| Application layer | FTP | SMTP | HTTP | ... |
| Transport layer | TCP | UDP | ||
| Network layer | IP ICMP | ARP | ||
| Data link layer | Ethernet | Token Ring | FDDI | ... |
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 protocolss 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 (2003), and has largely replaced all other LAN standards such as token ring, FDDI, and ARCNET.
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 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, 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, 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 tide of Ethernet products. In the process 3Com became a major company.
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.
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:
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.
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.
Most modern Ethernet installations use Ethernet switches as opposed to hubs.
Although the wiring is identical to hubbed Ethernet, with switches instead of hubs, 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.
There are four types of Ethernet frame:
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. IEEE 802.3 frames, but these are not common.
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.
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.
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.
History
General Description
CSMA/CD shared medium Ethernet
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.Ethernet hubs
Switched Ethernet
Ethernet frame types and the EtherType field
The different frame types have different formats and MTU values, but can coexist on the same physical medium.Varieties of Ethernet
Some early varieties of Ethernet
10 Mbit/s Ethernet
Fast Ethernet
Gigabit Ethernet
10 Gigabit Ethernet
10 gigabit Ethernet is very new, and it remains to be seen which of the standards will gain commercial acceptance.Related standards
See also: CHAOSnet