IP Encyclopedia > Ethernet

What Is Ethernet?

The earliest Ethernet standard was the DEC-Intel-Xerox (DIX) standard jointly developed by the Digital Equipment Corporation (DEC), Intel, and Xerox in 1982. After years of development, Ethernet has become the most widely used local area network (LAN) type, and many Ethernet standards have been put into use, including standard Ethernet (10 Mbit/s), Fast Ethernet (100 Mbit/s), Gigabit Ethernet (1000 Mbit/s), and 10 Gigabit Ethernet (10 Gbit/s).

Contents

Why Is Ethernet Required?

If multiple people speak at the same time at a large conference, the conference will become disordered and noisy, and people will not be able to follow what's being said. This naturally leads to a rule that when one person is speaking, others need to listen.

Similarly, when two or more connected devices on a shared network attempt to send data packets at the same time through a shared copper cable or optical cable, electrical pulses or photons forming the data packets will overlap (used to indicate bits 1 and 0 in the data packets). As a result, a data packet conflict occurs, and information cannot be exchanged. Therefore, a rule is required to guide the correct and effective transmission of information between devices. Ethernet technology provides this rule.

Ethernet uses carrier sense multiple access with collision detection (CSMA/CD) technology to solve the problem of data packet conflicts between communication parties. To put it simply, before you speak at a conference, make sure no one else is speaking. If you hear someone speaking while you are speaking, you stop speaking, listen, and wait until the other person finishes.

Therefore, more formally, Ethernet is the common name of the IEEE 802.3 standard based on the CSMA/CD protocol. CSMA/CD defines when information is transmitted, how detected conflicts are handled, and rules such as the transmission speed and required media.

Today, Ethernet has become the most commonly used computer LAN technology and is widely used in scenarios such as homes, enterprises, and academic institutions.

Origin and Development of Ethernet

Origin of Ethernet

In 1972, when Robert Metcalfe (father of Ethernet) was hired by Xerox, his first job was to connect computers in Xerox's Palo Alto Research Center (PARC) to the Advanced Research Projects Agency Network (ARPANET), progenitor of the Internet. In 1972 also, Robert Metcalfe designed a network to connect computers in the PARC. That network was based on the Aloha system (a radio network system) and connected to many computers in the PARC, so Metcalfe originally named it the Alto Aloha network.

The Alto Aloha network started operating in May 1973, and Metcalfe then gave it an official name Ethernet, which is the prototype of Ethernet. The network operated at a rate of 2.94 Mbit/s and used thick coaxial cable as transmission medium.

In June 1976, Metcalfe and his assistant David Boggs published a paper Ethernet: Distributed Packet Switching for Local Computer Networks. At the end of 1977, Metcalfe and his three co-workers patented their "Multipoint data communication system with collision detection." Since then, Ethernet was known to the public.

Development of Ethernet

Since the birth of Ethernet in the 1970s, Ethernet standards have been evolving continuously. After long-term development, Ethernet standards include standard Ethernet (10 Mbit/s), Fast Ethernet (100 Mbit/s), Gigabit Ethernet (1000 Mbit/s), and 10 Gigabit Ethernet (10 Gbit/s), as described in the following table.

Table 1-1 Development of Ethernet standards

Rate	Name	IEEE Standards
10 Mbit/s	Standard Ethernet	802.3
100 Mbit/s	Fast Ethernet	802.3u
1000 Mbit/s	Gigabit Ethernet	802.3z/ab
10 Gbit/s	10 Gigabit Ethernet	802.3ae

Currently, Ethernet is the most widely used LAN technology and replaces most of other LAN standards, such as token ring, Fiber Distributed Data Interface (FDDI), and Attached Resource Computer Network (ARCNET). After the rapid development of 100M Ethernet at the end of the last century, Gigabit Ethernet and even 10 Gigabit Ethernet are continuously expanding their application scope under the promotion of international organizations and leading enterprises.

Birth of Multi-Gigabit Ethernet

With the rapid development of enterprise bandwidth requirements, the mainstream Gigabit Ethernet is rapidly becoming the bottleneck of enterprise networks. The most direct solution is to upgrade the Ethernet bandwidth to 10 Gbit/s.

However, Cat5e and Cat6 twisted pair cables are deployed in most offices and enterprise network environments, and cannot support a transmission rate of 10 Gbit/s. If the Ethernet bandwidth is upgraded to 10 Gbit/s, the cables need to be upgraded at the same time. As a result, the network reconstruction cost is high.

To solve the preceding problems, the Multi-Gigabit Ethernet standard IEEE 802.3bz has emerged. It provides two rate specifications: 2.5 Gbit/s and 5 Gbit/s, which comply with the bandwidth features of Cat5e and Cat6 cables. In this way, Multi-Gigabit Ethernet is compatible with mainstream Cat5e and Cat6 cables on the existing network during a network upgrade, greatly reducing costs.

For details, see "Multi-Gigabit Ethernet."

