you understood my question exactly ! 😉
We did not know that the BTS is able to sent the ack in an earlier frame (frame 15 in your example) – we thought that the earliest acknowledge is possible in the next 51er frame. Do the UL and DL SDCCH frames have different “starting points” which leads to this “gap” ?
We made several investigations with different T200 settings. The SDCCH dropping seems to be reduced (SDCCH radio failures decrease in the counters) in Nokia and Siemens if the timer has larger values. From our point of view this must be related to bad HF conditions: for example the MS can’t hear the BTS, several retransmissions are done and the connection is ended because of Layer 2 failure (after N200+1*T200). If T200 is increased the chance to get better HF conditions (while MS is moving around) is higher because the “tear down time” for Layer 2 is increased as well.
Cheers and Thank you for replying