Ethernet

Ethernet is a packet-based 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 the present (2004), and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET.

History

Ethernet was originally developed as one of the many pioneering projects at Xerox PARC. A common story states that Ethernet was invented in 1973, when Robert 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, 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 (DIX), which was first published on September 30 1980. 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 tidal wave 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.

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

CSMA/CD shared medium Ethernet

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 to speak again, thus avoiding another collision. Exponentially increasing back-off times are used when there is more than one failed attempt to transmit.

Ethernet originally used a literally shared coaxial cable, with this cable winding around a building or campus to every attached machine. Computers were connected to an Attachment Unit Interface (AUI) transceiver, which in turn connected to the cable. Whilst a simple passive wire was highly reliable for small Ethernets, it was not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector could make the whole Ethernet unusable.

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 shared-medium Ethernet, since a misbehaving node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses. Use of a single cable also means that the bandwidth is shared, so that network traffic can slow to a crawl in scenarios such as the network and nodes restarting after a power failure.

Ethernet hubs

This problem was addressed by the invention of Ethernet hubs, which had a physical star topology, with multiple devices being wired back to a hub, and the hub then being either wired back to an original passive coax backbone, or to a higher-level hub. Instead of a coax connection or an AUI cable, hubbed 10BASE-T Ethernet uses Cat-3/Cat-5 cable and RJ45 connectors to connect endpoints to hubs.

However, in spite of the physical star topology, hubbed Ethernet networks still use CSMA/CD, with every packet being sent to every port on the hub, and only minimal cooperation from the hub in dealing with packet collisions.

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" were developed to maximize available bandwidth.

Switched Ethernet

Most modern Ethernet installations use Ethernet switches as opposed to hubs. Although the wiring is identical to hubbed Ethernet, switched Ethernet has several advantages over shared medium Ethernet including greater bandwidth and simplified wiring. Switched networks typically have a star topology, even though they still implement a single Ethernet "cloud" from the viewpoint of attached machines.

Initially, Ethernet switches work like Ethernet hubs, with all traffic being echoed to all ports. However, as the switch "learns" the end-points associated with each port, it ceases to send non-broadcast traffic to ports other than the intended destination. In this way, Ethernet switching can allow the full wire speed of Ethernet to be used by any given pair of ports on a single switch.

Since packets are typically only delivered to the port they are intended for, traffic on a switched Ethernet is slightly less public than on shared-medium Ethernet. However, as it is easy to subvert switched Ethernet systems by means such as ARP spoofing and MAC flooding, as well as for the network administrators to use monitoring functions to copy traffic from the network, switched Ethernet should still be regarded as an insecure network technology.

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Ethernet frame types and the EtherType field

Frames are the format of data packets on the wire.

There are five types of Ethernet frame:

Original Ethernet Version I (no longer used)
The Ethernet Version 2 or Ethernet II frame, the so-called DIX frame (named after DEC, Intel, and Xerox), this is the most common today, as it is often used directly by the Internet Protocol.
Novell's homegrown Variation of IEEE 802.3 ("raw 802.3 frame") without LLC
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.

Ethernet Type II Frame format

The most common Ethernet Frame format, type II

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.

EtherType	Protocol
0x0800	IP Internet Protocol (IPv4)
0x0806	Address Resolution Protocol (ARP)
0x8035	Reverse Address Resolution Protocol (RARP)
0x809b	AppleTalk (Ethertalk)
0x80f3	Appletalk Address Resolution Protocol (AARP)
0x8137	Novell IPX (alt)
0x8138	Novell
0x86DD	Internet Protocol, Version 6 (IPv6)

Type field (EtherType) for some common protocols

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.

Novell's "raw" 802.3 frame format was based on early IEEE 802.3 work. Novell used this as a starting point to create the first implementation of its own IPX Network Protocol over Ethernet. They did not use any LLC header but started the IPX packet directly after the length field. In principle this is not interoperable with the other later variants of 802.x Ethernet, but since IPX has always FF at the first byte (while LLC has not), this mostly coexists on the wire with other Ethernet implementations (with the notable exception of some early forms of DECnet which got confused by this).

