Tuesday, April 14, 2009

UE initiated Bearer Resource Modification : a.k.a Dedicated Bearer

Source : 3GPP TS 23.401 : Clause : 5.4.5 and 3GPP TS 29.274 Clause : 7.2.5

Some time back I wrote about Dedicated Bearer establishment in LTE. i was still under confusion about how dedicated bearers are established and how different are they from secondary PDP context. I closed that post with an open question can MME initiate a dedicated bearer. After much reading and lot of questions which readers asked me, I understand the behavior of Dedicated bearers to certain extent now. In this post I want to concentrate on UE requested Dedicated bearers, just like secondary PDP contexts yet so different. In GTP v1 UE initiates the secondary PDP context with a new TFT, but in LTE UE starts with Bearer Resource Modification procedure. (3GPP TS 29.274 Clause : 7.2.5)

Bearer resource modification procedure can do more, but lets just consider that we are trying to get a dedicated bearer established.

The UE sends a Request Bearer Resource Modification (LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the MME.

TAD : Traffic Aggregate Description indicate add packet filters operation. EPS Bearer Identity may not be sent here. Linked Bearer Identity is sent to indicate to which PDN connection additional bearer resource is linked to. PTI is dynamically allocated by UE.

The MME sends the Request Bearer Resource Modification (IMSI, LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the selected Serving GW. The MME validates the request using the Linked Bearer Id

The Serving GW sends the Request Bearer Resource Modification (IMSI, LBI, PTI, EPS Bearer Identity, QoS, TAD, Protocol Configuration Options) message to the PDN GW

If requested is accepted then dedicated bearer establishment is invoked. The PTI allocated by the UE is used as a parameter in the invoked Dedicated Bearer Activation Procedure to correlate it to the UE Requested Bearer Resource Modification Procedure. This provides the UE with the necessary linkage to what EPS Bearer to be used for the new traffic flow aggregate. The PDN GW shall not modify the QoS parameters requested by the UE  

Now go back to the dedicated bearer establishment post.

This takes one more step towards understanding bearers better. As usual comments are welcome.


Ankit said...

Can some one throw light on what the PCC interaction for this would be. The case that I am thinking of is, can a Bearer Resource Command ending on the PGW end up in a Create and Update Bearer Request (two messages which would both need the same sequence number and would have to include the PTI). The PCC can come back with rules and bearer binding may result in the creation and update of bearers. Any insight to this would be much appreciated.

Santosh said...

Considering there is already dedicated bearer established with a particular QCI, update of the same bearer will be ruled if bearer resource command comes with other QCI value.

First step for an update is to check the QCI, then contact PCC and take a decision whether to update the bearer or create a new bearer. Remember bearer resource command has Flow QoS which doesnt have ARP value, while create bearer request message has Bearer QoS which includes ARP. PGW will fetch this value PCC.

I dont fully understand your question, but above is what I think.

Rakesh said...

Does a UE/MME initiated bearer possibe in LTE?

Till now I have two thoughts, I never seen MME initiated bearer in specs.

And In testing I found it is there..

so it is confusing.

Santosh said...

Note that in LTE MME cannot initiate a bearer creation. A bearer creation is always PGW/SGW or UE initiated.

However MME can trigger bearer deletion and UE detach.

Adnan said...

Hi Santosh..
Do you know if we can initiate dedicated bearer from UE side via AT commands ?

Santosh Kumar Dornal said...

Hi Adnan

Never had a chance to work with LTE phone. I think it should be possible though.

Regards, Santosh

Rauf said...

Hi Santosh,

Can you tell why and when a UE initiate dedicated bearer? Are users aware that they asked for a dedicated bearer? Or the request come from application? or based on the application type and user subscription, network decide that a dedicated bearer should be opened for this application. I am just confused!


Santosh Kumar Dornal said...

Hi Rauf

I have write up for dedicated bearer and IMS almost ready. Will post over the weekend. It might help you to find some answers.

Cheers, Santosh

Anonymous said...

How does the UE initiated dedicated bearer activation work with PCRF interaction. With Bearer Resource command request, there should be interaction with PCRF (using Credit control request/answer). Based on this, PCEF shall trigger SGW with Create Bearer request, which inturn reaches MME, ENB. On the PGW side, after receiving Create bearer response, should it need to go towards PCRF again to update the details received in Create bearer response. Because, i expect that the EPS bearer id of the dedicated bearer is received only in Create bearer response, which needs to be communicated to the PCRF. If so, what messages should be used towards PCRF? Is it again Credit control request? or is this interaction with PCRF after receiving Create bearer response not required at all?

