Distinctive features of Ethernet technology. What is Ethernet

Ethernet—most networks these days are based on it. There are a large number of technologies that allow you to connect computers into a network. Each of them was developed at different times and is designed to solve a specific problem.

Ethernet technology covers the two lower layers of the OSI model at once. Physical and channel levels. Further we will talk only about the physical layer of the OSI model, i.e. about how data bits are transferred between two neighboring devices.

Currently, technology is used to build local networks FastEthernet, which is a new implementation of the technology Ethernet.

What is Ethernet

This technology was developed in 1970 by the Palo Alto Research Center, which belongs to the Xerox Corporation, and in 1980, the IEEE 802.3 specification was adopted on its basis.

The basic operating principle used in this technology is as follows. In order to begin transmitting data on the network, the computer's network adapter “listens” to the network for the presence of any signal. If it is not present, then the adapter begins data transmission, but if there is a signal, then the transmission is delayed for a certain time interval. The time of exclusive use of a shared medium by one node is limited by the time of transmission of one frame.

Frame - it is a unit of data exchanged between computers on an Ethernet network. The frame has a fixed format and, along with the data field, contains various service information, such as the recipient's address and the sender's address. After the sender adapter has placed the frame on the network, all network adapters begin to receive it. Each adapter analyzes the frame, and if the address matches their own device address (MAC address), the frame is placed in the internal buffer of the network adapter, but if it does not match, it is ignored.

In the event that two or more adapters, having “listened” to the network, begin to transmit data, a collision (collision). The adapters, having detected a collision, stop data transmission, and then, after “listening” to the network again, repeat data transmission at different intervals.

? NOTE. To receive a data packet that is destined for a particular adapter, it must accept all packets that appear on the network.

This method of accessing the data transmission medium is called CSMA/ CD(carrier-sense multiple access/collision detection) - multiple access with carrier detection.

What is Ethernet - collisions

As follows from the above, with a large number of computers on the network. and with intensive information exchange, the number of collisions grows very quickly. and as a result, network throughput decreases. It is possible that the throughput may drop to zero. But even in a network where the average load does not exceed the recommended one. This is 30-40% of the total bandwidth, the transmission speed is 70-80% of the nominal.

However, this problem has now been almost solved. Because they have developed devices capable of dividing data streams between those computers for which this data is intended. In other words, traffic between ports connected to the sending and receiving network adapters is isolated from other ports and adapters. Such devices are called switches (switch).

There are various implementations of this technology - Ethernet, Fast Ethernet, Gigabit Ethernet. For example, they can provide data transfer rates of 10, 100 and 1000 Mbit/s, respectively.

The IEEE 802.3 standard contains several specifications that differ in topology and type of cable used. For example, 10 BASE-5 uses thick coaxial cable. 10 BASE-2 is a thin cable. And 10 BASE-F, 10 BASE-FB, 10 BASE-FL and FOIRL use optical cable. The most popular specification is IEEE 802.3 100BASE-TX. In which a cable based on unshielded twisted pairs with RJ-45 connectors is used to organize the network.

Ethernet Network Implementations

The Ethernet specifications listed above can be described as follows. The first number in the specification name indicates the maximum data transfer rate. For example, “10” indicates a signal transmission rate of 10 Mbit/s. “Base” means the use of Baseband technology in the standard. B aseband- This is narrowband transmission. With this method of transmitting data over a cable, every bit of data is encoded. It is encoded by a separate electrical or light pulse. In this case, the entire cable is used as one communication channel. Those. simultaneous transmission of two signals is not possible.

Originally, the last section in the specification title was intended to indicate the maximum length. Cable segment lengths in hundreds of meters. This is without hubs or switches. However, for convenience and a more complete definition of the essence of the standard, everything has been simplified. And now the numbers in its name have been replaced with the letters T and F. Where T stands for twistedpair- twisted pair, and F denotes fiber optic.

Thus, nowadays one can find networks based on the following specifications:

• 10Base-2 - 10 MHz Ethernet on coaxial cable with a resistance of 50 Ohms, baseband. 10Base-2 is known as "thin Ethernet";
• 10Base-5 - 10MHzEthernet on a standard (thick) coaxial cable with a resistance of 50 Ohms, baseband;
• 10Base-T - 10MHz Ethernet over twisted pair cable;
• 100 Base-TX - 100MHz Ethernet over twisted pair cable.

A very significant advantage of the various Ethernet options is mutual compatibility. One that allows you to use them together on the same network. And in some cases, without even changing the existing cable system.

