Broadband Access Networks Network capacity is not always
Broadband Access Networks Network capacity is not always the same in both directions. The oldest access network application is telephony. With telephony, capacity is usually the same in both directions. Parties involved in a conversation talk, approximately, the same amount during a conversation. Of course, this is not always the case, and some conversations may be quite one-sided. Nevertheless, telephone systems have evolved with the assumption that capacity in both directions needs to be the same. However, many new applications, including most Internet games, require more capacity in the network to the user direction (the forward link) than in the user to the network direction (the reverse link). Typically, a player will receive a great deal of information about the state of the game from the game server on the forward link but will transmit relatively little information (mainly player actions) on the reverse link. Some access networks such as ADSL and cable modem networks naturally support this asymmetry. Their forward link capacity is much greater than their reverse link capacity. Other technologies, such as some cellular broadband networks, are capable of adapting to this asymmetry while other networks provide only symmetric capacity. Very few access networks are able to offer Quality of Service guarantees. That is, the network guarantees that important connection characteristics such as bit rate, delay and delay variation will be within agreed limits. Specification of these limits form a spectrum ranging from best effort networks where there are no guarantees through networks where guarantees are specified in terms of long-term averages, to networks where bit rates, error rates, delays and variation in delays are tightly specified. As we move across this spectrum, networks generally become less commonly available and more expensive to implement and to use. Access networks are also often a major source of latency and jitter. Latency is the delay experienced in a game before a command by the user is reflected in the game state. Jitter is the variation in delay. Networks where the same medium is shared among many users, such as wireless and cable modem networks, are more prone to variations in delay than other networks. Often users will want to access a game while they are mobile. Perhaps they are travelling on public transport, or they might be passengers in a car. Some access networks are able to support mobile users more effectively than others. Although there have been many interesting developments in Mobile Internet Protocol (IP) networks, cellular wireless networks are currently most effective in supporting mobile users. Some access networks are more convenient to access and to use than others. For example, connecting to a cellular network is usually trivial. The telecommunications company running the network provides direct access to the Internet. However, connecting through a Wireless Local Area Network (WLAN) may require significant reconfiguring of the device used to connect to the network. In this chapter, we provide an outline of the strengths and weaknesses of the most commonly used access network technologies. We discuss their design, capacity, potential to introduce latency and jitter, reliability, usage costs and how they are typically used. We hope that through understanding these networks, game developers and those deploying games will have a better understanding of the network constraints within which their games must operate. 8.2 Access Network Protocols and Standards In this section, we discuss some of the general characteristics of access networks.
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