Google has upgraded its Nexus One phone so it works on the same high-speed wireless network as Apple's iPhone, putting the two rivals on an even more direct collision course in the mobile market.

The Nexus One is unlocked, which means users can use it with a SIM card from most GSM operators worldwide. Currently the device is compatible with most 3G networks, including T-Mobile in the US. However, there are some carriers that have different 3G frequencies, such as AT&T in the US and Rogers Wireless in Canada, so users with these SIM cards can only access 2G or EDGE networks on their Nexus One.

Starting today, an additional version of the Nexus One is available from the Google web store that is compatible with AT&T?s 3G network. This new model can be purchased as an unlocked device without a service plan. In addition to AT&T?s 3G network, this device will also run on Rogers Wireless in Canada. And like the first version of the Nexus One, it can be used with most GSM operators globally.

Additionally, Nexus One devices can now be shipped to Canada from Google?s web store, and will work with a SIM from Rogers Wireless.

The switch to AT&T's faster system represents another step in Google's attempt to siphon sales from Apple's iPhone with its own version of a sleek mobile phone that relies on touch-screen technology.

The unsubsidized version of the Nexus One sells for $529. The iPhone sells for as little as $99 with a two-year commitment to subscribe to AT&T.