ethernet
(read ethernet, from lat. aether - ether) - packet data transfer technology, mainly local
.
Ethernet standards define wired connections and electrical signals on physical level, format
frames and media access control protocols - at the data link layer of the OSI model. ethernet basically
described by the IEEE 802.3 group standards. Ethernet has become the most common LAN technology in the middle
90s of the last century, replacing such obsolete technologies as Arcnet, FDDI and Token ring.
History of creation
Ethernet technology was developed along with many of the first projects of Xerox PARC Corporation.
It is generally accepted that Ethernet was invented on May 22, 1973, when Robert Metcalfe
wrote a memo to 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 pamphlet titled "Ethernet: Distributed Packet-Switching For Local Computer Networks".
Metcalfe left Xerox in 1979 and founded 3Com to promote 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
rivalry with two major patented technologies, token ring and ARCNET, which were soon buried under the oncoming waves of Ethernet products. In the process of struggle, 3Com has become a major company in this industry.
The standard of the first versions (Ethernet v1.0 and Ethernet v2.0) states 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:
- the ability to work in duplex mode;
- low cost of twisted pair cable;
- higher reliability of networks in the event of a cable failure;
- greater noise immunity when using a differential signal;
- the ability to power over the cable low-power nodes, such as 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 network cards, and sometimes even 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 network on ) - Carrier Sense Multiple Access and
collision detection (CSMA/CD, Carrier Sense Multiple Access with Collision Detection), bit rate
data 10 Mbps, packet size from 72 to 1526 bytes, data encoding methods are described. Working 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
physical layer can set more stringent restrictions, for example, to a thin coaxial segment
no more than 30 workstations can be connected, and no more than 100 workstations can connect to a thick coax segment). However
a network built on a single shared segment becomes inefficient long before reaching
node limit, mainly due to half-duplex operation.
In 1995, the IEEE 802.3u Fast Ethernet standard was adopted at 100 Mbps, and it became possible
full duplex operation. In 1997, the IEEE 802.3z Gigabit Ethernet standard was adopted at speeds
1000 Mbit/s for transmission over optical fiber and two years later for transmission over twisted pair.
Types of Ethernet
Depending on the data rate and transmission medium, there are several technology options.
Regardless of how the stack is passed network protocol and the programs work almost the same way in
all of the options listed below.
Most Ethernet cards and other devices support multiple baud rates,
using autodetection (autonegotiation) speed and duplex, to achieve the best
connections between two devices. If auto-detection does not work, the speed is adjusted to
partner, and the half-duplex transmission mode is turned on. For example, if the device has an Ethernet port
10/100 indicates that you can use 10BASE-T and 100BASE-TX technologies through it, and the port
Ethernet 10/100/1000 - Supports 10BASE-T, 100BASE-TX and 1000BASE-T standards.
Early modifications of Ethernet
- Xerox Ethernet - original technology, 3Mbps speed, existed in two versions Version 1 and Version 2, frame format latest version is still in widespread use.
- 10BROAD36 - not widely used. One of the first standards that allows you to work over long distances. Used a wideband modulation technology similar to that used
in cable modems. A coaxial cable was used as the data transmission medium. - 1BASE5, also known as StarLAN, was the first twisted-pair Ethernet technology. It worked at a speed of 1 Mbps, but did not find commercial use.
10 Mbps Ethernet
- 10BASE5, IEEE 802.3 (also called "Thick Ethernet") - the original development of technology with a data rate of 10 Mbps. Following an early IEEE standard, 50 ohm impedance (RG-8) coaxial cable is used, with a maximum segment length of 500 meters.
- 10BASE2, IEEE 802.3a (called "Thin Ethernet") - using 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 needs 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 using twisted pair for data transmission at a speed of 10 Mbps.
Later evolved into the 10BASE-T standard.
Although it is theoretically possible to connect more than
two devices operating in simplex mode, this scheme is never used for Ethernet, in
different from working with . Therefore, all twisted-pair networks use a star topology,
while, networks on coaxial cable are built on a "bus" topology. Terminators for work on
twisted-pair cables are built into each device, and it is not necessary to use additional external terminators on the line.
- 10BASE-T, IEEE 802.3i - 4-wire twisted-pair cable (two twisted pairs) category-3 or category-5 is used for data transmission. The maximum segment length is 100 meters.
- FOIRL - (an acronym for the English Fiber-optic inter-repeater link). The base standard for Ethernet technology that uses an optical cable to transmit data. The maximum transmission distance without a repeater is 1 km.
- 10BASE-F, IEEE 802.3j - The main term for a family of 10 Mbps ethernet standards using optical cable at a distance of up to 2 kilometers: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of these, only 10BASE-FL has become widespread.
