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by Dinesh Thakur Category: Communication Networks

Fast Ethernet is the name of the extension to 100 Mbit/s Ethernet network to 10 Mbit/s. This is the IEEE 802.3u working group that is at the origin. Access technique is the same as in the Ethernet Version 10 Mbit/s, but at a speed multiplied by 10. transported frames are identical. This increase in speed may conflict with the wiring system and the possibility or not there such important transit flows.

This is why three sub-standards have been proposed for the 100 Mbit/s:


• 100BaseTX IEEE 802.3, which requires two pairs of unshielded (UTP) Category 5 or two shielded pairs (STP) type 1.

• IEEE 802.3 100BaseT4, which requires four pairs of unshielded (UTP) Category 3, 4 and 5.

• IEEE 802.3 100BASE-FX, which requires two optical fibers.

The maximum distance between the two farthest points is greatly reduced compared to the version 10 Mbit/s. The minimum length of the frame is always 64 bytes; the transmission time is 5.12 microseconds. We deduce that the maximum distance that can be covered in that time is of the order of 1000 m, accounting for Fast Ethernet maximum length of approximately 500 m. As hubs of crossing time are relatively large, most manufacturers limit the maximum distance to 210 m for Fast Ethernet. The time between two frames or intergap is reduced to 0.96 microseconds.

This solution has the advantage of providing a good compatibility with version 10 Mbit/s, which links to the same hub stations to 10 Mbit/s and 100 Mbit/s. The connection cost of 100 Mbit/s is now the same as that of the conventional Ethernet, ten times slower.

Fast Ethernet networks often use Ethernet network interconnection networks 10 Mbit/s. The relatively limited distance covered by the Fast Ethernet, however, not allow him to "irrigate" a rather large company. Gigabit Ethernet, as we detail below, not to further solves this problem as a shared version. However, the switched version with more distance constraint, the switched Gigabit Ethernet is one of interconnect solutions for Fast Ethernet networks.

Another option to extend the coverage of the Ethernet network is to connect Fast Ethernet by bridges intended to filter frames using the MAC address. These bridges have the same functionality as switches, found today in large company’s transfer Ethernet frames for networks using Ethernet switches.


IEEE has designed two categories of Fast Ethernet: 100Base-X and 100BaseT4.

100Base-X uses two cables between the station and the hub and 100Base-T4 uses four cables between the station and hub.  

                Categories of Fast Ethernet

100 Base-TX

(a) 1OOBase-TX uses two pairs of category 5 unshielded twisted pair (UTP) or two pairs of shielded twisted pair (STP) cables to connect a station to the hub.

(b) One pair is used to carry frames from the station to the hub and the other to carry frames from the hub to the station.

(c) The distance between hub and station should be less than 100 meters.

(d) For this implementation, the MLT-3 scheme is used. However as MLT-3 is not a self synchronous line coding scheme, 4B/5B block coding is used to provide bit synchronization.

(e) This creates a data rate of 125 Mbps, which is fed into MLT-3 for encoding.

(f) The encoding and decoding is implemented in two steps as shown in Fig.

                                 Encoding in 100 Base TX

100 Base-FX  

(a) It users two pairs fiber-optic cables.

(b) One pair carries frame from the station to the hub and the other from hub to the station.

(c) The distance between the station and the hub (or switch) should be less than 2000 meters.

(d) It makes use of NRZ-I encoding scheme.

(e) As NRZ-I has a bit synchronization problem for long sequences, 100Base-FX uses 4B/5B block encoding that increases the bit rate from 100 to 125 mbps.

(f) The encoding scheme for 100Base-FX is shown in Fig.

                              Encoding in 100 Base FX

100 Base-T4

(a) It uses four pairs of category 3 UTP.

(b) Two of the four pairs are bi-directional, the other two are unidirectional.

(c) In each direction, three pairs are used at the same time to carry data as shown in fig.

(d) Encoding/decoding in 100Base-T4 is more complicated.

(e) As this implementation uses category 3 UTP, each twisted pair cannot easily handle more than 25 Mbaud.

(f) As one pair switches between sending and receiving, three pairs of UTP category 3 can handle only 75 Mbaud (25 Mbaud each).

(g) Thus it requires an encoding scheme that converts 100 Mbps to a 75 Mbaud signal. This is done by using 8B/6T (eight binary/six ternary) encoding scheme.

                                8B/6T encoding/decoding

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About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.SC (Computer Science), MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.

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