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 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.
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.
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.