- 10BASE-FL (Fiber Link) - Improved version of the FOIRL standard. The improvement concerned an increase in the length of the segment up to 2 km.
- 10BASE-FB (Fiber Backbone) - Now an unused standard, it was intended to combine repeaters into a backbone.
- 10BASE-FP (Fiber Passive) - A passive star topology that does not need repeaters - has never been used.
Fast Ethernet (Fast Ethernet, 100 Mbps)
- 100BASE-T is a generic 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 is used, in fact only two unshielded pairs of conductors are used, duplex data transmission is supported, distance up to 100 m.
- 100BASE-T4 is a category 3 twisted-pair standard. All four pairs of conductors are used, data transmission is in half duplex. Practically not used.
- 100BASE-T2 is a category 3 twisted-pair standard. Only two pairs of conductors are used. Full duplex is supported, with signals propagating in opposite directions on each pair. The transfer rate in one direction is 50 Mbps. Practically not used.
- 100BASE-SX is a standard that uses 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 only limited
the amount of attenuation in the optical cable and the power of transmitters, according to different materials from 2x to 10
kilometers - 100BASE-FX WDM is a standard using single mode fiber. The maximum length is only limited
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 one Latin
letter A(1310) or B(1550). Only paired interfaces can work in pairs: on the one hand, the transmitter
at 1310 nm, and on the other - at 1550 nm.
- 1000BASE-T, IEEE 802.3ab is a standard using category 5e twisted pair. 4 pairs are involved in data transmission. Data transfer rate - 250 Mbit / s over one pair. The PAM5 coding method is used, the fundamental frequency is 62.5 MHz. Distance up to 100 meters
- 1000BASE-TX was created by the Telecommunications Industry Association (Telecommunications
Industry Association, TIA) and published in March 2001 as "Physical Layer Specification
duplex Ethernet 1000 Mbps (1000BASE-TX) Category 6 balanced cabling
(ANSI/TIA/EIA-854-2001)" (Eng. "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 transmission and reception (one pair in each direction), which greatly simplifies the design
transceiver devices. Another significant difference in 1000BASE-TX is the lack of circuitry
digital interference compensation and return noise, resulting in complexity, power consumption
and the price of processors becomes lower than those of 1000BASE-T processors. But, as a consequence, for
stable operation of this technology requires a cable system High Quality, so 1000BASE-TX
Can only use Category 6 cable. Based this standard almost never created
products, although 1000BASE-TX uses a simpler protocol than the 1000BASE-T standard and therefore may
use simpler electronics. - 1000BASE-X is a generic term for standards with pluggable GBIC or SFP transceivers.
- 1000BASE-SX, IEEE 802.3z is a standard using multimode fiber. Travel range
signal without repeater up to 550 meters. - 1000BASE-LX, IEEE 802.3z is a standard using single-mode fiber. Travel range
signal without a repeater up to 5 kilometers.
used.- 1000BASE-CX - standard for short distances (up to 25 meters) using twinax cable
with a wave impedance of 75 ohms (each of the two waveguides). Superseded by 1000BASE-T and no longer
used. - 1000BASE-LH (Long Haul) is a standard using single-mode fiber. Travel range
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), uses CX4 copper cable and InfiniBand connectors.
- 10GBASE-SR - 10 Gigabit Ethernet technology for short distances (up to 26 or 82 meters, depending on
depending on cable type), multimode fiber is used. It also supports distances up to 300
meters using a new multimode fiber (2000 MHz/km). - 10GBASE-LX4 - uses wavelength multiplexing to support distances from 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 terms of speed and data format with 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.
With the advent of Ethernet, it began to be used as the main network connection. Let's take a closer look at the industrial applications of Ethernet.
What is Ethernet?
The local area network Ethernet (local-area network (LAN)) is the main one for communication between our computers, routers, printers. It has played an important role in the industrial world by becoming the established standard for IoT connectivity.
According to Cisco, in 2003 Ethernet made up about 85% of all LAN connections in the world. Industrial Ethernet differs from commercial Ethernet in that it uses Ethernet standards to manage and operate industrial networks.
The advent of Ethernet
ALOHAnet was the first wireless network for data transmission to which several computer systems divided within the University of Hawaii. Scientists tried to get independent data transmission nodes over the air to communicate with each other based on peer-to-peer technology without interference. ALOHAnet's solution was Collision Detection Multiple Access (CSMA/CD). This idea inspired Bob Metcalfe of Xerox to do more research in this area.
In the early days of Ethernet, there were two most common configurations: 10Base2 and 10Base5. The data transfer rate for both configurations was 10 Mbps using coaxial cable.
