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Ethernet, Ethernet

By software convention, the 16 bits after the destination and source address fields were a packet type field, but, as the paper says, "different protocols use disjoint sets of packet types", so those were packet types within a given protocol, rather than the packet type in current Ethernet which specifies the protocol being used.

It competed with two largely proprietary systems, Token Ring and Token Bus. To get over delays of the finalization of the Ethernet CSMA/CD standard due to the difficult decision processes in the "open" IEEE and due to the competitive Token Ring proposal strongly supported by IBM, support of CSMA/CD in other standardization bodies, i.e. ECMA, IEC and ISO was instrumental for its success. Proprietary systems soon found themselves buried under a tidal wave of Ethernet products. In the process, 3Com became a major company. 3COM built the first 10 Mbit/s Ethernet adapter (1983), followed quickly by Digital Equipment's Unibus to Ethernet adapter.

Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The methods used show some similarities to radio systems, although there are fundamental differences, such as the fact that it is much easier to detect collisions in a cable broadcast system than a radio broadcast. The common cable providing the communication channel was likened to the ether and it was from this reference that the name "Ethernet" was derived.

StarLAN was the first step in the evolution of Ethernet from a coaxial cable bus to a hub-managed, twisted-pair network. The advent of twisted-pair wiring dramatically lowered installation costs relative to competing technologies, including the older Ethernet technologies.

As with other IEEE 802 LANs, each Ethernet station is given a single 48-bit MAC address, which is used to specify both the destination and the source of each data packet. Network interface cards (NICs) or chips normally do not accept packets addressed to other Ethernet stations. Adapters generally come programmed with a globally unique address, but this can be overridden, either to avoid an address change when an adapter is replaced, or to use locally administered addresses.

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. Multipoint systems are also prone to very strange failure modes when an electrical discontinuity reflects the signal in such a manner that some nodes would work properly while others work slowly because of excessive retries or not at all (see standing wave for an explanation of why); these could be much more painful to diagnose than a complete failure of the segment. Debugging such failures often involved several people crawling around wiggling connectors while others watched the displays of computers running a ping command and shouted out reports as performance changed.

The network interface card interrupts the CPU only when applicable packets are received: the card ignores information not addressed to it unless it 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.

For coaxial-cable-based Ethernet, each end of the cable had a 50ohm () resistor attached. Typically this resistor was built into a male BNC or N connector and attached to the last device on the bus, or, if vampire taps were in use, to the end of the cable just past the last device. If termination was not done, or if there was a break in the cable, the AC signal on the bus was reflected, rather than dissipated, when it reached the end. This reflected signal was indistinguishable from a collision, and so no communication would be able to take place.

Repeaters could be used to connect segments such that there were up to five Ethernet segments between any two hosts, three of which could have attached devices. Repeaters could detect an improperly terminated link from the continuous collisions and stop forwarding data from it. Hence they alleviated the problem of cable breakages: when an Ethernet coax segment broke, while all devices on that segment were unable to communicate, repeaters allowed the other segments to continue working - although depending on which segment was broken and the layout of the network the partitioning that resulted may have made other segments unable to reach important servers and thus effectively useless.

These could be connected to each other and/or a coax backbone. A well-known early example was DEC's DELNI. These devices allowed multiple hosts with AUI connections to share a single transceiver. They also allowed creation of a small standalone Ethernet segment without using a coaxial cable.

This changed hubs from a specialist device used at the center of large networks to a device that every twisted pair-based network with more than two machines had to use. The tree structure that resulted from this made Ethernet networks more reliable by preventing faults with (but not deliberate misbehavior of) one peer or its associated cable from affecting other devices on the network, although a failure of a hub or an inter-hub link could still affect lots of users. Also, since twisted pair Ethernet is point-to-point and terminated inside the hardware, the total empty panel space required around a port is much reduced, making it easier to design hubs with lots of ports and to integrate Ethernet onto computer motherboards.

With bridging, only well-formed Ethernet packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. Bridges learn where devices are, by watching MAC addresses, and do not forward packets across segments when they know the destination address is not located in that direction.

Bridges also overcame the limits on total segments between two hosts and allowed the mixing of speeds, both of which became very important with the introduction of Fast Ethernet.

These variants have also seen substantial penetration in enterprise datacenter applications, but are rarely seen connected to end user systems for cost/convenience reasons. Their advantages lie in performance, electrical isolation and distance, up to tens of kilometers with some versions. Fiber versions of a new higher speed almost invariably come out before copper. 10 gigabit Ethernet is becoming more popular in both enterprise and carrier networks, with development starting on 40 Gbit/s and 100 Gbit/s Ethernet. Metcalfe now believes commercial applications using terabit Ethernet may occur by 2015 though he says existing Ethernet standards may have to be overthrown to reach terabit Ethernet.

A frame viewed on the actual physical wire would show Preamble and Start Frame Delimiter, in addition to the other data. These are required by all physical hardware. They are not displayed by packet sniffing software because these bits are removed by the Ethernet adapter before being passed on to the host (in contrast, it is often the device driver which removes the CRC32 (FCS) from the packets seen by the user).

The two formats were eventually unified by the convention that values of that field between 64 and 1522 indicated the use of the new 802.3 Ethernet format with a length field, while values of 1536 decimal (0600 hexadecimal) and greater indicated the use of the original DIX or Ethernet II frame format with an EtherType sub-protocol identifier.

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. The SNAP header allows EtherType values to be used with all IEEE 802 protocols, as well as supporting private protocol ID spaces. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.

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. This does not conform to the IEEE 802.3 standard, but since IPX has always FF at the first two bytes (while in IEEE 802.2 LLC that pattern is theoretically possible but extremely unlikely), in practice 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.

Since Netware 4.10, Netware now defaults to IEEE 802.2 with LLC (Netware Frame Type Ethernet_802.2) when using IPX. (See "Ethernet Framing" in References for details.) Mac OS uses 802.2/SNAP framing for the AppleTalk V2 protocol suite on Ethernet ("EtherTalk") and Ethernet II framing for TCP/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 0x0800 for IPv4 and 0x86DD for IPv6.

IP traffic cannot 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 can also be transmitted over Ethernet using IEEE 802.2 with LLC/SNAP, but, again, that's almost never used (although LLC/SNAP encapsulation of IPv6 is used on IEEE 802 networks).

Source: Wikipedia > Ethernet