FULL DUPLEX MODE

The Fast Ethernet technology standard also includes recommendations. Recommendations for enabling full duplex operation (full— duplexmode) when connecting the network adapter to the switch. Or when directly connecting switches to each other.

The essence of full-duplex mode is the ability to simultaneously transmit and receive data over two channels. Tx (channel from transmitter to receiver) and Rx (channel from receiver to transmitter). And at the same time, the transmission speed doubles and reaches 200 Mbit/s.

At the moment, almost all network equipment manufacturers claim the following. That their devices provide full-duplex operation. However, due to different interpretations of the standard, in particular how to manage the flow of personnel. It is not always possible to achieve correct operation of these devices. And also good speed performance.

Ethernet (reads Ethernet, from lat. aether - ether) - packet technology for transmitting data mainly local
.

Ethernet standards define wired connections and electrical signals at the physical layer, format
frames and media access control protocols - at the data link layer of the OSI model. Ethernet mostly
described by IEEE group 802.3 standards. Ethernet has become the most common LAN technology in the middle
90s of the last century, displacing such outdated technologies as Arcnet, FDDI and Token ring.

History of creation

Ethernet technology was developed along with many of Xerox PARC's early projects.
It is generally accepted that Ethernet was invented on May 22, 1973, when Robert Metcalfe
wrote a memo for the head of PARC on the potential of Ethernet technology. But the legal right to
Metcalfe received the technology a few years later. In 1976, he and his assistant David Boggs
published a brochure entitled "Ethernet: Distributed Packet-Switching For Local Computer Networks."

Metcalf left Xerox in 1979 and founded 3Com to market computers and local
computer networks (LAN). He managed to convince DEC, Intel and Xerox to work together and develop
Ethernet standard (DIX). This standard was first published on September 30, 1980. He started
competing with two major proprietary technologies: token ring and ARCNET - which were soon buried under the rolling waves of Ethernet products. In the process, 3Com became the dominant company in the industry.

The standard of the first versions (Ethernet v1.0 and Ethernet v2.0) indicates that as a transmission medium
coaxial cable is used, later it became possible to use twisted pair and optical
cable.

The reasons for switching to were:

• possibility of working in duplex mode;
• low cost of twisted pair cable;
• higher reliability of networks in case of cable failure;
• greater noise immunity when using a differential signal;
• the ability to power low-power nodes via cable, for example IP phones (Power over Ethernet, POE standard);
• lack of galvanic connection (current flow) between network nodes. When using a coaxial cable in Russian conditions, where, as a rule, there is no grounding of computers, the use of a coaxial cable was often accompanied by a breakdown of network cards, and sometimes even a complete “burnout” of the system unit.

The reason for switching to optical cable was the need to increase the length of the segment without repeaters.

Access control method (for a network on) - multiple access with carrier sense and
collision detection (CSMA/CD, Carrier Sense Multiple Access with Collision Detection), transmission rate
data 10 Mbit/s, packet size from 72 to 1526 bytes, data encoding methods are described. Operating mode
half-duplex, that is, the node cannot simultaneously transmit and receive information. Number of nodes in
one shared network segment is limited to a limit of 1024 workstations (specifications
the physical layer can impose more stringent restrictions, for example, on a thin coaxial segment
no more than 30 workstations can be connected, and no more than 100 can be connected to a thick coaxial segment). However
a network built on one shared segment becomes ineffective long before reaching
limit on the number of nodes, mainly due to the half-duplex mode of operation.

In 1995, the IEEE 802.3u Fast Ethernet standard with a speed of 100 Mbit/s was adopted and it became possible
work in full duplex mode. In 1997, the IEEE 802.3z Gigabit Ethernet standard was adopted with speed
1000 Mbit/s for transmission over optical fiber and another two years later for transmission over twisted pair.

Types of Ethernet

Depending on the data transfer rate and transmission medium, there are several technology options.
Regardless of the transmission method, the network protocol stack and programs work the same in almost all
all of the options listed below.

