a 3G phone that works on both Tmobile and AT&T?

Why aren’t there devices out there that can use both WCDMA 1700 and 1900 frequencies? Why do 3G phones have to be compatible with only one of either T-Mobile or AT&T?

Is there a technical reason preventing this? or are the reasons purely business driven?

R

RF engineering is a black art. The more frequencies a device has to cover, the more difficult it is to engineer, especially in a very small package. For example, an antenna that performs well at 1700 MHz may not perform well at 1900 MHz. Adding in another antenna not only would take up more space, but may affect the performance of the first antenna.

I’m far from an expert at this stuff, but it’s not hard for me to believe that it’s not practical (or maybe cost effective) to make a cell-phone sized device that acceptably supports all of the frequencies required for both AT&T and T-Mobile with current technology.

The phones in the US are mostly sold by the phone networks. The networks buy the phones from the manufacturer and specify that the phones are locked. There is a setting that controls what network the phone will connect to.

They are two basic frequencies for cell phones. around 800MHz and 1.9 Ghz. In the past you might have seen phones that did not operate at both frequencies. But now with big national networks that have been pieced together from smaller companies and various spectrum auctions. A phone that does not handle both frequencies is not very useful to the networks.

Looks like Motorola is trying to fix this - http://www.androidcentral.com/motorola-building-module-get-phones-both-t-mobile-att-3g.

My T Mobile G1 works on AT&T. T Mobile’s network is not that big so you get to roam for free on AT&T. (unless things have changed since Sept)

I suppose you’re referring to Voice connections. Most GSM phones in the market today are “Quad” band and have no problem working on different cell networks, even across countries.

My question was regarding 3G data connections. My research so far hasn’t yielded even a single mainstream phone - smart or otherwise that does both 1700 (TMo) and 1900 (ATT) bands. The TMo G1 US version does only 1700/2100 bands, which means your phone wont be able to connect to an ATT 3G network.

Moreover device manufacturers seem averse to producing GSM/CDMA hybrids. But then Blackberry has some models (I reckon they’re called "World Edition"s) that do this.

R

Yes, it’s entirely business (profit) driven. By ‘locking’ the phones, cell phone companies make it harder for you to switch to another company when you are dissatisfied with their service or prices.

There is no technical reason the phones can’t work with all cellphone networks. In fact, in Europe & most of the world they do so. You may have to replace the SIM card from Company A with one for Company B, but the phone itself will still work. Also, in many countries in the EU, ‘locking’ phones the way American cell phone companies do is illegal.

The frequencies a phone’s radio works on is a completely different issue than SIM locking. You can unlock an iPhone all you want, and the radio still won’t be able to do 3G on T-Mobile’s network – it physically does not support that frequency.

I dont think its a big challenge technically. There’s no shortage of quad-band phones. Most of this is business driven. I think in the case of the NexusOne it was just rush to market or AT&T not wanting in on the deal. The FCC needs to certify all radios and it was probably quicker to just get certified for T-Mobile.

In the US its expected for someone to buy a phone for a small fee and pay more per month to make up for it. In many countries the opposite is true. You pay a hefty fee for the phone and pay less per month for service, as you are just buying service not a subsidy for your phone. So why should US companies make these phones portable? Yes, it would be nice, but the US system lends itself to lock-in.

T-mobile recently is allowing European-like pricing. Buy a phone and get a cut on your monthly.

Nexus One isnt locked into T-mobile. It uses 2100mhz, which is the most common 3G UMTS frequency worldwide (I think). At launch google said its going to work in the UK, Singapore, and Hong Kong. Delaying the product just for AT&T doesnt make much sense, especially if AT&T’s relationshop with Apple means they might not even allow it. Google has announced a Vodaphone model for Europe and a CDMA model for Verizon in the US. I have a feeling that AT&T just told them to piss off.