About GSM/Wifi handover

Alan Jones junk at alan2.com
Thu Jul 26 16:13:00 CEST 2007

> It's called UMA and is documented in 3GPP TS 44.318 and 3GPP TS 43.318

Well, with a quick look at 44.318, which is titled "Generic Access (GA) to the A/Gb interface", I'd say that this is not a spec which is going to help us out. 43.318 is the technical realization of the same technology. 

This diagram displays the location of the A/Gb interfaces pretty clearly:

Anyways, the access to the MSC which those specs define is way lower (stack wise) than we have access to. If you look at figure 2 and 3 in 43.318, the stack is pretty clearly defined. The GA protocols just provide an alternate transport layer for the GSM Layer 3 messaging, on which messages from MM and friends travel, instead of going out over RR/LAPDM/GSM. This technology is more akin to a mobile handover between GSM and UMTS (Inter-RAT Handover), where all the signaling and mobility stuff stays the same, just the raw data transport (L1 and L2) changes.

I can say with almost absolute certainty that we will not be able to get this messaging without explicit support from the GSM baseband module... And in that case, I would fully expect that the baseband module would implement the GA and IPSec stuff itself (maybe in a case where it owns the WiFi controller) for security reasons.

Wally, you seem to be pretty familiar with 07.07, have you seen any extensions to support this technology from the high level commands? 

I honestly think that it will take a VoIP-based solution (supported by the service providers), since it is really the only true open telephony technology, in order to get the signaling and data transport to a high enough level that we could implement this sort of thing without explicit support in the baseband. With that said, from a network perspective, some level of support in the baseband would make things significantly smoother, so I would anticipate that any provider doing such thing would just expect the baseband chipset to implement at least some stuff. Keep in mind that generally, Handover is fully controlled by the network, with input from the MS.


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