Most Ethernet cards and other devices support multiple data rates,
using autonegotiation of speed and duplex to achieve the best
connections between two devices. If autodetection does not work, the speed is adjusted to suit
partner, and the half-duplex transmission mode is activated. For example, the presence of an Ethernet port in the device
10/100 means that you can work through it using 10BASE-T and 100BASE-TX technologies, and the port
Ethernet 10/100/1000 - supports 10BASE-T, 100BASE-TX and 1000BASE-T standards.
Early Ethernet Modifications

• Xerox Ethernet - the original technology, speed 3Mbit/s, existed in two versions Version 1 and Version 2, the frame format of the latest version is still widely used.
• 10BROAD36 - not widely used. One of the first standards allowing work over long distances. Used broadband modulation technology similar to that used
  in cable modems. Coaxial cable was used as a data transmission medium.
• 1BASE5 - also known as StarLAN, was the first modification of Ethernet technology to use twisted pair cables. It worked at a speed of 1 Mbit/s, but did not find commercial use.

10 Mbit/s Ethernet

• 10BASE5, IEEE 802.3 (also called "Thick Ethernet") - the initial development of technology with a data transfer rate of 10 Mbps. Following the early IEEE standard, it uses 50 ohm coaxial cable (RG-8), with a maximum segment length of 500 meters.
• 10BASE2, IEEE 802.3a (called "Thin Ethernet") - uses RG-58 cable, with a maximum segment length of 185 meters, computers connected one to another to connect the cable to the network
  the card needs a T-connector, and the cable must have a BNC connector. Requires terminators on each
  end. For many years this standard was the main one for Ethernet technology.
• StarLAN 10 - The first development that uses twisted pair cables to transmit data at a speed of 10 Mbit/s.

Later it evolved into the 10BASE-T standard.

Despite the fact that it is theoretically possible to connect more than one twisted pair cable (segment)
two devices operating in simplex mode, such a scheme is never used for Ethernet, in
difference from working with . Therefore, all twisted pair networks use a star topology,
while coaxial cable networks are built on a “bus” topology. Terminators for work on
twisted pair cables are built into each device, and there is no need to use additional external terminators in the line.

• 10BASE-T, IEEE 802.3i - 4 wires of a twisted pair cable (two twisted pairs) of category-3 or category-5 are used for data transmission. The maximum segment length is 100 meters.
• FOIRL - (acronym for Fiber-optic inter-repeater link). The basic standard for Ethernet technology, using optical cable for data transmission. The maximum data transmission distance without a repeater is 1 km.
• 10BASE-F, IEEE 802.3j - The main term for a family of 10 Mbit/s ethernet standards using optical cable over distances of up to 2 kilometers: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of the above, only 10BASE-FL has become widespread.
• 10BASE-FL (Fiber Link) - An improved version of the FOIRL standard. The improvement concerned an increase in the length of the segment to 2 km.
• 10BASE-FB (Fiber Backbone) - Currently an unused standard, intended for combining repeaters into a backbone.
• 10BASE-FP (Fiber Passive) - A “passive star” topology in which repeaters are not needed - has never been used.

Fast Ethernet (Fast Ethernet, 100 Mbit/s)

• 100BASE-T is a general term for standards that use . Segment length up to 100 meters. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2 standards.
• 100BASE-TX, IEEE 802.3u - development of the 10BASE-T standard for use in star topology networks. Category 5 twisted pair cable is used, actually only two unshielded pairs of conductors are used, duplex data transmission is supported, distance up to 100 m.
• 100BASE-T4 is a standard that uses twisted pair cable of category 3. All four pairs of conductors are used, data transmission occurs in half duplex. Practically not used.
• 100BASE-T2 is a standard that uses Category 3 twisted pair cables. Only two pairs of conductors are used. Full duplex is supported, with signals traveling in opposite directions on each pair. Transmission speed in one direction is 50 Mbit/s. Practically not used.
• 100BASE-SX is a standard using multimode fiber. The maximum segment length is 400 meters in half duplex (for guaranteed collision detection) or 2 kilometers in full duplex.
• 100BASE-FX is a standard using single-mode fiber. The maximum length is limited only
  the amount of attenuation in the optical cable and the power of transmitters, for different materials from 2x to 10
  kilometers
• 100BASE-FX WDM is a standard using single-mode fiber. The maximum length is limited only
  the amount of attenuation in the fiber-optic cable and the power of the transmitters. There are two interfaces
  species, differ in the wavelength of the transmitter and are marked either with numbers (wavelength) or with one Latin
  letter A(1310) or B(1550). Only paired interfaces can operate in pairs: on one side the transmitter
  at 1310 nm, and on the other - at 1550 nm.

