Today's article opens a new section on the blog, which will be called " networks". This section will cover a wide range of issues related to computer networks . The first articles of the rubric will be devoted to explaining some of the basic concepts that you will encounter when working with the network. And today we will talk about what components will be required to create a network and which ones exist. types of networks.
Computer network is a combination of computer and network equipment connected via communication channels into a single system. To create a computer network, we need the following components:
Computer networks are usually divided into two main types: global and local.
Local networks(Local Area Network - LAN) have a closed infrastructure before reaching Internet service providers. The term "local area network" can describe both a small office network and the network of a large factory covering several hectares. In relation to organizations, enterprises, firms, the term is used corporate network
– local network of a separate organization ( legal entity) regardless of the territory it occupies.
Corporate networks are networks of a closed type, access to them is allowed only to a limited circle of users (for example, company employees). Global networks are focused on serving any users.
Global network(Wide Area Network - WAN) spans large geographic regions and consists of many local area networks. Everyone is familiar with the global network, which consists of several thousand networks and computers - this is the Internet.
The system administrator has to deal with local (corporate) networks. Ordinary user computer connected to the local network is called workstation . A computer that shares its resources with other computers on a network is called server ; and the computer accessing the shared resources on the server is client .
There are various types of servers: file (for storing shared files), database servers, application servers (providing remote work programs on clients), web servers (for storing web content) and others.
Network load is characterized by a parameter called traffic. Traffic is the flow of messages in a data network. It is understood as a quantitative measurement of the number of data blocks passing through the network and their length, expressed in bits per second. For example, the data transfer rate in modern local networks can be 100Mbps or 1Gbps
Currently, the world has a huge amount of all kinds of network and computer equipment that allows you to organize a variety of computer networks. The whole variety of computer networks can be divided into several types according to various criteria:
By territory:
By way of connecting computers:
Control method:
According to the composition of computing facilities:
By type of transmission medium networks are divided into fiber-optic, with the transmission of information via radio channels, in the infrared range, through satellite channel etc.
You may come across other classifications of computer networks. Usually, system administrator one has to deal with local wired networks with centralized or decentralized control.
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Types of computer networks
Computer networks can be classified according to various criteria.
I. According to the principles of management:
1. Peer-to-peer - not having a dedicated server. In which control functions are alternately transferred from one workstation to another;
2. Multi-rank is a network that includes one or more dedicated servers. The remaining computers in such a network (workstations) act as clients.
II. By connection method:
1. "Direct connection"- two personal computers are connected by a piece of cable. This allows one computer (master) to access the resources of another (slave);
2. "Common bus" - connecting computers to one cable;
3. "Star" - connection through the central node;
4. "Ring" - serial connection of the PC in two directions.
III. By area coverage:
1. The local network(a network in which computers are located at a distance of up to a kilometer and are usually connected using high-speed communication lines.) - 0.1 - 1.0 km; LAN nodes are located within the same room, floor, building.
2. Corporate network(within are within the same organization, firm, plant). The number of nodes in the CVS can reach several hundred. At the same time, the corporate network usually includes not only personal computers, but also powerful computers, as well as various technological equipment (robots, assembly lines, etc.).
The corporate network makes it possible to facilitate the management of the enterprise and the management of the technological process, to establish clear control over information and production resources.
3. Global network(a network whose elements are separated from each other by a considerable distance) - up to 1000 km.
As communication lines in global networks, both specially laid (for example, transatlantic fiber optic cable) and existing communication lines (for example, telephone networks). The number of nodes in a hot water supply system can reach tens of millions. The global network includes separate local and corporate networks.
4. World Wide Web - unification of global networks (Internet).
TOPOLOGY OF COMPUTER NETWORKS
Network topology is the geometric shape and physical arrangement of computers in relation to each other. Network topology allows you to compare and classify different networks. There are three main types of topology:
1) Star;
2) Ring;
BUS TOPOLOGY
This topology uses a single transmission channel based on a coaxial cable, called a "bus". All network computers connected directly to the bus. At the ends of the bus cable, special plugs are installed - "terminators" (terminator). They are necessary in order to extinguish the signal after passing through the bus. The disadvantages of the "Bus" topology include the following:
Data transmitted over the cable is available to all connected computers;
In the event of a bus failure, the entire network ceases to function.
