What is Ethernet? Why is it called Ethernet?

Ethernet refers to the baseband LAN specification created by Xerox, which was jointly developed by Xerox, Intel and DEC. It is the most common communication protocol standard adopted by local area networks. Ethernet uses CSMA/CD (Carrier Sense Multiple Access and Collision Detection) technology and runs at the rate of 10m/s on various types of cables. Ethernet is similar to IEEE802.3 series standards. Include standard Ethernet (10Mbit/s), fast Ethernet (10mb it/s) and 10G Ethernet. They all conform to IEEE802.3.

Standard:

The contents specified in IEEE802.3 include physical layer wiring, electrical signals and media access layer protocols. Ethernet is the most widely used LAN technology at present, which has largely replaced other LAN standards. Such as token ring, FDDI and ARCNET. Following the rapid development of 100M Ethernet at the end of last century, with the promotion of international organizations and leading enterprises, the application scope of Gigabit Ethernet and even 10G Ethernet is also expanding.

Common 802.3 applications are:

10M: 10base-T (copper UTP mode),

100M: 100base-TX (copper UTP mode),

100base-FX (optical fiber line),

1000m: 1000base-T (copper UTP mode)

The general characteristics of Ethernet are summarized as follows:

* * * Enjoy media: All network devices use the same communication media in turn. ?

Broadcast domain: The frame to be transmitted is sent to all nodes, but only the addressed nodes will receive the frame. ?

CSMA/CD: Carrier Sense Multiple Access/Collision Detection in Ethernet to prevent twp or more nodes from transmitting at the same time. ?

MAC address: All Ethernet network cards (NICs) in the media access control layer use 48-bit network addresses. This address is unique in the world. ?

Ethernet basic network composition:

* * * Enjoy media and cables: 10base (twisted pair), 10Base-2 (thin coaxial cable) and 10Base-5 (thick coaxial cable). ?

Repeater or hub: A hub or repeater is a device used to receive a large number of Ethernet connections on network devices. The data obtained by the connected receiving side is reused and sent to all connected devices on the transmitting side to obtain transmission-type devices. ?

Bridge: The bridge belongs to the second layer equipment, which is responsible for dividing the network into independent conflict domains and obtaining network segments, so as to achieve the purpose of maintaining broadcasting and * * * enjoyment in the same domain/network segment. The bridge includes a table covering all network segments and forwarding frames to ensure normal communication behavior within and around the network segment. ?

Switches: Switches, like bridges, belong to Layer 2 devices and are multi-port devices. A switch supports functions similar to those of a bridge, but it has advantages over a bridge because it can temporarily connect any two ports together. The switch includes a switching matrix through which ports can be quickly connected or disconnected. Unlike hubs, switches only forward frames from one port to other connected target nodes, and do not include broadcast ports. ?

Ethernet protocol: The frame structure of Ethernet is provided in IEEE 802.3 standard. At present, Ethernet supports four transmission rates supported by optical fiber and twisted pair media:

10 Mbps _？ 10Base-T？ Ethernet (802.3)?

100 Mbps _ Fast Ethernet (802.3u)?

1000 Mbps _ Gigabit Ethernet (802.3z))?

10 Gigabit Ethernet _ IEEE? 802.3ae

history

The initial development of Ethernet technology came from one of many pioneering technical projects of Xerox Palo Alto Research Center. People usually think that Ethernet was invented in 1973, when robert metcalf wrote a memo to his PARC boss about the potential of Ethernet. But Metcalfe himself thinks that Ethernet didn't appear until several years later. 1976, Metcalfe and his assistant David Boggs published an article entitled "Ethernet: Distributed Packet Switching Technology in Local Computer Networks". At the end of 1977, Metcalfe and his collaborators obtained the patent of "Multipoint Data Communication System with Conflict Detection". Multicast transmission system is called CSMA/CD (Carrier Sense Multiple Access with Collision Detection), which marks the birth of Ethernet.

From 65438 to 0979, Metcalfe left Xerox Company and founded 3Com Company to develop personal computers and local area networks. 3com lobbied Digido, Intel and Xerox, hoping to realize the standardization and normalization of Ethernet with them. This universal Ethernet standard was released on September 30th, 1980 in/kloc-0. At that time, there were two popular non-public network standards, token ring network and ARCNET, which were rapidly shrunk and replaced under the impact of Ethernet tide. In the process, 3Com also became an international company.

