long SDCCH duration can be due to various things : high amount of FDR, Location updates, SMS, …
in your opinion, do you think the main problem lies in the SDCCH duration or in the TCH duration ? For me (maybe i’m wrong) it’s the TCH behaviour that shall be fixed first hand.
i think if you can know where the calls are going and why they’re going in these particular target cells, it will solve your problem of short TCH duration and, as a side effect, your problem of long SDCCH.