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Network Latency, Jitter and Loss Regardless of game genre, the realism of online game play depends on how well the underlying network allows game participants to communicate in a timely and predictable manner. In the previous chapter, we broadly reviewed the nature of modern IP network services. In this chapter we will discuss in more detail three characteristics of best effort Internet Protocol (IP) service latency, jitter and loss that have a significant impact on game play experience and game design. We will also look briefly at the technical methods Internet Service Providers (ISPs) can utilise to control these characteristics of their network services. 5.1 The Relevance of Latency, Jitter and Loss As noted in the previous chapter, IP packets carry information between sources and destinations on the network. Latency refers to the time it takes for a packet of data to be transported from its source to its destination. In many networking texts, you will also see the term Round Trip Time (RTT) in reference to the latency of a round trip from source to destination and then back to source again. In many cases the RTT is twice the latency, although this is not universally true (some network paths exhibit asymmetric latencies, with higher latencies in one direction than the other). In online gaming communities, the term lag is often colloquially used to mean RTT. Variation in latency from one packet to the next is referred to as jitter.There are a number of mathematically precise ways to define jitter, usually depending on the timescale over which the latency variation occurs and the direction in which it occurs. For example, a path showing an average 100 ms latency might exhibit latencies of 90 ms and 110 ms for every alternate packet fairly noticeable jitter in the short term, even though the long-term average latency is constant. Alternatively, the path might exhibit latency, that is, drifting 90 ms, 95 ms, 100 ms, 105 ms, 110 ms, 105 ms, 100 ms…and so on. For our purposes, it is sufficient to know that latency can fluctuate slowly or rapidly from one packet to the next. Figure 5.1 summarises the key difference between latency and jitter. Packet loss refers to (not surprisingly) the case when a packet simply never reaches its destination. It is lost somewhere in the network. A path s packet loss characteristic is often described in terms of packet loss rate or packet loss probability (ratio of the number of packets lost per number of packets sent). Networking and Online Games: Understanding and Engineering Multiplayer Internet Games Grenville Armitage, Mark Claypool, Philip Branch . 2006 John Wiley & Sons, Ltd

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