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LTE: What is NAS COUNT wrap around ?

+1 vote
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What does it mean for MME and UE ? what are action need to do ?

posted Oct 17, 2013 by Vikram Singh

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1 Answer

+2 votes

I hope I understood your question correctly -

There are two separate counters NAS COUNT: one related to uplink NAS messages and one related to downlink NAS messages. The NAS COUNT counters use 24 bit internal representation and are independently maintained by UE and MME. The NAS COUNT is constructed as a NAS sequence number (least significant bits) concatenated with a NAS overflow counter (most significant bits).

The NAS sequence number part of the NAS COUNT is exchanged between the UE and the MME as part of the NAS signalling. After each new or retransmitted outbound security protected NAS message, the sender shall always increase the NAS COUNT number by one. Specifically, the NAS sequence number is increased by one, and if the result is zero (due to wrap around), the NAS overflow counter is also incremented by one. The receiving side estimates the NAS COUNT used by the sending side. Specifically, if the NAS sequence number wraps around, the NAS overflow counter is incremented by one. (Check 124.301 section 4.4.3.5 for what happen if receiver estimates NAS COUNT wrap around)

answer Oct 17, 2013 by Luv Kumar
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