126 Networking and Online Games: Understanding and Engineering
126 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games to particular regions of coverage and localities within those regions, and finally to the leaves of the tree representing the residential receivers of the television signal. Cable television is built on the existing analogue TV distribution mechanism, and is thus intrinsically an analog medium at the consumer s end. Each television channel occupies approximately 6 MHz. If a coaxial cable network is used from the head-end to the customer premises, then the signal that is distributed occupies Radio Frequencies (RF) in the range of 50 to 450 MHz in the downstream direction. For hybrid fibre-coaxial cable systems that use fibre optic cable in all but the last hop, the frequency range available is much higher, typically from 50 to 750 MHz. In the upstream direction, a much smaller 5 to 42 MHz may be available. To use this analog medium as an Access Network for transporting digital communication, the digital signal must be modulated onto the analog carrier. An RF signal of 6 MHz can support bit rates in the megabit per second range, but to achieve this, there are a number of obstacles to overcome. The first difficulty is that cable television is primarily a downstream broadcast medium. Upstream capacity is often limited or nonexistent. The second is that RF transmission is often subject to electrical noise that can severely limit modulation efficiency, particularly in the upstream direction. Finally, even with 750 MHz available, there is not enough capacity to allow a full channel to every user. The capacity has to be shared between multiple users through an MAC sublayer. Dealing with the situation where there is no reverse channel is the most difficult. In this case, a hybrid solution where downstream communication is provided by the cable network and upstream communication is provided by the telephone network (using a suitable modulation system) is the most common solution, particularly where the network is solely coaxial cable based. Where the network is the more modern hybrid fibre-coaxial network, a reverse channel in the 5 to 42 MHz range, operating at a much lower rate than the downstream network, is a typical solution. In order to control individual usage, some ISPs impose rate caps on the downstream bit rate. This has been shown to have quite serious consequences for latency when the link is overloaded with data traffic [NGUY2004a]. Even modest rate caps, in conjunction with excess data traffic, can cause latency increases of 100 ms or more. Certainly, this is something that should be of concern to game players and those deploying networked games requiring low latencies. Standardisation of access networks using cable modems has been dominated by the industry-sponsored Data over Cable Service Interface Specification (DOCSIS). DOCSIS supports the delivery of Ethernet frames between the cable modem and the head-end. It implements a medium access control mechanism allowing shared access by multiple users. Figure 8.2 shows the main components in a simplified Data over Cable TV network. At the root of the tree that makes up the network is the head-end which provides connectivity to the Internet via a router or bridge. Each user connects to the branch which ultimately connects to the head-end. Cable networks are an important and effective broadband access network. However, since they are based on a shared medium and rate caps, they can be subject to arbitrary variations in bandwidth and consequent random variations in delay.
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