CSMA/CD is a modified version of CSMA.

A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governs the way the computers share the channel. This scheme is relatively simple compared to the token ring. When one computer on the network wants to send some data to another computer, the following algorithm is followed:

1. Start – If the wire (also referred as ethernet cable) is idle, start transmitting, else go to step 4.
2. Transmitting – 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.
3. End successful transmission – Report success to higher network layers; exit transmit mode.
4. Wire is busy – Wait until wire becomes idle – This will go back to step 1 and the loop continues until the wire is idle for data transmission.
5. Wire just became idle – Wait a random time, then go to step 1, unless maximum number of transmission attempts has been exceeded.
6. Maximum number of transmission attempt exceeded – Report failure to higher network layers; exit transmit mode.

Ethernet originally used a shared coaxial cable to connect every machine on the network. Computers were connected to an Attachment Unit Interface (AUI) transceiver, which in turn was connected to the cable. While 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 segment unusable. This presented a single point of failure at multiple locations.

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. The network interface card filters out information not addressed to it, interrupting the CPU only when applicable packets are received unless the card is put into “promiscuous mode“. This “one speaks, all listen” property is a security weakness of shared-medium Ethernet, since a 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 when, for example, the network and nodes restart after a power failure.

Note: Collision is an event that occurs when data is simultaneously transmitted over the same wire by two computers. Collision is very common on ethernet and should not be considered a problem.

There are several types of Ethernet frame:

1. Original Ethernet Version I (no longer used)
2. 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.
3. Novell’s homegrown Variation of IEEE 802.3 (“raw 802.3 frame”) without LLC
4. IEEE 802.2 LLC frame
5. IEEE 802.2 LLC/SNAP frame

In addition, Ethernet frames may optionally contain a IEEE 802.1Q tag to identify what VLAN it belongs to and its IEEE 802.1p priority (quality of service). This doubles the potential number of frame types.

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 Ether Type, and the use of the new frame format. This is now supported in the IEEE 802 protocols using the SNAP header.

Type field (EtherType) for some common protocols

IEEE 802.2 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 802.2 frame, allowing the coexistence of both standards on the same physical medium. All 802.2 frames have a logical link control (LLC) header. By examining this header, it is possible to determine whether it is followed by a SNAP (subnetwork access protocol) header. (Some protocols, particularly those designed for the OSI networking stack, operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented network services.) The LLC header includes two additional eight-bit address fields (called service access points or SAPs in OSI terminology); when both source and destination SAP are set to the value 0xAA, the SNAP service is requested.

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 802.2 with LLC (Netware Frame Type Ethernet_802.2) when using IPX. There is a classic series of Usenet postings by Novell’s Don Provan that have found their way into numerous FAQ’s and are widely considered the definitive answer to the Novell Frame Type jungle.

Mac OS uses 802.2/SNAP framing for the AppleTalk protocol suite on Ethernet (“EtherTalk”) and Ethernet 2 framing for TCP/IP.

The 802.2 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. In the past, many corporate networks supported 802.2 Ethernet to support transparent translating bridges between Ethernet and IEEE 802.5 Token Ring or FDDI networks. The most common framing type used today is Ethernet Version 2, as it is used by most Internet Protocol-based networks, with its EtherType set to 0×0800.

There exists an Internet standard () for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP headers. It is almost never implemented on Ethernet (although it is used on Token Ring and FDDI networks). IP traffic can not be encapsulated in IEEE 802.2 LLC frames without SNAP because, although there is an LLC protocol type for IP, there is no LLC protocol type for ARP. IP Version 6 over Ethernet is also standardized based on IEEE 802.2 with LLC/SNAP.

The IEEE 802.1Q tag, if present, is placed between the Source Address and the EtherType or Length fields. The first two bytes of the tag are the Tag Protocol Identifier (TPID) value of 0×8100. This is located in the same place as the EtherType/Length field in untagged frames, so an EtherType value of 0×8100 means the frame is tagged, and the true EtherType/Length is located after the tag. The TPID is followed by two bytes containing the Tag Control Information (TCI) (the IEEE 802.1p priority (quality of service) and VLAN id). The tag is followed by the rest of the frame, using one of the types described above.

Many Ethernet cards and switch ports support multiple speeds, using auto-negotiation to set the speed and duplex for the best values supported by both connected devices. If auto-negotiation fails, a multiple speed device will sense the speed used by its partner, but will assume half-duplex.

A 10/100 Ethernet port supports 10BASE-T and 100BASE-TX and a 10/100/1000 Ethernet port supports 10BASE-T, 100BASE-TX, and 1000BASE-T.

