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S1-AP Functions: E-UTRAN Radio Access Bearer (E-RAB) management Procedures (Setup, Modify and Release)

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Definition of S1:
S1: logical interface between an eNB and an EPC, providing an interconnection point between the E-UTRAN and the EPC. It is also considered as a reference point.

We know that S1-Application Protocol (S1-AP) provides the signalling service between E-UTRAN and the Evolved packet core (EPC).

Key features of S1-AP:

  • NAS messages encapsulation / decapsulation and delivery to NAS protocols.
  • S1-association and SON-related procedures and messages (e.g. S1-setup, eNB/MME configuration update/transfer, overload start/stop)
  • UE specific procedures and messages (e.g. registration/UE context related-, idle mode related- , handover related-, bearer related- signaling)
  • Part of MME emulator and SON-enabler

    S1-AP has various functions. One of them is E-RAB management function. Which consists of E-RAB setup, modify and release.

Definition of E-RAB:
An E-RAB uniquely identifies the concatenation of an S1 Bearer and the corresponding Data Radio Bearer.
When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer of the Non Access Stratum (NAS).

E-RAB Setup:
E-RAB set up request is initiated by MME towards E-NodeB. Based on the information contains this message.E-NodeB will assign the Data Radio Bearers to UE (Uu Interface). For each E-RAB and based on the E-RAB level QoS parameters IE the eNB shall establish a Data Radio Bearer and allocate the required resources on Uu. The procedure uses UE-associated signalling.

E-RAB setup request

E-RAB Modify:
The purpose of the E-RAB Modify procedure is to enable modifications of already established E-RABs for a given UE. This message is also initiated by MME, Whenever there is an requirement to change the QoS parameters for UE. MME will initiate this message and according to this E-NodeB will made the changes.

E-RAB Modification Request

E-RAB Release:
The purpose of the E-RAB Release procedure is to enable the release of already established E-RABs for a given UE.The E-RAB RELEASE COMMAND message shall contain the information required by the eNB to release at least one E-RAB in the E-RAB To Be Released List IE. If a NAS-PDU IE is contained in the message, the eNB shall pass it to the UE.
Upon reception of this message the E-NodeB will execute the release of data bearer associated with those e-rabs.
E-Rab release

posted Apr 24 by anonymous

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1> S1 Interface protocol which is the interface between E-EUTRAN and Core Network.
2> S1 Interface is a point - to - point logical interface which is based on IP Network.
3> S1 Interface includes both protocol for Control plane signalling and Data Plane.

S1 - Control Plane Interface

1> The interface between eNodeB and MME and responsible for control plane signalling.
2> It is also known as S1 - Application Part ( S1-AP / S1 - MME ) Protocol.
3> It uses S1-AP protocol and SCTP protocol as transport layer.

S1 - Data Plane Interface

1> The interface between eNodeB and S-GW is responsible for data transfer.
2> It is also known as S1 - User Part ( S1-U ) or GTP - U.
3> It uses GTP-U protocol and used UDP as transport layer protocol.

3GPP Specs to refer for S1 Interface

36.410 :- S1 General aspects and principles
36.411 :- S1 Layer 1
36.412 :- S1 Signalling transport
36.413 :- S1 Application protocol
36.414 :- S1 Data transport

S1 : Network Entity Interface View

1> S1 - Interfaces exist between E-EUTRAN(eNodeB) and Core Network (MME / S-GW)
2> eNB is responsible for MME selection and MME selects the S-GW and notifies to eNB.
3> Multiple S1-AP / S1-MME interface exist i.e A eNB can have multiple S1-AP to MMEs and A eNB can have multiple S1-U to S-GWs.

S1 Interface High Level View

4> Refer the upper image to see the interface's possible between multiple eNB, MME, S-GW.

S1 : Protocol Interface View

1> Protocol exist for both Control plane (eNB - MME) and User plane (eNB - S-GW).
2> Refer the following image to see the protocol interface.

LTE S1 Interface

3> S1 - AP uses SCTP as Transport layer and S1-U uses UDP as transport layer.
4> The standard receiving port for S1-AP over SCTP is 36412 and for S1-U/GTP-U over UDP is 2152.
5> L1 represents Physical layer and L2 represents Data link layer same as OSI model.