Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs. The most common use regards online gaming when the game doesn't respond in sync with the player's controls, usually due to a slow internet connection, server latency or overworked hardware.
Lag is also often used in reference to video games to describe to the delay (or latency) between an action by a player and the reaction of the game.
In distributed applications (such as MMORPGs), lag is often caused by communication latency, which is the time taken for a sent packet of data to be received at the other end. It includes the time to encode the packet for transmission and transmit it, the time for that data to traverse the network equipment between the nodes, and the time to receive and decode the data. This is also known as "one-way latency". A minimum bound on latency is determined by the distance between communicating devices and the speed at which the signal propagates in the circuits (typically 70–95% of the speed of light in vacuum). Actual latency is often much higher because of packet processing in networking equipment, and other traffic.
Lag in local video gamingEdit
All video games incur some lag, since once an input from the player is received, the game must compute the next frame of video and that video frame must be scanned out to a display device. But in general parlance, video game lag refers to delays that are noticeable to a player. The tolerance for lag depends heavily on the type of game. For instance, a strategy game or a turn-based gamewith a low pace may have a high threshold or even be mostly unaffected by high delays, whereas a twitch gameplay game such as a first-person shooter with a considerably higher pace may require significantly lower delay to be able to provide satisfying gameplay. But, the specific characteristic of the game matter. For example, fast chess is a turn-based game that is fast action and may not tolerate high lag. And, some twitch games can be designed such that only events that impact the outcome of the game introduce lag, allowing for fast local response most of the time.
Main article: Lag (online gaming)All online video games such as Evony, Halo, World of Warcraft, Final Fantasy XI, Call of Duty, Adventure Quest Worlds, RuneScape, League of Legends, Heroes of Newerth, War of Legends, Ace Of Spades , etc. incur online lag due to a combination of local and remote processing lag and communications latency, and user tolerance for lag depends highly upon the type of game, similarly as it does for local gaming. In general parlance, the round-trip network latency between a client game and the host server is referred to as the client's ping time.
Main article: Cloud gamingCloud gaming is a type of online gaming where the entire game is hosted on a game server in a data center, and the user is only running a thin client locally that forwards game controller actions upstream to the game server. The game server then renders the next frame of the game video which is compressed using low-lag video compression and is sent downstream and decompressed by the thin client. For the cloud gaming experience to be acceptable, the round-trip lag of all elements of the cloud gaming system (the thin client, the Internet and/or LAN connection the game server, the game execution on the game server, the video and audio compression and decompression, and the display of the video on a display device) must be low enough that the user perception is that the game is running locally. Because of such tight lag requirements, distance considerations of the speed of light through optical fiber come into play, currently limiting the distance between a user and a cloud gaming game server to approximately 1000 miles, according to OnLive, the only company thus far operating a cloud gaming service.
Cloud gaming is a very new technology, but early tests have shown that in practice, cloud gaming lag is only slightly higher than local console lag. For example, Unreal Tournament 3 incurs up to 133ms of lag on a console and was tested in July 2010 as incurring 150ms in lag on OnLive, resulting in the assessment in July 2010 that "Out of controlled conditions, OnLive has managed to get within spitting distance of console response times". In September 2010, reviewers reported a steady reduction in lag due to constant improvements in the technology, to the point where "the actions on screen were one-to-one with my input controls. In fact, I forgot that it wasn't running natively on my PC."