Network Latency, Jitter and Loss Statistical multiplexing assumes
Network Latency, Jitter and Loss First, at the physical layer all links experience a finite (albeit usually extremely low) rate of data corruption which we refer to as bit errors, and characterise by a link s bit error rate. Bit errors may be introduced by poor signal-to-noise ratios in the digital-to-analogueto-digital conversion process, resulting in erroneous encoding or decoding of data. Bit errors may simply be due to electrical glitches in hardware. Some link technologies encode additional information within each frame to enable limited reconstruction of a frame after one-or two-bit errors. This is known as forward error correction,(FEC). In any case, uncorrected bit errors are usually discovered through cyclic redundancy check (CRC) calculations at both the transmitting and receiving end of a link. The CRC is a 16or 32-bit value mathematically calculated during transmission and sent along with each frame, and then recalculated at the receiver. If the original and recalculated CRCs differ, the frame is discarded. Since this can occur anywhere along an IP path, there is no way to inform the end hosts why or how their packet was lost. Second, transient congestion can become so severe that queuing points along the path simply run out of space to hold new packets. When this happens new packets are simply dropped until the queue(s) have emptied enough to take new packets. This is the network s most aggressive form of self-protection against too much traffic. Some networks even employ proactive packet drop mechanisms that introduce a random, nonzero loss probability well before the queue is full. (This is referred to as active queue management, with the most well-known variant known as Random Early Detection (RED) [RFC2309]). Proactive packet dropping is intended to force TCP-based applications to slow down before congestion becomes too serious, but they have little effect on non-reactive UDP-based game traffic (except to cause packet loss). Third, dynamic routing changes do not always converge immediately on a fully functional and complete end-to-end path. When route changes occur, there can be periods of time (of tens of seconds to minutes) where no valid shortest path exists between previously connected sources and destinations. This manifests itself as unexpected packet loss affecting tens, hundreds or thousands of packets. 5.3 Network Control of Lag, Jitter and Loss Online games, particularly the real-time interactive genres, need greater control over network latency, jitter and loss than more traditional email, online chat and web surfing applications. In this section, we will briefly review the mechanisms that ISPs can deploy to control network conditions on behalf of game players, and the difficulties faced by ISPs in utilising these techniques effectively. One approach is for ISPs to ensure that their link and router capacities far exceed the offered traffic loads, and to utilise creative routing of traffic to ensure that no single router becomes a point of serious congestion. Attractive because of its conceptual simplicity, this approach tends to be practical only for large or core network operators who have flexible access to physical link infrastructures (for example, a telephone company that owns both an ISP and the underlying optical fibre between cities). The just deploy more bandwidth approach tends not to work where high speed technologies simply cannot be deployed in a cost-effective manner (for example, where 100-Mbps home LANs meet sub-1 Mbps broadband access links, as in Figure 5.2). ISPs and consumers must contemplate ways of prioritising access rather than simply hoping
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