Please explain me the scenario when eNB talk to S-GW?

Question

i would to know when eNB contact to SGW (using S1-U interface) in which scenario.

As i read eNB sends Attach requirest to MME & after that MME select the SGW based on TAI & PGW based on APN--- this is one scenario of establishing the session, but when S1-U interface will be used & when eNB talk to SGW is my doubt ...

Answer 1

Protocol stack point of view, S1-U is the interface between eNodeB and SGW and it is based on GTP-U. This interface is used for data flow. Now coming to your query, when an eNB talk to SGW. Answer is when an eNodeb wants to forward uplink data to network. Since SGW is immediate node which eNodeB knows for data plane so its forwards data to SGW using GTP user plane tunnels.

Each bearer has its own tunnel end identifier and it is communicated to eNodeB via MME by using Initial context setup request or E-RAB setup request messages.

References:
3GPP Technical Specification 36.413
3GPP Technical Specification 29.272

If you still have any doubt or want more clarification, please feel free to ask.

Answer 2

On behalf of Bart
First is downlink packet. The needed info for SGW to address the UE is shared through MME.