Sunday, October 4, 2009

LTE End to End Signalling

LTE Initial Attach/Default Bearer Establishment


LTE Dedicated Bearer Establishment


Thanks to Zahid of 3G4G blog for the RRC call flows. (here and here)


sagar said...

how the mapping between the QCI and the DSCP in the PGW

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.

Soumya Kanti Roy chowdhury said...

I see that you have shown ERAB Setup Request following Initial UE Message (in LTE Initial Attach Figure) between eNB and MME. When do you think Initial UE Context Setup Request/Response are exchanged?
Secondly, you have shown that Paging triggers the Inital Attach procedure (same Figure). Prior to Initial Attach, is there any context of the UE present in EPC so that the UE can be paged?

Santosh said...


I did see some ambiguity with UE Context Setup Request and ERAB setup request. 23.401 says Initial UE Context Setup Request should be sent from MME to eNB once the default EPS bearer has been established. How ever I couldnt find/understand the supporting behaviour in S1AP and NAS. Still working on it.

Paging doesnt trigger the Initial Attach, as far as I know. Its the means by which network advertises itself.

Thanks, Santosh

Soumya Kanti Roy chowdhury said...

Well, if you refer to 23.401 (8.7.0) section, it clearly says that Initial Context Setup Request (with NAS message 'Attach Accept' containing 'Activate default EPS bearer context request') will be sent from ENB to MME. This happens on receiving 'Create Session Response' from SGW, that you have correctly shown. For S1AP description, you can look into 36.300 (9.0.0) section 19 and 36.413 (8.7.0). I am interested in the ambiguities you mentioned in 23.401. Can you please tell me which part of the spec you found those?

You are absolutely correct that Paging does not trigger Initial Attach procedure. Initial Attach is done by UE itself. That is why you should remove the Paging messages from your first figure titled 'LTE Initial Attach/Default Bearer Establishment'. Paging initiates Service Request procedure that you will find in section 5.3.4 of 23.401 (8.7.0).

I must end with a note that I am following your fantastic blog of late and it does come in my help - no doubt for others also. Keep it up. Thank you very much,

Santosh said...

Thanks a lot Soumya for the clarifications. I shall take a look the specs and correct the call flows. As far as I remember I couldn't find Activate Default EPS bearer context request mentioned any where in 23.401. I just couldn't find the NAS message any where, may be I should look more closer :-)

I will also get rid of paging, it is giving an illusion that paging is causing the default bearer establishment.

Ambiguities in specs ... may be in a new post. Thanks again.

Soumya Kanti Roy chowdhury said...

You can refer to 24.301 (8.2.1) section; let me pick up a couple of lines from there "If the attach request is accepted by the network, the MME shall send an ATTACH ACCEPT message to the UE and start timer T3450. The MME shall send the ATTACH ACCEPT message together with an ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message contained in the ESM message container information element to activate the default bearer...". The next line of the spec says that the network may also initiate activation of dedicated EPS bearers, but I did not find under what circumstances it will do so. Yes, "Ambiguities in Spec" should be an excellent article - and it will create stir in many quarters.

Thank you very much for your responses as well,


Santosh said...

Cool Soumya! You got it right. That fact that was confusing me was if UE Initial Context can trigger Radio Bearer establishment or not. But taking a second look at 36.413 I understood that UE initial Context Request can setup the radio bearers.

This is a classic case of ambiguity to me atleast. 23.401 says just attach request shall establish default bearer, but 24.301 says we also need another NAS message to activate default bearer. But only when you look at 36.413 (S1AP) you will understand all the messages that are to be sent to activate default bearer.

Also I still cannot find which field of Attach Accept carries the IP address to be assigned to UE.

Anyway, the call flow is messed up, i will change and put a new one soon.

Thanks for the corrections.

Mega said...

Nice LTE signalling flow chart. Can you also add the NAS and AS security procedures (messages for authentication and ciphering)? Thanks.

Santosh said...

In couple of days..

tanuja said...

Hi ,
Can sowmya or Santosh help in understanding the PDN connectivity during the initial attach procedure with PDN Connectivity Request and default bearer esablishment procedure with it on Ref to 24.301 sec 6.5 and the QOS w.r.t Default bearer

Santosh said...

There is enhanced call flow here which is self explanatory. Let me know if you need any specific detail.

Emon said...


How does an application can initiate RRC request to get a dedicated bearer? Is there any API in the LTE modem to do that?



Santosh said...


Its not an RRC messages, it will be a NAS message. And yes, there will be a need for API to trigger a dedicated bearer from UE. Or network may initiated the bearer itself.


EMon said...

Dose those kind of API described or define any where?

Santosh said...

It is specific to UE implementation