Ethernet

Ethernet is a packet switched computer networking protocol which implements the physical layer and the media access controll (MAC) sublayer of thec OSI model. The most widespread standard in use during the 1990s and currently is IEEE's 802.3. In the original ethernet specification, all networked computers are connected together by a common wire or channel, sometimes refered to as the ether. (This is an oblique reference to the luminiferous aether through which 19th century physicists believed light traveled.) A scheme known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD) governs the way the computers share the channel.

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 to speak with one another (the air) and politely wait for each other to finish speaking before speaking themselves. If two guests start speaking at the same time, each stops speaking and waits for a short, random period of time. The hope is that by each choosing a random period of time, both people will not choose the same time to try and speak again, thus avoiding another collision. Exponential 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.

Originally developed in the 1960's for the Alohanet in Hawaii using radio, the scheme was quickly converted to wire systems, as the electronic control system is relatively simple compared to token ring or master controlled networks.

Ethernet as it is currently known was invented by Bob Metcalfe and D. R. Boggs while working at Xerox PARC in 1973.

Ethernet uses a system of globally unique addresses called MAC addresses to ensure that all systems in an Ethernet have distinct addresses.

Some early varieties of Ethernet

10BASE5 -- the original 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 empolyed 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.

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.
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-FL -- An updated version of the FOIRL standard.

Fast ethernet

100BASE-T -- A term for any of the three standard for 100Mbps ethernet over twisted pair calbe. 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.

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 gibabit 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 100Mbs ethernet over fiber; tt 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-to-the-desktop installations.
TIA 1000BASE-T -- Promoted by the Telecommunications Industry Association, it was a commercial failure, and no products exist. 1000BASE-T uses simpler protocol than the official 1000BASE-TX 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.

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