The maximum allowable stub length for 10Base2 was 185 feet using RG58 coaxial cable, also known as “Thin Ethernet”. 10Base 5 offered longer communication distances. However, the connection required a thick coaxial cable that was heavy and difficult to manage.
With this in mind, new technologies such as 10Base-FL have continuously evolved, allowing networks to use fiber optic media and extend transmission distances up to 2,000 feet. 10Base-T has become a popular option due to its ease of installation and the use of inexpensive, unshielded twisted pair (UTP) CAT3 cable. The distance between computers should not exceed 100 meters and each machine should have a standard RJ-45 connector. By the 1990s, Ethernet equipment with transfer rates up to 100 Mbps became available.
Today's computer standard implies that the device must have a network adapter that implements 100Base-TX. Category 5e UTP (CAT5) cables are also standard and use the same lengths as for 10base-T networks up to 100 feet or less. Networks that previously used coax cables are now being upgraded to fiber optics specifically for point-to-point communications. 100Base-FX uses two optical fibers for duplex point-to-point links that reach 2000 feet. Gigabit Ethernet or 1000 Mbps connections are available using twisted pair and fiber optic media.
Ethernet link layer
Ethernet defines physical layer layers and data transmission channels depending on the purpose of the network. IEEE 802.3 has become the main networking standard. the physical layer defines electrical signals, data transfer method, media, connector types, and network topology. Optical fiber or twisted pair can be used for data transmission. There are four various types data transfer at different speeds:
The link layer defines the media access method. Half-duplex communication is coupled in bus or star topologies: 10/100Base-T, 10Base2, 10Base 5 and others. They use Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This allows multiple hosts (computers) to have equal access to the network. All nodes in the Ethernet network are constantly monitoring the transmission of information.
The nodes wait for a response from the network before starting to transmit data. When they start transmitting data at the same time, the signals overlap each other, which may damage the originals. When the node detects that another device is also trying to send data, it detects a collision and stops transmitting data. An attempt to resume transmission is made after a certain interval. This method of data transfer makes it easy to add or remove nodes from the network.
After connecting, the node begins to receive and transmit information over the network. However, this can eventually lead to a decrease in throughput and an increase in the number of collisions. This makes Ethernet a probabilistic network. On point-to-point duplex Ethernet networks such as 10Base-FL or 100Base-FX, collisions are not a problem. This is due to the fact that there are only two nodes with the possibility of separate transmission and reception of data. This allows for simultaneous transmission and reception of information, which doubles the transmission rate.
The Ethernet frame defines the format of the data message sent over the network. The message format contains several fields of information, including the data to be transmitted. A data block is defined as the actual data to be sent and can contain between 46 and 1500 bytes of binary information. The data block length is determined and included in the message as a field for the receiver to determine how much of the message is data.
A MAC address is a six byte binary set number that includes source and destination information for nodes. The MAC address is included in every message and transmitted through the network, and each node on an Ethernet network has a unique MAC address.
The link layer defines the frame structure of received or transmitted messages. Switches, nodes and hubs can be added or removed from the network with ease, and this technology makes troubleshooting easy. These factors have made Ethernet connections the new standard for industrial networking solutions. The functions of the OSI layers indicate how the information will be transmitted.
There are seven layers in the OSI reference model. The bottom layers (1-4) focus on data communication, while 5-7 are applied. The lower level (1) is closest to the physical environment, so it is called the physical. The physical and lower layers of the data link are implemented in hardware and software, such as cables or Ethernet (layer 2).
Layer 3 is used for logical addressing and routing. The most common application is the use of the Internet Protocol (IP). Layer 4 is the transport layer, which ensures that data is transmitted without errors and in the correct sequence. It uses the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) to transfer data. Industrial Ethernet differs from commercial Ethernet in that it uses all the lower layers, not just 2.
The upper layers of the OSI reference model are used for applications and are generally only implemented for software. Layer 5 for session management. It is responsible for dialing and synchronizing session connectivity (that is, creating and managing sessions) between networks and applications.
Layer 6 is for using data representation. This layer represents the data and encoding type, and defines the symbols used. This ensures that data can be transferred across the network and between nodes, and that it is compressed and encoded. Layer 7 is for application use. It is used software for data preparation and interpretation. This is the topmost layer, closest to the user.
Connection types for Ethernet and industrial systems
The TCP/IP protocol, using Ethernet, makes it possible to increase the level of standardization. Historically, time-critical network applications have used deterministic networks. When using industrial Ethernet, it is important to keep in mind the speed and stability of the connection.
Determinism is the ability of a network to communicate over a forecast period. For motion control systems, this is essential, as data transfer from and to the device must be carried out on a regular basis. These networks are based on the concept of master/slave or handover.
