Saturday, June 6, 2009

LTE : Bearer ID

EPS Bearer ID is similar to NSAPI from GTPv1. There is not much of a difference. I tried to capture the EBI;s in the below call flow. Its pretty much self explanatory. EBI values are always assigned by MME. So MME sets a EBI value for default bearer and sends it to SGW in Create Session request. In the same way MME again assigns the EBI value to dedicated bearer in Create Bearer Response.


LTE_EBI.jpg

One thing to note is LBI, which is Linked Bearer Id. This is the default bearer ID. It used to link dedicated bearers with the default bearer. Bearer Context : EBI mean that EBI value is sent in Bearer Context IE. EBI Id is of 4 bits, which mean we can have 16 bearers. But there are 5 reserved values which I could never find out. So we will have altogether 11 bearers.

PTI is procedure transaction ID. PTI, LBI and TAD will give us a unique combination to create/delete a dedicated bearer.

Correct me if I am wrong. Comments are welcome.

15 comments:

Anonymous said...

Hi Santosh,

Do you remember the spec where it is mentioned that 5 values for EBI are reserved ?

also, Since there can be at the most 11 bearers - is there a limit on max Default bearers ?

Santosh said...

Hi there

Please refer to 3GPP TS 24.007 Section 11.2.3.1.5 for EBI.

There can be only one default bearer per UE per APN.

Cheers, Santosh

Anonymous said...

Hi Santosh,

I meant is there any restriction on the no of APNs per UE ?
(1 APN = 1 Defalut Bearer)

Santosh said...

Ian not sure about the number but I heard that a UE can connect to 3 apn's simultaneously.

girish said...

Hey folks, clarify me on this regard..
At any given point of time, UE can avail 11 EPS bearers, be it of any combination of default EPBs and dedicated EPBs..

For example, if the UE connects to 3 APNs then there would be 3 Default EPBs and hence posting a restriction of max 8 dedicated bearers shared among those APNs.!!

Santosh said...

That is correct!

king kori..:) said...

Hi Santosh,
is there a rule/protocol which says EBI number should start from 5?? Or is there number range with in that EBI will be assigned?

Santosh said...

Yes. EBI can only start at 5. EBI is a 4 bit value, so possible EBI's are 0 to 15. EBI 0 to 4 are reserved.

Sandeep said...

Does a UE request for a dedicated bearer without a default bearer????

Or if a UE has default bearer and it is not using this can it be possible to request for a dedicated bearer????????

Santosh said...

1. No

2. Yes

Sandeep said...

Thanks for reply.

can u please explain the 2nd point I can't understand properly ? Does P-GW allow to activate dedicated bearer as it is not using its default bearer and want for another bearer i.e one bearer is already with the UE which is empty.....

Santosh said...

Think of it from non usage point. As of now, dedicated bearers are mainly used for voip. So when a default bearer is created, a dedicated bearer is also created. A UE may just do voip using the dedicated bearer and may leave the default bearer idle (idle-not referring to UE state).

Sandeep said...

Thanks a lot Santosh.

Anonymous said...

Hi Santosh,

I can't find where it says that EBI for a dedicated bearer is unique per Ue. I.e. that the following example is not valid for one ue:

Def Bearer EBI = 5
Ded Bearer EBI = 6
Ded Bearer EBI = 7

Def Bearer EBI = 6
Ded Bearer = 7

If this is not valid, could you please tell me in what spec you found it.

Thanks in advance!

/Gus

Santosh said...

Hi Gus

Each bearer should have a unique bearer id, irrespective of default or dedicated. That is the base in core network, as all the resources are allocated per EBI.

Read initial few pages of 3GPPP TS 23.401

Regards, Santosh