The data sent to the different eNBs from BM-SC via MBMS-GW need to be sent through different eNBs providing overlapped coverage to a UE on the same radio resource (SFN/SF) which is configured for MBSFN transmission. Since BM-SC is not aware of the radio resource sequence on the eNBs, it estimates a transmission time but it needs to be ensured that this timed data packet goes on the same radio resource from all the eNBs in the same MBSFN Area. This is ensured through the time-stamp send along-with data in the SYNC PDU. This time-stamp is a value relative to a common time source and provides an indication to the eNBs regarding the next occurring MBSFN radio resource on which it is to be scheduled. The time reference is provided by the synchronization period and the time instance with respect to this time reference (synchronization period) is provided by the synchronization sequence. Why sync sequence - because it corresponds to a burst of broadcast data and so in effect the sync sequence is providing the start of this burst. The sync period is configured both at BM-SC and MCE by the operator and all eNBs and BM-SC are time-synchronized to each other within 10-16 usecs (so that they see the same time reference). Also, BM-SC for any arrive data packet, estimates the sync sequence based on transport latency to the eNBs of MBSFN area and other processing times.
So for eg: let us say GPS epoch (6th Jan, 1980, 0 hours) is taken as the start of sync period with a periodicity of 600 secs (10mins). And for the MBMS service, the bcast burst periodicity is 50 ms (i.e. sync sequence would be updated every 50ms, ie increment by 5 since granularity is 10ms). So, say 10 data packets arrive every 50ms to be broadcasted. All these 10 data packets would have the same sync sequence. Also, let us say in the current sync period ("X"), first of these 10 data packets arrive at 10 secs (=1000 * 10ms) and BM-SC has estimated a delay of 50ms (=5 * 10ms) to eNBs. Then BM-SC sets the sync sequence value (ie the timestamp of transmission of 1st packet of this burst, relative to the start of sync period "X") to 1000 + 5 = 1005 and sets the same as timestamp in SYNC PDU and sends to eNBs (over multicast transport). Now assuming that this packet arrives in 30ms at eNBs, then eNBs would buffer the data in SYNC PDU for at least another 20ms and after that in next MBSFN radio resource, sends the data packet over the radio interface to the UEs.
So in summary, synchronization period provides a time reference and synchronization sequence provides the start time of the burst (sequence) with respect to the current synch period.
