| §4.1 Local and global computer networks
Keywords:
Message
link
computer network
information transfer rate
the local network
global network
Earlier, we have already said that communication is one of the most important information processes . Information is transmitted from the source to the receiver in the form of some sequence of signals, symbols, signs. For example, during a direct conversation between people, sound signals are transmitted - speech; when reading a text, a person perceives graphic symbols - letters. The transmitted sequence of signals, symbols, signs is called a message.
A communication channel (transfer of information) is a system technical means and a signal propagation medium for transmitting messages from a source to a receiver. When people communicate directly, information is transmitted using sound waves, when talking on the phone - using acoustic and electrical signals distributed over communication lines, when reading - using light waves.
Any transformation of information coming from a source into a form suitable for its transmission over a communication channel is called coding. Currently, digital communication is widely used, when the transmitted information is converted into a binary code.
Insufficient technical quality communication channels and some other reasons can lead to distortion of the transmitted signal and loss of information. To avoid such situations, the code transmitted over the communication line is made redundant. Due to this, the loss of some part of the information during transmission can be compensated. Besides, in modern systems digital communication, all messages are divided into parts (packets, blocks). For each block, a checksum (the sum of binary digits) is calculated, which is transmitted along with this block. At the place of reception, the checksum of the received block is recalculated, and if it does not match the original amount, then the transmission of this block is repeated.
For centuries, mankind has used postal services to transmit letters; in the second half of the 19th century, sound transmission technology (telephone) was invented; Since the 1930s, telefax has been used to transmit images. Nowadays, for the transmission of texts, images, sound and many other types of information, computer networks- two or more computers connected by information transmission lines. With the advent of computer networks, it became possible to send a letter that arrives faster than a telegram, get an answer, find out the latest news, talk to a friend sitting at a computer hundreds of kilometers away, as if he were in the next room, book a plane ticket or a room in hotel, download desired program, ringtone or movie.
An important characteristic of a computer network is information transfer rate, or channel capacity. This value is defined as the amount of information in bits per second (bps) and in derived units: kilobits per second (1 Kbps = 1000 bps), megabits per second (1 Mbps = 1000 Kbps), gigabits per second (1 Gbps = 1000 Mbps).
Distinguish between local and global computer networks.
A local computer network unites computers installed in one room (for example, a school computer class) or in one building (for example, all computers located in a school building can be combined into a local network). A local network allows users to share access to computer resources, as well as to peripheral devices (printers, scanners, disks, modems, etc.) connected to the network.
Local networks are peer-to-peer and dedicated server.
In small local networks, all computers are equal, that is, each of them can use the resources of the other. Users independently decide which resources of their computer (files, folders, disks) to make available to the entire network. Such networks are called peer-to-peer.
In networks with a large number of users, it is undesirable that all of them have access to all computers on the network. When combining more than 10 computers, it is advisable to allocate the most powerful computer- server (English server - serving). On the hard drive of the server, files (data and programs) are placed that are accessed by other network computers - clients. In addition, all network users can have access to peripheral equipment connected to the server (for example, a printer or scanner).
Each computer connected to the local network must have a special board - network adapter. Its function is the transmission and reception of signals propagated through communication channels.
The connection of computers (their network cards) to the local network is carried out using various types cables (twisted pair, optical fiber - Fig. 4.1) or via wireless channels (such as Wi-Fi).
Rice. 4.1. Cables:
twisted pair and fiber
A twisted pair is two isolated copper wire twisted one relative to the other. This twisting of the wires reduces the effect of interference on the signals transmitted over this cable. A twisted pair connection consists of several twisted pairs (2 or 4) covered with a plastic sheath. Data transfer rate - from 10 Mbps to 1000 Mbps.
Fiber optic cable transmits light through glass fibre. This type of connection provides a very high transmission speed, the length of the channel is hundreds and thousands of kilometers, and it is absolutely not subject to electromagnetic interference. Data transfer rate - from 100 Mbps to 10 Gbps.
