HSS-initiated Detach
Figure 4 provides an illustration of how HSS initiates detach after detecting detach triggering.
Figure 4. Procedure for HSS-initiated Detach
Detach Triggering by HSS
When detach is triggered at HSS due to subscriber withdrawal, the HSS attempts to delete the user’s MM context and EPS bearer immediately.
1) [MME ← HSS] Detach Request
The HSS and the MME communicate with each other over S6a interface using Diameter protocol. The HSS requests the MME for detach of the user by sending a Cancel Location Request (CLR) message with the following parameters:
❷ EPS Session Termination
Upon receipt of the Cancel Location Request (CLR) message from the HSS, the MME releases all the resources previously allocated to the user. Steps for such procedure are the same as in MME-initiated detach (Figure 3) described in Chapter III, except that an additional action is required by the MME in Step 2). The MME needs to send the HSS a Cancel Location Answer message as a response to the request made in Step 1).
2) [MME → HSS] Responding to Detach Request
After receiving the Detach Accept message from the UE, and the Delete Session Response message from the S-GW, the MME sends a Cancel Location Answer message to the HSS as a response to the Cancel Location Request message sent in Step 1).
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