Let's say that there has been a handover from LTE to 3G of a particular UE. Once the handover is complete, the MME is no longer required to maintain the sessions/bearers that has been established for that particular UE. I have found the following call flow in a reference, which is not so reliable.
1. eNB ⇒ MME S1AP S1 UEContextReleaseRequest 2. MME ⇒ SGW GTPv2 S11 Release Access Bearers Request 3. SGW ⇒ MME GTPv2 S11 Release Access Bearers Response 4. MME ⇒ eNB S1AP S1 UEContextReleaseCommand 5. eNB ⇒ MME S1AP S1 UEContextReleaseComplete 6. SGSN ⇒ MME GTP Gn SGSN context request 7. MME ⇒ SGSN GTP Gn SGSN context response 8. SGSN ⇒ MME GTP Gn SGSN context acknowledgement 9. MME ⇒ MSC SGSAP SGs SGsAP-EPS-DETACH-INDICATION 10. MSC ⇒ MME SGSAP SGs SACK SGsAP-EPS-DETACH-ACK 11. MME ⇒ SGW GTPv2 S11 Delete Session Request 12. SGW ⇒ MME GTPv2 S11 Delete Session Response
Given the scenario, some of these calls make sense to me. For an instance, once the handover to 3G is done, there is no point of having the access bearers S1 between eNB and S-GW and S5/S8 bearer between S-GW and P-GW and so forth. So, the
Release Access Bearer Request in 2, and
Delete Session Request at 11 are sensible.
What I particularly don't understand is that, why are calls 6,7 and 8 there? What purpose do they serve? Or is this entire flow blatantly wrong? If it is blatantly wrong, where can I find something sensible to read about what really happens? I have combed through the 3GPP specs for weeks now, but still couldn't find answers to the questions above.
Okay, after looking at some PCAPs, the purpose of the calls 6,7 and 8 is also clear. Upon falling back to 3G, the SGSN is going to require the UE context at MME, in a format that SGSN can understand. To an SGSN, MME looks like another SGSN. So it requests, the context from there over S3 interface. However, the call flow above says the message exchange between MME and SGSN happens over Gn, that which contradicts with the the 3GPP spec, that which according to, the Gn interface is used to establish the connection between SGSN and GGSN. Now, the new question is going to be on the accuracy of the flow. Is this call flow accurate?