LTE - Indirect Tunnel during Handover

LTE is a high speed network with an assumption of always on connectivity to packet data network even when UE is moving at high speeds. So during the handovers it is assumed there is no packet loss. But when UE is moving from source network to target network, there is definitely a connection break and make. While the connection is broken and again made at target network, the data to UE is buffered at the source eNB and forwarded to target eNB once the handover is complete. If there is a direct link between source and target eNB then data buffered at the source eNB will sent over it to target eNB. Else indirect tunnel will be used . Below figure shows S1 Handover with SGW Relocation when Indirect Tunnel is used.
 

 This post specifically concentrates on how the buffered data will flow in case of indirect tunnel. Cristina in her blog has very clearly written about this, but I would like to make rather simple explanation

The links in “orange” are normal links and links in “black” form a indirect tunnel. The source eNB decides whether direct tunnel is present or not. If direct tunnel is not present, source eNB SHALL NOT include “Direct Path Forwarding Availability” IE in Handover Required message. To make things a bit simple I am assuming that MME has NOT been re-located, but considered that SGW is relocated. Once MME receives the “Handover Required” message it sees that target eNB is being served by another SGW. So MME will create a new session with target SGW and sends “Handover Request” message to target eNB. Target eNB shall respond with “Handover Request Success” and includes “indirect tunnel DL TEID” that is to be used by target SGW for indirect tunnel, along with normal S1-U DL TEID.  Once MME receives this message it shall forward “indirect tunnel DL TEID” to target SGW in “Create Indirect Tunnel Request message”. Now the target SGW gets to know the TEID which it should use for sending data over indirect tunnel and also gets ready to receive the data from source SGW. In Create Indirect Tunnel Response target SGW sends the “DL TEID” that source SGW should use to send data over indirect tunnel towards target SGW. This information is conveyed to source SGW by MME in “Create Indirect tunnel Request”. Now source SGW gets to know that indirect tunnel is created, so it sends a “UL TEID” that source eNB should use to buffered data over indirect tunnel to it. This TEID is sent to source eNB in handover command. This completes the indirect tunnel.

The data flow will be:-
- Source eNB buffered packets to Source SGW over UL TEID that source SGW sent.
- Source SGW sends packets to target SGW over DL TEID that target SGW has sent to MME, which was forwarded to source SGW.
- Target SGW sends packets to target eNB over DL TEID that target eNB has been sent to MME which was sent to target SGW.
 

I know this is bit confusing, but here it is.

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.