TOPOLOGY "RING"
Ring topology is characterized by the absence of connection endpoints; the network is closed, forming an inextricable ring through which data is transmitted. This topology implies the following transmission mechanism: data is transmitted sequentially from one computer to another until it reaches the recipient computer. The disadvantages of the "ring" topology are the same as those of the "bus" topology:
Public availability of data;
Damage resistance of the cable system.
STAR TOPOLOGY
In a network with a "star" topology, all computers are connected to a special device called a network hub or "hub" (hub), which performs the functions of data distribution. There are no direct connections between two computers on the network. Due to this, it is possible to solve the problem of public data availability, as well as increase the resistance to damage to the cable system. However, network functionality depends on the state of the network hub.
Carrier access methods in computer networks
Different networks have different procedures for exchanging data between workstations.
The International Institute of Electrical and Electronics Engineers (IEEE) has developed standards (IEEE802.3, IEEE802.4, and IEEE802.5) that describe methods for accessing network data channels.
The most common implementations of access methods are Ethernet, ArcNet, and Token Ring. These implementations are based respectively on the IEEE802.3, IEEE802.4 and IEEE802.5 standards.
Ethernet access method
This access method, developed by Xerox in 1975, is the most popular. It provides high data transfer speed and reliability.
For this method access uses the "common bus" topology. Therefore, a message sent by one workstation is received simultaneously by all other stations connected to the common bus. But the message is only for one station (it includes the address of the destination station and the address of the sender). The station to which the message is intended receives it, the rest ignore it.
The Ethernet access method is a Carter Sense Multiple Access with Collision Detection (CSMA/CD -Carter Sense Multiple Access with Collision Detection) method.
Before transmitting, the workstation determines whether the channel is free or busy. If the channel is free, the station starts transmitting.
Ethernet does not exclude the possibility of simultaneous transmission of messages by two or more stations. The hardware automatically recognizes such conflicts. After a collision is detected, the stations delay transmission for some time. This time is short and each station has its own. After a delay, transmission resumes.
In reality, conflicts lead to a decrease in network performance only if several tens or hundreds of stations are operating.
ArcNet access method
This method was developed by Datapoint Corp. It has also become widespread, mainly due to the fact that ArcNet equipment is cheaper than Ethernet or Token-Ring equipment.
ArcNet is used in local area networks with a star topology. One of the computers creates a special marker (message of a special kind), which is sequentially transmitted from one computer to another.
If a station wishes to send a message to another station, it must wait for the token and append the message to it, complete with the sender and destination addresses. When the packet reaches the destination station, the message will be "unhooked" from the marker and transferred to the station.
Token-Ring access method
The Token-Ring access method was developed by IBM and is designed for a ring network topology.
This method is similar to ArcNet in that it also uses a token passed from one station to another. Unlike ArcNet, the Token-Ring access method allows you to assign different priorities to different workstations.
Communication media, their characteristics
Coaxial cable
Coaxial cable was the first type of cable used to connect computers on a network. This type of cable consists of a central copper conductor covered with a plastic insulating material, which in turn is surrounded by a copper mesh and/or aluminum foil. This outer conductor provides grounding and shields the center conductor from external electromagnetic interference. When laying networks, two types of cable are used - "Thick coaxial cable" (Thicknet) and "Thin coaxial cable" (Thinnet). Networks based on coaxial cable provide transmission at speeds up to 10 Mbps. The maximum segment length ranges from 185 to 500 m depending on the type of cable.
"Twisted Pair"
Twisted pair cable is one of the most common types of cable today. It consists of several pairs copper wires covered with a plastic shell. The wires that make up each pair are twisted around each other, which provides protection against mutual interference. Cables of this type are divided into two classes - "shielded twisted pair" ("Shielded twisted pair") and "unshielded twisted pair" ("Unshielded twisted pair"). The difference between these classes is that shielded twisted pair is more protected from external electromagnetic interference, due to the presence additional screen copper mesh and/or aluminum foil surrounding the cable wires. Twisted-pair networks, depending on the cable category, provide transmission at speeds from 10 Mbps to 1 Gbps. The length of a cable segment cannot exceed 100 m (up to 100 Mbps) or 30 m (1 Gbps).
