Wednesday, August 3, 2011

Understanding ISR

GTP Version Fallback

I had a discussion with my colleagues about GTP fallback mechanisms. While 3G/2G allows GTP fallback from V1 to V0 over Gn interface the same is not true for S11 interface. LTE is designed all the way to speak GTPv2. However if you look at 3GPP TS 29.274 (section 7.10), there is an interesting section pointing to Fallback to GTPv1. Of course, refer to the latest revision of the spec as things have changed from older revisions.

A GTPv2 entity may fall back to GTPv1 when the other entity responds to a GTPv2 message with GTPv1 Version Not Supported message. This is a pretty much valid and straight forward case. If a UE is moving from UTRAN/GERAN to LTE using Tracking Area Update, then MME may request for UE context from SGSN over GTPv2 message if S3 interface is configured. But if SGSN doesn’t support S3 interface then it may send GTPv1 Version Not Supported to MME. MME in this case may fall back to GTPv1 and may request the context from SGSN over Gn interface.

The second scenario deals with S16 interface, about which I never ever thought of. S16 interface is between 2 SGSNs and is GTPv2 interface. If SGSN deployed is S4 SGSN then the two S4 SGSNs communicate over S16 using GTPv2. As far as I know S16 interface deployment is going to take a lot of time, but it is an interesting case. Now assume that UE is connected via S4 SGSN to a GGSN. Remember that though SGSN supports S4, the current scenario is that the UE is connected to a GGSN over GTPv1 interface. Now the UE moves to a new S4 SGSN using Routing Area Update. The new S4 SGSN may use S16 interface to fetch the UE context from old S4 SGSN. But the old S4 SGSN established the UE context over GTPv1. So the old S4 SGSN may send GTPv1 Version not supported to new S4 SGSN. The new S4 SGSN may now request the UE context over Gn using GTPv1. Thus the fallback to GTPv1.
This kind of detail in the specification is amazing and hats off to the GTP authors.