Vikas,
you are confusing a physical channel (RACH) and a message (CHANNEL REQUEST).
The RACH carries the CHANNEL REQUEST.
CHANNEL REQUEST is a 8-bits message (as you said).
FACCH is a physical channel. But it contains only 1 bit. So the message is either 0 or 1. If it’s = 1, then the content of the TCH should be decoded as signalling message (such as HO COMMAND)
SDCCH is a physical channel. It can carry many different messages (immediate assignment, etc…) you’ll find a description of each of them on the 3GPP website.