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A word on lag (or latency)

Everyone who has played a high-speed multiplayer game has experienced lag. Whether it was the sluggish frame-rate in GTA2 or the being killed without getting a chance to react in Tom Clancy's: Rainbow Six. Lag is really annoying, because it doesn't give all players the same chance. If you have a 56k modem and are playing against someone with a ASDL connection in Korea, and the server is in Korea also, you're bound to experience it. While your enemy moves lightening fast and is never where you shoot, you hardly move at all and by the time you realize you're being attacked, you're already dead.

This may be a reason to develop turn-based or puzzle games.

One way around lag is optimization. Here are a few quick tips:

• Don't send information every frame.
• Only send coordinates when you move or change direction.
• Don't send the entire display, only the parts which have changed.
• Don't allow more than 32 players at once.
• Players should be able to see an opponent's connection speed (33.6, 56k, ISDN, cable, ASDL, T1, etc.) before choosing to play with the opponent.

Even if you consider all the above aspects, you may not be able to get the speed you want or need for the game to run smoothly. The next step to reducing lag is dead-reckoning algorithms. Although some algorithms use statistics, others linear equations, others cubic splines, etc., they all work in the same way. The algorithm will predict what the player's next action is and will use its prediction until a packet has arrived that describes the action that the player actually performed. A good source for articles on dead-reckoning is http://www.GameDev.net.

The last way around lag that I know of is approximation. This is basically dead-reckoning, but not quite. A game that uses this is Baldur's Gate. In the manual it says something like, "If you happen to be situated close to another player's computer, you may notice that you screens appear a little differently.". This seems to be a good method for multiplayer RPGs. When you log on to a multiplayer game, the level is transferred to your machine. When you start playing, all that is ever sent to you are the important actions that a player or NPC has done. Each computer independently updates its map and some things which depend on a random number will appear differently. The advantage of this is that since we are only transferring the important data, we are cutting down on the load.

Next : Further reading, last minute stuff, and more, more, more