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Fast Ethernet (802.3u)
Fast Ethernet (IEEE 802.3u) is a variation of Ethernet standards that carries data traffic at 100 Mbps in local area networks. It was launched as the IEEE 802.3u standard in 1995 and remained the fastest network technology until the introduction of Gigabit Ethernet.
Fast Ethernet is commonly named 100BASE-X, where 100 represents the maximum throughput (100 Mbps), BASE denotes the use of baseband transmission, and X indicates the type of medium used (TX or FX).
Types of Fast Ethernet
The three main varieties of Fast Ethernet are 100BASE-T4, 100BASE-TX, and 100BASE-FX, each designed for different cabling infrastructures and distance requirements.
100BASE-T4
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Uses four pairs of UTP Category 3 cable − two pairs are bidirectional and two are unidirectional.
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Three pairs can transmit data simultaneously in each direction at 25 Mbaud each, totaling 75 Mbaud.
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Employs 8B/6T encoding scheme (eight binary/six ternary).
100BASE-TX
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Uses two pairs of UTP Category 5 or STP Type 1 cables − one pair for each transmission direction.
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Maximum distance between hub and station is 100 meters.
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Operates at 125 Mbaud using MLT-3 encoding with 4B/5B block coding.
100BASE-FX
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Uses two pairs of optical fiber − one pair for each transmission direction.
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Maximum distance between hub and station is 2000 meters.
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Operates at 125 Mbaud using NRZ-I encoding with 4B/5B block coding.
Fast Ethernet Comparison
| Standard | Cable Type | Max Distance | Encoding |
|---|---|---|---|
| 100BASE-T4 | 4-pair UTP Cat 3 | 100m | 8B/6T |
| 100BASE-TX | 2-pair UTP Cat 5 | 100m | MLT-3 + 4B/5B |
| 100BASE-FX | 2-strand Fiber | 2000m | NRZ-I + 4B/5B |
Frame Format
Fast Ethernet uses the same frame format as IEEE 802.3, maintaining backward compatibility while increasing transmission speed.
The frame fields include a 7-byte Preamble for synchronization, 1-byte Start of Frame Delimiter, 6-byte source and destination addresses, 2-byte Length field, variable Data field (46-1500 bytes), and 4-byte CRC for error detection.
Conclusion
Fast Ethernet (IEEE 802.3u) provided a crucial 10x speed improvement over traditional Ethernet, offering 100 Mbps throughput through multiple implementation options. Its backward compatibility and support for existing cabling infrastructure made it a successful bridge technology before Gigabit Ethernet adoption.
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