Monday, June 14, 2010

LTE Tidbits IV

Continuation to LTE Tidbits I, II, III


There have been several questions on how RRC and EMM/ECM states are in sync. To begin with, RRC and EMM/ECM states are two seperate layers, each layer provides service to other but doesnt have authority to modify each other. eNB, acts as a relay between UE and MME, i.e it will receives RRC messages from UE and puts them in S1AP messages towards MME. This means eNB will simple pass the NAS pdu's from UE to MME without modifying them. SO at UE, there are two layers to maintain, RRC state and NAS state. We cannot have a state where there no radio bearers for dedicated bearer, but the NAS layer has the EBI for dedicated bearer.

S1AP Initial UE message holds an extereme importance. It is sent only when UE first establishes an RRC connection with eNB. This message is what makes UE move from ECM idle to connected mode.

FAQ's:-

1. In a normal TAU procedure, when MME decides to change the SGW, how does the new SGW come to know about the IP and TEID of eNB for data downlink flow and how does the eNB come to know about SGW IP and TEID?

[S] When a SGW has changed during TAU, MME will send a create session request towards the new SGW. This CSReq is for establishing the control plane.Now in CSRes the new SGW will respond with its own user plane IP and TEID that is used by eNB for uplink. This info is propagated to eNB in Initial context setup message. Now eNB will create radio bearers and once the radio bearers are created it will send downlink user plane IP and TEID in intial context setup response message to MME. This info is send to SGW in modify bearer request.

2. In Initial Attach procedure, how does the eNB comes to know about the SGW IP and TEID?
[S] Same philosophy as above applies. The trick here is radio bearers will be created only once the network has setup the EPS bearers. Once EPS bearer is created using CSReq/Res, only then then eNB creates the radio bearers and this info is sent to SGW by MME in Modify bearer req/resp

3. If OI=0 in both CSR during initial attach and S1 based HO, why does the SGW not send CSR towards the PGW in S1 based HO?

[S]CSReq is used to create EPS bearers. In S1 HO case the EPS bearers are already created, so CSReq will not be sent to PGW. BUt since the SGW has changed, PGW should be informed of new SGW, so a modify bearer request is sent to PGW.

4 comments:

Asimananda said...

Regarding FAQ 1 :

I think the CSR contains the eNB IP and TEID details towards the SGW and thus SGW comes to know about eNB details.

As per 23.401, "If the new MME selected a new Serving GW it sends a Create Session Request (IMSI, bearer contexts, MME Address and TEID, Type, the Protocol Type over S5/S8, RAT type) message to the selected new Serving GW.".

I think the bearer context carries the eNB IP and TEID details (in S1-U eNodeB F-TEID IE), though I am not too sure about this.

Now when the UE is in IDLE mode, there is no need to let the eNB know about the SGW IP and TEID details (as there will be no data flow in IDLE mode). When user is in CONNECTED mode, the TAU happens after a handover normally and handover procedure is responsible to let eNB know about the SGW IP and TEID details.

Let me know if the above is correct.

Santosh said...

eNB FTEID is sent in CSReq only if X2 HO with SGW change is in progress. Else there is no way MME will know about eNB FTEID untill and unless eNB sends them to it.

Also standalone TAU procedure can be initiated by UE in Idle. Idle mode TAU can relocate SGW and also eNB as both SGW and eNB are freshly creating the bearers.

One thing to understand here is, when UE is moving
1. A handover will occur if UE is in ECM Connected/RRC Connected mode
2. A UE will initiate TAU to let network know of its position in RRC Idle/ECM Idle mode

Asimananda said...

as per the discussions in "Spec Ambihuity" section, I have some comments reg the FAQ-1....

UE IDLE mode, neither SGW nor the eNB need to know the details of each other as no data would be flowing.

UE CONNECTED mode, no element would be changed if TAU is initiated (user not moving), if user is moving and any change is there, handover will happen always before TAU.

I hope this time I found it right.

Santosh said...

Exactly! Now spread the word! :-)