Ethernet - Wikipedia, The Free Encyclopedia

8/14/2019 Ethernet - Wikipedia, The Free Encyclopedia

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A standard 8P8C (often called RJ45)

connector used most commonly on cat5

cable, a type of cabling used primarily in

Ethernet networks.

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Ethernet

From Wikipedia, the free encyclopedia

Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The nam

comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physica

Layer of the OSI networking model, through means of network access at the Media Access Control (MAC) /DatLink Layer, and a common addressing format.

Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting e

systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN

technology. It has been in use from around 1980[1] to the present, largely replacing competing LAN standards suc

as token ring, FDDI, and ARCNET.

Contents1 History

2 Standardization

3 General description

4 Dealing with multiple clients

4.1 CSMA/CD shared medium Ethernet

4.1.1 Main procedure

4.1.2 Collision detected procedure

4.2 Repeaters and hubs

4.3 Bridging and switching

4.4 Dual speed hubs

4.5 More advanced networks

5 Autonegotiation and duplex mismatch

6 Physical layer

7 Ethernet frame types and the EtherType field

7.1 Runt frames

8 Varieties of Ethernet

8.1 Early varieties8.2 10Mbit/s Ethernet

8.3 Fast Ethernet

8.4 Gigabit Ethernet

8.5 10-gigabit Ethernet

8.6 40 Gigabit Ethernet and 100 Gigabit Ethernet

9 Related standards

10 See also

11 References

11/7/2009 Ethernet - Wikipedia, the free encyclop

http://en.wikipedia.org/wiki/Ethernet 1/


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nternet ayer

IP (IPv4, IPv6) ICMP ICMPv6 IGMP

IPsec (more)

Link Layer

ARP/InARP NDP OSPF Tunnels (L2TP)

PPP Media Access Control (Ethernet, DSL,

ISDN, FDDI) (more)

12 External links

History

Ethernet was originally developed at Xerox PARC in 1973

1975.[2] In 1975, Xerox filed a patent application listing Robert

Metcalfe, David Boggs, Chuck Thacker and Butler Lampson asinventors (U.S. Patent 4,063,220

(http://www.google.com/patents?vid=4063220) : Multipoint data

communication system (with collision detection)). In 1976, after the system was deployed at PARC, Metcalfe and

Boggs published a seminal paper.[3]

The experimental Ethernet described in that paper ran at 3 Mbit/s, and had eight-bit destination and source addres

fields, so the original Ethernet addresses were not the MAC addresses they are today. By software convention, th

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.

Metcalfe left Xerox in 1979 to promote the use of personal computers and local area networks (LANs), forming

3Com. He convinced DEC, Intel, and Xerox to work together to promote Ethernet as a standard, the so-called

"DIX" standard, for "Digital/Intel/Xerox"; it specified the 10 megabits/second Ethernet, with 48-bit destination and

source addresses and a global 16-bit type field. The first standard draft was first published on September 30, 198

within IEEE. It competed with two largely proprietary systems, Token Ring and Token Bus. To get over delays o

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 (1981), followed quickly by Digital Equipment's Unibus to

Ethernet adapter.

Twisted-pair Ethernet systems have been developed since the mid-80s, beginning with StarLAN, but becoming

widely known with 10BASE-T. These systems replaced the coaxial cable on which early Ethernets were deploye

with a system of hubs linked with unshielded twisted pair (UTP), ultimately replacing the CSMA/CD scheme in

favor of a switched full duplex system offering higher performance.

Standardization

Notwithstanding its technical merits, timely standardization was instrumental to the success of Ethernet. It required

well-coordinated and partly competitive activities in several standardization bodies such as the Institute of Electrica

and Electronics Engineers (IEEE), the European Computer Manufacturers Association (ECMA), the International

Electrotechnical Commission (IEC) and finally the International Organization for Standardization (ISO).

In February 1980, IEEE started a project IEEE 802 for the standardization of Local Area Networks (LAN).

The "DIX-group" with Gary Robinson (DEC), Phil Arst (Intel) and Bob Printis (Xerox) submitted the so-called

"Blue Book" CSMA/CD specification as candidate for the LAN specification. Since IEEE membership is open to

all professionals including students, the group received countless comments on this brand-new technology.




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A 1990s network interface

card. This is a combination

card that supports both

coaxial-based using a

10BASE2 (BNC connector,

left) and twisted pair-based

10BASE-T, using an RJ45

(8P8C modular connector,

right).

