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Ether is a frame-based puter working technology for local area works (LANs). The name es from the physical concept of ether. It defines wiring and signaling for the physical layer, and frame formats and protocols for the media access control (MAC)/data link layer of the OSI model. Ether is mostly standardized as IEEEs 802.3. It has bee the most widespread LAN technology in use during the 1990s to the present, and has largely replaced all other LAN standards such as token ring, FDDI, and ARC.

History

Ether was originally developed as one of the many pioneering projects at Xerox PARC. A mon story states that Ether was invented in 1973, when Robert Metcalfe wrote a memo to his bosses at PARC about Ether's potential. But Metcalfe claims Ether was actually invented over a period of several years. In 1976, Metcalfe and his assistant Did Boggs published a paper titled Ether: Distributed Packet-Switching For Local puter works.

The experimental Ether described in that paper ran at 3 Mbps, and had 8-bit destination and source address fields, so Ether addresses weren't the global addresses they are today. 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 Ether, which specifies the protocol being used.

Metcalfe left Xerox in 1979 to promote the use of personal puters and local area works (LANs), forming 3. He convinced DEC, Intel, and Xerox to work together to promote Ether as a standard, the so-called "DIX" standard, for "Digital/Intel/Xerox"; it standardized the 10 megabits/second Ether, with 48-bit destination and source addresses and a global 16-bit type field. The standard was first published on September 30, 1980. It peted with two largely proprietary systems, token ring and ARC, but those soon found themselves buried under a tidal we of Ether products. In the process, 3 became a major pany.

Metcalfe sometimes jokingly credits Jerry Saltzer for 3's success. Saltzer cowrote an influential paper suggesting that token-ring architectures were theoretically superior to Ether-style technologies. This result, the story goes, left enough doubt in the minds of puter manufacturers that they decided not to make Ether a standard feature, which allowed 3 to build a business around selling add-in Ether work cards. This also led to the saying "Ether works better in practice than in theory," which, though a joke, actually makes a valid technical point: the characteristics of typical traffic on actual works differ from what had been expected before LANs became mon in ways that for the simple design of Ether.

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 Ether.

General deion

A 1990s Ether work interface card. This is a bo card that supports both coaxial-based 10BASE2 (BNC connector, left) and Twisted-pair-based 10BASE-T (RJ-45 connector, right).

Ether is based on the idea of peers on the work sending messages in what was essentially a radio system, captive inside a mon wire or channel, sometimes referred to as the ether. (This is an oblique reference to the luminiferous aether through which 19th century physicists incorrectly theorized that electromagic radiation treled.) Each peer has a unique 48-bit key known as the MAC address to ensure that all systems in an Ether work he distinct addresses. By default work cards e programmed with a globally unique address but this can generally be changed and there are a number of reasons for doing so.

Due to the ubiquity of Ether and the ever-reducing cost of the hardware needed to support it, most manufacturers build the functionality of an Ether card directly into PC motherboards.

Despite the huge changes in Ether from a thick coaxial cable bus running at 10 Mbps to point-to-point links running at 1 Gbps (see gigabit ether) and beyond, the different variants remain essentially the same from the programmer's point of view and are easily interconnected using readily ailable inexpensive hardware.

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

CSMA/CD shared medium Ether

A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governs the way the puters share the channel. Originally developed in the 1960s for the ALOHA in Hawaii using radio, the scheme is relatively simple pared to token ring or master controlled works. When one puter wants to send some information, it obeys the following algorithm:

Start - If the wire is idle, start transmitting, else go to step 4

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.

End successful transmission - Report success to higher work layers; exit transmit mode.

Wire is busy - Wait until wire bees idle

Wire just became 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 work layers; exit transmit mode

This works something like a dinner party, where all the guests talk to each other through a mon medium (the air). 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 oiding another collision. Exponentially increasing back-off times (determined using the truncated binary exponential backoff algorithm) are used when there is more than one failed attempt to transmit.

Ether originally used a shared coaxial cable winding around a building or campus to every attached machine. puters were connected to an Attachment Unit Interface (AUI) transceiver, which in turn connected to the cable. While a simple passive wire was highly reliable for small Ethers, it was not reliable for large extended works, where damage to the wire in a single place, or a single bad connector could make the whole Ether segment unusable. Coax was also prone to very strange failure modes when an electrical discontinuity reflected the signal in such a manner that some nodes would work just fine while others would work slowly due to excessive retries or not at all; these could be much more painful to diagnose than a plete failure of the segment. Debugging such failures often involved several people crawling around wiggling connectors while others watched the displays of puters running ping and shouted out reports as performance changed.

