Basic difference between R2 and C7 is in thier signalling. R2 has to have 16th TS for signalling reserved whether you need or dont. Where as in C7 there is no rule that a signalling slot has to be allocated. It can be used for traffic too. For example if you have 5 E1 and only 1 signalling is sufficient..you can use the other 4 E1s without alloting the TS for signalling.