10 Mbps Ethernet ( 10Mbit/Second)

1. 10BASE5 (also called Thick wire or Yellow Cable) — this is the original 10 Mbit/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 meters. Transceivers could be connected by a so-called “vampire tap”, which was attached by drilling into the cable to connect to the core and screen, or using N connectors at the end of a cable segment. 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.
2. 10BASE2 (also called Thin wire or Cheaper net) — 50 ohm RG-58 coaxial cable, of maximum length 200 meters, connects machines together, each machine using a T-adaptor to connect to its NIC, which has a BNC connector. Requires termination at each end. For many years this was the dominant 10 Mbit/s Ethernet standard.
3. Star LAN 10 — First implementation of Ethernet on twisted pair wiring at 10 Mbit/s. Later evolved into 10BASE-T.
4. 10BASE-T — runs over 4 wires (two twisted pairs) on a cat-3 or cat-5 cable up to 100 meters in length. A hub or switch sits in the middle and has a port for each node.
5. FOIRL — Fiber-optic inter-repeater link. The original standard for ethernet over fibre.
6. 10BASE-F — A generic term for the family of 10 Mbit/s ethernet standards using fiber optic cable up to 2 kilometers in length: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of these only 10BASE-FL is in widespread use.
7. 10BASE-FL — An updated version of the FOIRL standard.
8. 10BASE-FB — Initially intended for backbones connecting a number of hubs or switches, it is now obsolete.
9. 10BASE-FP — A passive star network that required no repeater, it was never implemented.

Fast Ethernet (100Mbit/Second)

Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s.
Fast Ethernet is no longer the fastest form of Ethernet: Gigabit Ethernet and the new 10 Gigabit Ethernet standards are 10 and 100 times faster, respectively.

1. 100BASE-T — A term for any of the three standards for 100 Mbit/s ethernet over twisted pair cable up to 100 meters long. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2. In practical scenarios, the cable can be extended up to 150 meters but it is not advisable.
2. 100BASE-TX — Similar star-shaped configuration to 10BASE-T. It also uses two pairs, but requires cat-5 cable to achieve 100Mbit/s.
3. 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.
4. 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.
5. 100BASE-FX — 100 Mbit/s ethernet over multimode fibre. Maximum length is 400 meters for half-duplex connections (to ensure collisions are detected) or 2 kilometers for full-duplex.

Gigabit Ethernet

Gigabit Ethernet (GbE) is a term describing various technologies for implementing Ethernet networking at a nominal speed of one gigabit per second.

Gigabit Ethernet is supported over both optical fiber and twisted pair cable. Physical layer standards include 1000BASE-T, 1 Gbit/s over cat-5e copper cabling and 1000BASE-SX for short to medium distances over fiber.

While it is currently deployed in high-capacity backbone network links (for instance, on a high-capacity campus network) its speed is largely not yet required for small network installations. Gigabit Ethernet has begun to penetrate the desktop (as of 2004), shipping standard on Apple’s Power Mac G5, the notebook (Apple’s Power Book), and is built into some high-end Pentium and Athlon motherboards. Desktop applications for it include professional video editing.

It is no longer the fastest Ethernet standard, with the ratification of 10 Gigabit Ethernet in 2002

1. 1000BASE-T — 1 Gbit/s over cat-5e or cat-6 copper cabling.
2. 1000BASE-SX — 1 Gbit/s over multi-mode fiber (up to 550 m).
3. 1000BASE-LX — 1 Gbit/s over multi-mode fiber (up to 550 m). Optimized for longer distances (up to 10 km) over single-mode fiber.
4. 1000BASE-LH — 1 Gbit/s over single-mode fiber (up to 100 km). A long-haul solution.
5. 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

Ten-gigabit Ethernet (XGbE or 10GbE) is the most recent (as of 2002) and fastest of the Ethernet standards.

IEEE 802.3ae defines a version of Ethernet with a nominal data rate of 10 Gbit/s, ten times faster than 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.

1. 10GBASE-CX4 — designed to support short distances over copper cabling, it uses InfiniBand 4x connectors and CX4 cabling and allows a cable length of up to 15 m.
2. 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.
3. 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.
4. 10GBASE-LR and 10GBASE-ER — these standards support 10 km and 40 km respectively over single-mode fiber.
5. 10GBASE-SW, 10GBASE-LW and 10GBASE-EW. These varieties use the WAN PHY, designed to inter operate with OC-192 / STM-64 SONET/SDH equipment. They correspond at the physical layer to 10GBASE-SR, 10GBASE-LR and 10GBASE-ER respectively, and hence use the same types of fiber and support the same distances. (There is no WAN PHY standard corresponding to 10GBASE-LX4.)
6. 10GBASE-T — Uses unshielded twisted-pair wiring.10GBASE-T should be ready by the Northern Hemisphere summer of 2006.

Ethernet, by far is the most widely used LAN standard. Every peer node on the network connected using ethernet has a 48-bit unique MAC address such that each system on the network can be distinguished.

History of Ethernet

Ethernet was originally developed as one of the many pioneering projects at Xerox PARC. The standard was first published on September 30, 1980. It competed with two largely proprietary systems, token ring and ARCNET, but those soon found themselves buried under a tidal wave of Ethernet products. In the process, 3Com became a major company.

The characteristics of typical traffic on actual networks differ from what had been expected before LAN’s became common in ways that favor the simple design of Ethernet. Metcalfe and Saltzer worked on the same floor at MIT’s Project MAC while Metcalfe was doing his Harvard dissertation, in which he worked out the theoretical foundations of Ethernet.