Wednesday, June 3, 2009

LTE: QoS (Part 2)

In the last post we understood TFT and QoS behavior in LTE. What's important to understand is bearers are all about achieving QoS. The whole concept of bearers is just to provide proper quality of service to user thus enhancing the quality of experience. So what we know at this point is a default bearer always exists once the UE is switched ON, until it is powered off. Any other bearer is a dedicated bearer which is associated with a TFT and QoS.

In this post lets try to understand few more terms like TAD and QCI (QoS Class Identifier). The concentration is on UE requested bearer creation/modification procedure.

Basically a UE may request for dedicated bearer. Lets say we are running a web browser on our mobile phone. Now UE wants to establish a dedicated bearer for that. So UE sends out a request for bearer creation procedure. MME here sends sends out a bearer resource command which includes LBI (Linked Bearer ID), PTI (Procedure Transaction ID), TAD (Traffic Aggregate Descriptor), QoS.

TAD is a partial TFT. It includes packet filters for a particular L7 activity. In our case as we are running HTTP, TAD will include packet filter for HTTP. Now MME will send this information to PGW. Now PGW understands that a dedicated bearer has to be created for running HTTP traffic. So it sends out a create bearer request with TFT (HTTP) and QoS. Here UE may request for particular QoS.

Lets look at QCI here. QCI field may be mapped to a diff serv. (Assured Forwarding?) . Say QCI value is set to 2 which indicates a particular class of assured forwarding, say AF12. We have MBR and GBR values in QoS which may be set to some value.

Once MME requests create bearer request it responds with a bearer response creating a dedicated bearer for HTTP activity with a QoS value (QCI: AF12). After this our mobiles should start displaying the web pages with a speed equal to Roger Federer's ace. :)

Now, you remember that your boss has asked something and you want to email him. So another dedicated bearer for this email? Ok... lets stop and understand this a bit clearly. If your http dedicated bearer QoS is fine for running email then we may just use it. But you need to indicate this to PGW. So we do a bearer modification (update?) procedure instead of bearer creation. MME here will send bearer resource command with TAD mapped to email traffic, say IMAP(?). But note here the QoS is same. (QCI still is AF12). PGW looks at this and thinks that may be I should update the dedicated bearer to include IMAP too. So it sends out Update bearer request with newly added TFT for IMAP. (We already have TFT for HTTP). MME responds to saying OK.

At this point we have two activities running on single dedicated bearer. That means when there is no QoS change a single bearer may be used to run various traffics. This brings us to square one where I said bearer is all about QoS. Once email is done, PGW may remove that TFT (IMAP) from dedicated bearer.

This was just a short post to look at things when no QoS is changed. I will try to analyze the whole QoS with various IE's in coming days.

I hope it was useful and as usual comments are greatly welcomed.

37 comments:

Duncan Salerno said...

How does this relate to APN's in the 2G/3G world?

As far as I understand it you can have secondary GTP contexts per APN to give different QoS. Is this similar to what's going on here, but they are secondary contexts to the default bearer?

So if I had a private APN for LTE, as before I could have secondary contexts that would have different QoS for different traffic, eg. corporate VoIP would be prioritized over web browsing, but could use the same APN?

Santosh said...

First, we establish both Primary and secondary PDP contexts to same APN. If you are using two APN's then you will have two primary PDP contexts, one for each APN.

You can compare the primary PDP context to default and secondary PDP context to dedicated bearers. The only difference is dedicated bearers in LTE are initiated by EPC where as secondary PDP contexts are UE initiated.

So you can have a default bearer established, over which a number of (Max 11?) dedicated bearers can be established each with different QoS.

Next, if you have a private APN, you can establish different dedicated bearers (secondary contexts) with different QoS values for running different traffic.

Duncan Salerno said...

Thanks for that. Will be interesting to see what APN's get used in practice - if the providers will just create a TFT on the default bearer for voice or a whole new APN.

TechnoTerp !! said...

do you knw if in handoffs, inter RNC seesion transfers referring to here, TFT information is exchanged ?

Can TFT settings on one RNC differ from other ?

Santosh said...

TFT is for the network. TFT can lead to special QoS mechanisms for the bearers. I am not sure if it will effect the radio at all. So if there is a handover, there should just update of the traffic paths or establishment of new bearers with the TFT's.

Confused? :)

TechnoTerp !! said...

im not talking abt radio resources here, TFT information will be per session, isnt it ? Meaning by, network related parameters for that session. So was wondering if for session transfers, does this information get propagated.

Thanks for responding quickly to my earlier post

Santosh said...

Yup. If the session transfers the same TFT's are to be maintained so that there is no change in QoS.

fahim said...

hi,
Can you tell me in same scenario when two websites are open and default bearer deactivated . How the dedicated bearers will be deactivated?

Santosh said...

If default bearer is deactivated the whole connection is lost, which means your UE is "dead" or "switched off" or "connection to a APN is deleted". Default bearer will not be deactivated unless all the dedicated bearers associated with it are deactivated.

fahim said...

hi, can u tell me . Does default bearer is used for packet transmission of voice only or dedicated bearer will be used for it.

Does default bearer send any data

Santosh said...

There is no restriction on any bearers. You can use default bearer to send any data (voice, video , etc etc).

Anonymous said...

Hi everybody!!

I would like to know the classification that is made in LTE and the parameters that define these classes of traffic. Is LTE as UMTS for example that clasify its traffic as background, interactive, streaming...

Santosh said...

Absolute co-incidence. I was just reading this.

