LTE - Indirect Tunnel during Handover

 哎。搬运工。。。有什么好屏蔽的么。

http://wired-n-wireless.blogspot.com/2010/11/lte-indirect-tunnel-during-handover.html

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.