Development of Metro Ethernet

With the birth of 10 Gigabit Ethernet, the transmission rate increases by 10 times based on the original Gigabit Ethernet, and the transmission distance greatly extends. Therefore, 10 Gigabit Ethernet is free from the limitation that Ethernet can be applied only to LANs. In addition, Ethernet is extended to metropolitan area networks (MANs) because of its advantages such as simple configuration, flexible networking, and low cost.

Metro Ethernet is a city-level network technology based on the Ethernet protocol. It usually covers dozens to hundreds of kilometers and extends the network coverage to the entire city through transmission media such as copper cables and optical fibers. It not only enables information to be transmitted more smoothly within a city, but also provides powerful technical support for scenarios such as Data Center Interconnect (DCI) and smart city construction.

As an enhanced version of Ethernet, Metro Ethernet overcomes the limitations of Ethernet in terms of distance and bandwidth by upgrading technologies and devices. Compared with a traditional LAN, a Metro Ethernet network connects different devices in a city to implement fast information exchange over a long distance, greatly improving network scalability. Compared with traditional WAN technologies, Metro Ethernet has advantages such as low costs, high bandwidth, and flexible deployment.

Ethernet Cable Standards

Introduction to Ethernet Cable Standards

The media used to transmit information on Ethernet networks include coaxial cables, twisted pair cables, and optical fibers. The following table lists the mature Ethernet physical layer standards.

Table 1-2 Ethernet cable standards

10M Ethernet Cable Standards	100M Ethernet Cable Standards	1000M Ethernet Cable Standards	10G Ethernet Cable Standards
10BASE-2 10BASE-5 10BASE-T 10BASE-F	100BASE-T4 100BASE-TX 100BASE-FX	1000BASE-SX 1000BASE-LX 1000BASE-TX	10GBASE-T 10GBASE-LR 10GBASE-SR

10M Ethernet Cable Standards

100M Ethernet Cable Standards

1000M Ethernet Cable Standards

10G Ethernet Cable Standards

10BASE-2

10BASE-5

10BASE-T

10BASE-F

100BASE-T4

100BASE-TX

100BASE-FX

1000BASE-SX

1000BASE-LX

1000BASE-TX

10GBASE-T

10GBASE-LR

10GBASE-SR

In these standards, 10, 100, 1000, and 10G indicate the operating rate, and BASE in the middle indicates that signals are transmitted in baseband mode. The number or letter following the hyphen (-) indicates a physical transmission mode. For example, T indicates that coaxial cables are used, TX indicates that twisted pair cables are used, and FX indicates that optical fibers are used. (For details, see the following table.)

10M Ethernet cable standards

The following table lists the 10M Ethernet cable standards defined in IEEE 802.3.

Table 1-3 10M Ethernet cable standards

Name	Cable	Maximum Transmission Distance
10BASE-5	Thick coaxial cable	500 m
10BASE-2	Thin coaxial cable	200 m
10BASE-T	Twisted pair cable	100 m
10BASE-F	Optical fiber	2000 m

The greatest limitation of coaxial cables is that devices are connected in series, so a single point of failure (SPOF) may cause a breakdown of the entire network. As a result, 10BASE-2 and 10BASE-5, which use coaxial cables, have fallen into disuse.

100M Ethernet cable standards

100M Ethernet is also called Fast Ethernet (FE). Compared with 10M Ethernet, 100M Ethernet has a faster transmission rate at the physical layer, but they have no difference at the data link layer.

The following table lists the 100M Ethernet cable standards.

Table 1-4 100M Ethernet cable standards

Name	Cable	Maximum Transmission Distance
100Base-T4	Four pairs of Category 3 twisted pair cables	100 m
100Base-TXs	Two pairs of Category 5 twisted pair cables	100 m
100Base-FX	Single-mode or multi-mode optical fiber	2000 m

10Base-T and 100Base-TX have different transmission rates, yet both apply to Category 5 twisted pair cables. 10Base-T transmits data at 10 Mbit/s, while 100Base-TX transmits data at 100 Mbit/s.

100BASE-T4 is rarely used now.

Gigabit Ethernet cable standards

Gigabit Ethernet is an extension of the Ethernet standard defined in IEEE 802.3. Based on the Ethernet protocol, Gigabit Ethernet increases the transmission rate to 10 times the FE transmission rate (100 Mbit/s), reaching 1 Gbit/s. IEEE 802.3z is used to transmit signals over optical fibers, and IEEE 802.3ab is used to transmit signals over twisted pair cables. The following table lists the Gigabit Ethernet cable standards.

Table 1-5 Gigabit Ethernet cable standards

Name	Cable	Maximum Transmission Distance
1000Base-LX	Single-mode and multi-mode optical fibers	316 m
1000Base-SX	Multi-mode optical fiber	316 m
1000Base-TX	Category 5e or Category 6 twisted pair cable	100 m

Gigabit Ethernet can be used to upgrade an existing Fast Ethernet network from 100 Mbit/s to 1000 Mbit/s.

Gigabit Ethernet uses 8B10B coding at the physical layer. In traditional Ethernet transmission technologies, the data link layer delivers 8-bit data sets to the physical layer, where they are processed and sent still as 8 bits to the physical link for transmission.