Wireless wifi connection provides data transfer rates up to 300 Mbps.
Local networks, uniting dozens of computers in a small area, do not provide joint access to information to users located at a considerable distance from each other (for example, in different settlements).
global computer network is a system of interconnected computers located on an arbitrarily far away from each other (for example, in different countries and on different continents).
Examples of global computer networks are regional and corporate networks. Regional computer networks provide an association of computers within the same region (city, region, region, country). Corporate computer networks are created to ensure the activities of various types of corporate structures that have territorially remote divisions (for example, banks with their branches).
The most famous and most extensive global computer network is the Internet. This network unites numerous local, regional and corporate networks, as well as computers of individual users, distributed throughout the world.
The basis of any global computer network are computer nodes and communication channels.
A node is a powerful computer that is permanently connected to the network. Subscribers are connected to computer network nodes - personal computers users or local networks.
A wide variety of physical channels are used for data transmission in global networks: electric cable; radio communication through repeaters and communication satellites; infrared rays (as in television remotes remote control); modern fiber optic cable; conventional telephone network.
An organization that provides users with a connection to the global network through their computers is called provider(English provider - supplier) of network services.
Task. The data transfer rate through some connection is 128,000 bps. How much time (in seconds) will it take to transfer a 625 KB file over this connection?
Computer network- these are two or more computers connected by information transmission lines.
local computer network combines computers installed in the same room or building, and provides users with the ability to share access to computer resources, as well as to peripheral devices connected to the network. Local networks are peer-to-peer and with a dedicated server.
global computer network- this is a set of interconnected computers located at an arbitrarily large distance from each other (for example, in different countries and on different continents).
1. Familiarize yourself with the presentation materials for the paragraph contained in the electronic supplement to the textbook. What can you say about the forms of presentation of information in the presentation and in the textbook? What slides would you like to add to your presentation?
2. How do you understand the meaning of the phrase: “The possibility of transferring knowledge, information is the basis for the progress of the whole society as a whole and each person individually”? Discuss this question in a group.
3. Since ancient times, people have exchanged information in various ways, notified of danger or transmitted important and urgent information. Prepare a short report about one of the previously used ways of transmitting information.
4. What is a computer network?
5. What is a communication channel? How is the bandwidth of a communication channel determined?
6. How does a peer-to-peer LAN work?
7. How does a local network with a dedicated server work?
8. What type of local network is installed in your computer class? What functions does it perform?
9. What networks are called global? Give examples of such networks.
10. What communication channels are used for data transmission in global computer networks?
11. The data transfer rate over a certain communication channel is 512,000 bps. Transferring a file over this channel takes 16 seconds. Determine the size of the file in kilobytes.
12. Find out the names of network service providers in your area.
13. Build a graph of relationships connecting the concepts discussed in this paragraph.
A local area network can be considered a connection of two or more devices using a cable, radio waves or optical signals, which makes it possible to exchange data between them. Devices located in the same room or building and interconnected are called a local computer network (LAN - Local Area Network). The number of devices connected to such a network is limited by the capabilities of the cabling system and network equipment used.
The connection between devices can be direct or using additional communication nodes.
Networks are backbone information structures, consisting of logical and physical layers or components, the main purpose of which is the exchange of information.
The physical layer is represented by network components that provide a physical connection between computers. These components are usually: network interface ( LAN card or network adapter card, standard or extended communication or parallel port or multiport card), network data transmission medium (coaxial cable, two-wire so-called twisted pair or fiber optic) and node elements (routers, concentrators, repeaters (repeaters, hubs (hub )), switches (switch)) and end elements (terminators, connectors, connectors, plugs).
