The World of Warcraft Client
To solve the technological issues while providing an interactive and graphically generous user experience, Blizzard chose to create their software with a "fat" client. This would be unlike browser-based technology, where the user interface was relatively "light" and the bulk of information was stored on the server. The original release of the World of Warcraft architecture required four gigabytes of hard drive space to store graphics, music, maps, animations, and practically every other aspect of the environment. This was a significant amount of hard drive space at the time, but storage technology was growing rapidly and capacity considerations soon became an issue that time and technology resolved on its own. Today, the storage requirement for the current World of Warcraft version is six gigabytes of hard drive space (although patches and add-ons can easily grow this storage requirement above eight gigabytes).
Architecting Bandwidth By distributing this graphically heavy "world" to the user's hard drive, Blizzard was able to build a complex environment that didn't require a huge amount of bandwidth. Even dialup users enjoyed the sights and sounds of Azeroth through this efficient communication between the Blizzard servers and the end user application. Even today, the minimum network requirements for the original World of Warcraft client remain a 56 Kbps dialup link. Dial-up users may be limited going forward, because the recent "Burning Crusade" expansion pack specifies a high-speed network link as a minimum requirement.
A major component of Blizzard's networking architecture is the requirement of real-time interaction. The World of Warcraft universe is "alive," with constant movement of characters, real-time chats, and incursions from enemy players and other nefarious creatures. The World of Warcraft client software is in constant communication to the Blizzard servers, and it's not unusual to have many network packets transferred in a single second. It's this ongoing "heartbeat" of communication that provides the ongoing ebbs and flows of activity around the player.
This constant level of communication is a tricky business. Even with high bandwidth links, extensive packet delays or "lag" can negatively affect the end user experience. In many ways, the World of Warcraft traffic flows are similar to those of a streaming media application, such as a Voice over IP telephone call or real-time video feed. Because of the real-time nature of Azeroth, there's little tolerance for lost packets and network congestion (regardless of the bandwidth).