you have to keep in mind that indicators and qos is not standard, so each vendor may have a different approach. So take my words with perspective…
TCH assignment is the phase during call setup when the BSS wants to shift the MS from SDCCH to TCH.
The TCH assignment is divided in 2 parts :
1- the BSS checks that a timeslot TCH is available in the serving cell (or in neighbor cells, with directed retry)
2- the BSS asks the MS to move onto the TCH timeslot and to send an initial confirmation message while on this TCH timeslot.
In part 1 : there is a TCH REQUEST, which leads to a TCH ALLOCATED (= available TCH) or a TCH REJECTED (= no TCH availble)
In part 2 : the MS will try to access this TCH. If the dialog BTS/MS onto this TCH is possible, this is a TCH SUCCESS. If the dialog is not possible, this is a TCH FAILURE.
See, there are three main counters here :
TCH REQUEST -> TCH ALLOCATED -> TCH SUCCESS.
In your vendor, they might have different names, but the spirit should be the same. Try to find on which Abis or A interface message they are linked.
The TCH CONGESTION occurs only in phase 1.
The TCH FAILURE occurs only in phase 2 (with most common problems such as RADIO or HARDWARE PROBLEMS)
The ratio TCH SUCCESS / TCH REQUEST gives you the TCH UNSUCCESS RATE, which the KPI you should monitor.
Do you need more details ? I’ll write you the protocol messages that are linked to requests, allocations and successes, to make it clearer, if you want.
I hope it helps,