Fiber optic cable
Fiber optic cables are the most advanced cable technology that provides high speed data transfer to long distances resistant to interference and eavesdropping. An optical fiber cable consists of a central glass or plastic conductor surrounded by a layer of glass or plastic coating and an outer protective sheath. Data transmission is carried out using a laser or LED transmitter that sends unidirectional light pulses through the center conductor. The signal at the other end is received by a photodiode receiver, which converts light pulses into electrical signals that can be processed by a computer. The transmission speed for fiber optic networks ranges from 100 Mbps to 2 Gbps. The segment length limit is 2 km.
Modern networks can be classified according to various criteria:
By remoteness of computers:
Local LAN (Local Area Network) - a network within an enterprise, institution, one organization. Computers are located at a distance of up to several kilometers and are usually connected using high-speed communication lines.
Regional MAN (Metropolitan Area Network) - unite users of the region, city, small countries. Telephone lines are used as communication channels. The distance between network nodes is from 10 to 1000 km.
Global WAN (Wide Area Network) - includes other global networks, local area networks, as well as computers that are separately connected to it.
According to the purpose and list of services provided:
- General use of files and printers - with the help of a special computer (file server, printer server), users' access to files and printers is organized.
General use of databases - with the help of a special computer (database server), user access to the database is organized.
Application of Internet technologies - Email, The World Wide Web, teleconferencing, videoconferencing, file transfer over the Internet.
By the way of organizing interaction:
- Peer-to-peer networks - all computers in a peer-to-peer network are equal, while any network user can access data stored on any computer. The main advantage of peer-to-peer networks is the ease of installation and operation. The main disadvantage is that in the conditions of peer-to-peer networks it is difficult to solve information security issues. Therefore, this method of organizing a network is used for networks with a small number of computers and where the issue of data protection is not a matter of principle.
- Networks with a dedicated server ( hierarchical networks) - when installing a network, one or more servers- computers that manage the exchange of data over the network and the distribution of resources. Any computer that has access to the server's services is called network client or workstation. The server itself can only be a client of a server at a higher level in the hierarchy. The hierarchical network model is the most preferable, as it allows creating the most stable network structure and more rationally allocating resources. Also, the advantage of a hierarchical network is a higher level of data protection.
The disadvantages of a hierarchical network, compared to peer-to-peer networks, include:
The need for an additional OS for the server.
Higher network installation and upgrade complexity.
The need to allocate a separate computer as a server
By server technology:
Networks with file-server architecture - used file server where most programs and data are stored. At the request of the user, they are sent required program and data. Information processing is performed at the workstation.
Networks with client-server architecture - data is exchanged between the client application and the server application. Data is stored and processed on a powerful server that also controls access to resources and data. The workstation receives only the results of the query.
According to the speed of information transfer computer networks are divided into low-, medium- and high-speed:
Low-speed networks - up to 10 Mbps;
Medium-speed networks - up to 100 Mbps;
High-speed networks - over 100 Mbps.
According to the type of transmission medium, networks are divided into:
Wired (coaxial cable, twisted pair, fiber optic);
Wireless with the transmission of information via radio channels or in the infrared range.
By topology (how computers are connected to each other):
Common bus;
A network topology refers to the physical or electrical configuration of the cabling and network connections.
In network topology, several specialized terms are used:
Network node - a computer or a network switching device;
Network branch - a path connecting two adjacent nodes;
Terminal node - a node located at the end of only one branch;
Intermediate node - a node located at the ends of more than one branch;
Adjacent nodes are nodes connected by at least one path that does not contain any other nodes.
Any computer network can be viewed as a collection of nodes. The configuration of physical connections is determined by the electrical connections of computers to each other and may differ from the configuration of logical connections between network nodes. Logical connections are data transfer routes between network nodes, are formed by appropriate equipment settings.
There are three main types of physical topology of local area networks:
Ring topology provides for the connection of network nodes with a closed curve, i.e. medium cable. In such a network, two and only two branches are attached to each node. Information is transmitted along the ring from node to node, usually in one direction. Each intermediate node between transmitter and receiver relays the sent message.
The receiving node recognizes and receives only messages addressed to it. In a network with a ring topology, special measures must be taken so that in the event of a failure or disconnection of a station, the communication channel between the other stations is not interrupted. The advantage of this topology is the ease of management, the disadvantage is the possibility of failure of the entire network if there is a failure in the channel between two nodes.