Your suggestions/opinions please.


Anonymous said...

For UE initiated dedicated bearer activation, how does interaction with PCRF happens? My assumption was that, PDN gateway after receiving Bearer Resource command(BRC) from SGW, should trigger
PCRF with Credit control request(CCR). Upon successful Credit control answer(CCA), Create bearer request is triggered towards SGW, which in turn goes towards MME, ENB.
Create Bearer response towards PDN gateway should contain the EPS Bearer ID for the dedicated bearer (apart from Linked EBS bearer id). After receiving Create bearer response at PDN gateway, whether there should be another interaction with PCRF to exchange the received EBS bearer id, is this required at first place? If so, what message should be used for this? Again CCR?
Could you please provide your views/comments.


Santosh Kumar Dornal said...

PGW sending CCR towards PCRF after Create bearer response is based on whether PCRF has requested for it in the charging rule. PCRF can send resource allocation notification in charging rule asking PCEF to update the rule status. And EBI is not sent to PCRF if PCEF is doing the bearer binding.

Kumar said...

Thanks for the info. In TS 23.203 (Figure 7.4), it has been mentioned that Provisional ACK (message number 15) should be sent to PCRF upon completion of IP CAN session signalling (in this case Create bearer response). Whether this should be done only based on event trigger received from PCRF in the earlier credit control answer?

If this provisional ACK is not requested by PCRF, then is the EPS Bearer ID (created by MME for dedicated bearer) not required for the PCRF?

Santosh Kumar Dornal said...

A rule failure is always reported to PCRF by the PCEF. However a rule success is intimated only upon PCRF request. PCRF sends a event trigger for rule report if AF has requested for it. Since this is 3GPP you can any way send success to PCRF even if it has not requested for, but it is not a good idea as there will be unnecessary signalling.

I dont think EBI is sent to PCRF, unless you are dealing with 3GPP-GPRS IPCAN type.

Anonymous said...

In BEARER RESOURCE MODIFICATION REQUEST message, EPS bearer identity for packet filter IE will contain the value of EPS bearer to which the TFA belongs.

Then what value the EPS bearer identity IE will carry (is it always 0)?

Santosh Kumar Dornal said...

EBI will be of the bearer that is being modified, for this message.

Kumar said...

Thanks for the info.
In case of UE initiated dedicated bearer activation, the received TFTs are sent towards the PCRF in IP-CAN Session Modification procedures using Packet-Filter-Information, Packet-Filter-Identifier with Packet-Filter-Operation set to "ADDITION". In TS 29.212, it has been mentioned, that in case, TFT information is received from PCEF part (of PDN gateway), PCRF sends the PCC rule information in Flow-Information AVP, which also contains the Packet-Filter-Identifier.

My query is that, will there be any mapping between the TFTs (received from UE, sent in Packet-Filter-Information) and the PCC rules sent by PCRF?
If so, is it the Packet-Filter-Identifier which is used for this mapping, as this information is sent to PCRF, and also being received back within Flow-Information AVP.

But, it has not been mentioned explicitly anywhere, that PCRF shall be sending the same Packet-Filter-Identifier (within the Flow-Information AVP) as received in Credit Control Request.

Can you please comment & provide your views on this.


Santosh Kumar Dornal said...

Along with packet filter information pcef will also send the qos info. PCRF does the mapping between QCI and Packet filters. Based the qci PCRF may choose to create a new bearer for the received packet filters or may choose to append packet filters to existing rule or may choose reject the request.

Anonymous said...


in the 24.301 spec for BEARER ALLOCATION REQUEST, it is mentioned that N/W can initiate modification or dedicated bearer allocation procedure.

Can you let me know does it depende on EPS BI value send in BAQ request from UE. Also in which spec it is mentioned.

Also can you give me example of default and dedicated bearer setup from N/W side

Anonymous said...

Can somebody throw some light why in the Charging-Rule-Definition AVP the Flow Description AVP is being created with value permit in/out from any to any sent back to the PCEF?
As I understand the Flow Description AVP is to set the QoS for a particular PCC rule and particular service flow.
Could this be something from the PCEF CCR-i triggering the PCRF to send this?