As you will no doubt have heard rumblings about, the 900MHz spectrum is being opened up globally for 3G use and some manufacturers are already shipping devices that are able to take advantage of this. But what does that actually mean?

Currently, mobile devices are typically referred to as either ‘tri-band’ or ‘quad-band’ and either ‘single mode’ or ‘dual mode’.
The terms tri-band and quad-band refer only to the bands which the device is able to use for GSM/GPRS service: tri-band = 900/1800/1900 MHz and quad-band=850/900/1800/1900 MHz.
The terms single mode and dual mode refer to whether the device is a GSM/GPRS-only device, or whether it is both GSM/GPRS and UMTS capable.

The frequencies that a device is capable of using for GSM/GPRS service tend to be referenced specifically, because different parts of the world use different bands, therefore it is important to know that a given device will work in your part of the world.
However, UMTS-capable handsets are simply referred to as ‘dual mode’ because UMTS uses the same frequency globally: 2100MHz, so there is no need to be any more specific (although having said the implementation of UMTS in North America only uses 2100MHz for downlink, the uplink channel differs in North America from the rest of the world).
The recent announcement that 900MHz is now being made available for UMTS service means that it will now be necessary to refer to devices as being both quad-band (GSM) and dual-band (UMTS).

NOTE - when devices are referred to as being ‘900MHz-capable’, they actually operate within a range of different frequencies around 900MHz, depending on which mobile operator the SIM is with. 900MHz is used as an umbrella term.

Why is this happening?

This decision is a global one, made by the ITU, the International Telecommunications Union, part of the UN, which is responsible for managing the global interoperability of mobile networks, and approved by the EU for implementation within Europe.

As you’ll know from my guide to mobile data technologies (http://blog.brightpointuk.co.uk/introduction-mobile-data-technologies), 3G network protocols are attractive not only to the customer because they provide higher data rates, but also to the mobile operators because they enable them to cram higher numbers of users onto base stations and make more efficient use of the available bandwidth, enabling simultaneous voice and data transmissions, etc.
The physical properties of the 900MHz spectrum from a radio perspective mean greater coverage and less signal attenuation than the 2100MHz band. It therefore makes sense to use 3G network protocols in preference to the older GSM standard wherever possible.
Essentially, GSM is now ‘old hat’ and this is the first step in the move to retire it altogether.

In the UK this has caused some controversy among the operators due to the fact that when mobile networks were first launched in the UK, backed by the government, the 900MHz spectrum was effectively given away to Vodafone and O2 for nothing.
Orange and T-Mobile rolled out later using the 1800MHz band.
Network-sharing agreements between operators mean that these distinctions are no longer as clear-cut in places, and Vodafone also own a small amount of frequency in the 1800 MHz region.
Tri-band and quad-band devices are able to use both of the frequencies so it is not a problem: both frequency-selection and cell hand-over happen transparently to the user.
The important bit is that when the 2100MHz frequency band was auctioned off by the government for 3G services, all networks (including Hutchison 3G, or Three) paid for their chunk on the understanding that this was the only frequency where 3G services would be permitted.
The news that the 900MHz band is now going to be allowed as well, means that Vodafone and O2 can effectively increase their 3G coverage for nothing. Well, not ‘nothing’ – it will cost billions and take years to update the network, but these operators have the option to do this – the others do not, and this is why they’re now complaining.
OFCOM are considering levying a charge to Vodafone and O2 to be able to use the 900MHz spectrum for 3G service, despite the fact that they already own the bandwidth.
Whatever the decision arrived at by the operators and OFCOM – this is happening from a technology standpoint and financial squabbles bear no impact on the technology or its deployment globally.

So what?

As with current services all of the clever radio stuff will happen in the background: if your device is a dual-band UMTS device, it will ‘know’ itself what frequency to use automatically.
When the 900MHz frequency allocation is 'turned off' for GSM use by the operator, then users will have no choice but to use a UMTS-capable device - which the networks will no doubt offer as part of the upgrade process. Although operators may decide to further divide up the chunk of the 900MHz spectrum allocation they own and run both 2G and 3G service concurrently while 2G handsets are slowly phased out.

As the networks deploy 3G services using the 900MHz band instead of 2100MHz, we may start to see older single-band 3G-capable devices similarly being replaced as they won't be able to fallback to GPRS service, and won't be able to use the 900MHz band for 3G service.

For reference, if you’re interested, the global radio frequency map for mobile phone usage looks like this:

At some point in the future the 1800MHz band will also be replaced as we head towards a 2G switch off entirely in Europe.