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A LAN (local area network) is often thought of as a small network, usually not exceeding a few kilometers.

However, assuming we have an unlimited supply of quality wires and chips, is it possible to build a huge LAN spanning more than dozens of kilometres?

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TP cables are specified to ~100 m. With repeaters, the limits from signal quality are in practice removed, and with intelligent devices lengths become more or less arbitrary (cf. the Internet). Your "common consensus" is probably that it's seldom needed to have larger areas covered. Building material, property costs, heating, etc. also weigh in. It is also good to limit the # of clients in an Ethernet segment since it runs on collisions, including other factors, but there is no physical limit. So to your first question: no –  Daniel Andersson Jun 18 '12 at 14:35

4 Answers 4

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The main aspect limiting cables length is CSMA/CD.

CSMA/CD requires a maximum cable length in conjunction with a minimum packet size. This assures the packet that is being transmitted "fills" all the cable so colisions can be detected.

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Since most modern networks are switch based and no longer use CSMA/CD (i.e. full duplex operation disables CSMA/CD), this doesn't really apply to networking. The distance limitations on network runs are based on the the physical characteristics of the cable and how the data is encoded onto the medium. –  YLearn Jul 27 '14 at 20:42

The main problem with longer LANs is signal attenuation. You can't avoid this only using "unlimited good wires" - you need powered active devices acting as signal repeaters to clean up and amplify the signal.

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It is the physical aspect that limits the length of a network cable. A complication of longer cables would be the need of additional power on the cable. Optical fibre has less problems with distance, but also has its limits.

It is always the distance that limits the length of the connection.

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Physical length slowly degrades signal strength and with reduced signal strength, harmonic distortion, crosstalk and common mode interference destroy the signal's intelligibility coming out the other end (signal to noise ratio). –  Fiasco Labs Jun 18 '12 at 14:55

The largest LAN that I know of belongs to San Diego City Schools. All internet traffic is funneled through a gateway that prohibits the students from accessing sites with mature content (and a bunch of other sites as well). From their gateway, the information goes to each of the schools in the district (by fiber I believe). As you can see from the map, the school district covers about 500 square miles.

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Good point, but how will this tally with criziot's answer? –  Pacerier Jul 22 '14 at 3:19
    
@Pacerier I think so, since 1 km is about 0.6 mi, the district is about 30 km by 40 km, so it spans a few dozen km. Well, maybe it's on the small size, but it's close. –  LDC3 Jul 22 '14 at 3:59

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