The use of Ethernet networks must be controlled at a level of no more than 10% or they will have insufficient performance. Network segmentation using routers and switches minimizes unwanted network traffic and reduces its consumption. Another way involves the use of new protocols (more high levels) combining prioritization and message synchronization to optimize the delivery time of information.
The result of these methods was the transition to the use of Ethernet for industrial control at the level of shops and sites. Ethernet is becoming more and more popular in industrial environments due to its low hardware cost and ease of installation. The use of bridges and high-speed switches increases network determinism. As a result, data rates of 1 Gbps, 10 Gbps, 100 Gbps are becoming more common.
Basic types of Ethernet connection
Modbus TCP/IP
The first industrial protocol based on Ethernet, introduced in 1999. Implemented based on the Modbus protocol, which was developed by Modicon in 1979.
Advantages:
- Standard Ethernet layers are used: TCP/IP hardware and transport layer;
- Open and relatively simple protocol;
Disadvantages:
- Not a hard real-time protocol;
Major supplier: Schneider Electric.
Factory automation technology: RTPS
EtherCAT
open source code, based on IEC 61158 and other similar standards. Advantages:
- Rigid real-time industrial protocol;
- Effective and simple communication;
Disadvantages:
- The total number of devices used is limited;
- Not intended for standard TCP/IP and EtherCAT packets;
Major supplier: Beckhoff.
Factory Automation Technology: Shared Frame
Ethernet/IP
Extends the DeviceNET concept.
Advantages:
- Use of transport layers of Ethernet networks (i.e. TCP and UDP);
Disadvantages:
- Networks can be overloaded with UDP messages if the settings are not correct;
Major supplier: Rockwell Automation.
Factory automation technology: CIP
Profinet
An application protocol that extends Profibus.
Advantages:
- Supports both standard and deterministic Ethernet traffic;
- Implements IEEE 1588 and Quality of Service (QoS) to add determinism;
Disadvantages:
- Real-time and isochronous real-time controlled by switches. QoS recommended;
Major supplier: Siemens.
Factory automation technology: PROFINET IO
FEDERAL AGENCY FOR EDUCATION
GOU VPO "MOSCOW STATE INSTITUTE
RADIO, ELECTRONICS AND AUTOMATION
(Technical University)"
Dagestan branch
Department of Computer Engineering
Course work
in the discipline "Computer Networks"
on the topic:
"The local network ethernet "
Performed: 4th year student
specialties VMKSiS
Isaeva P. M.
Checked: Feylamazova S. A.
Makhachkala 2011
1. Introduction…………………………………………….……………2
2. History of Ethernet……………………………………………………3
3. Ethernet networks…………………………………………………..…6
4. Servers……………………………………………………….....11
5. Equipment for local networks…………………………..15
6. Network topology……………………………………………….....16
7. General characteristics of local area networks....22
8. Ethernet security of the local network………………………...26
9. Bridges and commutations……………………………………...........29
10. Varieties of Ethernet…………………………………...32
11. Standardization…………………………………………...33
12. Conclusion………………………………………………..34
13. List of used literature………………………35
INTRODUCTION
Computer networks, also called computer networks, or data transmission networks, are the logical result of the evolution of the two most important scientific and technical branches of modern civilization - computer and telecommunication technologies. On the one hand, networks are a special case of distributed computing systems in which a group of computers performs a set of interrelated tasks in a coordinated manner, exchanging data in automatic mode. On the other hand, computer networks can be considered as a means of transmitting information over long distances, for which they use data coding and multiplexing methods that have been developed in various telecommunication systems.
Late 90s of the last century revealed a clear leader among LAN technologies - the Ethernet family, which included the classic 10 Mbps Ethernet technology, as well as Fast Ethernet 100 Mbps and Gigabit Ethernet 1000 Mbps. Simple operation algorithms predetermined the low cost of Ethernet equipment. A wide range of the hierarchy of speeds allows you to rationally build a local network, using the technology of the family that best meets the tasks of the enterprise and the needs of users. It is also important that all Ethernet technologies are very close to each other in terms of operation, which simplifies the maintenance and integration of these networks.
The relevance of this work is due to the importance of studying local computer systems for students of technical specialties as one of the cornerstone concepts of the subject "Computer Networks".
The aim of the work is to study the characteristics and features of the Ethernet local area network.
In accordance with the purpose of the work, the following tasks were set: definition of the concept of "local computer network”, a description of the main methods of building networks (network topology), a brief description of the main network protocols that provide consistent user interaction on the network, the study of such local area network technologies as Ethernet, Token Ring, FDDI, Fast and Gigabit Ethernet.