• 1000BASE-T, IEEE 802.3ab - a standard using Category 5e twisted pair cable. 4 pairs are involved in data transmission. Data transfer speed - 250 Mbit/s over one pair. The PAM5 encoding method is used, the fundamental frequency is 62.5 MHz. Distance up to 100 meters
• 1000BASE-TX was created by the Telecommunications Industry Association
  Industry Association (TIA) and published in March 2001 as the "Physical Layer Specification
  full-duplex Ethernet 1000 Mb/s (1000BASE-TX) symmetrical Category 6 cabling systems
  (ANSI/TIA/EIA-854-2001) "A Full Duplex Ethernet Specification for 1000 Mbis/s (1000BASE-TX)
  Operating Over Category 6 Balanced Twisted-Pair Cabling (ANSI/TIA/EIA-854-2001). Standard, uses
  separate reception and transmission (one pair in each direction), which significantly simplifies the design
  transceiver devices. Another significant difference between 1000BASE-TX is the absence of a circuit
  digital compensation of interference and return noise, resulting in complexity, power consumption
  and the price of processors becomes lower than those of 1000BASE-T standard processors. But, as a consequence, for
  Stable operation of this technology requires a high-quality cable system, so 1000BASE-TX
  Can only use Category 6 cable. Almost no products have been created based on this standard.
  products, although 1000BASE-TX uses a simpler protocol than the 1000BASE-T standard and therefore can
  use simpler electronics.
• 1000BASE-X is a general term for standards with pluggable GBIC or SFP transceivers.
• 1000BASE-SX, IEEE 802.3z is a standard using multimode fiber. Travel distance
  signal without a repeater up to 550 meters.
• 1000BASE-LX, IEEE 802.3z - a standard using single-mode fiber. Travel distance
  signal without a repeater up to 5 kilometers.
• 
  
  used.
• 1000BASE-CX - standard for short distances (up to 25 meters), using twinaxial cable
  with a characteristic impedance of 75 Ohms (each of two waveguides). Replaced by the 1000BASE-T standard and is no longer
  used.
• 1000BASE-LH (Long Haul) is a standard using single-mode fiber. Travel distance
  signal without a repeater up to 100 kilometers.

10 Gigabit Ethernet

The new 10 Gigabit Ethernet standard includes seven physical media standards for LAN, MAN and
WAN. It is currently covered by the IEEE 802.3ae amendment and should be included in the next revision
IEEE 802.3 standard.

• 10GBASE-CX4 - 10 Gigabit Ethernet technology for short distances (up to 15 meters), using CX4 copper cable and InfiniBand connectors.
• 10GBASE-SR - 10 Gigabit Ethernet technology for short distances (up to 26 or 82 meters, in
  depending on the cable type), multimode fiber is used. It also supports distances up to 300
  meters using new multimode fiber (2000 MHz/km).
• 10GBASE-LX4 - uses wavelength multiplexing to support distances of 240 to 300 meters over multimode fiber. Also supports distances up to 10 kilometers when using single-mode
  fibers.
• 10GBASE-LR and 10GBASE-ER - these standards support distances up to 10 and 40 kilometers
  respectively.
• 10GBASE-SW, 10GBASE-LW and 10GBASE-EW - These standards use a physical interface compatible
  in speed and data format with the OC-192 / STM-64 SONET/SDH interface. They are similar to 10GBASE-SR standards,
  10GBASE-LR and 10GBASE-ER respectively, as they use the same cable types and transmission distances.
• 10GBASE-T, IEEE 802.3an-2006 - adopted in June 2006 after 4 years of development. Uses
  shielded twisted pair. Distances - up to 100 meters.

The word Ethernet comes from two words “ether” or ether and “net” - network. That is, the translation will result in an ethereal network.

You need to understand that Ethernet and the Internet are completely different things. Thus, Ethernet is a technology by which information is transferred between computers connected to a local network. At the same time, the Internet is a global system of computer networks interacting with each other throughout the world. In essence, this is a worldwide information space created on the basis of the IP protocol.

Ethernet technology is used in industry, offices, cellular communications, and wherever data exchange between machines is implemented. Technology is a kind of substitute for radio broadcasting.

Specially developed broadcast standards. They are called protocols. These are Fast and Gigabit Ethernet, and the maximum 10G Ethernet. The latter is just developing. When transmitting information using 10 gigabit Internet technology, optical fiber will be used, unlike conventional gigabit Internet, which uses copper wire.

A little history

This technology appeared in 1973. But the standard itself was approved and developed only in 1980. And in 1981, the first transceiver or transceiver was released. In 1983, the IEEE 802.3 Ethernet technology standard appeared.