Novell Netware used this frame type by default until the mid nineties, and since Netware was very widespread back then (while IP was not) at some point in time most of the world's Ethernet traffic ran over "raw" 802.3 carrying IPX. Since Netware 4.10 Netware now defaults to IEEE 802.x with LLC (Netware Frame Type Ethernet_802.2) when using IPX. There is a Classical Series of Usenet Postings by Novell's Don Provan that have found their way into numerous FAQs and are widely considered the definitive answer to the Novell Frame Type jungle.

The 802.x variants of Ethernet are not in widespread use on common networks currently, with the exception of large corporate Netware installations that have not yet migrated to Netware over IP. 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 0x0800. There exists a well defined standard for encapsulating IP traffic in IEEE 802.3 frames with LLC/SNAP headers, but it is commonly not supported.

IP Version 6 over Ethernet is also standardized based on IEEE 802.x. with LLC/SNAP.

Novell IPX is probably the only network protocol that supports all 4 current frame types.

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 proprietary 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.
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 1 Mbit/s and was a commercial failure.
StarLAN 1 -- The first implementation of Ethernet on twisted pair wiring.

10 Mbit/s Ethernet

10BASE5 (also called Thickwire or Yellow Cable) -- this is the original 10Mbit/s implementation of Ethernet. The early IEEE standard uses a single 50-ohm coaxial cable of a type designated RG-8, of maximum length 500 metres, into which the transceiver was connected by a so-called "vampire tap", which attached itself by drilling into the cable to connect to the core and screen. An AUI cable then connected the transceiver to the Ethernet device. Largely obsolete, though due to its widespread deployment in the early days, some systems may still be in use. It requires precise termination at each end of the cable.
10BASE2 (also called Thinwire or Cheapernet) -- 50-ohm RG-58 coaxial cable, of maximum length 200 metres, connects machines together, each machine using a T-adaptor to connect to its NIC, which has a BNC connector. Requires terminators at each end. For many years this was the dominant 10 Mb/s ethernet standard..
StarLAN 10 -- First implementation of Ethernet on twisted pair wiring at 10 Mbit/s. 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. 10 Mbit/s.
FOIRL -- Fiber-optic inter-repeater link. The original standard for ethernet over fibre.
10BASE-F -- A generic term for the new family of 10 Mbit/s 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 100 Mbit/s 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. 100Mbit/s.
100BASE-T4 -- 100 Mbit/s ethernet over cat-3 cabling (as used for 10BASE-T installations). Uses all four pairs in the cable. Now obsolete, as cat-5 cabling is the norm. Limited to half-duplex.
100BASE-T2 -- No products exist. 100 Mbit/s ethernet over cat-3 cabling. Supports full-duplex, and uses only two pairs. It is functionally equivalent to 100BASE-TX, but supports old cable.
100BASE-FX -- 100 Mbit/s ethernet over fibre.

Gigabit Ethernet

1000BASE-T -- 1 Gbit/s over cat-5e or cat-6 copper cabling.
1000BASE-SX -- 1 Gbit/s over fiber.
1000BASE-LX -- 1 Gbit/s over fiber. Optimized for longer distances over single-mode fiber.
1000BASE-CX -- A short-haul solution (up to 25 m) for running 1 Gbit/s 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 26 m and 82 m depending on cable type. It also supports 300 m operation over a new 2000 MHz.km multi-mode fiber.
10GBASE-LX4 -- uses wavelength division multiplexing to support ranges of between 240 m and 300 m over deployed multi-mode cabling. Also supports 10 km over single-mode fiber.
10GBASE-LR and 10GBASE-ER -- these standards support 10 km and 40 km 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 capable of interoperating with it.

Wireless Ethernet (IEEE 802.11) -- Often running at 2 Mbit/s (802.11legacy), 11 Mbit/s (802.11b) or 54 Mbit/s (802.11g).
100BaseVG -- An early contender for 100 Mbit/s 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 100 Mbit/s ethernet over fiber; it is incompatible with the official 100BASE-FX standard. Its main feature is interoperability with 10BASE-FL, supporting autonegotiation between 10 Mbit/s and 100 Mbit/s operation -- a feature lacking in the official standards due to the use of differing LED wavelengths. It is targeted at the installed base of 10 Mbit/s 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.

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