not congested, and if there are no Abis failures, or other software failures.
This is counted as a “TCH ALLOCATION”.
If the cell is congested, the BSC doesn’t send the channel activation, and there is a “TCH ASSIGNMENT FAILURE CONGESTION”. The KPI “TCH ASSIGNMENT PREPARATION SUCCESS RATE” decreases.
Once the TCH is allocated, the BSC sends the Channel Assignment message to the MS. The MS then tries to hop onto this TCH ts.
If it is succesful, then it is counted as a “TCH SUCCESS”.
Otherwise, it is probably because there was a radio issue… then it is counted as TCH ASSIGNMENT FAILURE RADIO…. and the KPI “TCH ASSIGNMENT EXECUTION SUCCESS RATE” decreases.
These problems also occurs when the TRX on which the TCH is ALLOCATED suffers from imbalance. Then the executions are probably going to fail, because the imbalance actually translates into “radio problems”.
Hope it clear things up…