Wednesday, February 4, 2009

EPS Bearers -- Short Notes

I havent read anything for while, work is keeping me busy. Today i got a chance to get back to 3GPP Spec TS 23401 and I started reading the QoS in EPS bearers. Now LTE being all IP with no circuit swtching, the operation is little different from UMTS. In UMTS/GSM networks the phone will first get an IP address to do packet data. But in LTE its always IP. Lest see what EPS bearer is.

EPS bearer uniquely identifies traffic flow that recieves a common QoS treatment between a UE and a PDN GW.
Packet filters signalled in the NAS procedure are associated with unique packet filter identifier on per PDN connection basis. The EPS bearer Traffic Flow Template (TFT) is set of all packet fileters associated with EPS bearer.

One EPS bearer is estabalished when UE connects to PDN and that remains lifetime of that PDN connection to provide UE with always on IP connnectivity to PDN. THis is default bearer. Any additional bearer that is established to the same PDN is referred as dedicated bearer.

Up Link Traffic Flow Template (UL TFT) : - Set of uplink packet filters in TFT
Downlink Traffic Flow Template (DL TFT) : - Set of downlink packet filters in TFT
Every dedicted EPS bearer is associated with TFT.

More to follow, please correct me if I am wrong and comments are always welcome

Notes :

- S11 interface : MME and S-GW

- S5/S8 interface : S-GW and PDN

- Default and dedicated bearer

- Uplink TFT and Downlink TFT

- The decision to establish or modify a dedicated bearer can only be taken by the EPC and the bearer level QoS parameter values are always assigned by the EPC.

- No QoS negotiation between E-UTRAN and EPC during dedicated bearer establishment/modification is supported.

Will we support two TFT's? Both UL and DL TFT?


P said...

more info on EPS Bearers:

btw ur blogs r good and informational

Santosh said...

Hey ..thanks!

Anonymous said...

Any idea where I can get detailed LTE Call Flows including the QoS part?



Santosh said...

Hi Jim

There is some information on LTE Call flows and QoS in this blog itself. Please look at the blog archives. If you are looking for something specific let me know.

Cheers, Santosh

Anonymous said...

Hello Santosh,

Are bearers (the EPS bearer or any part of it like the radio bearers) bidirectional or unidirectional or can be both? Does this depend on whether it's the default vs. dedicated bearer?
GBR bearers require reserving resources, so it does matter more whether the bearer is bidirectional or not.
If bidirectional, then does the QoS profile associated with the bearer different for each direction?
23.401 has this comment: "A unidirectional EPS bearer is either associated with an UL TFT or a DL TFT that matches the unidirectional traffic
flow(s) and a DL TFT or an UL TFT in the other direction that blocks all traffic flows."


Santosh said...

Typically there are no qos policies enforced on default bearer, except for MBR. While dedicated bearers are GBR and they are assigned to TFT. TFT defines in which direction the qos values should be enforced.

Priyanka said...

Hi Santosh,

I am new to LTE and so my question may be very basic.I was reading on UL/DL TFT and have a question. I am still not ver clear how TFT can be used.After reading I interpreted that TFT is set of port number and the IP address of server and host.Spec 23.401 section states that "The UE routes uplink packets to the different EPS bearers based on uplink packet filters in the TFTs assigned to these EPS bearers. The UE evaluates for a match, first the uplink packet filter amongst all TFTs that has the lowest evaluation precedence index and, if no match is found, proceeds with the evaluation of uplink packet filters in increasing order of their evaluation precedence index....." Is the uplink packet filter an ID that maps to each set of the port no and IP address which is stored at UE and MME.Based on TFT then assigned by PCRF to the bearer,UE will know what Traffic filter ID to use for each bearer? Can you provide more details of how TFT will help?

Santosh said...

Execuse the delay in reply. Had been busy.

TFT contains a set of packet filter. A packet filter will distinguish a network flow.These flows are treated according to the SLAs. Packet filter can have IP addresses, ports or protocols. Based on PCRF the packet filters are associated to dedicated bearers. The dedicated bearers are intern treated specially in the network. UE will be sent UL packet filters, so that it can map the corresponding network flow to dedicated bearers,thus providing qos according to SLAs. Same is the case on the PGW, DL packet filters are mapped to corresponding dedicated bearers.

VIJAY said...

Do default bearers also have QoS rules and Charging rules ?? Can anyone through some light on this...

Santosh said...