History of Ethernet
Ethernet technology was developed along with many of the first projects of Xerox PARC Corporation. It is generally accepted that Ethernet was invented on May 22, 1973, when Robert Metcalfe ( Robert Metcalfe) wrote a memo to the head of PARC on the potential of Ethernet technology. But Metcalfe got the legal right to the technology a few years later. In 1976, he and his assistant David Boggs published a pamphlet called Ethernet: Distributed Packet-Switching For Local Computer Networks.
Metcalfe left Xerox in 1979 and founded 3Com to promote computers. He managed to convince DEC, Intel and Xerox to work together and develop the Ethernet standard (DIX). This standard was first published on September 30, 1980. It began a rivalry with two major proprietary technologies, token ring and ARCNET, which were soon buried under the oncoming waves of Ethernet products. In the process of struggle, 3Com has become a major company in this industry.
The standard of the first versions (Ethernet v1.0 and Ethernet v2.0) states that coaxial cable is used as the transmission medium, later it became possible to use twisted pair and optical cable.
The reasons for switching to twisted pair were:
Ability to work in duplex mode;
Low cost twisted pair cable;
Higher network reliability in the event of a cable failure;
Greater noise immunity when using a differential signal;
Ability to power over cable low-power nodes, such as 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 a coaxial cable) - multiple access with carrier sense and collision detection (CSMA / CD, Carrier Sense Multiple Access with Collision Detection), data rate 10 Mbps, packet size from 72 to 1526 bytes, are described data encoding methods. The mode of operation is half-duplex, that is, the node cannot simultaneously transmit and receive information. The number of nodes in one shared network segment is limited to a limit of 1024 workstations (physical layer specifications may set more stringent restrictions, for example, no more than 30 workstations can connect to a thin coax segment, and no more than 100 to a thick coax segment). However, a network built on a single shared segment becomes inefficient long before reaching the limit of 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 Mbps was adopted and it became possible to work in full duplex mode. In 1997, the IEEE 802.3z Gigabit Ethernet standard was adopted at 1000 Mbps for transmission over optical fiber, and two years later for transmission over twisted pair.
Ethernet is an evolving technology. The evolution has included higher bandwidth, improved media access methods, and changes to the physical environment. Ethernet has become complex network technologies which underlies most local area networks today. Coaxial cable has been replaced with point-to-point connected Ethernet repeaters or switches to reduce installation costs, improve reliability, and allow point-to-point management and troubleshooting. There are many Ethernet options in common use.
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 48-bit MAC address. MAC addresses are used to identify and determine the destination and source of each data packet. Cards network interface(NIC) or chips usually do not accept packets addressed to other Ethernet locations. Adapters come programmed with a globally unique address. Despite significant changes in Ethernet from the thickness of the coax bus cable running at 10 Mbps to point-to-point "working at 1 Gbps and beyond, all generations of Ethernet (with the exception of the early experimental version) use the same frame format (and , hence the same interface for higher layers), and can be easily bridged between each other.
Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it, and the limited panel space needed for twisted-pair Ethernet, most manufacturers now build a functional Ethernet card directly into computer boards, eliminating the need for a separate NIC.
Ethernet networks
When creating local networks, a hardware architecture called Ethernet is most often used. In its simplest form, an Ethernet network consists of a single cable, to which all network nodes are connected using connectors, connectors and transceivers. A simple Ethernet network is relatively inexpensive, which, combined with transfer rates of 10, 100, and even 1000 Mbps, greatly contributes to its popularity.
There are three types of Ethernet, conventionally called thick, thin and twisted pair. When using thin and thick Ethernet, data is transmitted through coaxial cables that differ in diameter and in the way they are connected to the computer. To connect a computer to a thin Ethernet cable, a special T-shaped connector (T-connector) is used, which is inserted into a break in the cable and plugs into a connector on the back of the computer. To connect a computer to a thick Ethernet cable, you need to drill a small hole in the cable and use a special piercing tool (vampire tap) to connect an auxiliary transceiver cable to it. One or more network nodes can be connected to the transceiver cable. A thin Ethernet cable can reach 200 meters in length, and a thick one can reach 500 meters. These varieties of Ethernet are called 10base-2 and 10base-5, respectively. The base link comes from the term "baseband modulations", which means that data is transmitted directly to the cable, bypassing the modem. The number at the beginning specifies the speed in Mbps, and the number at the end specifies the maximum cable length in hundreds of meters. When using twisted pair, a cable is used that consists of two pairs of copper wires. This usually requires the installation of an additional device called an active hub. Twisted pair is referred to as 10base-T (T - twisted pair, that is, "twisted pair"). For twisted pairs with a transmission rate of 100 Mbps, the designation 100base-T is used.