Bus topology one of the simplest, implemented using a cable to which all computers are connected. All signals transmitted by any computer on the network travel along the bus in both directions to all other computers.
Topology star uses a separate cable for each computer, routed from a central device called hub or concentrator. The hub translates signals from any of its ports to all other ports, causing the signals sent by one node to reach the rest of the computers. In such a network, there is only one intermediate node. A star-based network is more fault-tolerant than a bus-based network, since a cable failure directly affects only the computer to which it is connected, and not the entire network.
While small networks tend to have a typical star, ring, or bus topology, large networks tend to have random connections between computers. In such networks, it is possible to single out separate arbitrarily subnets with a typical topology, therefore they are called networks with mixed topology. The choice of a particular topology is determined by the scope of the network, the geographical location of its nodes and the dimension of the network as a whole.
Open systems interconnection model. The main task to be solved when creating computer networks is to ensure the compatibility of equipment in terms of electrical and mechanical characteristics and to ensure the compatibility of information support (programs and data) in terms of the coding system and data format. The solution to this problem belongs to the field of standardization. One example of solving this problem is the so-called open systems interconnection model OSI (Model of Open System Interconnections).
According to the OSI model, the architecture of computer networks should be considered at different levels (the total number of levels is up to seven). The top level is the application level. At this level, the user interacts with computing system. The lowest level is physical. It provides signal exchange between devices. Data exchange in communication systems occurs by moving them from the upper layer to the lower one, then transporting them, and finally playing them back on the client computer as a result of moving from the lower layer to the upper one.
Let's consider how data is exchanged between users located on different continents in the OSI model.
1. At the application level, using special applications, the user creates a document (message, picture, etc.).
2. At the presentation layer, his computer's operating system captures where the generated data is (in RAM, in a file on the hard drive, etc.) and provides interaction with the next layer.
3. At the session level, the user's computer interacts with a local or global network. The protocols of this layer check the user's rights to "go on the air" and transfer the document to the transport layer protocols.
4. At the transport level, the document is converted into the form in which data is supposed to be transmitted in the network used. For example, it can be cut into small packages of a standard size.
5. The network layer determines the route of data movement in the network. So, for example, if at the transport level the data was “sliced” into packets, then at the network level each packet must receive an address to which it should be delivered regardless of other packets.
6. The connection layer (Link layer) is necessary in order to modulate the signals circulating in the physical layer in accordance with the data received from the network layer. For example, in a computer, these functions are performed by a network card or modem.
The real data transfer occurs at the physical layer. There are no documents, no packages, not even bytes - only bits, that is, elementary units of data representation. Restoration of the document from them will occur gradually, when moving from the lower to the upper level on the client's computer.
The physical layer facilities lie outside the computer. In local networks, this is the equipment of the network itself. When communicating remotely using telephone modems, these lines telephone communication, switching equipment of telephone exchanges, etc.
On the computer of the recipient of information, the reverse process of converting data from bit signals to a document takes place.
The different protocol layers of the server and client do not communicate with each other directly, but they do communicate through physical layer. Gradually moving from the upper level to the lower one, the data is continuously transformed, “overgrown” with additional data, which are analyzed by the protocols of the corresponding levels on the adjacent side. It creates an effect virtual level interactions.
In order for different computers on the network to communicate with each other, they must “speak” the same language, that is, use the same protocol. A protocol is a “language” used to exchange data when working on a network.
There are many protocols, each of them performs different tasks. Different protocols are used at different layers of the OSI model.
ethernet is the Link Layer protocol used by most modern LANs. The Ethernet protocol provides a unified interface to the network media that allows operating system use several Network layer protocols to receive and transmit data simultaneously. token ring is an alternative to the "classic" Ethernet protocol at the Connection Layer.
To be able to transfer information over network communication channels, it is necessary to install a protocol for exchanging messages (packets). There are several such protocols. The most widely used are: NetBEUI , IPX/SPX , TCP/IP . Protocols NETBEUI And IPX/SPX- used in local networks. Protocols TCP/IP are the basic protocols of the global Internet.
The main components of the network are workstations, servers, transmission media (cables) And network hardware.
workstations network computers are called, on which network users implement applied tasks.
Network servers- these are hardware-software systems that perform distribution control functions network resources general access. A server can be any computer connected to the network that hosts resources used by other devices on the network. Rather powerful computers are used as the hardware part of the server.