In addition to CSMA/CD, Token Ring supported by IBM and Token Bus selected and henceforward supported

by General Motors were also considered as candidates for a LAN standard. Due to the goal of IEEE 802 to

forward only one standard and due to the strong company support for all three designs the necessary agreement o

a LAN standard was significantly delayed.

In the Ethernet camp, it put at risk the market introduction of Xerox Star computing system and 3Com's Ethernet

LAN products. With such business implications in mind, David Liddle (GM Xerox Office Systems) and Bob

Metcalfe (3Com) strongly supported a proposal of Fritz Rscheisen (Siemens Private Networks) for an alliance inthe emerging office communication market, including Siemens' support for the international standardization of

Ethernet (April 10, 1981). Ingrid Fromm, Siemens representative to IEEE 802 quickly achieved broader support

for Ethernet beyond IEEE by the establishment of a competing Task Group "Local Networks" within the European

standards body ECMA TC24. As early as March 1982 ECMA TC24 with its corporate members reached

agreement on a standard for CSMA/CD based on the IEEE 802 draft. The speedy action taken by ECMA

decisively contributed to the conciliation of opinions within IEEE and approval of IEEE 802.3 CSMA/CD by the

end of 1982.

Approval of Ethernet on international level was achieved by a similar, cross-partisan action with Fromm as liaison

officer between the International Electrotechnical Commission IEC TC83 and ISO TC97SC6, the International

Standard ISO/IEEE 802/3 was approved in 1984.

General description

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 referencethat the name "Ethernet" was derived.

From this early and comparatively simple concept, Ethernet evolved into the

complex networking technology that today underlies most LANs. The coaxial

cable was replaced with point-to-point links connected by Ethernet hubs and/or

switches to reduce installation costs, increase reliability, and enable point-to-

point management and troubleshooting. 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 Ethernettechnologies.

Above the physical layer, Ethernet stations communicate by sending each other data packets, blocks of data that

are individually sent and delivered. 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 interfac

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.

Despite the significant changes in Ethernet from a thick coaxial cable bus running at 10 Mbit/s to point-to-point lin




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running at 1 Gbit/s and beyond, all generations of Ethernet (excluding early experimental versions) share the same

frame formats (and hence the same interface for higher layers), and can be readily interconnected.

Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it, and the reduced

panel space needed by twisted pair Ethernet, most manufacturers now build the functionality of an Ethernet card

directly into PC motherboards, eliminating the need for installation of a separate network card.

Dealing with multiple clients

CSMA/CD shared medium Ethernet

Ethernet originally used a shared coaxial cable (the shared medium) winding around a building or campus to every

attached machine. A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governe

the way the computers shared the channel. This scheme was simpler than the competing token ring or token bus

technologies. When a computer wanted to send some information, it used the following algorithm:

Main procedure

1. Frame ready for transmission.

2. Is medium idle? If not, wait until it becomes ready and wait the interframe gap period (9.6 s in 10 Mbit/s

Ethernet).

3. Start transmitting.

4. Did a collision occur? If so, go to collision detected procedure.

5. Reset retransmission counters and end frame transmission.

Collision detected procedure

1. Continue transmission until minimum packet time is reached (jam signal) to ensure that all receivers detect th

collision.

2. Increment retransmission counter.

3. Was the maximum number of transmission attempts reached? If so, abort transmission.

4. Calculate and wait random backoff period based on number of collision

5. Re-enter main procedure at stage 1.

This can be likened to what happens at a dinner party, where all the guests talk to each other through a common

medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start

speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is generally

measured in microseconds). 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 time

(determined using the truncated binary exponential backoff algorithm) are used when there is more than one failed

attempt to transmit.

Computers were connected to an Attachment Unit Interface (AUI) transceiver, which was in turn connected to th

cable (later with thin Ethernet the transceiver was integrated into the network adapter). While a simple passive wir

was highly reliable for small Ethernets, it was not reliable for large extended networks, where damage to the wire i

a single place, or a single bad connector, could make the whole Ethernet segment unusable. Multipoint systems ar

also prone to very strange failure modes when an electrical discontinuity reflects the signal in such a manner that




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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 aping command and shouted out reports as performance changed.

Since all communications happen on the same wire, any information sent by one computer is received by all, even

that information is intended for just one destination. 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 aEthernet 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 restar

after a power failure.

Repeaters and hubs

For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size which depended on the

medium used. For example, 10BASE5 coax cables had a maximum length of 500 meters (1,640 ft). Also, as was

the case with most other high-speed buses, Ethernet segments had to be terminated with a resistor at each end. Fo

coaxial-cable-based Ethernet, each end of the cable had a 50 ohm ( ) resistor attached. Typically this resistor wabuilt into a male BNC or N connector and attached to the last device on the bus, or, if vampire taps were in use, t

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.