Ethernet technology is based on a monochannel. Those. it is a network with selection of information. Initially, the whole technology was developed for local networks connecting computers at a distance of 10-100 m. Now Ethernet technology allows you to build communication subnets that connect computers at a distance of 40 km.
ethernet ( ether- ether net- network). Where did this name come from? The technology behind Ethernet networks was originally developed for radio networks.
Early networks used a fixed transmission medium for transmission - coaxial cable, twisted pair.
When they say Ethernet, it usually means any of the variants of this technology. In a narrower sense, Ethernet is a networking standard based on the technologies of the experimental Ethernet Network, which Xerox developed and implemented in 1975 (even before the advent of personal computer). The access method was tested even earlier: in the second half of the 60s, the radio network of the University of Hawaii used various options for random access to a common radio environment, collectively called Aloha. In 1980, DEC, Intel, and Xerox jointly developed and published the Ethernet version II standard for a coaxial cable network. For this reason, the Ethernet standard is sometimes referred to as the DIX standard after the capital letters of the company names.
Based on the Ethernet DIX standard, the IEEE 802 - Institute of Electrical and Electronics Engineers (Institute of Electrical and Electronics Engineers) committee created a standard that describes networks - monochannels that operate on the same principle as Ethernet networks.
There are certain differences between the IEEE 802 standard and the original description of Ethernet. These differences relate to the frame format, some features of the protocols. These differences arose due to the fact that the DIX association, after the creation of the initial protocol, continued to work on improving transmission rates, increasing reliability. At the same time, the developers of the 802 standard followed commercial developments. In many points, the descriptions of Ethernet and IEEE 802 are the same. Therefore, with a slight correction, we can say that they are one and the same.
Why talk about a set of standards? The 802 group worked not only for standards for single-channel networks such as Ethernet, but also for cyclic networks, and now creates and develops standards for modern networks. In particular, 802.11 - WI-FI, 802.16 - WI-MAX. New standards are currently being developed.
The 802 standards set describes 2 layers: physical and channel. Moreover, the channel is divided into 2 levels: the lower one is level 2a and the upper one is level 2b.
Level 2a is the Media Access Control (MAC) level. It describes the features of access to networks with specific types of distribution media and various types access.
Layer 2b is the Logical Link Control (LLC) layer. It localizes functions common to all networks.
How are Ethernet networks designed and operated?
As we have already said, this is a mono channel, which, however, can be implemented in different ways.
There is a whole family of specifications that describe the operation of Ethernet networks in different transmission media. Initially, Ethernet networks based on a thick coaxial cable were described. A special device was connected to it - a transceiver (transmitter + receiver).
The transceiver is part of the network adapter that performs the following functions:
1) receiving and transmitting data from cable to cable,
2) detection of collisions on the cable,
3) electrical isolation between the cable and the rest of the adapter,
4) protection of the cable from incorrect operation of the adapter.
Through this device, a connection is made to the network adapter of the computer. Stations are connected through a certain fixed distance. On both sides of the coaxial cable, special plugs are installed - terminators.
This scheme For a long time it was the only one in existence. She scheme is described by the specification 10Base-5 . This technology was quite popular, but also expensive.
The network could consist of several such segments - several monochannels connected by repeaters (amplifiers), which, receiving frames from one port, amplified the signals and transmitted them further.
Thus, 10Base-5 is a 0.5-inch coaxial cable, called "thick" coax. It has a wave impedance of 50 ohms. The maximum segment length is 500 meters (without repeaters).
The advantages of the 10Base-5 standard include:
1) good protection of the cable from external influences,
2) a relatively large distance between nodes,
3) the ability to easily move workstation within the length of the AUI cable.
The disadvantages include:
1) the high cost of the cable,
2) the complexity of its laying due to its high rigidity,
3) the presence of a special tool for terminating the cable,
4) if the cable is damaged or the connection is bad, the entire network stops working,
5) it is necessary to foresee the cable connection to all possible places for installing computers
The next stage is the creation of networks based on a thin coaxial cable. Here, the transceiver functions were transferred to network adapters, and the cable is connected to the computer according to a simpler scheme.
The corresponding specification is called 10Base-2 .
10Base-2 is a 0.25 inch coaxial cable called "thin" coax. It has a wave impedance of 50 ohms. The maximum segment length is 185 meters (without repeaters).
Thus, the 10 in the name denotes the data bit rate of these standards - 10 Mb / s, and the word Base - short for baseband - a method of transmitting at a single base frequency of 10 MHz (as opposed to standards that use several carrier frequencies, which are called broadband - broadband).
