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Ethernet

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Ethernet is a packet switched computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and packet formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ethernet is mostly standardized as IEEE's 802.3. It has become the most widespread LAN technology in use during the 1990s to current, and has largely replaced all other LAN standards such as Token Ring, FDDI, and ARCNET.

Table of contents

1 History

2 General Description

3 Ethernet frame types and the EtherType field

4 Varieties of Ethernet

5 Some early varieties of Ethernet

6 10Mbps ethernet

7 Fast Ethernet

8 Gigabit Ethernet

9 10 gigabit Ethernet

10 Related standards

11 External Links

History

Ethernet was original developed as one of the many pioneering projects at Xerox PARC. A common story states that Ethernet was invented in 1973, when Bob Metcalfe[?] wrote a memo to his bosses at PARC about Ethernet's potential. Metcalfe claims Ethernet was actually invented over a period of several years. In 1976, Robert Metcalfe and David Boggs (Metcalfe's assistant) published a paper titled, Ethernet: Distributed Packet-Switching For Local Computer Networks.

Metcalfe left Xerox in 1979 to promote the use of personal computers and local area networks (LANs), forming 3Com. He successfully convinced DEC, Intel, and Xerox to work together to promote Ethernet as a standard. Competing with them at the time were the two largely proprietary systems, Token ring and ARCNET, but both would soon find themselves buried under a tide of Ethernet products. In the process 3Com became a major company.

General Description

Ethernet is based on the idea of peers on the network sending messages in what was essentially a radio system, captive inside a common wire or channel, sometimes referred to as the ether. (This is an oblique reference to the luminiferous aether through which 19th century physicists believed light traveled.) Each peer has a globally unique 48-bit key known as the MAC address to ensure that all systems in an Ethernet have distinct addresses.

A scheme known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD) governs the way the computers share the channel. Originally developed in the 1960s for the ALOHAnet in Hawaii using radio, the scheme is relatively simple compared to token ring or master controlled networks. When one computer wants to send some information, it obeys the following algorithm:

if the wire is idle, start transmitting, else go to step 4
[transmitting information] if detecting a collision, continue transmitting until the minimum packet time is reached (to ensure that all other transmitters and receivers detect the collision) then go to step 4.
[end of successful transmission] report success to higher network layers, exit transmit mode.
[wire is busy ] wait until wire becomes idle
[wire has just become idle] wait a random time, then go to step 1, unless maximum number of transmission attempts has been exceeded
[maximum number of transmission attempt exceeded] report failure to higher network layers, exit transmit mode

In practice, this works something like a dinner party, where all the guests use a common medium (the air) to speak with one another. Before speaking, each guest politely waits for the current guest to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time. The hope is that by each choosing a random period of time, both guests will not choose the same time to try and speak again, thus avoiding another collision. Exponentially increasing back-off times are used when there is more than one failed attempt to transmit.

Since all communications happen on the same wire, any information sent by one computer is received by all, even if that information was intended for just one destination. Most Ethernet-connected computers therefore must continually filter out information that is not intended for them. This "one speaks, all listen" property is a security weakness of Ethernet, since a misbehaving node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses. This security flaw is largely ameliorated by switched networking (the use of switches as opposed to hubs).

Ethernet as a shared medium works well when the level of traffic is low. Since the chance of collision is proportional to the number of transmitters and the data to be sent, the network gets extremely congested above 50% capacity. To resolve this, Ethernet switches have been developed to maximize available bandwidth.

Ethernet frame types and the EtherType field

There are four types of Ethernet frame:

Original Ethernet Version I (no longer used)
The Ethernet Version 2 or Ethernet II frame, the so-called DIX frame, this is the most common today, as it is often used directly by the Internet Protocol.
IEEE 802.x LLC frame
IEEE 802.x LLC/SNAP frame

The different frame types have different formats and MTU values, but can coexist on the same physical medium.

The original Xerox Version 1 Ethernet had a 16 bit length field, although the maximum length of a packet was 1500 bytes. This length field was soon re-used in Xerox's Version 2 Ethernet as a label field, with the convention that values between 0 and 1500 indicated the use of the original Ethernet format, but higher values indicated what became known as an EtherType, and the use of the new frame format. This is now supported in the IEEE 802 protocols using the SNAP header.

IEEE 802.x defined the 16 bit field after the MAC addresses as a length field again. As Ethernet I framing is no longer used, this allows software to determine whether a frame is an Ethernet II frame or an IEEE 802.x frame, allowing the coexistence of both standards on the same physical medium. All 802.x frames have an LLC field. By examining the LLC field, it is possible to determine whether it is followed by a SNAP field.

The 802.x variants of Ethernet are not in widespread use on common networks. The most common type used today is Ethernet Version 2, as it is used by most Internet Protocol-based networks, with its EtherType set to 0x8000. There exists techniques for encapsulating IP traffic in e.g. 802.3 frames, but these are not common.

Varieties of Ethernet

Other than the framing types mentioned above, most of the other differences between Ethernet varieties have all been variations on speed and wiring. Therefore, in general, network protocol stack software will work identically on most any of the following types.

The following sections provide a brief summary of all the official ethernet media types. In addition to these official standards, many vendors have implemented proprietory media types for various reasons—often to support longer distances over fiber optic cabling.