A greater length could be obtained by an Ethernet repeater, which took the signal from one Ethernet cable and

repeated it onto another cable. If a collision was detected, the repeater transmitted a jam signal onto all ports to

ensure collision detection. 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 thproblem 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 segmen

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.

People recognized the advantages of cabling in a star topology, primarily that only faults at the star point will result

in a badly partitioned network, and network vendors began creating repeaters having multiple ports, thus reducing

the number of repeaters required at the star point. Multiport Ethernet repeaters became known as "Ethernet hubs"

Network vendors such as DEC and SynOptics sold hubs that connected many 10BASE2 thin coaxial segments.

There were also "multi-port transceivers" or "fan-outs". These could be connected to each other and/or a coaxbackbone. 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 witho

using a coaxial cable.

Ethernet on unshielded twisted-pair cables (UTP), beginning with StarLAN and

continuing with 10BASE-T, was designed for point-to-point links only and all termination

was built into the device. 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




6/17

A twisted pair Cat-3

or Cat-5 cable is

used to connect

10BASE-T Ethernet

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.

Despite the physical star topology, hubbed Ethernet networks still use half-duplex and CSMA/CD, with only

minimal activity by the hub, primarily the Collision Enforcement signal, in dealing with packet collisions. Everypacket is sent to every port on the hub, so bandwidth and security problems aren't addressed. The total throughpu

of the hub is limited to that of a single link and all links must operate at the same speed.

Collisions reduce throughput by their very nature. In the worst case, when there are lots of hosts with long cables

that attempt to transmit many short frames, excessive collisions can reduce throughput dramatically. However, a

Xerox report in 1980 summarized the results of having 20 fast nodes attempting to transmit packets of various size

as quickly as possible on the same Ethernet segment.[4] The results showed that, even for the smallest Ethernet

frames (64B), 90% throughput on the LAN was the norm. This is in comparison with token passing LANs (token

ring, token bus), all of which suffer throughput degradation as each new node comes into the LAN, due to token

waits.

This report was controversial, as modeling showed that collision-based networks became unstable under loads as

low as 40% of nominal capacity. Many early researchers failed to understand the subtleties of the CSMA/CD

protocol and how important it was to get the details right, and were really modeling somewhat different networks

(usually not as good as real Ethernet).[5]

Bridging and switching

While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forwarded al

traffic to all Ethernet devices. This created practical limits on how many machines could communicate on anEthernet network. Also as the entire network was one collision domain and all hosts had to be able to detect

collisions anywhere on the network, and the number of repeaters between the farthest nodes was limited. Finally

segments joined by repeaters had to all operate at the same speed, making phased-in upgrades impossible.

To alleviate these problems, bridging was created to communicate at the data link layer while isolating the physical

layer. 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.

Prior to discovery of network devices on the different segments, Ethernet bridges (and switches) work somewhat

like Ethernet hubs, passing all traffic between segments. However, as the bridge discovers the addresses associate

with each port, it only forwards network traffic to the necessary segments, improving overall performance.

Broadcast traffic is still forwarded to all network segments. 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

Fast Ethernet.

Early bridges examined each packet one by one using software on a CPU, and some of them were significantly

slower than hubs (multi-port repeaters) at forwarding traffic, especially when handling many ports at the same time

This was in part due to the fact that the entire Ethernet packet would be read into a buffer, the destination address

compared with an internal table of known MAC addresses and a decision made as to whether to drop the packet




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or forward it to another or all segments.

In 1989 the networking company Kalpana introduced their EtherSwitch, the first Ethernet switch. This worked

somewhat differently from an Ethernet bridge, in that only the header of the incoming packet would be examined

before it was either dropped or forwarded to another segment. This greatly reduced the forwarding latency and th

processing load on the network device. One drawback of this cut-through switching method was that packets th

had been corrupted at a point beyond the header could still be propagated through the network, so a jabbering

station could continue to disrupt the entire network. The remedy for this was to make available store-and-forward

switching, where the packet would be read into a buffer on the switch in its entirety, verified against its checksumand then forwarded. This was essentially a return to the original approach of bridging, but with the advantage of

more powerful, application-specific processors being used. Hence the bridging is then done in hardware, allowing

packets to be forwarded at full wire speed. It is important to remember that the term switch was invented by

device manufacturers and does not appear in the 802.3 standard.

