Few Questions on Bearer

What is the EPS Bearer?

For E-UTRAN access to the EPC the PDN connectivity service is provided by an EPS bearer for GTP-based S5/S8, and by an EPS bearer concatenated with IP connectivity between Serving GW and PDN GW for PMIP-based S5/S8.

An EPS bearer uniquely identifies traffic flows that receive a common QoS treatment between a UE and a PDN GW for GTP-based S5/S8, and between UE and Serving GW for PMIP-based S5/S8.

EPS Bearer is a virutal connection between UE and PGW which identifies a data send and received between these two end points with specific QoS attributes.

The procedure used to establish an EPS Bearer is called “EPS Bearer Activation” procedure. Either endpoint can trigger this procedure. For example, the PDN-GW may trigger this procedure when it determines that a new VoIP call is requested and a new EPS Bearer should be established to support this call.

What is Default Bearer and Dedicated Bearer?

Default Bearer

One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of the PDN connection to provide the UE with always-on IP connectivity to that PDN. That bearer is referred to as the default bearer.

Dedicated Bearer

Any additional EPS bearer that is established for the same PDN connection is referred to as a dedicated bearer.

Are default bearers created on a per UE or a per PDN (Packet Data Network) basis?

Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers. The first default bearer will be established during the Attach process when the UE first powers up and the second will be done using Activate default EPS bearer context request procedure. When the second default bearer is established it will be dependent on the service and the UE.

LTE Bearer Service Architecture

- An E-RAB transports the packets of an EPS bearer between the UE and the EPC. When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer.

- A data radio bearer transports the packets of an EPS bearer between a UE and an eNB. When a data radio bearer exists, there is a one-to-one mapping between this data radio bearer and the EPS bearer/E-RAB.

- An S1 bearer transports the packets of an E-RAB between an eNodeB and a Serving GW.

- An S5/S8 bearer transports the packets of an EPS bearer between a Serving GW and a PDN GW.

- A UE stores a mapping between an uplink packet filter and a data radio bearer to create the binding between an SDF and a data radio bearer in the uplink.

- An UL TFT in the UE binds an SDF to an EPS bearer in the uplink direction. Multiple SDFs can be multiplexed onto the same EPS bearer by including multiple uplink packet filters in the UL TFT.

- A DL TFT in the PDN GW binds an SDF to an EPS bearer in the downlink direction. Multiple SDFs can be multiplexed onto the same EPS bearer by including multiple downlink packet filters in the DL TFT.

- A PDN GW stores a mapping between a downlink packet filter and an S5/S8a bearer to create the binding between an SDF and an S5/S8a bearer in the downlink.

- An eNB stores a one-to-one mapping between a data radio bearer and an S1 bearer to create the binding between a data radio bearer and an S1 bearer in both the uplink and downlink.

- A Serving GW stores a one-to-one mapping between an S1 bearer and an S5/S8a bearer to create the binding between an S1 bearer and an S5/S8a bearer in both the uplink and downlink.

Dedicated bearer activation in combination with the default bearer activation

Dedicated bearer activation in combination with the default bearer activation at attach or UE requested PDN connectivity

It is possible for the PDN GW to initiate the activation of dedicated bearers as part of the attach procedure or as part of the UE requested PDN connectivity procedure over E UTRAN. However, the result of the dedicated bearer activation procedure is separate from the Attach procedure, meaning that the result of the Attach procedure is not dependent on whether the Dedicated bearer activation procedure succeeds or not. On the other hand, the dedicated bearer activation may only be regarded as successful if the Attach procedure completes successfully.

The messages of the Dedicated bearer activation can be sent together with the messages of the Attach procedure or of the UE requested PDN connectivity procedure (i.e. Attach accept or PDN Connectivity Accept).

If the MME has sent an Attach Accept message towards the eNodeB, and then the MME receives a Create Bearer Request before the MME receives the Attach Complete message, the MME shall wait for the Attach procedure to complete before the MME continues with Dedicated Bearer Activation procedure.

It is possible that multiple dedicated bearers can simultaneously be activated in the signalling flow shown below.

Figure describes the activation of dedicated bearer(s) in combination with the default bearer activation either as part of the Attach procedure (with specific steps 1a, 7a, 10a) or as part of the UE requested PDN connectivity procedure (with specific steps 1b, 7b, 10b).

5. (On the P‑GW-S‑GW interface) Create Session Response message of the Attach procedure or UE‑requested PDN connectivity procedure is combined with Create Bearer Request message of the Dedicated Bearer Activation Procedure

6.(On the S‑GW-MME interface) Create Session Response message of the Attach procedure or UE‑requested PDN connectivity procedure is combined with the Create Bearer Request message of the Dedicated Bearer Activation Procedure

7a. For Attach procedure: If the MME receives a Create Session Response message combined with a Create Bearer Request message, the MME shall send the S1-AP Initial Context Setup Request message to the eNodeB, including the NAS parts for both the Attach Accept message of the Attach procedure and the Bearer Setup Request of the Dedicated Bearer Activation Procedure.

NOTE: The MME shall not send a Bearer Setup Request message of a new Dedicated Bearer Activation procedure to the eNodeB before sending the Attach Accept message of the Attach procedure to the eNodeB. If the MME has already sent the Attach Accept message of the Attach procedure to the eNodeB, the MME shall wait for the Attach Complete message to arrive before sending a separate Bearer Setup Request of a Dedicated Bearer Activation procedure.

7b. For UE requested PDN connectivity procedure: If the MME receives a Create Session Response message combined with a Create Bearer Request message, the MME shall send the S1-AP Bearer Setup Request message to the eNodeB, including the NAS parts for both the PDN Connectivity Accept message and the Bearer Setup Request of the Dedicated Bearer Activation Procedure.

8-9. The radio bearer establishment of the default and dedicated bearer(s) is performed in the same RRC message.

10a.  For Attach procedure: The eNodeB sends the S1-AP Initial Context Setup Response message to the MME.

10b. For UE requested PDN connectivity procedure: The eNodeB sends the S1-AP Bearer Setup Response message to the MME.

11. For the Attach procedure: The UE sends the eNodeB a Direct Transfer message containing the Attach Complete (Session Management Response for the Default Bearer) message as response of the attach procedure, and Direct Transfer messages containing the Session Management Responses of the dedicated bearer setup procedure.

For the UE requested PDN connectivity procedure: The UE NAS layer builds a PDN Connectivity Complete (Session Management Response) for the Default Bearer Activation and Dedicated Bearer Activation Procedures. The UE then sends Direct Transfer (PDN Connectivity Complete) message to the eNodeB.

The NAS messages to establish the EPS bearers shall be handled individually by the UE and be sent in separate RRC Direct Transfer messages.

12. The eNodeB sends an Uplink NAS Transport message to the MME, which contains the NAS messages from the RRC message in step 11. There may be multiple Uplink NAS Transport messages when the UE sends multiple RRC messages containing NAS messages in step 11.

13. Upon reception of the response messages in both step 10 and step 12, the Modify Bearer Request message of the Attach procedure or UE requested PDN connectivity procedure is combined with the Create Bearer Response message of the Dedicated Bearer Activation Procedure. After that, the Serving GW continues with sending a Create Bearer Response message to the PDN GW.

Ref: 3GPP-23401.920