The network adapter appeared a little later, in 1982. 1985 Ethernet II was launched in 2008, and five years later the familiar 10 BaseT technology appeared - twisted pair. And the latest twist in the history of technology is 1995 the year that Fast Ethernet or modern 100 BaseT was introduced.

How it works

Gigabit Ethernet technology works, unlike its predecessors, using a four-pair cable. This wire is the most reliable and protected from all kinds of collisions.

Data transmission is encrypted not two levels, but four (00, 01, 10, 11). It turns out that one frame contains two bits at once.

A frame is a package of eight headers, which contain the recipient and sender addresses, tasks for adapters for synchronous reception and transmission of information, checksum fields and the information itself. Now everywhere frame used format 802.3 Internet technology. It defines all these eight headings.

The transfer of information occurs as follows - information in one computer is formed into a frame, encoded and, through a network adapter, goes to the adapter of another device, where it decrypts it and sends it to the user’s screen in the form of the data he needs.

The picture shows two-level signal, which was used earlier and the four-level one is more modern.

This scheme is called pulse amplitude coding. It is designed to reduce the voltage frequency to 125 Megahertz. And the adapter itself selects its own transmitted signal from the common channel to receive a signal from another computer.

Ethernet - collisions

Ethernet collisions are errors that can occur during data transfer between personal devices. This word comes from the English collision - collision.

Most often, such errors occur because one station starts send information before others. That is, while the other computer is sending data and the information is in the middle of the path, the second device begins its transmission. As a result, information packets collide without reaching the target; devices, having listened to the protocols and detected such errors, interrupt the transmission. Such collisions often occurred when the connection was made via a coaxial Ethernet cable or via a twisted pair cable consisting of two pairs.

Nowadays, with full duplex mode, this rarely happens.

How does the connection happen?

Previously, connections between computers were made using coaxial cables, special adapters and transceivers, if thick and thin cables had to be connected. If even one cable was damaged, the entire network stopped working.

To transmit signals, twisted pair cable and RJ45 connectors are currently used, which are connected to computers and other peripheral devices, or a router. Nowadays fiber optic cable is becoming more widespread. Here the speed is, of course, many times greater. The advantage of fiber optics in its reliability and protection from all kinds of collisions.

When connecting to a network, each computer is equipped with an Ethernet controller or, as it is also called, a network card, which performs a kind of encryption and decryption of the information received and sent by it. And the Ethernet port is the input interface on the network card, which is usually called the lan port.

Types of Ethernet

There are several varieties of Ethernet network technology, each depending on the speed and transmission medium. Early varieties were as follows:

• Xerox Ethernet speed 3 megabits per second.
• 1base5 with speed 1 Mb/s, but used twisted pair.

Ten megabit Ethernet had the following modifications:

• 10base5 with speed 10 megabits using thick coaxial cable.
• 10base2. A thin cable was used, but terminators or adapters were needed at each end.
• 10baseT - Twisted pair was used, but the maximum wire length could only be 100 meters from the router.

Fast is divided into:

• 100 baseT – speed 100 Mb/s, use of twisted pair. Length – 100 meters from the router.
• 100base fx – speed 100 Mb/s. Length from 400 meters to 2 kilometers in full duplex.

Gigabit:

• 1000 base lx – use of optical fiber for data transmission. For single-mode, the length is 5 kilometers, and for multi-mode, 550 meters.
• 1000 base sx – also uses optical fiber, and the data transmission length is only 550 meters.
• 1000base T – 5e standard twisted pair cable is used to transmit information.

10 gigabit:

• 10gbase t - category 6e twisted pair cable is used.
• 10gbase lx4 – fiber optic is used. Single-mode - 10 kilometers. Multimode - 300 meters.
• 10 gbase cx4 – requires cx4 copper cable and InfiniBand connectors.

MAC address

The MAC address or address of a personal device, which is given to it during manufacture, is an identifier that defines a particular computer unit on the network.

It allows identify the host and supply him with certain data and information. Thanks to this, you can avoid certain collisions that may arise during the transmission of information. This way, the data will always go strictly to the computer to which it was assigned.

You can find it by opening the properties of your network adapter. It consists of a hexadecimal set of numbers and letters. It is assigned not only to PCs, but also to printers, routers, routers and other devices that operate on a local or worldwide network.

What Ethernet is became known in the seventies of the 20th century. This was invented by Robert Metcalf, who at that time worked for the Xerox Corporation. In the late 70s, Metcalf opened his own company, 3com, where the development of new technology was completed. Over time, it replaced the existing types of local area networks, and Metcalf's company became a leader in this field.

