System information

CHAPTER
Troubleshooting Ethernet 4-57
4
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 where 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 wishing 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. Backoff algorithms determine when
the colliding stations retransmit. CSMA/CD stations can detect collisions, so they know when they
must retransmit.
Both Ethernet and IEEE 802.3 LANs are broadcast networks. In other words, all stations see all
frames, regardless of whether they represent an intended destination. Each station must examine
received frames to determine whether the station is a destination. If it is a destination, the frame is
passed to a higher protocol layer for appropriate processing.
Differences between Ethernet and IEEE 802.3 LANs are subtle. Ethernet provides services
corresponding to Layers 1 and 2 of the OSI reference model, whereas IEEE 802.3 specifies the
physical layer (Layer 1) and the channel-access portion of the link layer (Layer 2), but does not
define a logical link control protocol. Both Ethernet and IEEE 802.3 are implemented in hardware.
Typically, the physical manifestation of these protocols is either an interface card in a host computer
or circuitry on a primary circuit board within a host computer.