LTE: Why does SGW include S5/S8 interface PGW-C F-TEID in Create Session Response message to MME ?

Question

As per LTE core network architecture, MME shares S11 interface with SGW and SGW has S5/S8 interface with PGW.
I went through Create Session Response message sent by SGW to MME and found PGW-C and PGW-U tunnel ids.
I want to know what MME does with these PGW-C and PGW-U tunnel Ids ?

Answer 1

During S1-Based/X2 handover SGW may change but PGW will remain same. In this case MME will send create Session Request(SGW control plane tunnel ID will be zero) with valid PGW tunnel Id's (pgw control plae tunnel Id and user plane tunnel Id) to new SGW. Then SGW will create S1-U uplink tunnel Id, S5-U downlink tunnel Id and SGW control plane tunnel Id. Now, SGW will send Modify bearer request(with received PGW-C tunnel Id from MME) with SGW S5 user plane tunnel Id to PGW and also it will send create session response with S1U user plane TEID to MME. PGW will respond back with modify bearer response to SGW.

So, In the above scenario, SGW need not be send create session request to PGW to create tunnels again for the same UE and APN. It will reduce the signaling. So, for each successful bearer creation(default/dedicated) - MME will store all all GTP tunnel ID's. (SGW control and user plane TEID's, PGW control and User plane TEID's)

Comments

Thanks Rajeswararao to provide such a detailed explanation on this. I believe only experts can answer such a tricky question.