ok, i got an estimation of how much net is used and it's <3kb/s per player if everybody has the same beatmaps.
If someone hasn't got the beat map, it ranges from 8mb (no video), 20mb (with video) and 80mb (with hd video) and you can download the beatmap through the game (but it'll be less of a hassle if people download the beatmaps before respawn or grab it off someone at the venue.)
Because the online service is public, you'd probly need to enable a password, so randoms don't enter.
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