E2E VoLTE call setup(4/4) - dedicated EPS bearer creation

While SIP signaling is in progress in VoLTE call setup, a dedicated EPS bearer is created for voice media transfer. The dedicated EPS bearer for voice is temporary one as it lasts only during the voice media session which is different from the default EPS bearer in that the default EPS bearer is persistent until the UE is detached from the LTE. The creation of dedicated EPS bearer is triggered by the P-CSCF sending service data information (i.e., AAR) to the PCRF. Once the dedicated EPS bearer is created for voice, there comes two EPS bearer created between the UE and the IMS APN, i.e., a default EPS bearer for SIP signaling and a dedicated EPS bearer for voice media. On top of this, there can be more dedicated EPS bearers created with different PCC rules and QoS according to service types.

I. Introduction

The P-CSCF converts the media information in the SDP into the service data information and sends it to the PCRF. The PCRF performs session binding between IP-CAN session and the Application Session, generates PCC rules and provisions them to the SGW/PGW. The SGW/PGW requests to the MME the creation of dedicated EPS bearer using the received PCC rules. The SGW/PGW also performs bearer binding between PCC rules and the to-be-created IP-CAN bearer. The creation of dedicated EPS bearer procedure is composed of sequential procedures of creating uplink S1 bearer, DRB (Data Radio Bearer) and downlink S1 bearer across UE, eNB and SGW/PGW.

NOTE the S5 interface between SGW and PGW is not depicted in the following practice for simplicity.

II. Creation of a dedicated EPS bearer for voice traffic

The following scenario shows the procedure of creating a dedicated EPS bearer during VoLTE call setup.

Fig 1. dedicated EPS bearer creation

[59] The SGW/PGW generates its own GTP-U TEID and initiates the procedure to create a dedicated EPS bearer by sending Create Bearer Request (CBR) to the MME. The CBR contains Bearer Context information of the dedicated EPS bearer to be created.

• (Linked) EPS Bearer ID: Indicate the default bearer associated with the PDN connection.
• Bearer Context: a set of information for dedicated EPS bearer to be created which contains EPS Bearer ID, Bearer TFT, GTP-U TEID and Bearer QoS.
• EPS Bearer ID : a requested EPS bearer ID to be created and shall be set to '0' at this stage.
• Bearer TFT : the uplink packet filters to be sent all the way down to the UE and applied by the UE when the UE sends out RTP and RTCP packet.
• SGW GTP-U TEID : the identifier of the SGW as an end point of the GTP-U tunnel.
• Bearer QoS : QoS for this dedicated EPS bearer which includes UL/DL MBR, UL/DL GBR and QCI.

Fig 2. IEs in Create Bearer Request

[60] Upon receiving the CBR, the MME allocates the EPS bearer ID and requests the UE to activate the dedicated EPS bearer by sending Activate dedicated EPS bearer context request towards the UE. The message is delivered to the UE being contained in the E-RAB Setup request and the RRC Downlink Direct Transfer over S1AP and RRC interfaces respectively. The E-RAB Setup request contains SGW GTP-U TEID and the E-RAB level QoS parameters e.g., ARP, UL/DL MBR, UL/DL GBR, etc.

• e-RAB ID: unique identifier of the E-RAB for the UE. Please note that the E-RAB ID=’5’ was for the default EPS Beaer with the Internet APN, ‘6’ was for default EPS bearer with the IMS APN and now, it is set to be ‘7’.
• e-RAB level QoS Parameters: QoS to be applied to an E-RAB, which contains QCI, ARP, MBR and GBR of the E-RAB. This is a copy of the received Bearer level QoS at step #59.
• SGW GTP-TEID: the identifier of the SGW at the end of GTP-U tunnel.

Fig 3. IEs in E-RAB Setup Request

  

The following snapshot shows the Active dedicated EPS bearer context request contained in the Non-Access-Stratum (NAS) PDU container.

• EPS QoS: QoS of an EPS bearer context which includes QCI, MBR and GBR of the EPS bearer
• Traffic Flow Template (TFT): the uplink packet filters to be sent all the way down to the UE and applied by the UE when it sends RTP and RTCP packet.
• Linked Transaction Identifier (TI): the identifier of the active PDP context from which PDP address for the new PDP context could be derived.
• Negotiatiated QoS: QoS of a PDP context

  

Fig 4. IEs in Activate dedicated EPS bearer context request

Upon receiving the E-RAB Setup request, the eNB allocates the required resources and establishes uplink S1 bearer with the SGW as part of the E-RAB establishment.

[61-62] The eNB sends the RRC Connection Reconfiguration request to the UE. The UE modifies the radio bearer accordingly and responds with the RRC Conenction Reconfiguration Complete.

[63] The eNB responds with the the E-RAB Setup response to the MME and it contains eNB GTP-U TEID which is allocated by the eNB.

• E-RAB ID: unique identifier of the E-RAB for one UE. This value is originally assigned by the MME and if this is already occupied, the UE can change it.
• eNB GTP-U TEID: the identifier of the eNB at the end of GTP-U tunnel. The eNB newly assigns the GTP-U TEID for this dedicated EPS bearer and it is delivered to the SGW via MME.