There are the following types network equipment:
Network cables (coaxial, consisting of two concentric conductors isolated from each other, of which the outer one has the form of a tube; cables on twisted pair, formed by two wires intertwined with each other; fiber optic and etc.).
Network cards (Network interface adapters) are controllers connected to motherboard computers designed to transmit signals to the network and receive signals from the network. A network cable is connected to the adapter connectors.
Hubs (Hub) are the central devices of a cable system or a network of a physical "star" topology, which, when a packet is received on one of its ports, forwards it to all the others. A hub with a set of different types of ports allows you to combine network segments with different cable systems. You can connect to the hub port both a separate network node and another hub or cable segment.
The following devices are used to connect local networks to each other:
Bridges- network devices that connect two separate segments limited by their physical length. Bridges also amplify and convert signals for another type of cable. This allows you to expand the maximum network size.
Bridges transfer data between networks in packet form without making any changes to them. The figure below shows three LANs connected by two bridges. In addition, bridges can filter packets, protecting the entire network from local threads data and passing out only those data that are intended for other network segments.
Gateways (Gateway) - software and hardware systems that connect heterogeneous networks or network devices. Gateways allow you to solve problems of different protocols or addressing systems. The gateway, unlike the bridge, is used in cases where the connected networks have different network protocols. A message arriving at the gateway from one network is converted into another message that meets the requirements of the next network.
Routers (router) are standard network devices that operate at the network level and allow forwarding and routing packets from one network to another. It allows, for example, to split large messages into smaller portions, thus ensuring the interaction of local networks with different packet sizes. A router can forward packets to a specific address (bridges can only filter out unwanted packets), choose the best way to pass the package.
firewalls (firewalls, firewalls ) is a software and/or hardware barrier between two networks that allows only authorized interconnections to be established, controls information entering and exiting the local network, and ensuring the protection of the local network by filtering information.
Most firewalls are built on classical models of access control, according to which a subject (user, program, process or network packet) is allowed or denied access to an object (file or network node) upon presentation of some unique element inherent only to this subject. In most cases, this element is the password. For a network packet, this element is addresses or flags in the packet header, as well as some other parameters.
Communication network- a system of nodes and connections between them. The nodes perform the functions of creating, transforming, storing and consuming the communication product. Connections (transmission channels, communication lines) are used to transfer the product between nodes. Depending on the type of product, material, energy, information networks are distinguished. Examples of real networks: road and rail links; water and gas supply.
Information network- a communication network in which the product of communications is information. Examples: telephone networks, television, radio broadcasting.
Computing, or computer network- an information network, the nodes of which are computers and other computing equipment. In addition to special networking hardware, networking software is also required. Through the interaction of computers on a network, a number of new possibilities become available.
First - sharing hardware and software resources. Yes, at public access to an expensive peripheral device (printer, plotter, scanner, fax, etc.), the costs per individual user are reduced. Similarly used network versions application software.
The second is the sharing of data resources. With the centralized storage of information, the processes of ensuring its integrity are greatly simplified, as well as Reserve copy which ensures high reliability. The presence of alternative copies on two machines at the same time allows you to continue working when one of them is unavailable.
The third is to speed up data transfer and provide new forms of user interaction in one team when working on a common project.
Fourth, the use of common means of communication between different applied systems(communication services, data, video, speech, etc.).
One of the important classification features of networks is their size. The size of the network influences the choice of equipment used and the transmission technologies used.
Local computing network(LAN, or LAN - Local Area Network) unites nearby computers within a limited area, premises, building. Distinctive features of the LAN are the minimum delay time and low error rate. LANs can be elements of larger-scale formations: a campus or corporate network (CAN - Campus Area Network), which unites local networks of closely spaced buildings; municipal network, or city-scale network (MAN - Metropolitan Area Network); regional or wide area network (WAN - Wide Area Network), covering a large area; global computer network(GWS, or GAN - Global Area Network), having the size of a country and a continent.
According to the method of network management, they are divided into peer-to-peer and with dedicated server(centralized control). In peer-to-peer networks, all nodes are equal - each node can act as both a client and a server. Under client refers to a hardware-software object requesting some services. And under server– a combination of hardware and software that provides these services. A computer connected to a local network, depending on the tasks performed on it, is called a workstation (workstation) or a server (server).