The next stage of development is the use of unshielded twisted pair (UTP) and a network based on a centralized structure.
The schemes considered above have rather low reliability. It is enough to break at least in one place, the entire network fails.
Hub works the same as a repeater. If a station wants to transmit information to one of the stations connected to the Hub(s), it forms a frame indicating the recipient's address, this frame is transmitted over a twisted pair cable to the Hub. Each station has its own separate port. The frame received by the Hub is then relayed to all other ports. Those. the logic of work remains the same - a monochannel is a network with information selection.
This solution is 10Base-T standard .
One version of the explanation of the letter T in the name suggests that at the initial stage of creating twisted-pair networks, different organizations and offices used existing telephone lines to connect a computer to one Hub (y).
Networks built on the basis of the 10Base-T standard have many advantages over coaxial Ethernet options. These advantages are associated with the division of a common physical cable into separate cable segments connected to a central communication device. And although logically these segments still form a common collision domain, their physical separation allows you to control their state and turn it off in case of a break, short circuit or network adapter failure on an individual basis. This circumstance greatly facilitates the operation of large Ethernet networks, since the hub usually automatically performs such functions, while notifying the network administrator of the problem.
10Base-F standard uses fiber optics as a data transmission medium. Functionally, a 10Base-F network consists of the same elements as a 10Base-T network - network adapters, a multiport repeater, and cable segments connecting the adapter to the repeater port. As with twisted pair, two fibers are used to connect the adapter to the repeater - one connects the Tx output of the adapter to the Rx input of the repeater, and the other connects the Rx input of the adapter to the Tx output of the repeater.
CSMA/CD method (IEEE 802.3)
Carrier-Sense-Multiply-Access with Collision Detection
Carrier Sense Multiple Access with Collision Detection
This method describes the logic of operation of monochannels with selection.
Quite often, in the description of this method, there are flowcharts of this kind.
Structural scheme CSMA/CD algorithm (MAC layer): when transmitting a frame by a station
Block diagram of the CSMA / CD algorithm (MAC layer): when a frame is received by the station
The name of the method stands for - Multiple access with carrier sense and collision detection.
Multiple access means that all stations connected to a mono channel are equal. How is the transmission managed? There is no centralized control, no special point from which control would be carried out. The network management function is distributed to all stations. Each station implements its own part of the overall algorithm.
Suppose some station wants to transmit a frame (frame). It contains the addresses of the recipient and the sender in the header, and the packet is stored in the information part. Inside the information part of the packet, a message is stored, in the information part of which, in turn, is stored, for example, an http request.
Can the transmission start? Purely theoretically - maybe. On the other hand, a situation may also arise when the channel is busy, i. some other station is already transmitting. Therefore, stations wishing to start transmission first analyze whether the channel is free or busy. Those. perform an operation "carrier listening". If the carrier is recognized (carrier-sense, CS), 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.
If the channel is free, the station starts transmission. All other stations that can also transmit listen to the channel status. And as soon as they discover that a transmission has begun, they begin receiving the transmitted signal, from which they collect 0 and 1. From 0 and 1, they already collect either the entire frame or its header and analyze it. Each station determines by its header whether a frame is intended for it. 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 over the cable. The address of the source station is also included in the original frame, so the destination station knows to whom to send the response. If the frame is not intended for her, then the frame or its header (depending on what has already been received) is erased and further reception is impossible.
The station that transmits the frame also receives and analyzes it. If the received signal coincides with the transmitted one, then this indicates that the same signal that this station transmits passes in the channel, and no one else interferes in this process, no distortion occurs. If this remains true until the end of the transmission, then the frame is considered to have been transmitted.
In order to correctly handle a collision, all stations simultaneously monitor the signals that appear on the cable. If the transmitted and observed signals differ, then a collision detection (CD) is detected. To increase the probability of immediate detection of a collision by all stations in the network, the collision situation is enhanced by sending to the network by stations that have begun transmitting their frames a special bit sequence called a jam sequence.
In various sources, there is a comparison of this CSMA / CD method with a conversation of several people in a dark room. There is no light, no one sees each other. Someone starts talking, everyone else is silent and listens. Or suddenly two people start talking at the same time. Naturally, they begin to interrupt each other and fall silent.
Upon detecting a collision, the transmitting station is required to stop transmitting and wait for a short random time interval, and then may attempt to transmit the frame again.
Theoretically, it could happen that they will wait for the same time and start simulcast again, again causing a collision. In order to minimize the likelihood of such situations, it was proposed to implement binary exponential delay algorithm .