The term “Ethernet” is made up of the words ether (ether) and net (network). Now we will tell you in more detail what Ethernet is and what are the main features of this type of network. This type of network has a star or linear structure with a speed of 10-100 megabits/second. Initially, Ethernet was based on coaxial cable, but over time the technology changed, and the network began to be built on the basis or twisted pairs. Now there are about thirty types of Ethernet networks, which differ in speed, topography, size and type of cable. Not all varieties have found commercial application. For those who want to know in detail what Ethernet is, we list the most popular technologies.

Xerox Ethernet is a technology based on coaxial cable with a maximum speed of 3 megabits per second. Modification of StarLan, in which the speed of such a connection was first used is low - only 1 megabit per second.

In 10BASE5 technology, coaxial cable transmits data at a speed of 10 megabits/second. The speed is exactly the same in StarLan10, but the coaxial cable here has been replaced with twisted pair. This technology subsequently evolved into a variation of 10BASE-T, which used four twisted pairs.
In the 100BASE-T modification based on twisted pair, the speed increased to one hundred megabits/second. This type was further developed. 100BASE-FX transmits data over a fiber optic cable over a distance of 10 kilometers at a speed of one hundred megabits/second. 1000BASE-T uses four wire pairs, and the distance is one hundred meters. In the 1000BASE-LH modification, the distance increased to 100 kilometers. The last two types have the highest speed, reaching 1000 megabits per second.

The Ethernet network, which includes all of the above modifications, is connected using a special controller integrated into the motherboard.

Now let's look at the advantages of this type of network. Its main advantage is its accessibility. The computer is constantly connected to the network, and there is no need to call the provider before accessing the Internet. Essentially, Ethernet can be called a dedicated line in which a modem is simply not needed. Another advantage is the high speed provided by the Ethernet protocol. Speed ​​is provided symmetrically, regardless of whether the file is downloading or sending. In addition, one Ethernet connection can form the basis of a corporate or local network, in which all computers will have access to the same high connection speed.

Security in a modern Ethernet network is also well organized. As a rule, providers provide the user with real IP addresses, which ensure the anonymity of the computer on the World Wide Web. Of course, not the last advantage of such a network is the extreme ease of connection. This does not require a modem or any special software; it is enough to have a network card, which is built into almost all motherboards. This simplicity and accessibility also explains the low cost of an Ethernet connection. It costs much less than connecting to the global network via a telephone modem.

Over time, this type of network will become even more accessible. There are already modifications that provide speeds of about 10 gigabits/second. By the middle of this decade, technology is expected to be released that will provide speeds equal to 1 terabits/second. With such exciting prospects, anyone who understands what Ethernet is will definitely want to connect to this network.

Ethernet is the most widespread local network standard today. The total number of networks currently using Ethernet is estimated at 5 million, and the number of computers running Ethernet network adapters installed is estimated at 50 million.

When people say Ethernet, they usually mean any of the variants of this technology. In a narrower sense, Ethernet is a network standard based on the technologies of the experimental Ethernet Network, which Xerox developed and implemented in 1975 (even before the advent of the personal computer). The access method was tested even earlier: in the second half of the 60s, the University of Hawaii radio network used various options for random access to the general radio environment, collectively called Aloha. In 1980, DEC, Intel, and Xerox jointly developed and published the Ethernet Version II standard for a network built over coaxial cable. Therefore, the Ethernet standard is sometimes called the DIX standard after the capital letters of the company names.

Rice. 3. LLC level primitives a, b, c - without connection, d - with connection

Based on the Ethernet DIX standard, the IEEE 802.3 standard was developed, which largely coincides with its predecessor, but there are still some differences. While the IEEE 802.3 standard distinguishes between the MAC and LLC layers, original Ethernet combines both of these layers into a single data link layer. Ethernet defines a configuration test protocol (Ethernet Configuration Test Protocol), which is not found in IEEE 802.3. The frame format is also somewhat different, although the minimum and maximum frame sizes in these standards are the same.

Depending on the type of physical medium, the IEEE 802.3 standard has various modifications - 10Base-5, 10Base-2, 10Base-T, 10Base-F.

Manchester code is used to transmit binary information over the cable for all variants of the physical layer of Ethernet technology.

All types of Ethernet standards use the same method of separating the data transmission medium - the CSMA/CD method.