At present, there is a clear structuring of networks into local and global ones, the process of integrating the first into the second, where networks with several hundred computers are still considered local, and global ones have tens of thousands of connected computer systems. The information exchange rate reaches 200 Mbps, and 10 Mbps is considered the basic initial and low-cost configuration. Now computer networks allow not only to transmit or receive information in the literal sense of this concept, but also provide a set of service capabilities, the list of which is constantly expanding. This and remote administration, distributed file systems, remote program execution, Email, remote printing, distributed databases, systems remote access And distributed systems management, search engines, teleconferencing and more.
Devices that are used both as control centers in the network and as storage media are called servers. If devices are located relatively close to each other and are connected using high-speed network adapters, then such networks are called local networks. When using a local area network, devices are usually located within the same room, building, or in several closely spaced houses. A local computer network, as a rule, unites no more than a hundred computer systems belonging to any one structure, and is corporate in nature, both in terms of its operation and the nature of system software.
The principles of organization and software protocols for local and global computer systems can be both different and absolutely the same. Therefore, it is impossible to classify a network as local or global only on the basis of the type of network interaction and underlying software.
Net is a group of computers connected to each other by a communication channel. The channel provides data exchange within the network (that is, data exchange between computers in a given group). The network may consist of two or three computers, or it may unite several thousand PCs. Physically, data exchange between computers can be carried out via a special cable, telephone line, fiber optic cable or radio channel.
Computers on the network can be connected:
Computers connected to a network can perform two functions: they can be workstations or servers.
Work station- this is any working computer on the network that is not a server, as a rule, users work behind them. Requirements for workstations are determined by the range of tasks of the station. Typically, the main requirements are the requirements for speed and the amount of RAM.
Servers- these are computers that manage the entire network and accumulate all the data of workstations. Servers can run in automatic mode- they stand without a keyboard and sometimes even without a monitor, but in any case, the servers perform the functions of network management and data concentration. Network Administrator- a person whose responsibilities include all issues related to the installation and operation of the network, as well as solving all problems related to the rights and capabilities of network users.
Typically, the largest and most powerful computer on the network is selected as the server. However, the development of computer technology is clearly leading to a decrease in internal components - the computer becomes faster and more economical. Therefore, in a short time, the server can become outdated faster than conventional computers, which are not subject to such high requirements.
It is customary to distinguish between local and global networks. In essence, the main difference between them is already clear from the names, but there are also some significant technological differences.
Local networks(from English local - local) - these are networks consisting of closely spaced computers, most often located in the same room, in the same building or in closely spaced buildings. Local computer networks covering a certain enterprise or firm and uniting heterogeneous computing resources in a single environment are called corporate(from English corporate - corporate, general). Examples: banking network, educational institution network.
The most important characteristic local networks is the data transfer rate, so computers are connected using high-speed adapters with a data transfer rate of at least 10 Mbps. Local networks use high-speed digital communication lines. In addition, local networks must be adaptable and flexible: users must be able to locate computers connected to the network where they need to, add or move computers or other devices, and disconnect them as needed without interrupting the network.
Combining computers into a single network provides network users with new opportunities that are incomparable with the capabilities of individual computers. The network is not an addition, but a multiplication of the capabilities of individual computers. A local network allows you to organize the transfer of files from one computer to another or others, share computing and hardware resources, combine distributed data processing on several computers with centralized storage of information, and much more. With the help of a computer local network, the collective use of technical resources is carried out, which has a beneficial effect on the psychology and behavior of the user not only on the network, but also in real life.
Topology of local networks
Topology is a network configuration, a way of connecting network elements (i.e. computers) to each other. There are three most common ways to connect computers to a local network: "star", "common bus" And "ring".
Star connection(Fig. 1). Each computer through a special network adapter is connected by a separate cable to the merging device. If necessary, several networks with a star topology can be combined together, while the network configuration is branched out.
Advantages: With a star connection, it is easy to troubleshoot the network.
Disadvantages: The connection is not always reliable, since the failure of the central node can bring the network to a halt.
Common bus connection(Fig. 2). All computers on the network are connected to the same cable; this cable is shared by all workstations in turn. With this type of connection, all messages sent by each individual computer are received by all other computers on the network.