Peer-to-peer LANs are quite easy to maintain, but they cannot provide adequate information protection with a large network size. The costs of organizing peer-to-peer computing networks are relatively small. However, with an increase in the number of workstations, the efficiency of using the network decreases sharply. Therefore, peer-to-peer LANs are used only for small workgroups - no more than 20 computers.
A dedicated server implements network management (administration) functions in accordance with specified policies - sets of rules for separating and restricting the rights of network participants. LAN with a dedicated server have good means data security systems are capable of supporting thousands of users, but require constant skilled maintenance by the system administrator.
Depending on the data transfer technology used, there are broadcast networks and networks with transmission from node to node. Broadcast transmission is mainly used in small networks, and in large ones - transmission from node to node.
In broadcast networks, all network nodes share a single communication channel. Messages sent by one computer, called packets, are received by all other machines. Each packet contains the address of the recipient of the message. If the packet is addressed to another computer, then it is ignored. Thus, after checking the address, the recipient processes only those packets that are intended for it.
Node-to-node networks consist of machines connected in pairs. In such a network, a packet passes through a series of intermediate machines to reach its destination. In this case, there are often alternative paths from the source to the recipient.
The way computers are connected together in a network is called topology. There are three most common topologies that are used in LANs. These are the so-called tire, ring And star-shaped structures.
In the case of a bus (linear) structure, all computers are connected in a chain using one common coaxial cable. If at least one of the sections of the network with a bus structure is broken, the entire network as a whole becomes inoperable. The fact is that then there is a break in the only physical channel necessary for the movement of the signal.
The ring structure is used mainly in Token Ring networks and differs from the bus one in that all computers are connected in pairs to each other, forming a closed circuit. Also, in the event of a failure of one of the network segments, the entire network fails.
In a star network, the central node to which all others connect is concentrator(Hub - "hub"). Its main function is to provide communication between computers on the network. This structure is preferable, because in case of failure of one of the workstations or the cable connecting it to the hub, all the others remain operational.
When building networks, cellular ( fully connected) a topology in which each node is connected to all other individual links. The cost of creating redundant channels is offset by high reliability - there are almost always several ways for signals to pass from the sender to the recipient, so when one channel is disabled, the signals can be transmitted through others.
There are the following switching methods data in information networks: circuit switching, packet switching And message switching.
When switching circuits, the entire connection path is first established - from the sender to the recipient. This path consists of several sections connected by switches and (or) multiplexers. All data is transmitted along the established route. When the transfer is completed, the connection is terminated. An example is a telephone conversation: the channel is busy all the time of the conversation, even if the subscribers are silent. The transmission rate on such a channel is limited to the area with the lowest bandwidth.
In the second method, messages are broken into packets of a fixed length, which can be delivered over the network by independent routes, ensuring a uniform load on the network. In this case, packets of different messages can be transmitted over one channel. As an example, let's take an analogy: at rush hour, a group of students gets from the dormitory to the university on different vehicles, each in their own way.
Message switching resembles packet switching, but at a higher level (in this case, message switching nodes can be connected by both a circuit-switched network and a packet-switched network). The main difference is that the size of the data block is determined not by technological limitations, but by the content of the information in the message. It could be Text Document, email, file. Example - a group of tourists follows the route, at each point the composition of the group is checked. This scheme transmits messages that do not require an immediate response, such as e-mail messages.
15.3 OSI/ISO networking model
The operation of network equipment is impossible without interrelated standards. Harmonization of standards is achieved both through consistent technical solutions and through the grouping of standards. Each specific network has its own basic set of protocols - the "language" of data transmission. Protocol- formalized rules for the interaction of several computers, which can be described as a set of procedures that determine the sequence and format of messages exchanged between network components that are at the same level, but in different nodes.
International Organization for Standardization ISO (International Standards Organization) was proposed model computer network architecture OSI(Open System Interconnection - communication of open networks). This model, which most users try to adhere to, divides the communication functions in the network into seven levels. Data is exchanged by moving it on the sender's computer from the upper level to the lower one, then transporting it over a communication channel and converting it back on the recipient's computer from the lower level to the upper one.
The highest level - application layer(Application Layer - applied) is the interface between application programs and processes of the OSI model.
The presentation layer (Presentation Layer) defines the format for data exchange, serves to encrypt, compress and encode data.
