Wednesday, March 24, 2010

Piggybacking in LTE

I observed an interesting feature in 3GPP TS 23.401. Feature is defined in Annex F. Now I wonder are Annexes really that important?

In LTE we always have a default bearer. Dedicated bearer may be created on need basis. Looking at the way industry is moving it is pretty evident that VOIP will be mainly used for voice over LTE. Operators would like to provide a good QoS for a voip call so they would want a dedicated bearer to handle the same. Also with LTE phones VOIP will be a very basic feature and would become a must on every LTE device. This means the network infrastructure should be capable enough to support dedicated bearer for every LTE phone and a dedicated bearer has to be activated as soon as the UE connects to the network to provision VOIP.

A dedicated bearer may be requested by UE or can be triggered by network based on rules and charging functions. Considering the above it would be nice to have both default and dedicated bearer established while UE is being attached to the network. Annex F exactly defines the same.

During the LTE UE initial attach, a create session request is sent to PGW. PGW responds with create session response. Note that dedicated bearer creation is initiated by PGW by sending create bearer request to SGW. Now 3GPP TS 29.274 provisions a method where a GTP message can be piggybacked to other. During initial attach, PGW can send a create bearer request piggybacked to the create session response indicating that it has initiated a dedicated bearer creation. Just a flag needs to be set in the GTP-C header of create session response (Octect 1 bit 5 from LSB) and create bearer request can be appended. There will be only one UDP header though. Once MME receives the create session response with piggybacked create bearer request it will initiate both default and dedicated bearer activation in single shot.

I find this mechanism very cool as lot of processing in PGW can be avoided with respect to PCRF interaction for dedicated bearer creation. This piggybacking can be enabled based on a IMSI or based on APN that is UE connecting to. Interesting!

Any thoughts?


Anonymous said...

hi Santosh

Interesting. However the method which you outline may have issues in case one operator wants to implement some control/restriction on in-roamers in the network only , allowed to do part of the things.

While in such scenarios the operator may just allow default bearers to be created for signalling , but have a control of creation of the dedicated bearer.

what are your thoughts on it ?



Santosh said...

Hi Abhay

What you said is true. But the logic can be enabled based on a check, say IMSI. It might not make sense when the user is in roaming but in the home network piggybacking can be enabled based on the imsi or APN.

Cheers, Santosh

Anonymous said...

It is likely that any LTE deployment will likely be via PCRF/SPR which should leverage policy to restrict roamers from sourcing dedicated bearers.