Advantages: in a "common bus" topology, the failure of individual computers does not bring the entire network to a halt.
software file operating driver
Flaws: it is somewhat more difficult to find a fault in the cable, and if the cable (single for the entire network) breaks, the operation of the entire network is disrupted.
Ring connection(Fig. 3). Data is transferred from one computer to another; at the same time, if one computer receives data intended for another computer, then it transfers them further (along the ring).
Advantages: load balancing, the possibility and convenience of cable laying.
Flaws: physical restrictions on the total length of the network.
The composition of the hardware and software depends on the scheme. The topology is chosen based on the needs of the enterprise. If the enterprise occupies a multi-storey building, then the scheme can be applied in it "snowflake"(Fig. 4), which has file servers for different workgroups and one central server for the entire enterprise.
3.5. Local Area Networks
Local area network (LAN) called the joint connection of several separate computers to a single data transmission channel. The concept of LAN (eng. LAN - Local Area Network) refers to geographically limited (territorially or production) hardware and software systems in which several computer systems are interconnected using appropriate means of communication.
LAN provides the possibility of simultaneous use of programs and databases by several users, as well as the ability to interact with other workstations connected to the network. Through a LAN, the system combines personal computers located at many remote workplaces that share equipment, software and information. Workplaces of employees are no longer isolated and are combined into a single system.
The most important characteristic of a LAN is the speed of information transfer. Ideally, when sending and receiving data over the network, the response time should be almost the same as if they were received from the user's PC, and not from anywhere else on the network.. This requires data transfer at a rate 10 Mbps and above. The following speeds are actually achieved:
· Coaxial cable - 10¸ 50 Mbaud;
· Twisted pair - up to 10 Mbaud;
· Special twisted pair category 5 - up to 100 Mbaud;
· Optical fiber - up to 1 Gbaud;
· Telephone line - from 2400 baud to 56 kbaud;
· Satellite 10,000 computers at the same time and a speed of about 1 Mbaud.
LAN components: network devices and means of communication.
The LAN implements the principle of modular organization, which allows you to build networks of various configurations with different functionality. The main components of which the network is built are the following:
transmission medium – coaxial cable, telephone cable, twisted pair, fiber optic cable, radio, etc.;
workstations- PC, workstation or actual network station. If the workstation is connected to the network, it may not require either a hard drive or floppy disks. However, in this case, you need a network adapter - a special device for remote download operating system from the network;
interface boards – network boards for organizing the interaction of workstations with the network;
servers– separate computers with software that perform control functions network resources public access;
network software .
Let's take a closer look at some of these network components.
Servers
A network may have one or more servers. Various servers can be used to manage the operation of the network ( network servers), storing information in the form of files ( file servers), searching and extracting information from databases ( database servers), information distribution ( mail servers ), network printing ( print servers), etc. Server disks are accessible from all other network workstations if users have the appropriate permissions.
The interaction of the server with workstations occurs approximately according to the following scheme. As necessary, the workstation sends a request to the server to perform some action: read data, print a document, send email and so on. The server performs the requested action and issues an acknowledgment.
Transmission medium
Transmitting media are characterized by the speed and range of information transmission and reliability.
Twisted pair, coaxial cable, fiber optic lines are most often used as means of communication in a LAN. When choosing a transmission medium, the following indicators must be taken into account:
· information transfer rate;
· range transfer of information;
· security of information transfer;
· reliability of information transmission ;
· the cost of installation and operation.
Simultaneous fulfillment of the requirements for the transmission medium is a difficult task. So, for example, a high data transfer rate is often limited by the maximum allowable distance for reliable data transfer, while ensuring the necessary level of protection of the transmitted data. The cost of communication means affects the possibility of building and expanding the network.