Since all munications happen on the same wire, any information sent by one puter is received by all, even if that information was intended for just one destination. The work 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 Ether, since a node on an Ether work can eesdrop on all traffic on the wire if it so chooses. Use of a single cable also means that the bandwidth is shared, so that work traffic can slow to a crawl when, for example, the work and nodes restart after a power failure.

Ether repeaters and hubs

As Ether grew, the Ether hub was developed to make the work more reliable and the cables easier to connect.

For signal degradation and timing reasons, Ether segments he a restricted size which depends on the medium used. For example, 10BASE5 coax cables he a maximum length of 500 metres (1,640 feet). A greater length can be obtained by using an Ether repeater, which takes the signal from one Ether cable and repeats it onto another cable. Repeaters can be used to connect up to five Ether segments, three of which can he attached devices. This also alleviates the problem of cable breakages: when an Ether coax segment breaks, all devices on that segment are unable to municate; repeaters allowed the other segments to continue working.

Like most other high-speed buses, Ether segments must be terminated with a resistor at both ends. For coaxial cable, each end of the cable must he a 50-ohm resistor and heatsink attached, called a terminator and affixed to a male N or BNC connector. If this is not done, the result is the same as if there is a break in the cable: the AC signal on the bus will be reflected, rather than dissipated, when it reaches the end. This reflected signal is indistinguishable from a collision, and so no munication can take place. A repeater electrically isolates the segments connected to it, regenerating and retiming the signal. Most repeaters he an "auto-partition" function, which partitions (removes from service) a segment when it has too many collisions or collisions that last too long, so that the other segments are not affected by the broken one. The repeater reconnects the segment when it detects activity without collisions.

People recognized the usefulness of cabling in a star topology, and work vendors started creating repeaters hing multiple ports. Multi-port repeaters are now known as hubs. Hubs can be connected to other hubs and/or a coax backbone.

The first hubs were known as "multiport transceivers" or "fanouts". The best-known example is DEC's DELNI. These devices allow multiple hosts with AUI connections to share a single transceiver. They also allow creation of a small standalone Ether segment wit

hout using a coax cable.

work vendors such as DEC and SynOptics sold hubs which connected many 10BASE-2 thin coaxial segments.

Coaxial cable is used to transmit 10BASE-2 Ether

The development of Ether on unshielded twisted-pair cables (UTP), beginning with StarLAN and continuing with 10BASE-T eventually made Ether over coax obsolete. These variations allowed unshielded twisted-pair Cat-3/Cat-5 cable and RJ45 telephone connectors to connect endpoints to hubs, replacing coaxial and AUI cables. Hubs made Ether works more reliable by preventing problems with one cable or device from affecting other devices on the work. Twisted-pair Ether resolves the termination problem by making every segment point-to-point, so termination can be built into the hardware rather than requiring a special external resistor.

A Twisted pair 10BASE-T Cable is used to transmit 10BASE-T Ether

Despite the physical star topology, hubbed Ether works are half-duplex and still use CSMA/CD, with only minimal cooperation from the hub in dealing with packet collisions. Every packet is sent to every port on the hub, so bandwidth and security problems aren't addressed. The total throughput of the hub is limited to the speed of a single link, either 10 or 100 Mbit/s, minus the overhead for preambles, inter-frame gaps, headers, trailers, and padding. Collisions also reduce the total throughput, especially when the work is heily loaded. In the worst case when there are lots of hosts with long cables that transmit many short frames, excessive collisions that seriously reduce throughput can happen with loads as low as 50%. A more typical configuration can tolerate higher loads before collisions seriously reduce throughput.

Bridging and Switching

While repeaters could isolate some aspects of Ether segments, such as cable breakages, they still forward all traffic to all Ether devices. This creates significant limits on how many machines can municate on an Ether work. To alleviate this, bridging was created to municate at the data link layer while isolating the physical layer. With bridging, only well-formed packets are forwarded from one Ether 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. Control mechanisms like spanning-tree protocol enable a collection of bridges to work together in coordination.

Early bridges examined each packet one by one, and were significantly slower than hubs (repeaters) at forwarding traffic, especially when handling many ports at the same time. In 1989 the working pany Kalpana introduced their EtherSwitch, the first Ether switch. An Ether switch does bridging in hardware, allowing it to forward packets at full wire speed.

Most modern Ether installations use Ether switches instead of hubs. Although the wiring is identical to hubbed Ether, switched Ether has several advantages over shared medium Ether including greater bandwidth and better isolation from misbehing devices. Switched works typically he a star topology, even though they may still implement a single Ether shared medium from the viewpoint of attached machines, if they use the half-duplex option. Full-duplex Ether in the 10BASE-T and later standards is not a shared-medium system.

Initially, Ether switches work like Ether 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, Ether switching can allow the full wire speed of Ether 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 Ether is slightly less public than on shared-medium Ether. Despite this, switched Ether should still be regarded as an insecure work technology, because it is easy to subvert switched Ether systems by means such as ARP spoofing and MAC flooding, as well as for work administrators to use monitoring functions to copy traffic from the work.

