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Dedicated Bearer Establishment (simple call flow)

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  1. The PDN GW uses this QoS policy to assign the EPS Bearer QoS, i.e., it assigns the values to the bearer level QoS parameters QCI, ARP, GBR and MBR. The PDN GW sends a Create Dedicated Bearer Request message (IMSI, PTI, EPS Bearer QoS, TFT, S5/S8 TEID, LBI, Protocol Configuration Options) to the Serving GW, the Linked EPS Bearer Identity (LBI) is the EPS Bearer Identity of the default bearer. Procedure Transaction Id (PTI) parameter is only used when the procedure was initiated by a UE Requested Bearer Resource Modification Procedure

  2. The Serving GW sends the Create Dedicated Bearer Request (IMSI, PTI, EPS Bearer QoS, TFT, S1-TEID, LBI, Protocol Configuration Options) message to the MME. If the UE is in ECM-IDLE state the MME will trigger the Network Triggered Service Request from step 3 .In that case the following steps 4-7 may be combined into Network Triggered Service Request procedure or be performed standalone.

  3. The MME selects an EPS Bearer Identity, which has not yet been assigned to the UE. The MME then builds a Session Management Request including the PTI, TFT, EPS Bearer QoS parameters (excluding ARP), Protocol Configuration Options, the EPS Bearer Identity and the Linked EPS Bearer Identity (LBI). If the UE has UTRAN or GERAN capabilities, the MME uses the EPS bearer QoS parameters to derive the corresponding PDP context parameters QoS Negotiated (R99 QoS profile), Radio Priority, Packet Flow Id and TI and includes them in the Session Management Request. MME then signals the Bearer Setup Request (EPS Bearer Identity, EPS Bearer QoS, Session Management Request, S1-TEID) message to the eNodeB.

  4. "Radio Level" Messages exchanges

  5. The eNodeB acknowledges the bearer activation to the MME with a Bearer Setup Response (EPS Bearer Identity, S1-TEID) message. The eNodeB indicates whether the requested EPS Bearer QoS could be allocated or not.

  6. The eNodeB sends an Uplink NAS Transport (Session Management Response) message to the MME.
    Upon reception of the Bearer Setup Response message in step 7 and the Session Management Response message in step 9, the MME acknowledges the bearer activation to the Serving GW by sending a Create Dedicated Bearer Response (EPS Bearer Identity, S1-TEID) message.

  7. The Serving GW acknowledges the bearer activation to the PDN GW by sending a Create Dedicated Bearer Response (EPS Bearer Identity, S5/S8-TEID) message

posted Feb 6, 2014 by Jai Prakash

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I'm still curious, dedicated bearer will be created only when the UE make a call in VoLTE or what ?

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Bearer is a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. In short, bearer is a set of network parameter that defines data specific treatment .

Default Bearer in LTE
When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached. Default bearer is best effort service and comes with an IP address. UE can have additional default bearers as well.
Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer.

Bearers in LTE

Dedicated Bearer
Dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR).
For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services