Friday, November 13, 2009

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?


Nickname unavailable said...

I'm thinking of a few instances here.

Short-messages to the device to establish a dedicated bearer for something like message retrieval (aka Visual Voicemail or push traffic).

Presence or something SIP specific.

Lawful Intercept?

Diagnostics? Not uncommon to have diagnostics built into the device (UE Code) for customer care to interact with the mobile. Seems like an opportunity.

I'm just getting involved in building out an EPC in support of an LTE deployment so I'm learning as I go.

Martin said...

Hi Santosh,

have a look here, maybe that's what you are looking for:


Santosh said...

Thanks to both for the comments.

Martin, thats quite an interesting post. I shall dig deep into it.

And I wish luck to the anonymous poster with LTE deployment. :-)


shanky said...

Any body know how many dedicated bearer can be establish on one UE . i know 11 but why only 11 why not more than 11 ...i am confused to this so suggest me any answer