  

Fig 5. IEs in E-RAB Setup Response

[64] The UE responds with the Activate dedicated EPS bearer context response to the MME which is wrapped in the RRC Uplink Direct Transfer and S1AP Uplink NAS Transport on RRC and S1AP interfaces respectively.

• E-UTRAN Cell Group Identifier (CGI): globally unique identifier of a cell (PLMN ID + ECI)
• Tracking Area Identifier (TAI): globally unique identifier of a tracking area [PLMN ID + TAC]

Fig 6. IEs in Activate dedicated EPS bearer context response

  

[65] The MME responds to the SGW/PGW by sending Create Bearer Response. The Create Bearer Response contains the eNB GTP-U TEID which was received at step #63. Upon receiving the Create Bearer Response, the SGW establishes downlink S1 bearer towards the eNB.

Fig 7. IEs in Create Bearer Response

II. PDN connectivity and EBI allocation

The following shows the summary of all the procedures shown in the VoLTE call setup procedures, in which steps (1) to (4) are performed at the background automatically.

(1)    UE turend on.

(2)    PDN connection with the Internet APN: the QCI of the default EPS bearer = ‘9’, EBI = ‘5’.

(3)    PDN connection with the IMS APN: the QCI of the default EPS bearer = ‘5’, EBI = ‘6’.

(4)    IMS registration through the default EPS bearer with IMS APN.

(5)    The dedicated EPS bearer creation for voice upon request for VoLTE service: the QCI of the default EPS bearer = ‘1’, EBI = ‘7’.

Fig 8. PDN connection and EBI allocation

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REFERENCES

[1] 3GPP TS25.331, "Radio Resource Network (RRC); Protocol specification", v12.3.0, Sep 2014

[2] 3GPP TS24.301, "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage3", v12.4.0, Mar 2014

[3] 3GPP TS24.008, "Mobile radio interface Layer 3 specification; Core network protocols; Stage3", v13.4.0, Dec 2015

[4] 3GPP TS29.274, "3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS); Tunneling Protocol for Control Plane (GTPv2-C); Stage3", v13.0.0, Dec 2014

[5] 3GPP TS36.331, "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification", v12.3.0, Sep 2009

[6] 3GPP TS36.413, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP)", v12.3.0, Sep 2014

Last updated: 1 Jan 2016

VoLTE: PDN connectivity request vs handover

The PDN connectivity procedure is to request the setup of a default EPS bearer  to a PDN[TS24.301]. The PDN can either be the default PDN or an additional PDN. When it is the default PDN, the UE establishes the default EPS bearer as a part of the process of initial attach. If it is a subsequent PDN, this procedure will establish additional default EPS bearer with that PDN. As such the UE supports multiple default EPS bearers in multiple PDNs.

When the PDN connectivity procedure does collide with the handover procedure, the EPS shall be able to handle the situation based on the principle of best effort. The MME may pause the bearer creation procedure and resume when the handover is completed or the MME shall reattempt the E-RAB setup request after the handover is completed[TS23.401].

The following is the case where the PDN connectivity procedure collides with the X2 handover.

The above is the case the PDN connectivity request is sent for the second PDN. In this example, the MME proceeds the activation of the default EPS bearer by sending the Activate default EPS bearer Context request over S1AP while the X2 handover is in progress at the eNB. The eNB rejects the E-RAB Setup request with its cause value set to "X2-handover-triggered". Upon receiving the rejection to the E-RAB Setup request, the MME shall wait until the X2 handover is completed. Once it is completed, the MME re-attempts the activation of the default EPS bearer request. The following shows the S1AP procedure where the E-RAB setup request is rejected while subsequent PDN connectivity procedure.

The same mechanism is also applied to the S1 handover case as shown below.

In this case, the MME requests activation of the default EPS bearer as a normal procedure before receiving the Handover Required while the eNB already sent the Handover Required and subsequently receives the Activate default EPS bearer Context request. If the handover is only in preparation stage, the eNB may be able to cancel the handover and continues the E-RAB setup procedure. Instead, in this example, the eNB rejects the E-RAB setup request with its cause value set to "S1 intra system handover triggered". Upon rejection, the MME hold the state until the S1 handover is completed and resumes by reattempting the activation of the default EPS bearer.

NOTE the MME may pause between the step #b and the setp #6  as it is already aware of the S1 handover in progress. However, from implementation point of view, it looks better to pause after the setp #6 as it is a common place both for S1 and X2 handover.

If the timer at MME expires and the PDN connectivity couldn't be completed, it is expected that the UE reattempts the PDN connectivity procedure. In order to request connectivity to an additional PDN, the UE shall start timer T3482 after sending PDN connectivity request and enter the state of PROCEDURE TRANSACTION PENDING[TS24.301]. The T3482 stops when the UE receives the Activate default EPS bearer Context request or if expires, the UE reattempts the PDN connectivity procedure.

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