3G's HSPA (high speed packet access) scheme for fast mobile data can be great, but like so many other networks, it's a contended medium and as such, it won't always be fast.

That's one of the conclusions I drew from the HSPA demo at this week's Sony Ericsson launch. The phone maker had its new 3G modem plugged into a variety of devices - including Sony's fascinating Vaio UX490 ultra-mobile PC - and networks, but the speeds exhibited didn't even hit 1Mbit/s, never mind the 7.2Mbit/s that Vodafone's service is theoretically capable of.

In fact, it didn't look a lot better than the 384kbit/s that's genuinely possible with non-HSPA 3G phones.

Part of the problem was probably coverage - we were deep inside an old cinema that's now a "venue", and the walls looked pretty thick - but there's more to it than that.

There's also contention for the airwaves, not just from the other demo kit but also from the many journalists and Sony Ericsson staff around. Run a scan for Bluetooth devices at an event like this, and it's not unusual to turn up twice as many phones and PCs as there are people present.

And then there is contention for the backbone. It's all very well being able to run multiple 7.2Mbit/s links over the air, but if the backhaul from the base station is only 10Mbit/s, the data is not going to get through.

It's similar to the problem that ISPs have hit with wired connections. They calculated their acceptable contention ratios, but didn't foresee bandwidth-heavy apps such as BitTorrent and media downloads in general, so now they're annoying their customers by putting download caps on their services.

So yes, the convenience of wireless broadband will win fans to begin with - a big plus is quite simply that you don't have to wait at home for the BT technician to come and install stuff.

But over time, and as usage picks up, the question is whether the networks can genuinely carry the load.