LTE Detach and its Types -Part-5

6. EPS Entity Information: Before/After Detach

This chapter explains how information in EPS entities is changed after “EMM Case 2: Detach” procedures are performed. All the information in each entity is categorized into UE ID, UE Location, Security and EPS Session/Bearer information (see [2] for more information).

6.1 Before Detach

According to the EMM Case 2 scenario, a user stays in ECM/RRC-Connected state before he detaches or is detached from the network. Therefore, before detach, all the EPS entities have the same information they initially had after initial attach in EMM Case 1 (see [2] for more information). Figure 5 lists the information stored in each EPS entity before detach is performed.

Figure 5. Information in EPS Entities before Detach

6.2 After Detach

After detach, information that can be used for the user’s fast and secured attach next time is stored in UE and MME. All other contexts related to the user, such as NAS security context, GUTI and TAI information allocated by MME, etc., are deleted. Figure 6 lists the information elements stored in each EPS entity after “EMM Case 2: Detach” procedures. Ones that are to be deleted after detach are shown in gray, provisioned ones that are to be kept are in black, and ones to be used in next attach are in blue.

Figure 6. Information in EPS entities after Detach

Information elements kept for next attach are stored as follows:

At UE:

• UE ID: GUTI allocated by MME at the time of initial attach or TA updates
• UE Location: The last TA that UE visited before detach
• Security: NAS security context information that UE used before detach

At MME:

• UE ID: IMSI that UE provided at its initial attach, and GUTI allocated by MME at the time of UE’s initial attach or TA updates
• UE Location: The last TA that UE visited before detach. The TAI list allocated to UE may be kept as well.
• Security: NAS security context information that UE used before detach
• EPS Session/Bearer: Subscribed profile received from HSS at the time of UE location registration. The UE-AMBR value determined by MME at the times of EPS bearer establishment, and the APN-AMBR value used in UE-AMBR may be kept as well.

At HSS:

• UE Location: The last MME UE was registered at before detach

The End

We have learned how a user, while connected to the LTE network, detaches/is detached from the LTE network (“EMM Case 2” in [1]). We categorized detach cases depending on the entity that detects detach triggering, and looked into the detach procedures in each case. We also checked, after detach, what kinds of information elements are stored at UE, MME and HSS for later use at next attach. The subsequent document will discuss how S1 resource is released when a user, after initial attach, stays inactive for a certain period of time (“EMM Case 3. S1 Release due to User Inactivity” in [1])