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.However, for large-scale installations, either thick Ethernet or twisted pair is moreappropriate.For example, the Ethernet at GMU's Mathematics Department originally chose thickEthernet because it is a long route that the cable must take so traffic will not be disrupted each time ahost is added to the network.Twisted pair installations are now very common in a variety of installations.The Hub hardware is dropping in price and small units are now available at a price that is attractive toeven small domestic networks.Twisted pair cabling can be significantly cheaper for large installations,and the cable itself is much more flexible than the coaxial cables used for the other Ethernet systems.Thenetwork administrators in GMU's mathematics department are planning to replace the existing networkwith a twisted pair network in the coming finanical year because it will bring them up to date withcurrent technology and will save them significant time when installing new host computers and movingexisting computers around.One of the drawbacks of Ethernet technology is its limited cable length, which precludes any use of itother than for LANs.However, several Ethernet segments can be linked to one another using repeaters,bridges, or routers.Repeaters simply copy the signals between two or more segments so that all segmentstogether will act as if they are one Ethernet.Due to timing requirements, there may not be more than fourrepeaters between any two hosts on the network.Bridges and routers are more sophisticated.Theyanalyze incoming data and forward it only when the recipient host is not on the local Ethernet.Ethernet works like a bus system, where a host may send packets (or frames) of up to 1,500 bytes toanother host on the same Ethernet.A host is addressed by a six-byte address hardcoded into the firmwareof its Ethernet network interface card (NIC).These addresses are usually written as a sequence oftwo-digit hex numbers separated by colons, as in aa:bb:cc:dd:ee:ff.A frame sent by one station is seen by all attached stations, but only the destination host actually picks itup and processes it.If two stations try to send at the same time, a collision occurs.Collisions on anEthernet are detected very quickly by the electronics of the interface cards and are resolved by the twostations aborting the send, each waiting a random interval and re-attempting the transmission.You'll hearlots of stories about collisions on Ethernet being a problem and that utilization of Ethernets is only about30 percent of the available bandwidth because of them.Collisions on Ethernet are a normal phenomenon,and on a very busy Ethernet network you shouldn't be surprised to see collision rates of up to about 30percent.Utilization of Ethernet networks is more realistically limited to about 60 percent before you need to start worrying about it.[2]Other Types of HardwareIn larger installations, such as Groucho Marx University, Ethernet is usually not the only type ofequipment used.There are many other data communications protocols available and in use.All of theprotocols listed are supported by Linux, but due to space constraints we'll describe them briefly.Many ofthe protocols have HOWTO documents that describe them in detail, so you should refer to those if you'reinterested in exploring those that we don't describe in this book.At Groucho Marx University, each department's LAN is linked to the campus high-speed backbonenetwork, which is a fiber optic cable running a network technology called Fiber Distributed DataInterface (FDDI).FDDI uses an entirely different approach to transmitting data, which basically involvessending around a number of tokens, with a station being allowed to send a frame only if it captures atoken.The main advantage of a token-passing protocol is a reduction in collisions.Therefore, theprotocol can more easily attain the full speed of the transmission medium, up to 100 Mbps in the case ofFDDI.FDDI, being based on optical fiber, offers a significant advantage because its maximum cablelength is much greater than wire-based technologies.It has limits of up to around 200 km, which makes itideal for linking many buildings in a city, or as in GMU's case, many buildings on a campus.Similarly, if there is any IBM computing equipment around, an IBM Token Ring network is quite likelyto be installed.Token Ring is used as an alternative to Ethernet in some LAN environments, and offersthe same sorts of advantages as FDDI in terms of achieving full wire speed, but at lower speeds (4 Mbpsor 16 Mbps), and lower cost because it is based on wire rather than fiber.In Linux, Token Ringnetworking is configured in almost precisely the same way as Ethernet, so we don't cover it specifically.Although it is much less likely today than in the past, other LAN technologies, such as ArcNet andDECNet, might be installed.Linux supports these too, but we don't cover them here [ Pobierz całość w formacie PDF ]