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Fast Ethernet

Fast Ethernet

Question:
What are the various flavors of “Fast Ethernet” and how do they work?

Answer:
“Fast Ethernet” refers to 100 Mbps Ethernet networks. Fast Ethernet provides 10 times the bandwidth of traditional 10 Mbps networks. There are two main schools in fast Ethernet territory: 100Base-X and 100VG-AnyLAN.

100Base-X requires Category 5 cabling and is based on the CSMA/CD Ethernet standard. This scheme accelerates transfer rates by shortening cable distances and by taking advantage of the Category 5 data-grade cabling standard. It is highly compatible with existing 10 Mbps Ethernet segments because the underlying access principle, CSMA/CD, remains the same. In order to combine 10 Mbps segments with 100 Mbps, we simply need to match speeds while exchanging the packets.

100VG-AnyLAN can use Category 3 voice-grade cabling schemes with four pairs and is based on the Demand Priority access methodology. The framing format, however, stays the same. The Ethernet hub acts as the arbitrator and will decide how workstations access the network. A priority scheme ensures that time-critical transfers such as voice and video get enhanced priority. 100 Mbps is obtained primarily via an enhanced signaling scheme.

While 10Base-T uses two pairs to receive and transmit, 100VG-AnyLAN uses all four pairs. 25MHz signals are transmitted on each of the pairs. Contrast this with 20Mhz signals that are divided on the two wire pairs for 10Base-T.

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