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Chapter: Computer Networks : Data Link Layer

Fast Ethernet

Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel.

Fast Ethernet


Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. The goals of Fast Ethernet can be summarized as follows:


1. Upgrade the data rate to 100 Mbps.

2. Make it compatible with Standard Ethernet.

3. Keep the same 48-bit address.

4. Keep the same frame format.

5. Keep the same minimum and maximum frame lengths.


1. MAC Sublayer


A main consideration in the evolution of Ethernet from 10 to 100 Mbps was to keep the MAC sublayer untouched. However, a decision was made to drop the bus topologies and keep only the star topology. For the star topology, there are two choices, as we saw before: half duplex and full duplex. In the half-duplex approach, the stations are connected via a hub; in the full-duplex approach, the connection is made via a switch with buffers at each port.


2. Implementation


Fast Ethernet implementation at the physical layer can be categorized as either two-wire or four-wire. The two-wire implementation can be either category 5 UTP (100 Base-TX) or fiber-optic cable (100Base-FX). The four-wire implementation is designed only for category 3 UTP (l00Base-T4).


3. Encoding


Manchester encoding needs a 200-Mbaud bandwidth for a data rate of 100 Mbps, which makes it unsuitable for a medium such as twisted-pair cable. For this reason, the Fast Ethernet designers sought some alternative encoding/decoding scheme. However, it was found that one scheme would not perform equally well for all three implementations.


a. 100 Base-TX uses two pairs of twisted-pair cable (either category 5 UTP or STP). Forthis implementation, the MLT-3 scheme was selected since it has good bandwidth performance. However, since MLT-3 is not a self-synchronous line coding scheme, 4B/5B block coding is used to provide bit synchronization by preventing the occurrence of a long sequence of 0s and 1s. This creates a data rate of 125 Mbps, which is fed into MLT-3 for encoding


b. 100 Base-FX uses two pairs of fiber-optic cables. Optical fiber can easily handle highbandwidth requirements by using simple encoding schemes. The designers of 100Base-FX selected the NRZ-I encoding scheme for this implementation. However, NRZ-I has a bit synchronization problem for long sequences of 0s.


c. A 100 Base-TX network can provide a data rate of 100 Mbps, but it requires the use of category 5 UTP or STP cable. This is not cost-efficient for buildings that have already been wired for voice-grade twisted-pair (category 3). A new standard, called 100Base-T4 was designed to use category 3 or higher UTP.


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