After a collision occurs, time is divided into discrete intervals − delay intervals(slot time) 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, the collision window. Collision window is equal to the time of two round trips between the most remote network nodes, the worst case delay in which the station can still detect that a collision has occurred. The backoff interval is chosen to be equal to the value of the collision window plus some additional delay value to ensure:
backoff interval = collision window + additional delay
The backoff interval value in the 802.3 standard is defined as 512 bit intervals or 51.2 µs, 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 a shorter length, the station may transmit the frame and not have time to notice the occurrence of a collision due to the fact that the signals distorted by the collision will reach the station in worst case after the transfer is completed. Such a frame will simply be lost.
After the first collision, each station waits either 0 or 1 slot before trying to transmit again. If two stations collide and choose the same pseudo-random number, they will collide again. After the second collision, each station randomly selects 0, 1, 2, or 3 slots from the set (2 2 slots) and waits again. At the third collision (the probability of such an event after a double collision is 1/4), the intervals will be selected in the range from 0 to 2 3 - 1.
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 value of the range grows only up to 10 attempts, and then the range remains equal to , that is, . After 16 collisions in a row, the controller admits defeat and returns an error to the computer. Further restoration is handled by higher levels.
|
After the system starts, it is in the listening state. Suppose there is a request to send a frame. The station enters the waiting state. If the channel is busy, then this wait may take a long time, or it may happen that the station immediately switches to the transmission state. It depends on whether the environment is busy. If the transmission is successful, there are no collisions, then the command "transmission completed" the station goes into the listening state. And if a collision occurs, then the station from the transmission state goes into the delay state, where the delay calculation is performed. At the end of the delay, when the "delay time expired" event occurs, the station goes back to the idle state. At the end of the reception, the "frame received" event occurs, which puts the station into the listening state. In the event of a collision at the reception, the station also goes into the listening state.
What is Ethernet and how does it work?
Ethernet is what most networks are based on these days. There are a large number of technologies that allow you to connect computers in 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 link layers. Further, we will only talk about the physical layer of the OSI model, i.e. about how data bits are transferred between two neighboring devices.
Currently, local area networks are built using technology Fastethernet, which is a new implementation of technology ethernet.
What is Ethernet
This technology was developed in 1970 by Xerox's Palo Alto Research Center and adopted the IEEE 802.3 specification in 1980.
The basic principle of operation used in this technology is as follows. In order to start transferring data on the network, the computer's network adapter "listens" on the network for any signal. If it is not there, then the adapter starts data transmission, if there is a signal, then the transmission is delayed for a certain period of time. The time of exclusive use of the shared environment by one node is limited to the time of transmission of one frame.
Frame - 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 accept it. Each adapter analyzes the frame, and if the address matches their own device address (MAC address), the frame is placed in the network adapter's internal buffer, if it does not match, then 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 transmitting data, and then, having “listened” to the network again, repeat the data transmission at different intervals.
? NOTE. To receive a data packet that is intended for a particular adapter, it must accept all packets that appear on the network.
This method of access to 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 an intensive exchange of information, the number of collisions grows very quickly. and as a result, network throughput drops. 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 transfer rate is 70-80% of the nominal.
However, this problem has now been almost solved. Because they have developed devices that can share data streams between the computers for which this data is intended. In other words, traffic between ports connected to the transmitting 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 rates of 10, 100, and 1000 Mbps, respectively.
The IEEE 802.3 standard contains several specifications that differ in topology and the 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 for networking.
Ethernet implementations
The Ethernet specifications listed above can be described as follows. The first number in the name of the specification indicates the maximum data rate. For example, "10" indicates a signal transmission rate of 10 Mbps. "Base", means the use of Baseband technology in the standard. B aseband is a narrowband transmission. With this method of data transmission over a cable, each 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 name of the specification was intended to display the maximum length. Cable segment lengths in hundreds of meters. This is without hubs and switches. However, for convenience and a more complete definition of the essence of the standard, everything was simplified. And now its name has been replaced by the letters T and F. Where T stands for twistedpair- twisted pair, and F stands for optical fiber.
Thus, it is currently possible to meet networks based on the following specifications:
- 10Base-2 - 10 MHz Ethernet on 50 ohm coaxial cable, baseband. 10Base-2 is known as "thin Ethernet";
- 10Base-5 - 10MHz Ethernet 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 them to be used 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 the switches are directly connected to each other.
The essence of the full-duplex mode is the possibility of simultaneous transmission and reception of 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 Mbps.
On the this moment almost all manufacturers of network equipment declare the following. That their devices provide full-duplex operation. However, due to different interpretations of the standard, in particular the ways of managing the flow of personnel. It is not always possible to achieve the correct operation of these devices. And also good speed performance.
Found help - support the project!
It's easy to do this:
Share the link with your friends on social networks!
In contact with
