Re-Synchronization mechanism is a rare.It happens when difference of sequence number at HSS and UE/SIM is not in valid range. Before we go in detail of it, it would be better we just scroll through 3G Authentication.
Here we shall explain Re-synchronization with example. As we know Sequence Number is of 48 bits
SQN=(SEQ (43-Bits)+ IND(3-Bits))
Ideally HSS should have high sequence number(SEQ) at any (IND)index. Lets assume if sequence number difference is equal to and more than 2, then it is needed to synchronize again with HSS. This range is called DELTA range that is defined by SIM vendor.
During re-synchronization process UE/SIM send it's own sequence number to HSS, HSS updates it own end, generates fresh Authentication Vectors (AV) on updated Sequence Number, then to UE to authenticate Identity and Network again.
Re-synchronization is a very useful process by which we can Sync sequence number at HSS with SIM because we can not update sequence number at SIM because it is stored in non-volatile memory. Ideally both HSS and SIM should have Identical Sequence Number for an Index, but it is not always true because of following causes that could create Re-Synchronization.
1) As we know controlling entity (MME/SGSN) can ask for multiple Authentication Vectors. There are chances that some vectors are not used and got staled, this will increase the sequence number for some IND (Index) at HSS but not at SIM shall result in Re-Synchronization if difference reaches to 2. I.e. Sequence Number for a Index at HSS very high.
2) Sequence Number at SIM/UE for a Index is Higher than the HSS. Only possible to simulate in LAB network.
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