4.1. Access method csma/cd

Ethernet networks use a method of accessing the data transmission medium called carrier-sense-multiply-access with collision detection (CSMA/CD).

This method is used exclusively in networks with a common bus (which includes the radio networks that gave rise to this method). All computers on such a network have direct access to a common bus, so it can be used to transfer data between any two network nodes. The simplicity of the connection scheme is one of the factors that determined the success of the Ethernet standard. It is said that the cable to which all stations are connected operates in multiple access (multiply-access, MA).

All data transmitted over the network is placed in frames of a certain structure and provided with a unique address of the destination station. The frame is then transmitted over the cable. All stations connected to the cable can recognize the fact of frame transmission, and the station that recognizes its own address in the frame headers writes its contents to its internal buffer, processes the received data and sends a response frame along the cable. The source station's address is also included in the original frame, so the destination station knows who to send the response to.

With the described approach, a situation is possible when two stations simultaneously try to transmit a data frame over a common cable (Fig. 3). To reduce the likelihood of this situation, immediately before sending a frame, the transmitting station listens to the cable (that is, receives and analyzes the electrical signals occurring on it) to detect whether a data frame from another station is already being transmitted along the cable. If carrier-sense (CS) is recognized, then the station postpones the transmission of its frame until the end of someone else’s transmission, and only then tries to transmit it again. But even with this algorithm, two stations can simultaneously decide that there is no transmission on the bus at a given moment in time, and begin to simultaneously transmit their frames. They say what happens collision, since the contents of both frames collide on a common cable, which leads to information distortion.

To correctly handle a collision, all stations simultaneously monitor the signals appearing on the cable. If the transmitted and observed signals differ, then the collision detection (CD). To increase the likelihood of immediate detection of a collision by all stations on the network, the collision situation is strengthened by sending a special sequence of bits to the network by stations that have begun transmitting their frames, called jam sequence.

Once a collision is detected, the transmitting station must stop transmitting and wait for a short, random time interval, and then may attempt to transmit the frame again.

From the description of the access method it is clear that it is probabilistic in nature, and the probability of successfully obtaining a common medium at its disposal depends on the network load, that is, on the intensity of the need for frame transmission in stations. When developing this method, it was assumed that the data transfer rate of 10 Mb/s is very high compared to the needs of computers for mutual data exchange, so the network load will always be small. This assumption often remains true today, but real-time multimedia applications have emerged that require much higher data rates. Therefore, along with classic Ethernet, the need for new high-speed technologies is growing.

The CSMA/CD method defines the basic timing and logical relationships that guarantee the correct operation of all stations in the network:

A pause of 9.6 µs must be maintained between two frames of information sequentially transmitted over a common bus; this pause is needed to restore the network adapters of the nodes to their original state, as well as to prevent exclusive seizure of the data transmission medium by one station.

When a collision is detected (the conditions for its detection depend on the physical medium used), the station issues a special 32-bit sequence (jam-sequence) to the medium, which enhances the collision phenomenon for more reliable recognition by all network nodes.

After a collision is detected, each node that was transmitting a frame and encountered a collision attempts to retransmit its frame after some delay. The node makes a maximum of 16 attempts to transmit this frame of information, after which it refuses to transmit it. The delay value is chosen as a uniformly distributed random number from an interval whose length increases exponentially with each trial. This algorithm for choosing the delay value reduces the likelihood of collisions and reduces the intensity of sending frames to the network when it is heavily loaded.

Rice. 3. Scheme of collision occurrence in the CSMA/CD random access method (tp - signal propagation delay between stations A and B)

Clear recognition of collisions by all network stations is a necessary condition for the correct operation of the Ethernet network. If any transmitting station does not recognize a collision and decides that it transmitted the data frame correctly, then this data frame will be lost, since the frame information will be distorted due to the overlap of signals during a collision, it will be rejected by the receiving station (most likely due to checksum mismatch). Of course, most likely the corrupted information will be retransmitted by some upper-level protocol, for example, a transport or application protocol that works with connection establishment and numbering of its messages. But retransmission of the message by upper-level protocols will occur after a much longer time interval (tens of seconds) compared to the microsecond intervals that the Ethernet protocol operates. Therefore, if collisions are not reliably recognized by Ethernet network nodes, this will lead to a noticeable decrease in the useful throughput of this network.

