Saturday, January 30, 2010

LTE End to End Signaling:- Part 2

I wrote about LTE End to End Signaling some time back. Based on public demand I enhanced the call flows a bit :-). Here are two calls showing Initial Attach and UE initiated detach procedures.

LTE UE Initial Attach


LTE_UE_Initial_Attach.jpg

This is a huge call flow and there is a lot of processing involved. I would like give a brief out line of messages.

S1 Setup: This is where eNB is attached to the network. As long the eNB is functioning the S1 setup remains.

RRC Connections: Once UE comes up a RRC connection is established for communication with the network.

NAS: After RRC is established then begins the NAS signaling. UE sends Attach request along with PDN connectivity request to network. Attach is for attaching to the network and other message message is for establishing the bearers.

HSS: This is Home Subscriber System and it understands diameter protocol. Once MME receives Attach Request, it queries HSS for authentication details. HSS sends the authentication vectors to MME in Authentication Info Answer.

Authentication/Security: Networks requests UE for Auth vectors. Once UE provides the same MME compares the same with what HSS has sent. If they match UE is authenticated. Next is security. After the security all the NAS messages are encrypted using the security algorithms that were exchanged.

Bearers: After security network creates the EPS bearers first. (GTP messages). Then the radio bearers are created and RRC connections are modified accordingly. Once the radio bearers are created eNB downlink addresses are sent to SGW in GTP messages.


LTE UE Initiated Detach


LTE_UE_Detach.jpg

UE decides to detach from the network. Its sends a detach request message to network. Network deletes the EPS bearers then the radio bearers are torn down. Finally RRC connection is released.

I hope the messages are correct. Comments are always welcome.


26 comments:

Anonymous said...

Cool blog as for me. I'd like to read something more concerning this topic. The only thing your blog misses is a few pics of any devices.
Nickolas Karver
Cell jammer

Anonymous said...

Another comment about PMIP from the SGW during UE initial attach: you have only 1 arrow for the Bind Update Update/Ack to the SGW.
You should have two messages, at least: the Bind Update first, then the Binding Ack.

Santosh said...

You are right. I couldnt fit in two arrows because of lack of space. :-)

Hence displayed request and response with single two way arrow.

Anonymous said...

Hi,

Could you please do the same call flows for the Dedicated bearer establishment including Authentication + security

Thanks

Santosh said...

Sure. When I get some time.

Ramakrishnan said...

why does Attach complete and Inital context setup response piggybacked?
why this is not done inSgsn

Santosh said...

Attach complete is not sent in Initial Context Setup. Attach complete is send in Ulpink NAS transport message.

Saraswati said...

After a bearer(say default bearer during attach) is established for an UE in LTE then will the APN use the UE IP as the DestIP in DL packets mandatorily or can the APN use nearest ASP IP as the DEST IP in DL packets.

Santosh said...

PGW will direct the traffic with UE IP encapsulated in GTP tunnel. eNB will deliver it to UE.

Anonymous said...

Hello,

The Dedicated bearer establishmen is used for what? specific services? when this is used instead of the standard bearer?
You could tell me what release was to seek information to make the diagrams?

Thank you for your attention

Santosh said...

Everything in this blog is from Release 8 as of now.

Dedicated bearers are used to specific services like, voip, email, web browsing etc. There is no restriction on when a dedicated bearer should be created for what service. It is upto the service provider.

Anonymous said...

Hello Santosh.

Could you tell me what is the diference between x2-handover and Intra-MME/SGW handover?or is the same process?

Thank you for your attention

Santosh said...

X2 Handover takes place when the 2 eNBs can communicate over X2 link. If the X2 link doesnt exist between eNBs then S1 handover takes place which can be intra-MME/SGW or inter MME/SGW.

Anonymous said...

Hello Santosh.

Do you know any simulator that allows to simulate the process of mobility of the LTE, even if through some changes?

Thanks

Santosh said...

As far as I know there are no open source simulators. You can check with Catapult or Nethawk for the test tools.

I think both organizations have loads of simulators to test LTE networks.

Anonymous said...

Hello!

Could you explain how the eNB see the resources they have available, or where I can get this information.

thanks

Santosh said...

I believe its upto the eNB makers. You can check with 36 series specifications.

Anonymous said...

Hello!!

I would like have your opinion relatively my ideia.

when the UE connect to the eNB it's created a default bearer between SGW and PGW, this bearer is transparently for the eNB. Regarding the eNB, the UE indicates how many resources he need, which will be placed if possible. Depending of the type of services used by the UE, the core decides creates a dedicated bearer or not.

Do you agree?

Thanks

Santosh said...

Yes!

Anonymous said...

Hello!

In my last doub, I said that the default/dedicated bearer is only created between SGW and PGW, but actually this bearer is created between UE<---->eNB<---->SGW<-->PGW and this is called EPS bearer. This bearer is created to give some QoS.
This bearers only are created if the UE has allocated the necessary fisical resources in eNB.

It's really or i'm wrong?

Thanks

Santosh said...

Yes! You are correct.

Anonymous said...

Hello!

Sorry, but i have another doub.
When the default bearer is created, it is only composed by EPS bearer or also by Radio bearer, S1 bearer and S5/S8 bearer and EPS bearer is only an abstraction to upper layers?
When it's necessary a dedicated bearer, is created news bearers (EPS,Radio,S1 and S5/S8 bearers if EPS is only an abstraction to upper layers or only an Radio,S1 and S5/S8 bearers if EPS isn't an abstraction to upper layers) or the existing bearers are modified?

Thanks

Santosh said...

Ok! Here we go. When we say a bearer has been established for UE, that means Radio bearer, S1 Bearer and S5 bearers have been created. These bearer created to carry the traffic from UE to internet.

On UE, the traffic originating will be mapped to a radio bearer. This radio bearer traffic will be mapped to corresponding bearer in S1 and S5. These bearers not abstraction. They are real. Radio bearers mean radio resources have been allocated. In the same way in EPS the bearers are treated with different QoS schemes.

Hope this helped.

Amit said...

Hi Santosh...congrats!! very nice Blog for a newbei like me..a quick question - in this end-to end LTE call flow which messages relates to ESM and EMM sessions.

Santosh said...

Hi Amit

I dint differentiate between EMM and ESM here, refer to 3GPP TS 24.301 for the messages.

Regards, Santosh

RigRam. said...

Hi Santhosh,

After the Authentication, LU should happen right? With all subscription details, before MME sends create session to SGW?
Thanks for your post..it is always very insightful and simple..