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Speed limits

Posted on 8 Jul 2008 at 14:54

Paul Ockenden clears up the confusion surrounding the options available for mobile data from the UK phone networks.

One of this column's regular correspondents, Richard, has emailed to say: "I've found it very confusing dealing with the difference between UMTS, GPRS/EDGE, HSDPA etc. My data dongle has settings for UMTS Only or UMTS Preferred. And ditto with EDGE. I'm not sure what performance to expect from each option and which is the best strategically. Perhaps a future Mobile & Wireless column could explain the issues here?"

Richard makes a very good point, and I should give myself a small slap on the wrist here - I often write about things like HSDPA and UMTS, assuming that everyone knows what I'm on about, but I'm sure that to some it just looks like I'm playing acronym bingo! To start making sense of mobile data, it's worth having a quick recap of where we are and how we got here.

The first really usable mobile data standard was something called CSD (Circuit Switched Data). Prior to that, people had tried to make data connections via their phones using dial-up modems, sending buzzes and beeps over the GSM (and even the pre-GSM analogue) networks, but this was always pretty unreliable. CSD introduced a proper digital data channel, although the outbound interface from the mobile network was still via an emulated dial-up modem rather than direct IP. Still, it meant you could at last use your phone for data connections, albeit at a pathetic 9.6Kb/sec (and in these megabit broadband days it's easy to forget just how slow 9.6K was).

Although some networks offered higher-speed versions of CSD, essentially by gluing together several of these 9.6K channels, the next evolutionary leap in mobile data was the launch of GPRS (general packet radio service). Sometimes described as 2.5G, GPRS introduced a proper data channel - CSD simply fudged things by stuffing data into time slots allocated for GSM voice traffic. As a result, it raised speeds to 114Kb/sec, which was a real eye-opener for those of us who'd been stuck with CSD. The other big difference was that GPRS directly supported IP traffic, with no dial-up modems in sight. GPRS is hardly fast by today's standards and is getting long in the tooth, but it's still very much used: most BlackBerry phones work over GPRS, and if your office or house has a "RedCare" monitored alarm system, the chances are the backup signalling path uses GPRS.

There's a tweak available to make GPRS faster called Enhanced Data rates for GSM Evolution or EDGE (a name thought up to fit the acronym if ever I saw one). EDGE offers a maximum throughput of around 230Kb/sec, almost 3G speed, and the mobile network operators can deploy it relatively cheaply, in many cases by merely upgrading the firmware in their cell towers, since the hardware already supports it. This being so, I'm always surprised that some networks choose not to deploy it: for example, you won't find EDGE on T-Mobile in the UK, although it's supported on many of the firm's overseas networks. Instead, T-Mobile decided to concentrate on rolling out 3G in the UK, the next stage of evolution.

What we usually call 3G is correctly called UMTS (universal mobile telecommunications system), but if you ask for a UMTS phone in your high-street phone shop, I guarantee you'll get a blank look, so like that phone shop guy I'll continue to call it 3G. (Note for any pedants reading: I do know that EDGE is officially classed as a 3G standard because of its speed, so hold the email!) 3G is the lowest standard you should settle for in a mobile device nowadays, operating up to a theoretical maximum of 384Kb/sec; just a shade faster than EDGE, although you'll often not notice much difference. What's important is that 3G runs over a different network from GSM and GPRS on different radio frequencies, sometimes even from different cell towers, although often they're shared, and in a given location you might get a brilliant GSM signal but no 3G or (more rarely) the reverse. Seeing a strong five-bar mobile signal at your house or office does not guarantee that you're also going to get a good 3G signal...

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