Why not?

dhanu said...

hi, i want to learn PDCP, i am new to LTE ....from where should i start

R2 said...


How do you map SRBs and DRBs in default and dedicated bearers?

A maximum of eleven bearers are specified in 3GPP. Does this max number includes SRBs?

Santosh said...

AFAIK, SRBs are for signalling. Based on the number of bearers established so many DRBs should be created. In case a DRB is not created for established bearer UE may implicitly release the bearer.

Sandeep said...

Dear Santosh,

We are looking for a solution where a separate (first) router is connected to LTE UE (functioning as a second router) at one of the port. There are some information (such as priority bits etc) , carried in Ethernet frame, which is conveyed by first router to LTE UE. Is it possible to define mapping of certain Ethernet frame parameter to UL TFT in LTE UE? For example, if the priority bit in Ethernet frame is X, then LTE UE should ask for QCI with Guaranteed Bit Rate and if it is Y, then LTE UE should ask for QCI with N-GBR.


Santosh said...

Hi Sandeep

Going by your last statement "if the priority bit in Ethernet frame is X, then LTE UE should ask for QCI with Guaranteed Bit Rate and if it is Y, then LTE UE should ask for QCI with N-GBR.", I think you achieve this by writing an application on UE. Note that TFTs are defined at layer 3 and above. But your application on UE can request for a bearer with what ever qci/GBR you want upon certain conditions, for eg in your case when it receive a Ethernet frame X, and if networks grants it you are all set.

Sandeep said...

Thank you Santosh for your feedback. You cleared one of my doubts whether TFT is defined at IP level or can go up to Layer-2. Thanks for that.

The case is now more clarified by one of our customer. The ethernet frame is using VLAN Tag with 802.1p priority information. It is assumed that first router (non-LTE) will add certain priority for VoIP traffic (Voice Bearer) and certain for best effort traffic & the question was whether the VLAN tags can be honoured by LTE UE in such a way to request GBR bearer for Voice traffic?
Based on your response, I believe it is only possible by running a application in LTE UE .. right?

Sandeep said...

Dear Santosh,

Further to my query, is it possible for UL-TFT to have packet filter definition for DSCP based priorities? For example, if the first router (non-LTE) marks VLAN Tag Priority & also DSCP / TOS in IP packets, is it possible for LTE CPE to recognize such marking and request certain QCI?

Thanks for your help

Santosh said...

Hi Sandeep

For your first comment - Yes, writing an app on UE should help.

Second commnet -
Like I said you can make your UE request for a bearer with any qci, as long as it is valid. So you can write a logic in UE asking for a bearer when it receives a packet with Vlan tag priority etc..etc. But note that UE can only request, your network will actually award the bearer. So you need to have similar rules configured in the PCRF.

Sandeep said...

Dear Santosh,

I would like to clarify two items related to bearers:

1. What is the difference between a default bearer (which is always Non-GBR type), but limited by UE-AMBR (or APN-AMBR) as part of the subscriber's subscription AND Dedicated Bearer of Non-GBR type (this two can be considered as part of UE-AMBR and APN-AMBR) ?? -- Is only QoS (QCI) is the difference?

2. Is there a way to limit the maximum and minimum bandwidth per subscriber (UE) (& APN if APN is used) ? The standard talks about UE-AMBR and APN-AMBR, but both are for Non-GBR. The GBR bandwidth requirement is over and above this? Is there any parameter which can restrict - minimum and maximum - throughput of subscriber including all bearers & bearer types?


Santosh said...

Hi Sandeep

1. UE/APN AMBR are for limiting default bearer only. If you establish multiple default bearer to the same APN then UE AMBR will act upon to see if the APN AMBR is not exceeded.

2. For dedicated bearers quota is allocated outside of APN/UE AMBR. The quota is dictated by PCRF.

Regards, Santosh

rajeshini said...

I've one query on the release of default bearer. The Default bearer is cleaned up when UE Context Release procedure is performed by the EPC.

1. When does the EPC initiate UE Context Release procedure?
2. How do the Always-On PS connection is achieved if the default E-RAB resources get released on the S1-U interface?

Adnan Basir said...

Hi Friends,

I have been trying to find answer to this but not successful yet.. Please let me know if you know

"How does User data on physical resource block is mapped to a particular DRB or EPS bearer ? In other words if the eNB recieves User plane data on PDSCH, how does it know which EPS bearer this data is for ? How this mapping works ? "

Thanks in Advance