Ethernet plug:

Metcalfe once joked that Jerry Salz contributed to the success of 3Com. In an influential paper co-authored with others, Saltzer pointed out that token ring network is superior to Ethernet in theory. Influenced by this conclusion, many computer manufacturers either hesitate or decide not to use Ethernet interface as the standard configuration of their machines, so 3com has the opportunity to make a fortune by selling Ethernet cards. This situation has also led to another saying that "Ethernet is not suitable for theoretical research, but only suitable for practical application". Maybe this is just a joke, but it illustrates a technical point: usually, the actual data flow characteristics in the network are different from those estimated before the popularization of LAN, and it is precisely because of the simple structure of Ethernet that LAN is popularized. Metcalfe and Saltzer used to work on the same floor of the Massachusetts Institute of Technology (MIT) MAC project, when he was doing his graduation thesis at Harvard University, during which he laid the theoretical foundation of Ethernet technology.

This standard defines the cable types and signal processing methods used in local area network (LAN). Ethernet transmits data packets between interconnected devices at the rate of 10~ 100Mbps. Twisted-pair 10 Base T Ethernet has become the most widely used Ethernet technology because of its low cost, high reliability and 10Mbps. Direct spread spectrum wireless Ethernet can reach 1 1Mbps, and the products provided by many manufacturers can communicate with each other using common software protocols, which has the best openness.

Standard Ethernet:

At first, the throughput of Ethernet was only 10Mbps, and CSMA/CD (Carrier Sense Multiple Access/Collision Detection) access control mode with collision detection was adopted. This early 10Mbps Ethernet is called standard Ethernet, which can be connected by thick coaxial cable, thin coaxial cable, unshielded twisted pair, shielded twisted pair, optical fiber and other transmission media. And in IEEE? In the 802.3 standard, different physical layer standards are formulated for different transmission media. In these standards, the front number represents the transmission speed in Mbps, and the last number represents the length of a single-segment network cable (reference unit is 100m), Base stands for baseband, and Broad stands for broadband.

10BASE-5 uses a thick coaxial cable with a diameter of 0.4 inch and an impedance of 50 Ω, also known as thick cable Ethernet, with a maximum segment length of 500m m.. Baseband transmission mode, with bus topology. The main hardware devices of 10BASE-5 networking are: thick coaxial cable, Ethernet card with AUI socket, repeater, transceiver, transceiver cable, terminator, etc.

10BASE-2 adopts a thin coaxial cable with a diameter of 0.2 inch and an impedance of 50 Ω, also known as thin cable Ethernet, with a maximum length of 185m, baseband transmission mode and bus topology; The main hardware equipment of 10BASE-2 networking includes: thin coaxial cable, Ethernet card with BNC socket, repeater, T-connector, terminator, etc.

10BASE-T uses twisted pair, and the maximum segment length is1100mm ... The topological structure is star; The main hardware devices of 10BASE-T networking include: unshielded twisted-pair cables of class 3 or 5, Ethernet cards with RJ-45 sockets, hubs, switches, RJ-45 plugs, etc.

1base-5 adopts twisted pair, with the maximum segment length of 500m and the transmission speed of1Mbps; ;

10bbroad-36 adopts coaxial cable (RG-59/U CATV), with the maximum network span of 3600m and the maximum segment length of 1800m, which is a broadband transmission mode;

10BASE-F adopts optical fiber transmission medium, and the transmission rate is 10Mbps.

The working principle of 1. Ethernet and IEEE802.3

In broadcast-based Ethernet, all workstations can receive information frames sent to the network. Each workstation should confirm whether the information frame is sent to itself, and once it is confirmed to be sent to itself, it will be sent to a higher protocol layer.

In the Ethernet accessed through CSMA/CD transmission medium, any CSMA/CD LAN workstation can access the network at any time. Before sending data, the workstation should monitor whether the network is blocked, and only when the network is idle can the workstation send data.

In the competition-based Ethernet, any workstation can send data as long as the network is idle. When two workstations find that the network is idle and send out data at the same time, a conflict will occur. At this time, both transmission operations are destroyed, and the workstation must retransmit after a certain time. When to retransmit is determined by the delay algorithm.

