LTE: What is the significance of S5 interface between L-GW and SGW ?

Question

I have one doubt over S5 interface. Interface between SGW and PGW in lte core known as S5 interface. Now I found again S5 interface between L-GW( which is co-located with HeNB ) and SGW of core network. Can someone please explain why this interface is required ?

Answer 1

The S5 interface between the L-GW and S-GW is required to support a necessary set of procedures from the S5 interface between the S-GW and the P-GW, including Paging, Create Session Request.
Procedures for setting up the S5 interface between the L-GW and S-GW, the subset of procedures to be supported on this interface, and details of IEs to be included in the exchanged messages have to be defined.

Answer 2

For implementation of Local IP Access(LIPA) feature in a femto cell network, collocated LGW is used. It basically acts as a PGW but just connected to a local network. So when ue wants to connect to this local network, the data path is UE-HeNB-LGW-Network. It is a offloading method to reduce core network traffic. But signalling messaging are done by core network.

Now LGW is connected to SGW via S5 interface. It has a mostly dormant tunnel ,which is only used when a data packet for a IDLE ue arrives from the local network to LGW . After finding that UE is idle LGW buffers the data and sent a Dummy packet to sgw to trigger paging. SGW informs MME , which informs HeNB to awake the idle ue. after Radio bearer is established. LGW sends the buffered data.

To SGW, the whole process is transparent in the sense that it does not know that it is a LGW and as if a pgw was sending data to initiate paging.