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. Despite this, switched Ethernet should still be regarded as an insecur

network technology, because it is easy to subvert switched Ethernet systems by means such as ARP spoofing and

MAC flooding. The bandwidth advantages, the slightly better isolation of devices from each other, the ability to

easily mix different speeds of devices and the elimination of the chaining limits inherent in non-switched Ethernethave made switched Ethernet the dominant network technology.

When a twisted pair or fiber link segment is used and neither end is connected to a hub, full-duplex Ethernet

becomes possible over that segment. In full duplex mode both devices can transmit and receive to/from each other

at the same time, and there is no collision domain. This doubles the aggregate bandwidth of the link and is

sometimes advertised as double the link speed (e.g. 200 Mbit/s) to account for this. However, this is misleading a

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 link's bandwidth can be used and that segment length is not limited by th

need for correct collision detection (this is most significant with some of the fiber variants of Ethernet).

Dual speed hubs

In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices. Hubs suffered from the

problem that if there were any 10BASE-T devices connected then the whole network needed to run at 10 Mbit/s

Therefore a compromise between a hub and a switch was developed, known as a dual speed hub. These devices

consisted of an internal two-port switch, dividing the 10BASE-T (10 Mbit/s) and 100BASE-T (100 Mbit/s)

segments. The device would typically consist of more than two physical ports. When a network device becomes

active on any of the physical ports, the device attaches it to either the 10BASE-T segment or the 100BASE-T

segment, as appropriate. This prevented the need for an all-or-nothing migration from 10BASE-T to 100BASE-T

networks. These devices are hubs because the traffic between devices connected at the same speed is notswitched.

More advanced networks

Simple switched Ethernet networks, while an improvement over hub based Ethernet, suffer from a number of issue

They suffer from single points of failure. If any link fails some devices will be unable to communicate with

other devices and if the link that fails is in a central location lots of users can be cut off from the resources

they require.




8/17

It is possible to trick switches or hosts into sending data to your machine even if it's not intended for it (see

switch vulnerabilities).

Large amounts of broadcast traffic, whether malicious, accidental, or simply a side effect of network size ca

flood slower links and/or systems.

It is possible for any host to flood the network with broadcast traffic forming a denial of service attac

against any hosts that run at the same or lower speed as the attacking device.

As the network grows, normal broadcast traffic takes up an ever greater amount of bandwidth.

If switches are not multicast aware, multicast traffic will end up treated like broadcast traffic due tobeing directed at a MAC with no associated port.

If switches discover more MAC addresses than they can store (either through network size or throug

an attack) some addresses must inevitably be dropped and traffic to those addresses will be treated

the same way as traffic to unknown addresses, that is essentially the same as broadcast traffic (this

issue is known as failopen).

They suffer from bandwidth choke points where a lot of traffic is forced down a single link.

Some switches offer a variety of tools to combat these issues including:

Spanning-tree protocol to maintain the active links of the network as a tree while allowing physical loops foredundancy.

Various port protection features, as it is far more likely an attacker will be on an end system port than on a

switch-switch link.

VLANs to keep different classes of users separate while using the same physical infrastructure.

Fast routing at higher levels to route between those VLANs.

Link aggregation to add bandwidth to overloaded links and to provide some measure of redundancy,

although the links won't protect against switch failure because they connect the same pair of switches.

Autonegotiation and duplex mismatchMain articles: Autonegotiation and Duplex mismatch

Many different modes of operations (10BASE-T half duplex, 10BASE-T full duplex, 100BASE-TX half duplex,

) exist for Ethernet over twisted pair cable using 8P8C modular connectors (not to be confused with FCC's

RJ45), and most devices are capable of different modes of operations. In 1995, IEEE standard 802.3u

(100baseTX) was released, allowing two network interfaces connected to each other to autonegotiate the best

possible shared mode of operation. This works well for a network in which every device being set to autonegotiat

The autonegotiation standard contained a mechanism for detecting the speed but not the duplex setting of an

Ethernet peer that did not use autonegotiation. An autonegotiating device defaults to half duplex, when the remote

does not negotiate, as the remote peer is assumed to be a hub (which always has autonegotiation disabled and

supports only half duplex mode). If the remote is operating in half duplex mode this works. But if remote is in full

duplex mode, this generates a duplex mismatch. When two interfaces are connected and set to different "duplex"

modes, the effect of the duplex mismatch is a network that works, but is much slower than its nominal speed, and

generates more collisions. The primary rule for avoiding this is to never set one end of a connection to a forced full

duplex setting and the other end to autonegotiation.