2. Differences between Ethernet and IEEE802.3 services

Although there are many similarities between Ethernet and IEEE802.3 standards, there are also some differences. The services provided by Ethernet correspond to the first and second layers of the OSI reference model, while the services provided by IEEE802.3 correspond to the channel access parts of the first and second layers of the OSI reference model (that is, a part of the second layer). IEEE802.3 does not define logical link control protocol, but defines several different physical layers, while Ethernet only defines one.

Each physical layer protocol of IEEE802.3 can be characterized from three aspects, namely, the speed of local area network, signal transmission mode and physical medium type.

Ethernet is a baseband LAN technology developed in 1970s. It uses coaxial cable as network medium and CSMA/ CD mechanism, and the data transmission rate reaches 10MBPS. But now Ethernet is more used to refer to various local area networks using CSMA/CD technology. The frame format of Ethernet is consistent with IP, which is especially suitable for transmitting IP data. Ethernet has the advantages of simplicity, convenience, cheapness and high speed.

The name Ethernet comes from a scientific hypothesis: sound travels through the air, but what about light? It can spread in outer space without air. So, some people say that light passes through a substance called ether. Later, Einstein proved that ether did not exist at all.

The difference between Ethernet and Internet;

The main difference: Ethernet is a local area network, which can only connect nearby devices. The Internet is a wide area network, through which we can connect to the United States to get news.

Both are networks for connecting computers, but their ranges are different. Ethernet is limited to a certain distance, we can have hundreds of Ethernet; But the Internet is the largest wide area network. We only have one internet, so the internet can be said to be a network in the network.

Internet is a very large international system, which can connect networks all over the world. Private, public, academic or commercial networks or government networks can be interconnected and share resources. Figuratively speaking, the Internet is the network we use to open webpages, send emails, listen to music and watch movies online. It contains a wide range of information, and now we are used to it.

On the other hand, Ethernet basically only allows a few local computers to connect with each other. There is a set of technical support for sending messages between computers. Generally speaking, computers connected to Ethernet are all in the same building or nearby. But with the development of Ethernet cable, the scope of Ethernet can be extended to ten kilometers. However, because they are all interconnected by network cables, it is not realistic to connect to far places.

To be more practical, Ethernet is to connect your computer and notebook to the cat, and then connect to the Internet through the cat, so that you can Skype with friends abroad. So, your computer, notebook and cat form an Ethernet. As you can imagine, there are thousands of Ethernet in the world. Enterprises use Ethernet to connect all computers to the main server.

Ethernet can have one or several administrators. Some parts of the Internet may be owned by administrators, but no administrator can control the whole Internet.

Another difference is security. Ethernet is relatively safe because it is a closed internal network and outsiders have no authority. But the internet is open and everyone can browse it.

The following mainly introduces four different Ethernet frame formats.

Every Ethernet frame starts with a 64-bit (8-byte) leading character, as shown in figure 1. Among them, the first 7 bytes are called Preamble, including 16 hexadecimal number 0xAA, and the last 1 bytes are the frame start identifier 0xAB, which marks the beginning of an Ethernet frame. The leading character is used to synchronize the receiving node and prepare to receive the data frame.

Figure 1 Ethernet frame leading character

In addition, the definitions of fields in Ethernet frames of different formats are different and incompatible with each other. The following are the respective frame formats.

Ethernet II

That is, DIX 2.0: Ethernet standard frame format formulated by Xerox, DEC and Intel in 1982, as shown in Figure 2.

Figure 2 Original Frame Format of Ethernet 802.3

Ethernet Type II Ethernet frame has a minimum length of 64 bytes (6+6+2+46+4) and a maximum length of 15 18 bytes (6+6+2+ 1500+4). The first 12 bytes respectively identify the MAC address of the source node that sent the data frame and the MAC address of the target node that received the data frame. (Note: after ISL encapsulation, it can reach 1548 bytes, and after 802. 1Q encapsulation, it can reach 1522 bytes).

The next two bytes identify the upper layer data type carried by the Ethernet frame. For example, 16 hexadecimal number 0x0800 represents IP protocol data, 16 hexadecimal number 0x809B represents AppleTalk protocol data, and 16 hexadecimal number 0x8 138 represents Novell type protocol data.

