Mapping between P-TMSI and GUTI during inter RAT handover

Section describes usage of GUTI/P-TMSI as part of Inter RAT Handover.

GUTI:

GUTI is an identifier for the UE allocated by MME so that IMSI is not required to be transmitted over radio interface (where it is susceptible to interception). Structure of GUTI is designed such that it is possible to identify the node (MME) that allocated it.

GUTI = MCC + MNC + MME ID + M-TMSI

MME ID identifies an MME in the operator’s network and M-TMSI identifies a UE served by that MME.MME ID contains MME group id which is used to identify MME pool and MME code which is used to indicate a node within the pool

PTMSI:

P-TMSI (Packet domain TMSI) is an identifier for the UE allocated by SGSN. P-TMSI contains NRI (Network resource identifier) which is used to identify the node that allocated it. In 3G, LAC (present as part of RAI) is used to identify the SGSN pool and NRI is used to identify a node within the pool.

Note:  Mapping between GUTI and P-TMSI/RAI is done such that the information regarding the allocating node is still retained.

Usage of GUTI/P-TMSI during Inter-RAT handover (4G – 3G- 4G):

•  As part of Attach procedure, MME allocates a GUTI to the UE (Attach Accept).  
•  UE moves from 4G to 3G network.
• UE includes a P-TMSI and RAI values derived from GUTI as part of RAU request to SGSN as old P-TMSI/old RAI.
• SGSN uses the P-TMSI to identify the associated MME and requests the MME to provide UE context information.
•  MME maps the P-TMSI to GUTI to identify the subscriber and passes the necessary information to SGSN.
• Once the context transfer is done, SGSN assigns a P-TMSI.
• UE stores this information as part of its 3G subscriber context and uses for all subsequent 3G procedures.
• When UE moves from 3G to 4G, UE sends GUTI information mapped from P-TMSI as part of TAU request (old GUTI). Further since UE has a valid GUTI, UE sends this information as part of TAU request as additional GUTI.
• MME uses P-TMSI and RAI mapped from GUTI to identify the responsible SGSN and sends a request to perform context transfer including the P-TMSI.
• SGSN uses P-TMSI to identify the subscriber and passes necessary information to MME.
• Further MME checks if the GUTI passed as additional GUTI is valid. If it is not valid, a new GUTI is allocated for the subscriber and sent as part of TAU accept.
• It must be noted that MME is aware that it communicates with a 3G SGSN. However SGSN communicates to MME as if it is communicating to another SGSN. Hence only MME will do the mapping (P-TMSI <-> GUTI) in the network whenever necessary.
• The values of LAC and MME group id shall be disjoint, so that they can be differentiated. The most significant bit of the LAC is set to zero and the most significant bit of MME group id is set to one. This is required because MME should be able to identify from the old GUTI if it needs to talk to SGSN or another MME.