It's a standalone TiVo series 1 unit. We don't have DirecTiVo over here in the UK. The closest thing would be Sky+ but that's from Sky (satelite broadcasting company) and doesn't use a TiVo unit.
The sub checking is done by the unit connecting to the TiVo servers over the internet and requesting the guide data. They're identified by a serial number inside the unit itself.

- Trevor