The MSC is part of the core network, and it handles circuit switched calls/data, the SGSN is also part of the core network, and it handles packet switched data.

The MSC is part of the core network, and it handles circuit switched calls/data, the SGSN is also part of the core network, and it handles packet switched data. When AT&T is telling you not to use your femtocell for data, it isn't just because it puts an extra strain on their network, its also significantly slower than if you just used the wifi you probably already have to begin with. The extra hops it has to go through - from the femtocell, through your ISP, to AT&T's SGSN, GGSN, then out to AT&T's ISP, make it seem silly to want to use a femtocell for data. In addition, I don't see any reason they can't charge you for the femtocell, even though it is using your ISP's resources you already paid for, it still adds value to your service. The SIP gateways that providers like T-Mobile use seem like a better alternative to femtocell's anyways. With these, your voice and data both go over wifi, and only the voice will enter the core network. When you need to be paged on a mobile termination, the core network knows you are registered as a SIP guy, so it just sends you a SIP INVITE which reaches you over your wifi. When you roam back into the cell coverage area, it goes back to normal.