All parameters of the Ethernet protocol are selected in such a way that during normal operation of network nodes, collisions are always clearly recognized. It is for this that the minimum length of the frame data field must be at least 46 bytes (which, together with service fields, gives a minimum frame length of 72 bytes or 576 bits). The length of the cable system is selected in such a way that during the transmission of a frame of minimum length, the collision signal would have time to propagate to the farthest node in the network. Therefore, for a data transfer rate of 10 Mb/s, used in Ethernet standards, the maximum distance between any two network nodes should not exceed 2500 meters.

As the frame rate increases, which is the case with new standards based on the same CSMA/CD access method, such as Fast Ethernet, the maximum network length decreases in proportion to the increase in transmission rate. In the Fast Ethernet standard it is 210 m, and in Gigabit Ethernet it is limited to 25 meters.

Regardless of the implementation of the physical medium, all Ethernet networks must satisfy two access method restrictions:

the maximum distance between any two nodes should not exceed 2500 m,

there should not be more than 1024 nodes in the network.

In addition, each physical environment option adds its own restrictions to these constraints, which must also be met.

Let us clarify the basic parameters of the operations of transmitting and receiving Ethernet frames, briefly described above.

A station that wants to transmit a frame must first use the MAC node to pack the data into a frame of the appropriate format. Then, to prevent signals from mixing with those of another transmitting station, the MAC node must listen for electrical signals on the cable and, if it detects a 10 MHz carrier frequency, delay transmitting its frame. After the end of cable transmission, the station must wait a short additional pause, called interframe gap, which allows the destination node to receive and process the transmitted frame, and then begin transmitting its frame.

Simultaneously with the transmission of frame bits, the node's transmitting and receiving device monitors the bits received over the common cable in order to detect a collision in time. If a collision is not detected, then the entire frame is transmitted, after which the MAC level of the node is ready to receive the frame from the network or from the LLC level.

If a collision is detected, the MAC node stops transmitting the frame and sends a jam sequence that reinforces the collision condition. After sending a jam sequence to the network, the MAC node makes a random pause and re-attempts to transmit its frame.

In case of repeated collisions, there is a maximum possible number of frame retransmission attempts (attempt limit), which is equal to 16. When this limit is reached, a frame transmission error is recorded, the message of which is transmitted to the upper-level protocol.

In order to reduce the intensity of collisions, each MAC node randomly increases the duration of the pause between attempts with each new attempt. The time schedule for the duration of the pause is determined based on truncated binary exponential backoff algorithm. The pause is always an integer number of so-called grace intervals.

Delay interval (slot time)- this is the time during which the station is guaranteed to know that there is no collision in the network. This time is closely related to another important network timing parameter - collision window. The collision window is equal to the time it takes for a signal to travel twice between the most distant network nodes - the worst-case delay case at which the station can still detect that a collision has occurred. The delay interval is chosen equal to the collision window plus some additional delay value to guarantee:

backoff interval = collision window + additional delay

In 802.3 standards, most time intervals are measured in the number of interbit intervals, the value of which for a bit rate of 10 Mb/s is 0.1 μs and is equal to the transmission time of one bit.

The backoff interval in the 802.3 standard is defined as 512 bit intervals, and this value is calculated for a maximum coaxial cable length of 2.5 km. The value 512 also determines the minimum frame length of 64 bytes, since with frames of shorter length a station may transmit a frame and not have time to notice the fact that a collision has occurred due to the fact that signals distorted by the collision will reach the station in the worst case after the transmission is completed. Such a frame will simply be lost.

The pause time after the Nth collision is assumed to be equal to L delay intervals, where L is a random integer uniformly distributed in the range . The size of the range increases only up to the 10th attempt (remember that there cannot be more than 16), and then the range remains equal, that is, . The values ​​of the main parameters of the 802.3 frame transmission procedure are given in Table 1.

Table 1.

Given the above parameters, it is not difficult to calculate the maximum performance of an Ethernet segment in units such as the number of transmitted packets of minimum length per second (packets-per-second, pps). The number of Ethernet packets processed per second is often used to indicate the internal performance of bridges and routers, which introduce additional latency between nodes. Therefore, it is interesting to know the net maximum performance of an Ethernet segment in an ideal case where there are no collisions on the cable and no additional delays introduced by bridges and routers.

Since the size of the minimum length packet including the preamble is 64+8 = 72 bytes or 576 bits, its transmission takes 57.6 μs. By adding the interframe interval of 9.6 μs, we obtain that the period of the minimum packets is 67.2 μs. This corresponds to the maximum possible Ethernet segment throughput of 14880 p/s.