you forgot to mention the direction of the HO : which side is ALU ? is the problem in both direction ? which vendor is on the other side ?
the usual suspect is the core network… especially if there wasn’t any change on the BSS.
In the core network, they might have performed an upgrade of some kind, but wouldn’t consider it as a possible cause of problem 🙂
anyway, since HO CMD is sent to MS, I think all routing is well performed within your NGN. It looks like the channel request from the MS is not what the NGN expected… If I were you, I would double-check with NGN if they did anything at the hour when the problem appears
-> yes, btw, did the problem start on 3 BSC at the same hour on same day ? that could help the diagnostic 🙂
-> are you able to see the problem in the qos stats? can you verify that the problem is contained to only 3 BSC ?
-> check again the direction of the problematic ho’s.