Ethernet


Ethernet is the most popular network architecture for local area networks (LANs).

Ethernet
Ethernet

What is Ethernet?

The most popular network architecture for local area networks (LANs). Ethernet was originally developed by Xerox in the 1970s and was proposed as a standard by Xerox, Digital Equipment Corporation (DEC), and Intel in 1980.

A separate standardization process for Ethernet technologies was established in 1985 in the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard known as Project 802.

The IEEE standard was then adopted by the International Organization for Standardization (ISO), making it a worldwide standard for networking. Because of its simplicity and reliability, Ethernet is by far the most popular networking architecture used today.

It is available in three different speeds:

  • 10 Mbps, which is simply called Ethernet
  • 100 Mbps, which is called Fast Ethernet
  • 1000 Mbps or 1 Gbps, which is an emerging standard called Gigabit Ethernet

Index (on this page)




Ethernet Specifications

Ethernet specifications define the functions that occur at the physical layer and data-link layer of the Open Systems Interconnection (OSI) reference model and package data into frames for transmission on the wire. Ethernet is a baseband networking technology that sends its signals serially one bit at a time. It operates in half-duplex mode, in which a station can either transmit or receive, but cannot do both simultaneously.



Ethernet uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) media access control method for determining which station can transmit at any given time over the shared medium. In an Ethernet network, each station (computer) listens to the network and transmits data only if no other stations are currently using the network. If the wire is free of signals, any station that wants to can contend (try to take control of) the network in order to transmit a signal. Ethernet networks are thus based on the concept of contention and operate on a first-come, first-served basis, rather than relying on a master station that controls when other stations can transmit. If two stations try to transmit data at the same time, a collision occurs, and both stations stop transmitting. They wait a random interval of time (measured in milliseconds) and then try again.

The more stations on an Ethernet network, the higher the number of collisions, and the worse the performance of the network. Typical performance of a 10-Mbps Ethernet network with around 100 stations will support a bandwidth of only about 40 to 60 percent of the expected value of 10 Mbps. One way of solving the problem of collisions is to use Ethernet switches to segment your Ethernet network into smaller collision domains.




Ethernet Encapsulation

Ethernet stations transmit their data over the wire in packages called frames. An Ethernet frame has a minimum size of 64 bytes and a maximum size of 1518 bytes. A total of 18 bytes are used for information such as source and destination addresses, network protocol being used, and other frame overhead. Thus, the maximum payload size (amount of data carried) for an Ethernet frame is 1500 bytes. There are four different Ethernet encapsulation methods by which Ethernet packages data into a frame:

  • Ethernet II (used for TCP/IP)
  • Ethernet 802.3 (called Raw 802.3 in Novell networking, and used for connectivity with NetWare 3.11 and earlier)
  • Ethernet 802.2 (also called Ethernet 802.3/802.2 without SNAP [subnetwork access protocol], and used for connectivity with NetWare 3.12 and later)
  • Ethernet SNAP (also called Ethernet 802.3/802.2 with SNAP, and created for compatibility with Macintosh and TCP/IP systems)

Ethernet can use virtually any physical networking topology and cabling system (medium). Although a star topology (stations wired in a star-like fashion to a central hub) is often used from the physical point of view, all Ethernet networks are logical bus topology networks at heart.



One station places a signal on the bus, and that signal travels to every other station on the bus.

Ethernet Speeds, Types, Standards, and Specs

Ethernet is available in three different speeds and can be further differentiated by media and other considerations, as shown in the following table.

SpeedType of EthernetIEEE StandardsIEEE Specs
10 MbpsEthernet10Base2
10Base5
10BaseF
10BaseT
802.3
100 MbpsFast Ethernet100BaseFX
100BaseT
100BaseT4
100BaseTX
802.3u
1000 Mbps or 1 GbpsGigabit Ethernet1000BaseCX
1000BaseLX
1000BaseSX
1000BaseT
802.3z

Ethernet media specifications such as 10BaseT look strange and obscure but can be easily interpreted. For example, 10BaseT means 10 -Mbps baseband transmission over twisted-pair cabling media.