Interoperability problems lead some network administrators to manually fix the mode of operation of interfaces on

network devices. What would happen is that some device would fail to autonegotiate and therefore had to be set




9/17

into one setting or another. This often led to duplex setting mismatches. In particular, when two interfaces are

connected to each other with one set to autonegotiation and one set to full duplex mode, a duplex mismatch result

because the autonegotiation process fails and half duplex is assumed. The interface in full duplex mode then

transmits at the same time as receiving, and the interface in half duplex mode then gives up on transmitting a frame.

The interface in half duplex mode is not ready to receive a frame, so it signals a collision, and transmissions are

halted, for amounts of time based on backoff (random wait times) algorithms. When both packets start trying to

transmit again, they interfere again and the backoff strategy may result in a longer and longer wait time before

attempting to transmit again; eventually a transmission succeeds but this then causes the flood and collisions to

resume.

Because of the wait times, the effect of a duplex mismatch is a network that is not completely 'broken' but is

incredibly slow. This bad behaviour can be tolerated on low traffic link, but is really dramatic under heavy

bandwidth transfer attempt, and can lead to a complete stop of the traffic.

While autonegotiation is not required for 10/100 Mbit/s, it is recommended as default behaviour by IEEE 802.3u.

However, 1000baseT devices require autonegotiation to be active to elect the clock master (source of timing).

Enabing autonegotiation on every node eases transition from 10/100Mbit/s to 1000baseT switch and LAN. There

are no disadvantages of keeping autonegotiation active on all devices, because complete physical link behaviours

are controlled through autonegotiation (speed, duplex, clock master and flow control). For example, to force asingle speed link you can keep negotiation on, but negotiate only one speed. So the old method with autonegotiatio

off is deprecated everywhere, on switch and LAN cards.

Physical layer

Main article: Ethernet physical layer

The first Ethernet networks, 10BASE5, used thick yellow cable with vampire taps as a shared medium (using

CSMA/CD). Later, 10BASE2 Ethernet used thinner coaxial cable (with BNC connectors) as the shared

CSMA/CD medium. The later StarLAN 1BASE5 and 10BASE-T used twisted pair connected to Ethernet hubswith 8P8C modular connectors (not to be confused with FCC's RJ45).

Currently Ethernet has many varieties that vary both in speed and physical medium used. Perhaps the most commo

forms used are 10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize twisted pair cables and 8P8C

modular connectors (often called RJ45). They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively. However

each version has become steadily more selective about the cable it runs on and some installers have avoided

1000BASE-T for everything except short connections to servers.

Fiber optic variants of Ethernet are commonly used in structured cabling applications. These variants have also see

substantial penetration in enterprise datacenter applications, but are rarely seen connected to end user systems forcost/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. 1

gigabit Ethernet is becoming more popular in both enterprise and carrier networks, with development starting on 4

Gbit/s [6][7] 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. [8

A data packet on the wire is called a frame. 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




10/17

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 table below shows the complete Ethernet frame, as transmitted, for the MTU of 1500 bytes (some

implementations of gigabit Ethernet and higher speeds support larger jumbo frames). Note that the bit patterns in t

preamble and start of frame delimiter are written as bit strings, with the first bit transmitted on the left (notas byte

values, which in Ethernet are transmitted least significant bit first). This notation matches the one used in the IEEE

802.3 standard. One octet is eight bits of data (i.e., a byte on most modern computers).

802.3 MAC Frame

Preamble

Start-of-

Frame-

Delimiter

MAC

destination

MAC

sourceEthertype/Length

Payload

(Data and

padding)

CRC32Interframe

gap

7 octets of

10101010

1 octet of

101010116 octets 6 octets 2 octets

461500

octets4 octets 12 octets

641518 octets

721526 octets

After a frame has been sent transmitters are required to transmit 12 octets of idle characters before transmitting the

next frame. For 10M this takes 9600 ns, 100M 960 ns, 1000M 96 ns.

From this table, we may calculate the maximum net bit rate of 10 Mbit/s Ethernet to be approximately 9.75 Mbit/

assuming a continuous stream of maximum-sized packets (containing 1500 payload bytes each):

10/100M transceiver chips (MII PHY) work with four bits (one nibble) at a time. Therefore the preamble will be

instances of 0101 + 0101, and the Start Frame Delimiter will be 0101 + 1101. 8-bit values are sent low 4-bit and

then high 4-bit. 1000M transceiver chips (GMII) work with 8 bits at a time, and 10 Gbit/s (XGMII) PHY works

with 32 bits at a time.




