132 Networking and Online Games: Understanding and Engineering
132 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games Working group 802.11e has developed both the DCF and PCF schemes to introduce a more effective Quality of Service mechanism. The Enhanced Distributed Coordination Function (EDCF) distinguishes between high priority traffic and low priority traffic. High priority traffic waits a shorter period of time than low priority traffic before transmitting. This scheme is simple to implement but provides only relative guarantees of QoS. The second mechanism developed by 802.11e is the Hybrid Coordination Function (HCF). This is based on the PCF but allows the definition of traffic classes. Different traffic classes can be defined to provide different QoS based on characteristics such as bandwidth and maximum jitter. These are controlled by modifying the frequency with which a station generating a particular traffic class is polled and the maximum number of frames that a station may transmit in response to a poll. 8.6 Cellular Networks Cellular voice networks such as Global System for Mobile (GSM) and cdmaOne have been commonplace now for over twenty years, but cellular networks able to support packet data at broadband rates are only just becoming widely available. Generally, when compared with other wireless technologies, broadband cellular networks provide much greater coverage, lower latency and seamless handover but at lower bit rates and often, at very high usage costs. Important developments in broadband cellular access networks are Enhanced Data Rates for Global System for Mobile Equipment (EDGE), General Packet Radio System (GPRS), CDMA2000 and Universal Mobile Telecommunications System (UMTS). EDGE and GPRS are sometimes referred to as 2.5G networks, while CDMA2000 and UMTS are sometimes referred to as 3G networks. These technologies can provide broadband wireless access across a much wider area than is possible with other wireless technologies. However, they require the deployment of a great deal of infrastructure, the purchase of bandwidth licenses at often staggeringly high prices, and sophisticated end-user equipment. Consequently, usage costs tend to be high. Because of their expense and strict bandwidth licensing requirements, cellular networks are usually deployed by large telecommunications companies or specialised wireless communications companies. Comparatively low cost, privately deployed cellular networks are, at least for now, not possible. 8.6.1 GPRS and EDGE GPRS is derived from the GSM telephony system. Its purpose is to make efficient use of the GSM network for data purposes with a minimum deployment of additional hardware. GPRS is now commonly deployed wherever GSM telephony systems are deployed. However, while GPRS is an excellent packet data network, its delay performance is very poor. It is intended for the efficient communication of nonreal-time communications such as emails and web browsing [PAHL2002]. The GPRS standard defines four delay classes. Class 1 specifies a mean delay of less than two seconds, class 2 a delay of less than 15 seconds, class 3 a delay of less than 75 seconds and class 4 does not specify any delay. Clearly, delays of this magnitude make it unsatisfactory for real-time games. Nevertheless it may be of use to players of nonreal-time games. EDGE is not so much an alternative network as an improvement to the modulation scheme used in the GSM air interface. By adapting the bit rate to the quality of the link,
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