UMTS:
Within the UMTS Radio Access Network (UTRAN) the so-called “counting” or “re-counting” procedures can be executed to determine the number of terminals in each cell. “Counting” is initiated by the RNC as soon as the RNC needs to know the amount of active UEs that want to receive a specific MBMS service. This is used to determine the optimum transmission bearer, Point-to-Multipoint (P-t-M), Point-to-Point (P-t-P) or no transmission at all for a given MBMS service in the considered cell.
The need for counting is indicated on the MCCH channel per MBMS service. For the counting, the UEs need to reveal their interest to receive an MBMS transmission to the RNC. In case a large number of UEs wishes to receive the service, one has to avoid that all UEs start signaling at the same time, in order to prevent network overload. Therefore, a probabilistic response to the counting indication is used.
When the MBMS counting indication is received, the UE performs a random decision that with a probability equal to the probability factor contained in the counting indication message will generate a counting response from the UE, or with the complementary probability will not generate a counting response. In the latter case the UE continues monitoring the MCCH until the end of the MCCH modification period and if another counting indication is received repeats the random decision procedure. If the decision is to generate a counting response, the UE uses existing signaling procedures to indicate its presence (RRC connection establishment for idle UEs and Cell Update for UEs in URA-PCH state).
The MBMS counting procedure is terminated if the UE detects that the network has stopped sending the counting indication on the MCCH.
For LTE procedure should be same but I am not very sure.