Some early varieties of Ethernet

Xerox Ethernet[?] -- the original Ethernet implementation, which in turn had two versions, Version 1 and Version 2, during its development. The version 2 framing format is still in common use.
10BASE5 (also called Thicknet) -- this early IEEE standard uses a single coaxial cable into which you literally tapped a connection by drilling into the cable to connect to the core and screen. Largely obsolete, though due to its widespread deployment in the early days, some systems may still be in use.
10BROAD36 -- Obsolete. An early standard supporting ethernet over longer distances. It utilized broadband modulation techniques similar to those employed in cable modem systems, and operated over coaxial cable.
1BASE5 -- An early attempt to standardize a low-cost LAN solution, it operates at 1Mbps and was a commercial failure.
StarLAN 1 -- The first implementation of Ethernet on twisted pair wiring.

10Mbps ethernet

10BASE2 (also called ThinNet or Cheapernet) -- 50-ohm coaxial cable connects machines together, each machine using a T-adaptor to connect to its NIC. Requires terminators at each end. For many years this was the dominant ethernet standard 10Mbps.
StarLAN 10 -- First implementation of Ethernet on twisted pair wiring at 10Mbps. Later evolved into 10BASE-T.
10BASE-T -- runs over 4 wires (two twisted pairs) on a cat-3 or cat-5 cable. A hub or switch sits in the middle and has a port for each node. This is also the configuration used for 100BASE-T and Gigabit ethernet. 10Mbps.
FOIRL -- Fiber-optic inter-repeater link. The original standard for ethernet over fibre.
10BASE-F -- A generic term for the new family of 10Mbps ethernet standards: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of these only 10BASE-FL is in widespread use.
10BASE-FL -- An updated version of the FOIRL standard.
10BASE-FB -- Intended for backbones connecting a number of hubs or switches, it is now obsolete.
10BASE-FP -- A passive star network that required no repeater, it was never implemented

Fast Ethernet

100BASE-T -- A term for any of the three standard for 100Mbps ethernet over twisted pair cable. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2.
100BASE-TX -- also uses two pair, but requires cat-5 cable. Similar star-shaped configuration to 10BASE-T. 100Mbps.
100BASE-T4 -- 100Mbps ethernet over Category 3 cabling (as used for 10BASE-T installations). Uses all four pairs in the cable. Now obsolete, as Category 5 cabling is the norm. Limited to half-duplex.
100BASE-T2 -- No products exist. 100Mbps ethernet over Category 3 cabling. Supports full-duplex, and uses only two pairs. It is functionally equivalent to 100BASE-TX, but supports old cable.
100BASE-FX -- 100Mbps ethernet over fibre.

Gigabit Ethernet

1000BASE-T -- 1Gbps over cat-5 copper cabling.
1000BASE-SX[?] -- 1Gbps over fiber.
1000BASE-LX[?] -- 1Gbps over fiber. Optimized for longer distances over single-mode fiber.
1000BASE-CX[?] -- A short-haul solution (up to 25m) for running 1Gbps ethernet over special copper cable. Predates 1000BASE-T, and now obsolete.

10 gigabit Ethernet

The new 10 gigabit ethernet standard encompasses seven different media types for LAN, MAN and WAN. It is currently specified by a supplementary standard, IEEE 802.3ae, and will be incorporated into a future revision of the IEEE 802.3 standard.

10GBASE-SR[?] -- designed to support short distances over deployed multi-mode fiber cabling, it has a range of between 26m and 82m depending on cable type. It also supports 300m operation over a new 2000MHz.km multi-mode fiber.
10GBASE-LX4[?] -- uses wavelength division multiplexing to support ranges of between 240m and 300m over deployed multi-mode cabling. Also supports 10km over single-mode fiber.
10GBASE-LR[?] and 10GBASE-ER[?] -- these standards support 10km and 40km respecively over single-mode fiber.
10GBASE-SW[?], 10GBASE-LW[?] and 10GBASE-EW[?]. These varieties use the WAN PHY, designed to interoperate with OC-192 / STM-64 SONET/SDH equipment. They correspond at the physical layer to 10GBASE-SR, 10GBASE-LR and 10GBASE-ER respecively, and hence use the same types of fiber and support the same distances. (There is no WAN PHY standard corresponding to 10GBASE-LX4.)

10 gigabit Ethernet is very new, and it remains to be seen which of the standards will gain commercial acceptance.

Related standards

These networking standards are not part of the IEEE 802.3 Ethernet standard, but support the ethernet frame format, and are cabable of interoperating with it.

Wireless Ethernet (IEEE 802.11) -- Often running at 2Mbps and 11Mbps.
100BaseVG -- An early contender for 100Mbps ethernet. It runs over Category 3 cabling. Uses four pairs. Commercial failure.
TIA 100BASE-SX -- Promoted by the Telecommunications Industry Association[?]. 100BASE-SX is an alternative implementation of 100Mbps ethernet over fiber; it is incompatible with the official 100BASE-FX standard. Its main feature is interoperability with 10BASE-FL, supporting autonegotiation between 10Mbps and 100Mbps operation -- a feature lacking in the official standards due to to the use of differing LED wavelengths. It is targeted at the installed base of 10Mbps fiber network installations.
TIA 1000BASE-TX -- Promoted by the Telecommunications Industry Association[?], it was a commercial failure, and no products exist. 1000BASE-TX uses a simpler protocol than the official 1000BASE-T standard, but requires Category 6 cabling.

Mbps = Megabits per second
Gbps = Gigabits per second

It has been observed that Ethernet traffic has self-similar properties, with important consequences for traffic engineering.