11/17

Ethernet frame types and the EtherType field

Main article: Ethernet II framing

There are several types of Ethernet frames:

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 non-standard variation of IEEE 802.3 ("raw 802.3 frame") without an IEEE 802.2 LLC header.

IEEE 802.2 LLC frame

IEEE 802.2 LLC/SNAP frame

In addition, all four Ethernet frames types 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 encapsulation is defined in the IEEE 802.3ac

specification and increases the maximum frame by 4 bytes to 1522 bytes.

The different frame types have different formats and MTU values, but can coexist on the same physical medium.

The most common Ethernet Frame format, type II

Versions 1.0 and 2.0 of the Digital/Intel/Xerox (DIX) Ethernet specification have a 16-bit sub-protocol label field

called the EtherType . The new IEEE 802.3 Ethernet specification replaced that with a 16-bit length field, with thMAC header followed by an IEEE 802.2 logical link control (LLC) header. The maximum length of a frame wa

1518 bytes for untagged (1522 for 802.1p or 802.1q tagged) classical Ethernet v2 and IEEE802.3 frames. 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.[9] This convention allows software to determine whether a frame is an Ethernet II frame or an IEEE 802

frame, allowing the coexistence of both standards on the same physical medium. See also Jumbo Frames.

By examining the 802.2 LLC 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, operatedirectly 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'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




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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 notab

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 carryin

IPX. Since Netware 4.10, Netware now defaults to IEEE 802.2 with LLC (Netware Frame Type Ethernet_802.

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 I

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 0x0800 for IPv4 and 0x86DD for IPv6.

There exists an Internet standard for encapsulating IP version 4 traffic in IEEE 802.2 frames with LLC/SNAP

headers.[10] It is almost never implemented on Ethernet (although it is used on FDDI and on token ring, IEEE

802.11, and other IEEE 802 networks). 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 Versio

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).

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 0x8100. This is located in the same place

as the EtherType/Length field in untagged frames, so an EtherType value of 0x8100 means the frame is tagged, an

the true EtherType/Length is located after the Q-tag. The TPID is followed by two bytes containing the Tag Contr

Information (TCI) (the IEEE 802.1p priority (quality of service) and VLAN id). The Q-tag is followed by the rest

of the frame, using one of the types described above.

Runt frames

A runt frame is an Ethernet frame that is less than the IEEE 802.3 minimum length of 64 bytes. Possible causes are

collision, underruns, bad network card or software. [11][12]

Varieties of Ethernet

Early varieties

10BASE5: original standard uses a single coaxial cable into which you literally tap a connection by drilling

into the cable to connect to the core and screen. Largely obsolete, though due to its widespread deploymen

in the early days, some systems may still be in 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 cabl

1BASE5: An early attempt to standardize a low-cost LAN solution, it operates at 1 Mbit/s and was a




13/17

commercial failure.

10Mbit/s Ethernet

10BASE2 (also called ThinNet or Cheapernet): 50 coaxial cable connects machines together, each

machine using a T-adaptor to connect to its NIC. Requires terminators at each end. For many years this wa

the dominant Ethernet standard 10 Mbit/s.

10BASE-T: runs over four wires (two twisted pairs) on a Category 3 or Category 5 cable. A hub or switchsits 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

Main article: Fast Ethernet

100BASE-T: A term for any of the three standard for 100 Mbit/s Ethernet over twisted pair cable. Include

100BASE-TX, 100BASE-T4 and 100BASE-T2. As of 2009, 100BASE-TX has totally dominated the

market, and is often considered to be synonymous with 100BASE-T in informal usage.

100BASE-TX: 100 Mbit/s Ethernet over Category 5 cable (using two out of four pairs). Similar star

shaped configuration to 10BASE-T.

100BASE-T4: 100 Mbit/s Ethernet over Category 3 cable (as used for 10BASE-T installations).

Uses all four pairs in the cable, and is limited to half-duplex. Now obsolete, as Category 5 cables arethe norm.

100BASE-T2: 100 Mbit/s Ethernet over Category 3 cable. Uses only two pairs, and supports full-

duplex. It is functionally equivalent to 100BASE-TX, but supports old cable. No products supportin

this standard were ever manufactured.

100BASE-FX: 100 Mbit/s Ethernet over fiber.

Gigabit Ethernet

Main article: Gigabit Ethernet

1000BASE-T: 1 Gbit/s over unshielded twisted pair copper cabling (at least Category 5, with Category 5e

strongly recommended).

1000BASE-SX: 1 Gbit/s over short range multi-mode fiber.

