Saturday, November 21, 2009

LTE S1 Handover: Indirect tunnel

I wrote about LTE S1 handover in the white paper. However I dint cover this interesting concept. I am working on next revision of white paper and will make it public soon.

X2 based handover is used when there is X2 link between source and target eNB's. This also means that EPC should be just informed about change in eNB FTEID's for downlink. If we look deep, the downlink packets are still sent to source eNB until EPC receives a handover complete notification and the target eNB FTEID's. The buffered downlink packets are sent to target eNB by source eNB later which are then sent to UE. These packets are directly sent over the X2 link. This means there is direct tunnel formed between source and target eNB for data forwarding.

Now I started looking at a S1 based handover. I saw something called Indirect Tunnel.


Now in S1 handover we assume that there is no X2 link between source and target eNB. Also I am assuming that SGW has changed during the handover. So the downlink packets buffered at the source eNB during handover execution should be sent all the way to Source SGW, then to target SGW and then to target eNB. To do this target MME sends a GTP based message Create Indirect Data Forwarding Tunnel Request to target SGW. In this message the target eNB FTEID for downlink are sent. In the same way the message is sent from source MME to source SGW. This will create a indirect tunnel between source and target eNB. After this the buffered packets are sent from source eNB to target eNB which are later sent to UE.

Sounds good? Anything to add? Please feel free.


Santosh.M said...

Hi .. Thanks for the info.. Can you add more info about the tunnel creation and the teid exchange during the creation of indirect tunnel.



Hi....I have doubt regarding eGTP tunnel echo request message as mentioned in 29.281- R9 spec section 7.2.2...which says...
"For the GTP-U tunnel setup between two nodes for forwarding user traffic, e.g. between eNodeBs for direct forwarding over X2, Echo Request path maintenance message shall not be sent except if the forwarded data and the normal data are sent over the same path."

Is it possible normal and forwarded data path be same...Plz suggest

Abhishek Saurabh

Santosh said...

I guess so!

Anonymous said...

Hi Santosh,

Is it possible that the source and Target MME and SGW are same in case of S1-Handover.


Santosh said...


Anonymous said...

Hey Santosh,
Can you plz. cover scenarios when to do S1 handover when already X2 link is present.

Santosh said...

You can do S1 handover even X2 link is present
- when target eNB doesnt respond to X2AP messages
- when the X2 handover has failed for some reason (over X2 link).

Anonymous said...

Hi Santosh,
Please let me know.
When eNodeB has two IP Address(S1,X2), target eNodeB knows how whether Indirect or direct Data Forwading will be used?
I think that Handover Request message don't have IE for direct/indirect data forwarding .