In GSM there is basic two type of frequency hopping .
1) software hopping in which ur timeslot of Particular TX will hop
2) Hardware hopping in which ur Frequency will change on TRX basis.
Hopping we are implementing to avoid fading only.It is not neccesary to implement hopping in network. if u have very less frequency then it is very useful.
There is manh hopping seq. u kn define so out of that many seq. which seq. u want to apply to particular TRX is given in HSN(Hopping sequence no.) and MAIO is provided when u want to give offset in particular TRX or sector otherwise it can be default.
if any query let me know.