Information Technology Reference
In-Depth Information
4
CHAPTER
Troubleshooting Ethernet
Ethernet was developed by Xerox Corporation's Palo Alto Research Center (PARC) in the 1970s.
Ethernet was the technological basis for the IEEE 802.3 specification, which was initially released in
1980. Shortly thereafter, Digital Equipment Corporation, Intel Corporation, and Xerox Corporation
jointly developed and released an Ethernet specification (Version 2.0) that is substantially compatible
with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest market share of any
local-area network (LAN) protocol. Today, the term
Ethernet
is often used to refer to all carrier sense
multiple access collision detect (CSMA/CD) LANs that generally conform to Ethernet specifications,
including IEEE 802.3.
When it was developed, Ethernet was designed to fill the middle ground between long-distance,
low-speed networks and specialized, computer-room networks carrying data at high speeds for very
limited distances. Ethernet is well suited to applications on which a local communication medium must
carry sporadic, occasionally heavy traffic at high peak data rates.
Ethernet and IEEE 802.3
Ethernet and IEEE 802.3 specify similar technologies. Both are CSMA/CD LANs. Stations on a
CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations “listen”
to the network to see if it is already in use. If it is, the station wanting to transmit waits. If the network
is not in use, the station transmits. A collision occurs when two stations listen for network traffic, “hear”
none, and transmit simultaneously. In this case, both transmissions are damaged, and the stations must
retransmit at some later time. Back-off algorithms determine when the colliding stations retransmit.
CSMA/CD stations can detect collisions, so they know when they must retransmit. This access method
is used by traditional Ethernet and IEEE 802.3 functions in half-duplex mode. (When Ethernet is
operated in full-duplex mode, CSMA/CD is not used.) This means that only one station can transmit at
a time over the shared Ethernet.
This access method was conceived to offer shared and fair access to multiple network stations/devices.
It allows these systems fair access to the Ethernet network through a process of arbitration by dictating
how stations attached to this network can access the shared channel. It allows stations to listen before
transmitting and can recover if signals collide. This recovery time interval is called a slot time and is
based on the round-trip time that it takes to send a 64-byte frame the maximum length of an Ethernet
LAN attached by repeaters. Another name for this shared LAN is a
collision domain
. For half-duplex
