Causes
While a single-player game maintains the main game state on the local machine, an online game requires it to be maintained on a central server in order to avoid inconsistencies between individual clients. As such, the client has no direct control over the central game state and may only send change requests to the server, and can only update the local game state by receiving updates from the server. This need to communicate causes a delay between the clients and the server, and is the fundamental cause behind lag. While there may be numerous underlying reasons for why a player experiences lag, they can be summarized as insufficient hardware in either the client or the server, or a poor connection between the client and server.
Hardware related issues causes lag due to the fundamental structure of the game architecture. Generally, games consists of a looped sequence of states, or "frames". During each frame, the game accepts user input, performs necessary calculations (AI, graphics etc.). When all processing is finished, the game will update the game state and produce an output, for example in the form of a new image on the screen and/or a packet to be sent to the server. The frequency at which frames are generated is often referred to as the frame rate. As the central game state is located on the server, the updated information must be sent from the client to the server in order to take effect. In addition, the client must receive the necessary information from the server in order to fully update the state. Generating packets to send to the server and processing the received packets can only be done as often as the client is able to update its local state. Although packets could theoretically be generated and sent faster than this, it would only result in sending redundant data if the game state cannot be updated between each packet. A low frame rate would therefore make the game less responsive to updates and may force it to skip outdated data.
Conversely, the same hold true for the server. The frame rate (or tick rate) of the server determines how often it can process data from clients and send updates. This type of problem is difficult to predict and compensate for. Apart from enforcing minimum hardware requirements and attempting to optimize the game for better performance, there are no feasible ways to deal with it.
Perhaps the most common type of lag is caused by network performance problems. Losses, corruption or jitter (an outdated packet is in effect a loss) may all cause problems, but these problems are relatively rare in network with sufficient bandwidth and no or little congestion. Instead, the latency involved in transmitting data between clients and server play a significant role. Latency varies depending on a number of factors, such as the physical distance between the end-systems, as a longer distance means additional transmission length and routing required and therefore higher latency. Routing over the Internet may be extremely indirect, resulting in far more transmission length (and consequential latency) than a direct route, although the cloud gaming service OnLive has developed a solution to this issue by establishing peering relationships with multiple Tier 1 network Internet Service Providers and choosing an optimal route between server and user. In addition, insufficient bandwidth and congestion, even if not severe enough to cause losses, may cause additional delays regardless of distance. As with the hardware issues, packets that arrive slowly or not at all will make both the client and server unable to update the game state in a timely manner.
Online game systems utilizing a wireless network may be subject to significant lag, depending on the architecture of the wireless network and local electromagnetic interference impacting that network. Although radio propagation through air is faster than light through optical fiber, wireless systems are often shared among many users and may suffer from latency incurred due to network congestion, or due to network protocols that introduce latency. And, in the event of electromagnetic interference, transmitted packets may be lost, requiring a retransmission which also incurs latency.
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