Thursday, April 12, 2012

Hubs

Ethernet hubs connect multiple twisted-pair Ethernet devices together . They work at the physical layer ( the lowest or first layer ) . They repeat the signals received by each port out to all of the other ports . Hubs can therefore be a simple repeaters . Due to this design , only one port can successfully transmit at a time . If two devices transmit at the same time , they corrupt each other's transmissions , and both must back off and retransmit their packets later. This is known as a collision , and each host remains responsible for detecting collisions , and each host remains responsible for detecting collisions during transmissions , and retransmitting its own packets when needed .

When problems such as excessive collisions are detected on a port , some hubs can disconnect ( partition) that port for a while to limit its impact on the rest of the networkk . While a port is paritioned , devices attached to it cannot communicate with the rest of the network . Hub-based networks are generally more robust than coaxial Ethernet ( also known as 10base2 or ThinNet) , where misbehaving devices can be disable the entire segment , But hubs are limited in their usefulness, since they can easly becomes points of congestion on busy networks .

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