Not a lot of people know that Wolfenstein: Enemy Territory had a number of networking advances over its predecessor Return to Castle Wolfenstein, but DOOM 3's networking model was completely different, and so these features (such as anti-lag code for better sniping) couldn't be ported over. Instead, we needed a completely new approach; we wanted everyone on the battlefield to see exactly the same thing, and almost as importantly, we wanted everyone on the battlefield to see as far as possible.

The first new idea was called "Area of Relevance." Somewhat like "Level of Detail" for graphics (or Stephen Hawking's Time Cones -- depending on your preference), it's a set of circles that emanate out from you, and the amount of data that we send back depends on what can actually affect you at a distance. A good example of this is the sniper scope. When you're not using the scope, we send less information about players that are a long way away from you -- for example, the exact direction their head is facing -- but when you zoom in with the scope, we tighten that cone (and area of relevance) and again send this data with more regularity.

Another new feature is split-stream networking. All our data is split into two streams, with things that happen at great regularity that the client can reliably predict (such as certain vehicle physics results) being sent less often, while things that happen less often (such as the firing of an artillery gun) are sent real-time. We halved the bandwidth used with this feature alone.