Components for Building Computers From Scratch/Cables
QUESTION: 10Base T network uses cat. 3 UTP cable. And transfers Data 10Mbps.
10Base 2 network uses Thinnet Coaxial Cable. And it transfers Data 10Mbps.
So why not just use 10Base T or just 10Base 2 what is the diferance?
ANSWER: 10BaseT generally uses Category 5 (or 5e/6/etc) 8-conductor cabling for forwards compatibility with 100BaseTX or GbE support. It also serves a pragmatic purpose of keeping voice and data lines physically separated - there's no chance of accidentally plugging the 8P8C connector that Category 5/6 cabling uses into a telephone jack, or vice versa.
As far as why use UTP over Coaxial, different network topologies and support - coaxial can handle longer runs but requires different considerations when wiring (clients are connected via BNC T-connectors and only 30 nodes can be connected per segment). In general Ethernet via twisted-pair is easier and cheaper to deploy (and it came about later - remember that not all of the various network cabling standards for Ethernet were introduced simultaneously), and also when new standards are released, they are generally inclusive (so a 100Mbit network can support 10Mbit devices); especially if not using hubs (which is certainly the case today). In the real world (that is, today), twisted-pair is so common and equipment that supports it so prevalent and cheap that it isn't generally worth bothering with other kinds of connections unless you need to support an old machine for some purpose. That isn't to say coaxial networking was "wrong" - it's simply older technology that has been passed-over as time has progressed. It served a purpose in the 1980s and early 1990s, but isn't practical for modern networking, especially large or complex installations.
Wikipedia has a comparison between the two available here:
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QUESTION: OK you say 10Base T networks use CAT. 5 Cable but when I look up 10Base T networks they say it uses CAT. 3?
10BaseT can be run over Category 3 cabling, but in general practice this is not done; Category 5 (or 6) is most frequently used for Ethernet these days (and usually 100-1000Mbit; but it will support 10Mbit). Most "modern" (in the last 10-15 years) equipment that supports 10Mbit will accept 8p8c connections for Category 5/6 as well. It's simply a "progression of time" thing - back in the 1980s you would have seen Category 3; but not today.