Few more things are LTE below. Find part 1 here.
UE network attach:
We all know how important IMSI is and we wouldn't want to spill it out anytime. This requirement brings in a second type of attach procedure in LTE. Usually attach request from UE contains IMSI so that network can validate the same. Once network knows the UE's IMSI, it will generate a Globally Unique Temporary Identifier (Find details here) and assign it to UE. So for further communication UE can use GUTI instead of IMSI.
Why this? The Attach Request sent to the network is not encrypted. Security procedures are done only after network receives Attach Request. This means if IMSI is sent in Attach Request its susceptible to be known to outsider. To avoid this UE may send Old GUTI to network in Attach Request(The term "Old" is used here because it is assumed that UE is moving from one MME to other MME). Once the network receives that GUTI it will map the IMSI from it. Problem solved. What if the network doesn't know the GUTI, then it will have to ask the UE to send its IMSI. Network sends out an Identity Request to UE asking for its IMSI, to which UE will respond with Identity Response with its IMSI. Note that this message is not encrypted.
GUTI is assigned by the network. Which means UE should send its IMSI to network at-least once. If UE moves from one MME to other then its responsibility of MME to get the UE's IMSI from other MME based on the GUTI which UE has sent. Identity Request (for IMSI) should be sent only if MME fails to retrieve the IMSI.
Inter Rat Handovers:
There is special section in 3GPP TS 23.401 which we tend to ignore. Appendix. :-) If we look at inter handover requirements it is quite clear that SGSN's should be upgrade with new version of GTP protocol. But this Appendix has a different information altogether. Refer to Appendix 5 in the spec. Checkout the Routing and Tracking Area update procedures. They both speak of GTPv1 rather than GTPv2. This means MME has to support GTPv1 too. This sucks. Anyway, I still have my fingers crossed about inter RAT handovers. Hope they will be seems less.
More later. Comments/Corrections are welcome.
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