3GPP TS 23.401 Annex E -- Might help!

Anonymous said...

Thank you very much. I will have a look.

mpgonza

Anonymous said...

Is it possible taht they call LTE as UMTS bearer??? TS 23.107

Santosh said...

I dont think so. UMTS bearer might mean bearer for UMTS network in LTE, not sure again.

vkkstyle said...

hi,

How TFT is used in UE side ?
What part of layer in UE will enforce these TFT parameters on to the packet?
if possible can any one tell me the link of spec .......

krishna kishore

Anonymous said...

how can we map between the QCI and DSCP

Santosh said...

Each QCI value can be mapped to IP type of service (TOS). Eg:- QCI =1 can be mapped to AF31, QCI=2 can be AF32. It depends on what kind of service you want to give for a QCI. QCI is merely a identifier.

Anonymous said...

Is there a TFT that is being added to the dedicated bearer for IMAP traffic or it is just a packet filter that being added to the existing TFT because as per my knowledge, there is one-to-one relationship between a TFT and dedicated bearer. Please correct me if I am wrong.

Santosh said...

There are two things here, if the network feels that existing dedicated bearer qos is enough to serve IMAP then it will update the dedicated bearer packet filter. Else network may initiate a new dedicated bearer for IMAP.

Anonymous said...

In any case, there will be only one TFT per dedicated bearer (which in turn may contain many packet filters), is this statement correct?

Santosh said...

Yes. As far as I know the statement is correct.

Anonymous said...

Then there is slight confusion regarding the statements in your post that say "So it sends out Update bearer request with newly added TFT for IMAP. (We already have TFT for HTTP)" and "Once email is done, PGW may remove that TFT (IMAP) from dedicated bearer." which I understood as there would be 2 TFTs in a dedicated bearer. Please correct me if I am wrong.

Santosh said...

You are correct. The TFT should be replaced with packet filter in the statement.

The post was written keeping UE request bearer creation/modification in mind. This means that UE will send TAD(partial TFT) to network and network shall decide whether to update the exiting TFT with packet filter or to create a TFT.

Anonymous said...

Yes, that is more clear. Thanks Santosh, your posts have been very helpful in understanding LTE/SAE as I just have started working on it.

Santosh said...

You are welcome!

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Davide said...

Hi,

Thank you for the nice post. Two questions, please:

1) In a previous comment, you wrote:

"You can compare the primary PDP context to default and secondary PDP context to dedicated bearers. The only difference is dedicated bearers in LTE are initiated by EPC where as secondary PDP contexts are UE initiated. "

So, this means there is no innovation in going from 3G to LTE about QoS. I thought this concept of being able of putting different traffic on different bearers via TFT was related to LTE only.

What is the improvement in LTE compared with 3G then?

2) LTE introduced the concept of labeled QoS profile. What is it and what is the improvement from 3G?

Santosh said...

Hi

1. TFT thing is coming from 3G. You can secondary PDP contexts mapped to a TFT.
2. I am quite not sure of the improvements from 3G. But visible change is QoS is simplified in LTE. We have too many fields for QoS mapping in 3G but in LTE they are made simple. More over the network takes full control of QoS, which means QoS negotiation is not possible in LTE.

Thanks, Santosh

Emon said...

Hi there,

You wrote the UE can ask for a dedicated bearer for HTTP traffic. Could please explain how the UE can initiate the process of getting dedicated bearer for any specific data traffic?

BR

Emon

Santosh said...

Refer to 3GPP TS 23.401 section 5.4.5

Santosh

GAURAV said...

If my application(for IMS voice call)requires QOS that EPS is not ready to give then in that case EPS will reject the dedicated bearer creation? or will there be any negotiation?

Santosh said...

There will be no negotiation...u may request for anything, but what PCRF decides will be enforced.

Ram Asokan said...

Hi,

There is a maximum of 11 EPS bearers between an UE IP and APN. one or more of these EPS bearers can be mapped to a DRB. Since there can be a max of only 8 DRBs, how is it possible to use all 11 EPS bearers?

Regards

Anonymous said...

Hi Santosh,
I am quite new to this LTE QoS and TFT. So based on your example for UL HTTP, questions...
1. Lets say, if this is the very first time HTTP app that UE wants to use, how does UE or App decide to use Dedicated bearer? There is already Default Bearer, why not to use it? Who does decide to use Dedicated Bearer for specific app (in this case HTTP). Is it hard coded in UE for Layer 7? or is it provided to UE during Initial Attach? As far as I know, in Default Bearer setup, ONLY QCI, ARP and GBR/MBR is supplied to UE. THere is NO UL TFT information provided.
2. Continuing the above question, when UE or App has decided to use Dedicated bearer for HTTP, UE first has to have to have knowledge of UL TFT? So for the very first time of HTTP use, how does UE get knowledge of UL TFT? When UE or user clicks on HTTP browser, does it send some signalling data to MME hence to PGW which decides that this requires Dedicated Bearer? And then PGW initiates Create Dedicated Bearer Request?

Santosh Kumar Dornal said...

Hi

Quite a few questions there.

- Network tells UE when to use dedicated bearer and for what traffic based on the UE subscription.
- A dedicated bearer may be created during initial attach itself with a TFT.
- A default bearer may also be associated with a TFT.
- Default bearer doesnt have MBR/GBR.
- UE starts mapping data to a radio bearer as and when it receives a TFT, else everything is sent over default beare.
- PCRF/PGW decides when and with what filters a dedicated bearer has to be created. This depends on network config and user subscription