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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