What is the S11 interface in LTE? 

Earlier, we discussed some interfaces (e.g., s6a ) in LTE-EPC for providing data services to mobile phone subscribers. In this tutorial, we will discuss one more interface, which is known as s11. It is between MME and the SGW.

s11 GTP interface in LTE
s11 GTP interface in LTE

Both MME and SGW are in the roaming network. The s11 uses the GTP-C V2 protocol. It is similar to the Gn interface between PGW and SGW for control plane signaling with additional messages for device status, paging, etc.

GTP is a tunneling protocol that sets up a logical tunnel between eNodeB and SGW for data. Similar to other LTE interfaces, GTP uses standard UDP port 2123 for the control plane and 2152 for the data plane.

How does MME select an SGW for a subscriber (UE)?

MME interacts with the SGWs in a one-to-many fashion. This means it can use different SGWs to manage sessions. The serving SGW selection is made based on the tracing area code. A single SGW is assigned to all eNodeBs in tracking.

When a UE enters that tracking area, MME transmits a session and additional signaling to the assigned SGW.

What is S11’s role in a data session and UE management?

The UE connects to the MME via eNodeB to obtain a data service. This is accomplished by establishing a session with a default bearer.

The first step in creating a session is to authenticate and do an Update Location with the HSS over the diameter-based S6a interface to obtain authentication, APN, and QoS information.

With APN, it can determine the PGW, and then MME starts a GTP session and modification with the PGW.

List of messages for session management and default bearer creation.

  1. Create Session request/response.
  2. Delete Session request/response.
  3. Modify-Bearer request/response.

List of messages for paging and UE status handling.

  • Downlink data notification – To start paging at UE
  • Release access bearer – For a UE when it’s in sleep mode.

Other protocol messages.

Procedures on the S11 interface.

  • Handover (S1, X2, and UE)
  • Default Bearer Creation with a session.
  • Delete a session along with the default bearer.
  • Create a dedicated bearer.
  • Modify a dedicated bearer.
  • Delete a dedicated Bearer.