What are GANs (Generic Access Networks) doing in the femtocell?

I have previously written about GANs (Generic Acces Networks) in the article Driving Network Convergence with GANs (Generic Access Networks)  

So then you may ask what are GANs doing in the femtocell section? Wasn’t the femtocell technology established as an alternative to dual-mode solutions? To understand the logic here, you know that despite UMA (Unlicensed Mobile Access) being originally developed to support GSM traffic over unlicensed spectrum, its concept and technology can be successfully applied to licensed air interfaces as long as IP is used for transport and broadband backhaul is available.

The main distinction between GAN dual-mode and femtocell implementations is that with the latter, a GAN client is supported in femtocell rather than in the handset. As shown in the illustration below, in this approach the GAN Up interface encapsulating A and Gb, is established between a GAN-compliant femtocell and GANC in the operator core infrastructure, which converts GAN signaling to regular GSM traffic.

GAN femtocells make a lot of sense. For one, this approach is in large part standards based, with all the typical benefits of standards-based solutions. Operators embracing GAN now have both dual-mode VoIP and femtocell deployment options and therefore can more finely tailor their solutions to specific markets and subscriber segments.

Generic Access Network as defined in 3GPP is a current 3GPP standard that may be used to support Femtocells’ Home NodeBs.

The figure below illustrates HNB functional architecture utilizing GAN Iu mode as a basis for that architecture. The key consideration for the architecture is the functional splitting of the traditional RNC role between the HNB and Home NodeB Gateway (HNBGW). In this architecture, HNB is responsible for the radio aspects and the HNB GW is responsible for Core Network (CN) connectivity.