A new type of Ethernet technology that solves the problems of collisions and has twice the bandwidth of traditional Ethernet is called full-duplex Ethernet. Full-duplex Ethernet uses two pairs of wires with Ethernet switches to allow stations to simultaneously send and receive data without collisions. On a 10BaseT wired network, each full-duplex Ethernet station would have a transmission bandwidth of 20 Mbps.

Ethernet framing format

Ethernet frame format

The various Ethernet framing formats are incompatible with each other, so if you have a heterogeneous Ethernet network, you need to specify the correct frame type in order for machines running Microsoft Windows NT to see your Novell NetWare servers.



Windows NT allows you to select Auto Detect from the Frame Type drop-down list on the NWLink IPX/SPX-Compatible Transport protocol configuration property sheet if you don’t know what frame type your NetWare servers are using. (In Windows 2000, select the check box next to Auto Frame Type Detection in the NWLink IPX/SPX/NetBIOS-Compatible Transport protocol configuration property sheet.)

You might also need to configure your routers for the proper frame type. Older Cisco routers running Internetwork Operating System (IOS) version 10 or earlier do not support Ethernet 802.3/802.2 with SNAP.

Troubleshooting Tips for Ethernet Media Problems

ProblemSuggestion
No link integrityCheck that you have not mismatched 10BaseT and 100BaseT (or 100BaseTX and 100BaseT4) cables, hubs, or network interface cards; make sure no crossover cables are used for station-to-hub cable connections.
Too much noiseCheck for damaged cables; make sure you are using category 5 (CAT5) cabling (or enhanced category 5 cabling for 100BaseT) and that all your cabling interface components (patch panels, wall plates, terminal blocks, and so on) are CAT5 certified.
Too many collisions (greater than 0.1 percent of total frames on the network)Check for unterminated cables using a time-domain reflectometer; use a protocol analyzer to look for a jabbering transceiver (a network interface card that is continually broadcasting); and make sure you don’t have any cables exceeding the maximum specified length.

The Begining and Evolution of Ethernet

Ethernet was developed at Xerox PARC between 1973 and 1974. The idea was first documented in a memo that Robert Metcalfe wrote on May 22, 1973, where he named it after the luminiferous aether once postulated to exist as an “omnipresent, completely-passive medium for the propagation of electromagnetic waves.” In 1975, Xerox filed a patent application listing Metcalfe, David Boggs, Chuck Thacker, and Butler Lampson as inventors. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper.

Metcalfe left Xerox in June 1979 to start 3Com. He convinced Digital Equipment Corporation (DEC), Intel, and Xerox to work together to promote Ethernet as a standard. As part of that process, Xerox agreed to relinquish their ‘Ethernet’ trademark. The first standard was published on September 30, 1980, as “The Ethernet, A Local Area Network. Data Link Layer and Physical Layer Specifications“. This so-called DIX standard (Digital Intel Xerox) specified 10 Mbit/s Ethernet, with 48-bit destination and source addresses and a global 16-bit Ethertype-type field. Version 2 was published in November 1982 and defines what has become known as Ethernet II. Formal standardization efforts proceeded at the same time and resulted in the publication of IEEE 802.3 on June 23, 1983.



Since then, Ethernet technology has evolved to meet new bandwidth and market requirements. In addition to computers, Ethernet is now used to interconnect appliances and other personal devices. As Industrial Ethernet, it is used in industrial applications and is quickly replacing legacy data transmission systems in the world’s telecommunications networks.

Ethernet has evolved to include higher bandwidth, improved medium access control methods, and different physical media. The coaxial cable was replaced with point-to-point links connected by Ethernet repeaters or switches.

Original Ethernet
Original Ethernet




In a modern Ethernet, the stations do not all share one channel through a shared cable or a simple repeater hub; instead, each station communicates with a switch, which in turn forwards that traffic to the destination station. In this topology, collisions are only possible if station and switch attempt to communicate with each other at the same time, and collisions are limited to this link.

Modern Ethernet
Modern Ethernet

Furthermore, the 10BASE-T standard introduced a full-duplex mode of operation which became common with Fast Ethernet and the de facto standard with Gigabit Ethernet. In full-duplex, switch and station can send and receive simultaneously, and therefore modern Ethernets are completely collision-free.

See also




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