Let us consider in more detail the properties of some transmission media.
twisted pair
Twisted two-core wired connection(twisted pair), the cheapest among the transmission media. Allows you to transfer information at speeds up to 10 Mbps, easy to grow, low noise immunity. The cable length does not exceed 1000 m at a transmission rate of 1 Mbps. To increase the noise immunity of information, a shielded twisted pair is used, placed in a sheath similar to the screen of a coaxial cable. The price of such a pair is close to the price of a coaxial cable.
Coaxial cable
Coaxial cable is used for communication over distances of up to several kilometers, has good noise immunity at an average price. The information transfer rate is from 1 to 10 Mbps, in some cases it reaches 50 Mbps. Coaxial cable can be used for broadband transmission of information.
Broadband coaxial cable.
Such a coaxial cable is weakly susceptible to interference, easy to build, but has a high price. The data transfer rate reaches 500 Mbps. To transmit information over a distance of more than 1.5 km in the base band, it is necessary repeater(amplifier), while the distance of stable transmission is increased to 10 km. For a LAN with a bus or tree topology, the cable must have a Terminator (terminating resistor).
Ethernet - cable
Thick Ethernet
Coaxial cable with a wave impedance of 50 ohms (thick Ethernet. or yellow cable (yellow cable)). Uses a 15-pin standard switch. The maximum allowed transmission distance without a repeater does not exceed 500 m, and the total length of the Ethernet network is 3000 m. Thick Ethernet, due to the backbone topology, uses only one terminator at the end. In terms of noise immunity, it is an expensive alternative to conventional coaxial cable.
Slim Ethernet
Coaxial cable with impedance of 50 ohms (thin Ethernet) and data rate of 10 7 bps, cheaper than thick Ethernet.
LANs with thin Ethernet cable are low cost, require minimal scalability, and do not require additional shielding. The cable is connected to the network boards of workstations using tee connectors ( T connectors ) with compact bayonet connectors (CP-50). When connecting thin Ethernet segments, repeaters are required. The distance between workstations without repeaters cannot exceed 300 m, and the total length of the network cannot exceed 1000 m.
Fiber optic cable
The most expensive transmission medium for a LAN is fiber optic cable, also called fiberglass cable. The speed of information transfer through it reaches several gigabits per second with an allowable length of more than 50 km. The noise immunity of a fiber optic cable is very high, therefore, LANs based on it are used where electromagnetic interference occurs and information is required to be transmitted to long distances without the use of repeaters. Networks are resistant to eavesdropping because the branching technique in fiber optic cables is very complex. Typically, LANs based on fiber optic cable are built on a star topology.
Characteristics of typical transmission media are given in the table.
| Indicators | Transmission medium |
||
| twisted pair | Coaxial cable | Fiber optic cable |
|
| Price | low | Medium | High |
| Building | Very simple | problematic | problematic |
| Listen protection | bad | Good | Very good |
| grounding | No | Required | No |
| Interference immunity | Low | High | Very high |
Topology of the IVS
Topology, i.e. LAN connection configuration attracts more attention than other characteristics of the network. This is due to the fact that it is the topology that largely determines the most important properties of the network, such as, for example, reliability and performance.
There are different approaches to the classification of LAN topologies. According to one of them, local area network configurations are divided into two main classes: broadcast And consecutive .
IN broadcast configurations, each PC transmits signals that can be perceived by other PCs. Such configurations include a common bus, a tree (connecting several common buses using repeaters), a star with a passive center. The advantage of this class of configurations is the ease of networking.
IN successive configurations, each physical sublayer transmits information to only one PC. Such configurations include a star with an intellectual center, a ring, a hierarchical connection, a snowflake. The main advantage is the simplicity of the software implementation of the connection.
To prevent collisions in the transmission of information, it is most often used temporary separation method , according to which each connected workstation V certain moments time, the exclusive right to use the information transmission channel is granted. Therefore, the requirements for network bandwidth under increased load, i.e. with the introduction of new workstations, are reduced.
