Tuesday, March 17, 2009

Dedicated Bearers in LTE

After a long list of questions and debates I finally understand how dedicated bearers are established in LTE. The following is what TS 23.401 has to say about dedicated bearer establishment.

Here is how dedicated bearer is established.

  • 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
  • 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.
  • 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.
  • I will leave the rest of the "Radio Level" Messages.
  • 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.
  • 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.
  • 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
Open Question: Can MME or S-GW initiate a dedicated bearer? Is so how?


Anonymous said...

For the open question...
I believe it can only be user initiated or network initiated, I am not sure if it would make sense for the mme or s-gw to initiate a dedicated bearer. I.e. A SIP call. Someone would try to reach you or you would try to reach somone, but not the mme or s-gw, unless one of them is a smart machine trying to call you :P (I've watched too much terminator)

Santosh said...

Hey...thanks for the comments. I read bit more about dedicated bearers here and posted it here.


Coming to Terminator, Iam a big fan too. I beleive the Terminator age is coming true as I get lot of automated phone calls orginated by "Smart" crap advertising machines. :)

Anonymous said...

why do you need the lbi ? each pdn has a separate tunnel any way. so a default-bearer-create on a tunnel is meant for that pdn.

although, 29.274 says that the param is mandatory. it should be optional...

Santosh said...

LBI is not used in creation of default bearer. LBI is used during dedicated bearer creation to link the default bearer.

And the spec says LBI is mandatory in Create Bearer Request message. Mind you that this message is used to create dedicated bearer not the default bearer.

Anonymous said...

LBI is nothing but the bearer id of default bearer for the perticular APN.

Rashtrapathy said...

what might trigger the unexpected condition for loss of dedicated bearer in the network?