Hi olivier,
the channel required is an abis message –> a vendor can modify quite extensively the 3GPP in order to fit its needs.
For example, in case of an 11-bits access burst (= egprs channel request, with a radom access ref of 11 bits), the RA field is not big enough to carry the whole info (only 8 bits !)
So an optional field at the end of the Channel Required might be used.
It’s vendor dependent.
Cheers,
Pix