Different topologies implement different principles of information transfer . In broadcast it information selection, in successive information routing.
In a broadband LAN, workstations receive a frequency on which they can send and receive information. The transmitted data is modulated on the respective carrier frequencies. The technology of broadband communications allows you to simultaneously transport a fairly large amount of information in a communication medium.
Star topology .
Network topology in the form stars with an active center inherited from area mainframes , where the host machine receives and processes all data from terminal devices as an active data processing node. All information between peripheral workstations passes through the central node of the computer network.
Bandwidth network is determined by the computing power of the central node and is guaranteed for each workstation. Collisions, i.e. There are no collisions in the data transfer.
The cabling of the topology is relatively simple insofar as each workstation is connected to a central site, however, the cost of laying communication lines is high, especially when the central site is not geographically located in the center of the topology.
When expanding the LAN, it is impossible to use previously made cable connections: a separate cable must be laid to the new workstation from the central node of the network.
Star topology with good central node performance is one of the fastest topologies LAN, since the transfer of information between workstations takes place over dedicated lines used only by these workstations. The frequency of requests for information transfer from one station to another is low compared to other topologies.
Fig 1. Star topology
The performance of a star topology LAN is primarily determined by the parameters of the central node, which acts as network server. It can be a network bottleneck. If the central node fails, the operation of the network as a whole is disrupted.
In a LAN with a central control node, it is possible to implement an optimal mechanism for protecting against unauthorized access to information.
Ring topology.
In a ring network topology, LAN workstations are interconnected in a circle. The last workstation is connected to the first one, i.e. communication link closes in a ring.
Laying communication lines between workstations can be quite expensive, especially if the workstations are geographically located far from the main ring.
Messages in the LAN ring circulate in a circle. The workstation sends information to a certain address after receiving a request from the ring. The transfer of information turns out to be quite efficient, since messages can be sent one after another. So, for example, you can make a ring request to all stations. The duration of information transfer increases in proportion to the number of workstations included in the LAN.
Rice. 2. Ring topology
The main problem of ring topology is that each workstation must participate in the transmission of information, and if at least one of them fails, the entire network is paralyzed. Faults in the cable system are easily localized.
Extending a network with a ring topology requires that the network be shut down because the ring must be broken. There are no special restrictions on the size of the LAN.
A special form of ring topology is logical ring .
Physically, it is mounted as a connection of star topologies. Individual stars are switched on with the help of special switches (eng. Hub - concentrator), which in Russian is also sometimes called a “hub”. Depending on the number of workstations and the length of the cable between workstations, active or passive hubs are used. Active hubs additionally contain an amplifier for connecting from 4 to 16 workstations. The passive hub is exclusively a branching device (for a maximum of three workstations). The management of a single workstation in a logical ring is the same as in a normal ring. Each workstation is assigned an address corresponding to it, to which control is transferred (from the oldest to the youngest and from the youngest to the oldest). The disconnection occurs only for the downstream (nearest) node of the computer network, so that only in rare cases can the operation of the entire network be disrupted.
Bus topology
In a LAN with a bus topology, the main transmission medium ( tire) – common for all workstations. The functioning of the LAN does not depend on the state of an individual workstation, i.e. workstations can be connected to the bus or disconnected from it at any time without disturbing the network as a whole.
Rice. 3. Bus topology
However, in the simplest network Ethernet with a bus topology uses a thin Ethernet cable with a tee connector as the transmission medium ( T -connector), so the expansion of such a network requires a bus break, which leads to disruption of the network. More expensive solutions involve installing instead T - connectors of passive plug boxes.
Since the expansion of a LAN with a bus topology can be carried out without interrupting network processes and breaking the communication environment, the removal of information from the LAN and, accordingly, listening to information is carried out quite easily, as a result of which the security of such a LAN is low.
