10BASE2 (also known as cheapernet, thin Ethernet, thinnet, and thinwire) is a variant of Ethernet that uses thin coaxial cable (RG-58A/U or similar, as opposed to the thicker RG-8 cable used in 10BASE5 networks), terminated with BNC connectors. During the mid to late 1980s this was the dominant 10 Mbit/s Ethernet standard, but due to the immense demand for high speed networking, the low cost of Category 5 Ethernet cable, and the popularity of 802.11 wireless networks, both 10BASE2 and 10BASE5 have become increasingly obsolete, but still exist in many locations.
The name 10BASE2 is derived from several characteristics of the physical medium. The 10 comes from the maximum transmission speed of 10 Mbit/s (millions of bits per second). The BASE stands for baseband signalling, and the 2 supposedly represents the maximum segment length of 200 meters, but as a matter of fact, it can only run up to 185 metres. (The IEEE rounded 185 up to 200 to come up with the name 10Base2, to fit with the general standard).
10BASE2 coax cables had a maximum length of 185 meters (607 ft). The maximum practical number of nodes that can be connected to a 10BASE2 segment is limited to 30. In a 10BASE2 network, each segment of cable is connected to the transceiver (which is usually built into the network adaptor) using a BNC T-connector, with one segment connected to each female connector of the T.
As was the case with most other high-speed buses, Ethernet segments had to be terminated with a resistor at each end. Each end of the cable had a 50 ohm (Ω) resistor attached. Typically this resistor was built into a male BNC and attached to the last device on the bus. This is most commonly connected directly to the T-connector on a workstation though it does not technically have to be. A few devices such as Digital's DEMPR and DESPR had a built-in terminator and so could only be used at one physical end of the cable run. If termination was missing, or if there was a break in the cable, the AC signal on the bus was reflected, rather than dissipated, when it reached the end. This reflected signal was indistinguishable from a collision, and so no communication would be able to take place.
Terminators had a metallic chain attached to them for grounding purposes, however many people never understood how to properly ground cabling and thus grounded the terminators at both ends rather than just one end. This caused many of the grounding loop problems during that era which caused network outages and/or data corruption when swells of electricity traversed the coaxial cabling's outer shield on its path to the ground with the least resistance.
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