The variable length data field is followed by a 4-byte frame check sequence (FCS), and the data from the target MAC address field to the data field is checked by a 32-bit CRC cyclic redundancy check.

Ethernet 802.3 raw

The standard frame format of dedicated Ethernet published by Novell in 1983 is shown in Figure 3.

Figure 3 Original Frame Format of Ethernet 802.3

In the original Ethernet frame of Ethernet 802.3, the type field in the original Ethernet II frame is replaced by the "total length" field, which indicates the length of the subsequent data field, and the value range is 46~ 1500.

The next two bytes are the fixed hexadecimal number 0xFFFF of 16, which identifies the frame as a Novell Ethernet data frame.

Ethernet 802.3 SAP

The Ethernet frame format of Ethernet version 802.3 published by IEEE in 1985 is shown in Figure 4.

Fig. 4 Ethernet 802. 3 SAP frame format

As can be seen from Figure 4, in the Ethernet 802.3 SAP frame, the 2 bytes 0xFFFF in the original Ethernet 802.3 raw frame is changed to the DSAP and SSAP of each 1 byte, and the "control" field of 1 byte is added at the same time, which constitutes the header of the 802.2 logical link control (LLC). LLC provides connectionless (LLC type 1) and connection-oriented (LLC type 2) network services. LLC 1 is used for Ethernet, and LLC2 is used for IBM SNA network environment.

The new 802.2 LLC header includes two service access points: the source service access point (SSAP) and the target service access point (DSAP). They are used to identify the upper layer data types carried by Ethernet frames, such as 16 hexadecimal digit 0x06 for IP protocol data, 16 hexadecimal digit 0xE0 for Novell type protocol data, and 16 hexadecimal digit 0xF0 for IBM NetBIOS type protocol data.

As for the "control" field of 1 byte, it is basically not used (generally set to 0x03, which means the 802.2 unnumbered data format with no connection service is adopted).

Ethernet 802.3 snapshot

The Ethernet frame format of Ethernet 802.3 SNAP released by IEEE in 1985 is shown in Figure 5.

Fig. 5 Ethernet 802. 3 capture frame format

The main differences between Ethernet frame formats of Ethernet 802. Ethernet frame formats of 3 SNAP and Ethernet 802. 3 SAP is as follows:

The contents of 2 bytes in DSAP and SSAP fields are fixed, and their values are 16 hexadecimal digits 0xAA.

The content of the "Control" field 1 byte is fixed, and its value is 16 hexadecimal number 0x03.

Added snapshot field, which consists of the following two items:

A 3-byte Organization Unique Identifier (OUI ID) field is added, and its value is usually equal to the first 3 bytes of the MAC address, that is, the network adapter manufacturer code.

The 2-byte Type field is used to identify the upper layer data type carried by the Ethernet frame.

Ethernet can use various connection media, including coaxial cable, twisted pair and optical fiber. Among them, twisted pair is mostly used to connect the host to the hub or switch, and optical fiber is mainly used for cascading between switches and point-to-point links between switches and routers. Coaxial cable, as an early main connecting medium, has been gradually eliminated.

Pay attention to distinguish between straight lines and crossed lines in twisted pair.

Straight-through cables shall be used for the following connections:

Switch to the router Ethernet port.

Computer to switch

Computer to hub

Crossover cables are used to directly connect the following devices in the LAN:

Switch to switch

Switch to hub

Hub to hub

Router-to-router Ethernet port connection

Computer to computer

Ethernet port from computer to router

CSMA/CD*** Enjoy Media Ethernet

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)? [2]? Technology provides a way for multiple computers to share a channel. This technology first appeared in ALOHAnet developed by the University of Hawaii in1960s. It uses radio waves as the carrier. This method is simpler than token ring network or main control network. When a computer wants to send information, it must follow the following rules:

Start:? If the line is idle, start transmission, otherwise go to step 4.

Send:? If a collision is detected, continue to send data until the minimum message time is reached (ensure that all other repeaters and terminals detect the collision), and then go to step 4.

Transmission successful:? Report the transmission success to the high-level network protocol and exit the transmission mode.

Busy: Wait until the line is free? The line enters an idle state-wait for a random time, and then enter step 1 unless the maximum number of attempts is exceeded.

Maximum number of transmission attempts exceeded:? Report the transmission failure to the high-level network protocol and exit the transmission mode.

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