Characteristics of topologies of computer networks are given in the table.
| Characteristic | Topology |
||
| Star | Ring | Tire |
|
| Price extensions | Low | Medium | Medium |
| Joining subscribers | passive | Active | passive |
| Defence from bounce | Low | Low | High |
| Listen protection | Good | Good | bad |
| Behavior at high | Good | bad | bad |
| Work in real time | Good | Good | bad |
| Wiring cable | Good | bad | Good |
Tree topology.
It is formed by various combinations of the LAN topologies discussed above. The base of the tree (root) is located at the point where communication lines (tree branches) gather.
Networks with a tree structure are used where it is impossible to directly apply the basic network structures. To connect workstations, devices called concentrators .
There are two types of such devices. Devices to which a maximum of three stations can be connected are called passive hubs. To connect more devices, you need active hubs with signal amplification.
Types building a LAN methods of information transfer.
Token Ring network
This standard was developed by IBM. Unshielded or shielded twisted pair or optical fiber is used as the transmission medium. Data transfer rate from 4 Mbps to 16 Mbps. As access control method workstations to the transmission medium used marker ring (Token Ring). The main provisions of the method:
¨ LAN ring topology;
¨ the workstation can transmit data only after receiving the token, i.e. permission to transfer information;
¨ at any given time, only one station in the network has this right.
To LAN To k e n Ring uses three main types of packets:
¨ packet control/data (Data/Command Frame);
¨ marker (Token);
¨ reset packet (Abort).
Control/Data package . With the help of such a packet, data or network control commands are transmitted.
Marker.The station can start transmitting data only after receiving such a packet. There can be only one marker in the ring and, accordingly, only one station with the right to transmit data.
Reset package.Sending such a packet causes the transmission of information to stop.
Network To k e n Ring allows computers to be connected in a star topology.
Arknet local network.
Arknet (Attached Resource Computer NETWork) is a simple, inexpensive, reliable and flexible LAN architecture. Developed by Datapoint Corporation in 1977. Subsequently, the license for Arcnet was acquired by SMS Corporation (Standard Microsystem Corporation), which became the main developer and manufacturer of equipment for Arcnet networks. Twisted pair, 93 ohm coaxial cable and fiber optic cable are used as transmission medium. The data transfer rate is 2.5 Mbps. When connecting devices in apply bus and star topologies. Access control method stations to the transmission medium - marker tire (Token Bus). The method includes the following rules:
¨ devices connected to the network can only transmit data if they receive permission to transmit (token);
¨ at any given time, only one station in the network has this right;
Work principles
The transfer of each byte in Arcnet is carried out by sending an ISU (Information Symbol Unit - information transfer unit), consisting of three service start / stop bits and eight data bits. At the beginning of each packet, the initial delimiter AB (Alegt Burst) is transmitted, which consists of six service bits. The start delimiter functions as the preamble of the packet.
Arcnet defines 5 types of packets:
1. ITT package(Information To Transmit) - an invitation to transmit. This message transfers control from one network node to another. The station that received the packet ITT , acquires the right to transfer data.
2. FBE package(Free Buffeg Enquiries) - a request for readiness to receive data. This packet checks the node's readiness to receive data.
3. data package.This message is used to transfer data.
4. ASK package (ACKnowledgements) - confirmation of receipt. Acknowledgment of readiness to receive data or confirmation of receipt of a data packet without errors, i.e. response to FBE and data packet.
5. NAK package(Negative AcKnowledgements) unwillingness to receive. Node not ready to receive data in response to FBE or receiving a packet with an error.
Ethernet LAN
The Ethernet specification was proposed by Xerox in the late seventies. Later, Digital Equipment Corporation (DEC) and Intel joined this project. In 1982, the specification for Ethernet version 2.0 was published. Based on Ethernet, the IEEE 802.3 standard was developed.
Basic principles of work
¨ bus topology at the logical level;
¨ all devices connected to the network are equal, i.e. any station can start transmission at any time (if the transmission medium is free);
¨ data transmitted by one station is available to all stations on the network.