When only a single device (anything but a hub) is connected to a switch port, full-duplex Ether bees possible. In full duplex mode both devices can transmit to each other at the same time and there is no collision domain. This doubles the aggregate bandwidth of the link and was sometimes advertised as double the link speed (e.g. 200 Mbit/s) to account for this. However, this is misleading as performance will only double if traffic patterns are symmetrical (which in reality they rarely are). The elimination of the collision domain also means that all the links bandwidth can be used (collisions can occupy a lot of bandwidth as links get busy) and that segment length is not limited by the need for correct collision detection (this is most significiant with some of the fiber variants of ether).

It is essential that both the switch port and the device connected to it use the same duplex setting. Most 100BASE-TX and 1000BASE-T devices support auto-negotiation, where they signal the speed and duplex to use. However, if auto-negotiation is disabled or not supported, the duplex must be set by auto-detection or manually on both the switch port and the device to prevent duplex mismatch, a mon cause of problems with Ether (the device set to half-duplex will report late collisions and the device set to full-duplex will report runts). Many low-end switches lack the ability for manual speed and duplex setting, so ports always try to auto-negotiate. When auto-negotiation is enabled but does not succeed (e.g., because the other device does not support it), auto-detection sets the port to half-duplex. The speed can be automatically sensed, so connecting a 10BASE-T device to a 10/100 switch port with auto-negotiation enabled will correctly result in a half-duplex 10BASE-T connection. But connecting a device configured for full duplex 100 Mbit operation to a switch port configured to auto-negotiate (or vice versa) will result in a duplex mismatch.

Even when both ends of a cable are capable of autosensing speed and duplex settings, it is very mon for them to guess wrongly and fall back to 10 Mbit mode. Therefore, if performance is worse than expected, one should check whether a puter has put itself into 10 Mbit mode, and if one knows the other end is 100 Mbit capable, manually force it into the correct mode.

Problems also occur when two nodes try to operate at speeds faster than the cable can support, such as attempting 100BASE-T on Category 3 cable or 1000BASE-T on Category 3 or Category 5 cable. Unlike ADSL and conventional dialup modems, which perform an elaborate "training" sequence to determine the maximum data rate supported by the link, Ether nodes merely exchange speed capability messages and choose the highest speed supported by both ends. No attempt is made to see if the link can actually run at that speed, so if it's beyond the cable's capability, then the link will fail. The solution is to force either or both ends down to a speed supported by the cable.

Dual speed hubs

In the early days of Fast Ether, fast ether switches were relatively expensive devices. However, hubs suffered from the problem that if there were any 10baseT devices connected then the whole system would he to run at 10 Mbit. Therefore a promise between a hub and a switch appeared known as a dual speed hub. These effectively split the work into two sections, each acting like a hubbed work at its respective speed then acted as a two port switch between those two sections. This meant they allowed mixing of the two speeds without the cost of a Fast Ether switch.

Ether frame types and the EtherType field

Frames are the format of data packets on the wire.

There are several types of Ether frame:

The Ether Version 2 or Ether II frame, the so-called DIX frame (named after DEC, Intel, and Xerox); this is the most mon today, as it is often used directly by the Inter Protocol.

Novell's homegrown variation of IEEE 802.3 ("raw 802.3 frame") without IEEE 802.2 LLC

IEEE 802.2 LLC frame

IEEE 802.2 LLC/SNAP frame

In addition, Ether 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 he different formats and MTU values, but can coexist on the same physical medium.

The most mon Ether Frame format, type II

It is claimed that some older (Xerox?) Ether specification had a 16-bit length field, although the maximum length of a packet was 1500 bytes. Versions 1.0 and 2.0 of the Digital/Intel/Xerox (DIX) Ether specification, however, he a 16-bit sub-protocol label field called the EtherType, with the convention that values between 0 and 1500 indicated the use of the original Ether format with a length field, while values of 1536 decimal (0600 hexadecimal) and greater indicated the use of the new frame format with an EtherType sub-protocol identifier.

IEEE 802.3 defined the 16-bit field after the MAC addresses as a length field again, with the MAC header followed by an IEEE 802.2 LLC header. The convention described earlier allows software to determine whether a frame is an Ether II frame or an IEEE 802.3 frame, allowing the coexistence of both standards on the same physical medium. All 802.3 frames he an IEEE 802.2 logical link control (LLC) header. By examining this header, it is possible to determine whether it is followed by a SNAP (subwork access protocol) header. (Some protocols, particularly those designed for the OSI working stack, operate directly on top of 802.2 LLC, which provides both datagram and connection-oriented work services.) The L

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