4. MME-initiated Detach
Figure 3 displays how MME-initiated explicit detach is performed. The detach procedure for this type of detach begins when detach triggering is detected at MME, and thus the MME sends a Detach Request message to the UE. The procedure ends when the resources previously allocated to the UE’s EPS session are released.
Figure 3. Procedure for MME-initiated Detach
Detach Triggering by MME
Below is a description of the procedures to be performed after MME detects detach triggering, and before the EPS session termination procedure is carried out. If the user is in Idle state at this time, the MME performs paging to establish S1 signaling connection (Detailed paging procedures will be explored in our technical document “EMM Procedure 4. Service Request due to New Traffic”, and hence will not be discussed here).
1) [UE ← MME] Detach Request
As it is explicit detach, the MME sends a Detach Request message to request the UE for detach. Message parameters are as follows in case a detach request is sent by MME to UE:
In case of implicit detach, however, MME does not send a Detach Request message to UE.
2) [MME] Handling Security Context
After sending the Detach Request message to the UE, the MME stores the current NAS security context in use before deleting the EPS session. Next time the UE re-attaches, the MME can use the stored context again and skip the authentication and NAS security setup procedures for the user.
3) [UE] Noticing Detach Intent and Handling Security and Bearer Contexts
After receiving the Detach Request message from the MME, the UE becomes aware of the MME’s intent to detach. It checks the type of the intended detach to see whether or not re-attach is required after detach. Then it stores the current NAS security context and deletes the EPS bearer context.
❷ EPS Session Termination
Once the MME stored the NAS security context upon perceiving detach triggering, it requests the P-GW for termination of the user’s EPS session. This request triggers PCEF (P-GW)-initiated EPS termination, releasing all the network/radio resources allocated to the user, as to be described below.
(1) EPS Bearer Release and PCC Rule Removal
Through Steps 4) ~ 13), the MME requests for termination of the user’s EPS session, the PCRF deletes PCC rule upon the request, and S5 bearer resources are released, as in Steps 4) ~ 13) in Chapter III. In case of a detach type that requires re-attach, the MME can save the current UE-AMBR value in Step 5) so that the UE can establish an EPS bearer faster next time it re-attaches.
4) [UE → MME] Acknowledging Detach
After storing the NAS security context and deleting the EPS bearer context upon receipt of the Detach Request message from the MME in Step 1), the UE sends the a Detach Accept message as a response to the request in Step 1). In case of implicit detach, Steps 1), 14) and 16) are skipped.
(2) S1 Signaling Connection Release
In this phase, the MME releases any unreleased resources (S1 signaling connection, RRC connection and UE context left in the eNB) after receiving the Detach Accept message from the UE and the Delete Session Response message from the S-GW. Steps in this phase are the same as Steps 15) ~ 18) in Chapter III, except that in case the detach type is set as “re-attach required”, UE re-attaches the network after RRC connection is released.
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