LTE Dedicated Bearers: The Big Question!

I wrote about LTE dedicated bearers some time back(here , here and here ). Just when I thought I got every thing right I realized that I am back to square 1.

We know that dedicated bearers are network initiated. But the big question is when does the network decide to initiate the dedicated bearer. Many people have asked me about this so I thought of putting down what I knew.

In LTE there is a bearer which is always established until UE is shut down and it is called default bearer. Well UE can ask for a dedicated bearer by sending out a bearer allocation request to the network. Once network receives the UE request then we will have dedicated bearer. This is one case.

What we noticed is there is a trigger for dedicated bearer. Now if we look at the spec for dedicated bearer creation, it immediately starts with Create Bearer Request. Many are confused with what could be the trigger for this message. Next para should help.

Second case is:- default bearer is established. Network now wants to have all the http traffic put in separate dedicated bearer. So once the default bearer is established, network might ask UE to run the http traffic on dedicated bearer. Note here that there is no trigger for dedicated bearer, which means network is configured in such a way that all http shall run on dedicated bearers with a particular qos. Even if UE is running http of default bearer it may be asked to switch to dedicated bearer. If UE is not capable enough to run a http then it may reject the dedicated bearer request also it may reject the request if it thinks default bearer is fine for http.

Now the confusion here is IMS calls. Many say how will network initiate a dedicated bearer for IMS calls. The big confusion is with reception of IMS call. Well, this is what I think. IMS needs an application to be running UE, a special app. If UE is intelligent enough then it will request a dedicated bearer for IMS call while it is registering with network or when the IMS app on UE starts running. Else PGW may look at the packets that are being sent by UE (deep packet inspection?) and initiate a dedicated bearer by it self. The point here I want to make is we shouldnt think that network will create a dedicated bearer only when UE receives an IMS call, the bearer is created much ahead of it.

If we consider that network is modifying or creating a new bearer for already running traffic, then what about the UE application's connection state. Will the connection be reset as the bearer has to be switched. I dont know, an UE engineer should answer this. How ever this dedicated bearer is still pretty confusing, in fact many are confused with what could be triggers for a dedicated bearer creation. Any ideas?

LTE End to End Signalling

LTE Initial Attach/Default Bearer Establishment

LTE Dedicated Bearer Establishment

Thanks to Zahid of 3G4G blog for the RRC call flows. (here and here)

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?