Just how Microsoft plans to challenge Apple's iPod is becoming a little clearer.

Microsoft filed some specifications for the Zune digital-media player with the Federal Communications Commission on Thursday. Among the details: Toshiba will make the device; it will have a built-in wireless networking antenna and an FM radio tuner; it will have a 30-gigabyte hard drive and a 3-inch screen.

To compare, Apple's 30-GB iPod has a 2.5-inch screen and doesn't come with an FM tuner or a wireless antenna. Users can, however, buy an FM tuner add-on for the iPod. Apple also offers six other iPod models.

Microsoft has big plans for the wireless networking feature. According to the filing, Zune users will be able to send songs, photos, playlists and albums to other Zune devices. And users will be able to act as a virtual disc jockey using a feature that allows them to stream music to up to four other Zune devices.

However, at least initially, it doesn't look like Zune users will be able to download songs and other media directly from the Internet. The FCC filing says that they'll have to upload music, pictures and movies to the device from their computers when not getting them from other Zune users.

The new documents don't give any details on the music service Microsoft will offer in conjunction with the Zune. However, if I had to guess, the service will be based on a subscription model. That's likely the only way the company would be able to allow Zune users to send songs to each other without raising fears at the music labels about illegal copying and trading.

If Microsoft offers a subscription-based service, that too will be a point of differentiation with Apple's iTunes, which sells songs without subscriptions.