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4.4 Thin Coaxial Physical Topology

Thin Coaxial Cable Stubs

Note that the BNC Tee is connected directly to the female BNC on the interface, with no intervening piece of thin coaxial cable. The standard notes that the length of the "stub" connection from the BNC MDI on the interface to the coaxial cable should be no longer than four centimeters (1.57 inches), to prevent the occurrence of signal reflections which can cause frame errors.

While longer stub cables inserted between the BNC Tee and the Ethernet interface may seem to work, they actually create signal reflections which cause electrical noise and result in frame errors. Frames lost due to frame errors are typically detected and retransmitted by the application software. Therefore, the system may appear to work when stubs are used since a small level of frame loss is not usually noticed right away.

FIGURE 4.3 Thin coax stub cable is incorrect

However, a large enough frame error rate can cause a high number of retransmissions and make the network appear to slow down. The electrical noise and frame loss get worse as the traffic level increases, causing the response time of applications over the network to plummet just when the demand is highest. You can avoid these problems by making sure that there are no stub cables in your thin Ethernet system.

Quick Reference Guide to 10BASE2 Thin Ethernet - 04 SEP 95
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