1000BASE-LX: 1 Gbit/s over long range 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




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Main article: 10 Gigabit Ethernet

The 10 gigabit Ethernet family of standards encompasses media types for single-mode fibre (long haul), multi-mod

fibre (up to 300 m), copper backplane (up to 1 m) and copper twisted pair (up to 100 m). It was first standardise

as IEEE Std 802.3ae-2002, but is now included in IEEE Std 802.3-2008.

10GBASE-SR: designed to support short distances over deployed multi-mode fiber cabling, it has a range

between 26 m and 82 m depending on cable type. It also supports 300 m operation over a new2000 MHzkm multi-mode fiber.

10GBASE-LX4: uses wavelength division multiplexing to support ranges of between 240 m and 300 m ove

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 respectively 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 respectively, and hence use the same types of fiber and

support the same distances. (There is no WAN PHY standard corresponding to 10GBASE-LX4.)

10GBASE-T: designed to support copper twisted pair was specified by the IEEE Std 802.3an-2006 which

has been incorporated into the IEEE Std 802.3-2008.

As of 2009, 10 gigabit Ethernet is predominantly deployed in carrier networks, where 10GBASE-LR and

10GBASE-ER enjoy significant market shares.

40 Gigabit Ethernet and 100 Gigabit Ethernet

Main article: 100 Gigabit Ethernet

As of 2009, 40 Gigabit Ethernet and 100 Gigabit Ethernet (100GbE) standards are still in draft status.

Related standards

Networking standards that are not part of the IEEE 802.3 Ethernet standard, but support the Ethernet fram

format, and are capable of interoperating with it.

LattisNetA SynOptics pre-standard twisted-pair 10 Mbit/s variant.

100BaseVGAn early contender for 100 Mbit/s Ethernet. It runs over Category 3 cabling. Uses fo

pairs. Commercial failure.

TIA 100BASE-SXPromoted by the Telecommunications Industry Association. 100BASE-SX isan 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/

fiber network installations.

TIA 1000BASE-TXPromoted 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 so the electronics can be cheaper, but requires Category 6 cabling.

G.hnA standard developed by ITU-T and promoted by HomeGrid Forum




15/17

(http://www.homegridforum.org) for high-speed (up to 1 Gbit/s) local area networks over existing

home wiring (coaxial cables, power lines and phone lines). G.hn defines an Application Protocol

Convergence (APC) layer that accepts Ethernet frames and encapsulates them into G.hn MSDUs.

Networking standards that do not use the Ethernet frame format but can still be connected to Ethernet using

MAC-based bridging.

802.11A standard for wireless local area networks (LANs), often paired with an Ethernet

backbone.

802.16A standard for wireless metropolitan area networks (MANs), including WiMAX

10BaseSEthernet over VDSL

Long Reach Ethernet

Avionics Full-Duplex Switched Ethernet

TTEthernet Time-Triggered Ethernet for design of mixed-criticality embedded systems

Metro Ethernet

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

engineering.

See also

ALOHAnet

Broadband Internet access

Chipcom

List of device bandwidths

Chaosnet

Ethernet Automatic Protection Switching

Ethernet crossover cableEthernet Way versus IEEE Way

Fully switched network

Green Ethernet

Fiber media converter

AUI, GBIC, MII and PHY

Network isolator

Power line communication

Power over Ethernet

Spanning tree protocol

Virtual LANWake-on-LAN

Synchronous Ethernet

References

1. ^ "History of Ethernet (http://www.c isco.com/univercd/cc/td/doc/cisintwk/ito_doc/ethernet.htm) ". Cisco

Systems. http://www.c isco.com/univercd/cc/td/doc/cisintwk/ito_doc/ethernet.htm. Retrieved 2008-02-22.

2. ^ "Ethernet Prototype Circuit Board (http://americanhistory.si.edu/collections/object.cfm?key=35&objkey=96) ".

Smithsonian National Museum of American History. http://americanhistory.si.edu/collections/object.cfm?




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key=35&objkey=96. Retrieved 2007-09-02.

3. ^Ethernet: Distributed Packet-Switching For Local Computer Networks (http://www.acm.org/classics/apr96/)

4. ^ Shoch, John F. and Hupp, Jon A. (December 1980). "Measured performance of an Ethernet local network

(http://portal.acm.org/citation.cfm?doid=359038.359044#abstract) ". Communications of the ACM(ACM Press)

23 (12): 711721. doi:10.1145/359038.359044 (http://dx.doi.org/10.1145%2F359038.359044) . ISSN: 0001-0782

http://portal.acm.org/citation.cfm?doid=359038.359044#abstract.

