For more UMA answers, see my more recent article.
The carrier's server is called an UMA Network Controller, (UNC). From the perspective of the phone, the UNC looks just like a regular cell tower, and it talks to it in the same way as a cellular base station, except that everything is wrapped up and forwarded over the Internet. Communicating in this way has some important differences from the way your laptop accesses the Internet
When you are using a cell phone, the idea is that your IP address will stay the same no matter which cellular tower you are at. So, if you are loading a web page and driving down the highway at 120 km/h, you might switch from cell tower to cell tower, but your IP address will remain the same, and your web page will still load. The carriers accomplish this by giving you an IP address in their core network. When you ask for a web page, your request is forwarded through your cell phone company's servers. Your cell phone company actually downloads the web page for you, and then sends it to your phone.
The same thing happens with UMA. You might rove-in to your WiFi connection, but your IP address will remain the same. Your device is still directly connected to the carrier's core network, and the web page loads through your carrier's servers.
So if you wanted to load Skype on your phone to try and make free phone calls, forget it. It would cost you more in data usage charges than you'd save. Also, it's probably technically impossible, due to the amount of extra work your phone has to do.
When your laptop is transmitting data, the data is broken up into small chunks, called IP packets. These IP packets can then be transmitted directly over the Internet.
Over UMA, however, the situation is very different. IP packets over UMA are transmitted using the same techniques as if they were going over a cell tower. That means that after your web browser forms an IP packet, it has to be transformed into a form that is recognizable by carrier's servers. The packet will first be broken up into smaller chunks, called frames. Each frame will then have extra information added to it, called headers, that is needed to be understood by your carrier's network. The extra information is not so much, but what is really costly is the security.
You many be familiar with IPSec already. It's used by a lot of companies that issue their employees laptops. If you have to work from home, you might have some kind of security key, and to log in, you'll start up an application called a VPN Client, and then boom, it's as if you were sitting in your cubical at work, except that you're at home in your underwear.
UMA phones use the same technology. To connect, they form an IPSec tunnel into the carrier's network. Instead of a password, the phone checks that your SIM card is valid and up to date before letting you on.
IPSec provides great security. The packets are encrypted, and it's pretty much impossible to figure out what they mean, what web pages you're browsing, or what you are saying in your phone calls. However, it has a huge cost in terms of overhead. Each packet has to have extra headers added, and then it's encrypted. This encryption can expand the packets by as much as 30%. This means that your web pages will take 30% longer to load vs. using your laptop, even under the best of conditions.
I filed this patent to try to mitigate the problem.
You see, it's all part of the strategy to get you to use your cell phone at home. Carriers would much prefer you to use your cell phone all the time, so they can squeeze more revenue out of you. This would be beneficial for the consumer too, because rather than paying for a cell phone plus a landline, you ditch your landline and just pay a little more for your cell phone.
But if everybody did this, without UMA, it wouldn't work, because cell towers can only support a few dozen calls at the same time. UMA is a relatively cheap add-on to a carrier's infrastructure, so it makes sense to add it. Adding a new base station to cover dead spots in a neighbourhood costs a quarter million dollars. A WiFi access point, at wholesale rates, costs maybe $30.
There are a few reasons behind this. UMA specifications were only finalized as recently as 2005, and unlike the mature GSM specifications, they leave much open to interpretation. They don't address things like when your phone is supposed to rove in and rove out. There isn't an easy way to figure out if your Wifi connection is stronger than your cell tower. Your typical tower transmits at several watts of power because it has to reach tiny cell phones up to 30 km away, but your typical access point transmits at only a tiny fraction of that power. Your cell phone can't just choose the stronger one. There just isn't an easy way to decide which one to use. If your phone chooses to use the Wifi access point, but it's too weak to be used, then quality will suffer.
Quality of service is also a problem. If your laptop is downloading movies using bittorent, and you're trying to make a phone call, it just isn't going to work. Theoretically, a technology called Quality of Service is supposed to fix problems like this, but the technology just isn't there today in 2007. Most access point deployed don't support it at all, or they say they support it but it is completely inadequate. So if you are planning on making internet calls while watching videos online, plan on getting an up-to-date AP with decent QoS.
Finally, most people's access points use the default settings set at the factory. That means that they will be using the same WiFi channel, and two of them placed close together (as in an apartment building) will cause interference. Other appliances like Microwaves will also cause a degradation of the signal.