In contrast, on the optical fiber-based Gigabit Ethernet, the physical layer maps the 8-bit data sets transmitted from the data link layer to 10-bit data sets before sending them out.

10 Gigabit Ethernet Cable Standards

10 Gigabit Ethernet is currently defined in supplementary standard IEEE 802.3ae, which will be combined with IEEE 802.3 later. The following table lists the 10 Gigabit Ethernet cable standards.

Table 1-6 10 Gigabit Ethernet cable standards

Name	Cable	Maximum Transmission Distance
10GBASE-T	CAT-6A or CAT-7	100 m
10GBase-LR	Single-mode optical fiber	10 km
10GBase-SR	Multi-mode optical fiber	Several hundred meters

100 Gigabit Ethernet cable standards

The standard for 40 Gigabit/100 Gigabit Ethernet is defined in IEEE 802.3ba, which was published in 2010. 100 Gigabit Ethernet will be widely used as network technologies develop.

Key Ethernet Technologies

Network Layers of Ethernet

Ethernet uses passive media to transmit information in broadcast mode. It defines protocols used on the physical layer and data link layer, interfaces between the two layers, and interfaces between the data link layer and upper layers.

Physical layer
The physical layer determines basic physical attributes of Ethernet, including data coding, time scale, and electrical frequency.

The physical layer is the lowest layer in the Open Systems Interconnection (OSI) reference model and is closest to the physical medium (communication channel) that transmits data. Data is transmitted on the physical layer in binary bits (0 or 1). Transmission of bits depends on transmission devices and physical media, but the physical layer does not refer to a specific physical device or a physical medium. Actually, the physical layer is located above a physical medium and provides the data link layer with physical connections to transmit original bit streams.

Data link layer
The data link layer is the second layer in the OSI reference model, located between the physical layer and the network layer. The data link layer obtains services from the physical layer and provides services for the network layer. The basic service that the data link layer provides is to reliably transmit data from the network layer of a source device to the network layer of an adjacent destination device.

The physical layer and data link layer of Ethernet depend on each other. Therefore, different working modes of the physical layer must be supported by corresponding data link layer modes. This hinders Ethernet design and application.

Some organizations and vendors propose to divide the data link layer into two sub-layers: the Media Access Control (MAC) sub-layer and the Logical Link Control (LLC) sub-layer. In this way, different physical layers correspond to different MAC sub-layers, and the LLC sub-layer can be completely independent, as shown in the following figure.

Hierarchical structure of the Ethernet data link layer

CSMA/CD

Concept of CSMA/CD
Ethernet was originally designed to connect computers and other digital devices through a shared physical line. The computers and digital devices can access the shared line only in half-duplex mode. Therefore, a mechanism of collision detection and avoidance is required to prevent multiple devices from contending for the line. This mechanism is called carrier sense multiple access with collision detection (CSMA/CD).

The concept of CSMA/CD is described as follows:
- Carrier sense (CS)
  Before transmitting data, a station checks whether the line is idle to reduce chances of collision.
- Multiple access (MA)
  The data sent by a station can be received by multiple other stations at the same time.
- Collision detection (CD)
  If two stations transmit electrical signals at the same time, the voltage amplitude doubles the normal amplitude as signals of the two stations accumulate. This situation results in collision.
  
  The stations stop transmission after detecting the collision, and resume the transmission after a random delay.
CSMA/CD working process
1. A station continuously detects whether the shared line is idle.
  - If the line is idle, the station sends data.
  - If the line is in use, the station waits until the line becomes idle.
2. If two stations send data at the same time, a collision occurs on the line, and signals on the line become unstable.
3. After detecting the instability, the station immediately stops sending data.
4. The station sends a series of disturbing pulses. After a period of time, the station resumes the data transmission.
  The station sends disturbing pulses to inform other stations, especially the station that sends data at the same time, that a collision occurred on the line.
  
  After detecting a collision, the station waits for a random period of time, and then resumes the data transmission.

Duplex Mode of Ethernet

The physical layer of Ethernet can work in half-duplex or full-duplex mode.

Half-duplex mode
The half-duplex mode has the following features:
- Data can only be sent or received at any time.
- The CSMA/CD mechanism is used.
- The maximum transmission distance is limited.
Hubs work in half-duplex mode.
Full-duplex mode
After Layer 2 switches replace hubs, the shared Ethernet changes to the switched Ethernet, and the half-duplex mode is replaced by the full-duplex mode. As a result, the transmission rate of data frames increases significantly, with the maximum throughput doubled.

The full-duplex mode fundamentally solves the problem of collisions on Ethernet networks and eliminates the need for CSMA/CD.

The full-duplex mode has the following features:
- Data can be sent and received at the same time.
- The maximum throughput is theoretically twice that of the half-duplex mode.
- This mode extends the maximum transmission distance of the half-duplex mode.
Currently manufactured network adapters, Layer 2 devices, and Layer 3 devices support the full-duplex mode, except hubs.

To implement the full-duplex mode, the following hardware requirements must be met:
- Network adapter chips that support the full-duplex mode
- Physical media with separate data transmission and receiving channels
- Point-to-point connection