5. ^ Boggs, D.R., Mogul, J.C., and Kent, C.A. (August 1988). "Measured capacity of an Ethernet: myths and reality

(http://portal.acm.org/citation.cfm?doid=52325.52347#abstract) ".ACM SIGCOMM Computer Communication

Review (ACM Press) 18 (4): 222234. doi:10.1145/52325.52347 (http://dx.doi.org/10.1145%2F52325.52347) .

ISBN 0-89791-279-9. http://portal.acm.org/citation.cfm?doid=52325.52347#abstract .

6. ^ "Consideration for 40 gigabit Ethernet

(http://grouper.ieee.org/groups/802/3/hssg/public/may07/duelk_01_0507.pdf) " (PDF). IEEE HSSG.

http://grouper.ieee.org/groups/802/3/hssg/public/may07/duelk_01_0507.pdf.

7. ^ "40 gigabit Ethernet answers (http://grouper.ieee.org/groups/802/3/hssg/public/may07/kipp_01_0507.pdf) "

(PDF). IEEE HSSG. http://grouper.ieee.org/groups/802/3/hssg/public/may07/kipp_01_0507.pdf .

8. ^ "Bob Metcalfe on the Terabit Ethernet (http://www.lightreading.com/tv/tv_popup.asp?doc_id=146223) ".

http://www.lightreading.com/tv/tv_popup.asp?doc_id=146223. 080224 lightreading.com

9. ^ LAN MAN Standards Committee of the IEEE Computer Society (20 March 1997). IEEE Std 802.3x-1997 and

IEEE Std 802.3y-1997. The Institute of Electrical and Electronics Engineers, Inc.. pp. 2831.

10. ^ RFC 1042

11. ^ "Glossary of Terms - R (Zarlink Semiconductor) (http://products.zarlink.com/Glossary/r.htm) ".

http://products.zarlink.com/Glossary/r.htm. 071227 products.zarlink.com

12. ^ "sys/dev/tx/if_txreg.h (http://fxr.watson.org/fxr/source//dev/tx/if_txreg.h?v=RELENG62#L137) ".

http://fxr.watson.org/fxr/source//dev/tx/if_txreg.h?v=RELENG62#L137. 071227 fxr.watson.org

Metcalfe, Robert M. and Boggs, David R. (July 1976). "Ethernet: Distributed Packet Switching for Local

Computer Networks (http://portal.acm.org/citation.cfm?

id=360253&dl=ACM&coll=ACM&CFID=39370057&CFTOKEN=52797288) ". Communications of the ACM

19 (5): 395405. doi:10.1145/360248.360253 (http://dx.doi.org/10.1145%2F360248.360253) .

http://portal.acm.org/c itation.cfm?id=360253&dl=ACM&coll=ACM&CFID=39370057&CFTOKEN=5279728

the original Metcalfe and Boggs paper on Ethernet.

Digital Equipment Corporation, Intel Corporation, Xerox Corporation (September, 1980). The Ethernet: A

Local Area Network (http://portal.acm.org/citation.cfm?id=1015591.1015594) .

http://portal.acm.org/citation.cfm?id=1015591.1015594. Version 1.0 of the DIX specification.

Boggs, David R. and Mogul, Jeffrey C. and Kent, Christopher A. (1988). "Measured capacity of an Ethernet:

myths and reality (ftp://gatekeeper.dec.com/pub/DEC/WRL/research-reports/WRL-TR-88.4.pdf) " (PDF).

SIGCOMM88 Symposium proceedings on Communications architectures and protocols. pp. 222234.

doi:10.1145/52324.52347 (http://dx.doi.org/10.1145%2F52324.52347) . on the issue of Ethernet bandwidt

collapse.

IEEE 802.3-2008 standard (http://standards.ieee.org/getieee802/802.3.html)

Don Provan (1993-09-17). "Ethernet Framing (news:[email protected]) ".

comp.sys.novell (news:comp.sys.novell) . (Web link)

(http://groups.google.com/group/bit.listserv.novell/browse_thread/thread/d00a24530625714c) . a classicseries 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 ..

External links

Get IEEE 802.3 (http://standards.ieee.org/getieee802/802.3.html)

IEEE 802.3 (http://www.ieee802.org/3/)

Frame Size Support of Existing Devices (http://www.ieee802.org/3/frame_study/0409/braga_1_0409.pdf)

Embedded Ethernet (http://microcontroller.com/Embedded_Internet/) list of major suppliers




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