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UMA's dirty secrets
Posted on: 2007-07-24 19:57:10

For more UMA answers, see my more recent article.

What's UMA?

Recently, many carriers have started offering UMA, or WiFi phones. These are cell phones with WiFi capabilites. Don't be fooled -- you won't be able to get free calls and run skype on them. The UMA technology is meant to extend the carrier's cellular network into your home using your broadband internet connection.

How does UMA work?

An UMA phone operates just like a regular cell phone. It can talk to cellular base stations. But it is dual mode, and it also has a WiFi radio on board. When it finds a WiFi access point, it will attempt to connect to your carrier's servers over the Internet. If the connection is successful, it will "Rove in" and begin sending everything over the Internet.

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

Hands in your pocket

When you browse the web from your laptop, the data flows from your laptop to the web site you are visiting, with nobody in between. It is different when you are browsing with a cell phone, however. With a cell phone, you get assigned an IP address in your carrier's core network. The IP address is how your handset is identified on the network. For example, when you browse a web site, the IP address lets the web server know who to send the web page back to.

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.

UMA efficiency

Another important difference between browsing using your laptop and the cell phone is efficiency. Because your laptop is directly connected to the internet, it has a much greater advantage in terms of speed. Your tiny cell phone, however, is burdened with extra protocols that make loading web pages a very costly operation.

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.


Your UMA phone has a direct pipe into your carrier's core network. This requires a lot of security, because your carrier doesn't want just anyone to have this kind of access. So your phone communicates using a special protocol known as IPSec.

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.

UMA Advantages

If UMA is so inefficient, why use it at all? I am a strong supporter of UMA, despite its flaws. It's great for the consumer, because it gives you better coverage when you are at home. Also, it affects the pricing of your phone. Many carriers have special discounts, or even unlimited calling when you are on UMA.

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.


Early generation handsets, like the Samsung and Nokia, have a few problems. I have read reviews on the Internet and apparently they were horrible and people are asking for their money back.

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.

But wait...

Some cell phones can send data natively over WiFi, without going over carrier's servers. The carrier has no way to track this data, so you can send as much as you want. But in practice, you really have no way of knowing if it is using Wifi directly, or the UMA connection.

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2008-04-24 01:53:15
quite helpful for a quick understanding of UMA basics

usman aslam

2008-05-12 23:26:13
very informative page. Thanks. Just got my BB curve works like a charm on UMA. I think with faster internet speeds the QOS problem will just disapear.


2008-06-24 22:25:36
i just wondering...is there's any way that wifi calls and i'm not refer to skype,can be made from a iphone?i know at&t doesn't have that kind of phone or service.the only one that i know is t-mobile.but sme got the iphone working with the t-mobile sim card.it's that possible?

Michael Flaherty

2008-07-15 09:17:34
I like the UMA technology but it's obvious to me that T-Mobile does not understand the technology. Their UMA signal has been going down a lot lately, and according to the forums, this is widespread, but T-Mobile continues to not accept responsibility for the problem.

Crackberry forums are screaming about this.

sterlob red

2008-08-27 20:50:08
great summary. this finally put to bed my misunderstanding as to the wifi-uma-skype possibilities.



2009-02-03 08:21:34
I have a Nokia 6301 uma phone supplied by orange uk. The orange signal in my area is very poor so the phone connects to my base station and I must say works brilliantly! I do only use it for making and receiving calls (thats what phones are suppose to do)When my broadband drops out you can notice the difference instantly so in this case if you live in a poor reception area I would recommend. Oh yes dont take any bullshit from orange that you have to pay extra mins on my mobile tariff which is free!!!

John B

2009-10-12 18:09:16
I am on Orange in the UK and my phone used to have UMA (I say used to as I got rid of the feature).

It wasn't straightforward and involved flashing the firmware back to OEM (ie getting rid of the Orange firmware).

Why did I do this - simple - data charges!

Most mobile plans over here (well those with Orange at least) do not have generous data allowances. There was no way - with the Orange firmware - to use pure WiFi, it had to be UMA!

Having flashed the phone I now have the choice of pure Wifi or standard Orange cellular connectivity and no UMA (I tested this by putting an inactive SIM in the phone and still got WiFi).

I can understand why UMA may work for some people but for me it has no benefits and would actually cost me money.

Once my contract is up I'm seriously considering O2 as they don't generally fill their phones with garbage firmware (my phone now is much faster and has better battery life on the OEM firmware).


2009-10-31 11:30:16
UMA is the best kept secret in the cell phone industry. I'm overseas constantly for my job and it allows me to talk on my cell phone all over the world for an unlimited time without incurring roaming charges. Roaming overseas is usually 1 to 2 dollars a minute. With UMA, it's completely free. It's amazing. I can't believe the carriers let us get away with this.


2010-01-06 14:41:34
Thank You for your post. It is very helpful. I was upset that the nexus one did not support UMA. After reading your post it seems like if I were to browse the internet using wi-fi, the data would be transmitted through T-Mobile's data network, not over native wi-fi, which would slow it down. That would be a major disadvantage. Thanks for clearing things up.


2010-03-07 13:39:18
my phone signal is crap i got a uma phone from orange uk and only after 5 phones was told its my wireless box??????? any ideas?


2010-04-16 09:06:50
Thank You , if you're still there how about a little more info for us elderly trying to learn & understand on our own. Thanks great piece.


2010-05-07 18:47:58
i have blackberry 8900 from Tmobile usa.. i use UMA network when i am out of minutes. and all my calls are free... also i use this phone/feature outside of usa to make calls, and NOT be charged of roaming. all you do is go to ur network setup menue, and Turn off Tmobile tower, and turn on WIFI and search and connect to a wifi network.


2010-05-15 10:00:50
For me, the biggest benefit of UMA is international travel. I travel around the world and with UMA I can connect to the hotels WiFi and call home. I don't need a calling card or some other expensive international plan.

Jim Anderson

2012-07-05 14:19:09
This is so clear that even five years later I'm hoping that you will update it. Of course, carriers did exactly what you predicted they would do when they eliminated the exclusion for talk minutes over UMA, forcing customers into unlimited calling plans. I am also interested in your views about (1) are the same IP ports in use for UMA and other VOIP related services for example that may be offered by my cable broadband provider e.g. bundled home phone service; (2) unscheduled dropping of UMA by the phone when EDGE (I am a T-Mobile user on a Blackberry 9780) signal is minimal or non-existent...

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