Session layer (Session Layer) performs the functions of coordination of communication between workstations. The layer provides the creation of a communication session, control over the transmission and reception of message packets, and termination of the session.
The Transport Layer divides or assembles messages into packets when more than one packet is in the process of being transmitted or received, as well as controlling the order in which the message components pass. In addition, at this level, network layers of various incompatible networks are negotiated through gateways. Guarantees delivery of packets without errors, in the same sequence, without loss and duplication with acknowledgment.
The Network Layer provides the translation of logical address names into physical ones. Based on specific network conditions, service priority, routing is carried out, that is, the choice of a data packet transmission route in the network, and data flow control in the network (data buffering, error control when establishing a connection).
The data link layer (Data Link) defines the rules for using the physical layer by network nodes. This level is divided into two sublevels: Media Access Control, associated with network access and management, and Logical Link Control, associated with the transmission and reception of user messages. It is at the Data Link level that data is transmitted in frames, which are blocks of data containing additional control information. Error correction is performed automatically by resending the frame. In addition, the correct sequence of transmitted and received frames is also ensured at this level.
The lowest- physical layer(Physical Layer) defines the physical, mechanical and electrical characteristics of communication lines. At this level, the data coming from the link layer is converted into signals, which are then transmitted over communication lines. In local networks, this transformation is carried out using network adapters, in global networks modems are used for this purpose.
Each level actually interacts only with neighboring levels (upper and lower), virtually only with a similar level at the end of the line. Real interaction is the direct transfer of information, in which the data remains unchanged. Virtual interaction - mediated interaction and data transfer, and the data may change during the transfer.
Physical communication really takes place only at the lowest level. Horizontal connections between all other levels are virtual, they are actually carried out by the transfer and transformation of information first down, sequentially to the lowest level, where the real transfer takes place, and then at the other end - reverse transfer upwards sequentially to the corresponding level.
The basis TVS classification the most characteristic functional, informational and structural features are laid down.
According to the degree of territorial dispersal network elements (subscriber systems, communication nodes) distinguish between global (state), regional and local computer networks (WAN, RCS and LAN).
By the nature of the implemented functions networks are divided into computing (the main functions of such networks are information processing), informational (for obtaining reference data at the request of users), information and computing, or mixed, in which computing and information functions are performed in a certain, non-constant ratio.
By way of management TVS are divided into networks with centralized(the network has one or more governing bodies), decentralized(each AS has the means to manage the network) and mixed management, in which, in a certain combination, the principles of centralized and decentralized control are implemented (for example, under centralized control, only tasks with the highest priority associated with the processing of large amounts of information are solved).
On the organization of information transfer networks are divided into networks with information selection and information routing. In networks with a selection of information, built on the basis of a monochannel, the interaction of the AU is carried out by selecting (selection) the data blocks (frames) addressed to them: all the AUs of the network have access to all the frames transmitted in the network, but only the AUs to which they are intended take a copy of the frame. In networks with information routing Multiple routes can be used to transfer frames from a sender to a receiver. Therefore, with the help of communication systems of the network, the problem of choosing the optimal (for example, the shortest time to deliver a frame to the addressee) route is solved.
By type of data transfer organization networks with information routing are divided into networks with circuit (channel) switching, message switching and packet switching. There are networks in operation that use mixed systems data transmission.
According to the topology those. configurations of elements in TVS, networks are divided into two classes: broadcast (Fig. 11.1) and serial (Fig. 11.2). Broadcast configurations and a significant part of the serial configurations (ring, star with an intelligent center, hierarchical) are characteristic of the LAN. For global and regional networks, the most common is arbitrary (mesh topology). The hierarchical configuration and the “star” have also found application.
IN broadcast configurations at any time, only one workstation (subscriber system) can work to transmit a frame. Other PCs in the network can receive this frame, i.e. such configurations are typical for a LAN with information selection. The main types of broadcast configuration are common bus, tree, star with passive center. The main advantages of a LAN with a common bus are the ease of network expansion, the simplicity of the management methods used, the absence of the need for centralized management, and the minimum cable consumption. A tree topology LAN is a more advanced version of a bus topology network. A tree is formed by connecting several buses with active repeaters or passive multipliers (“hubs”), each branch of the tree is a segment. The failure of one segment does not lead to the failure of the others. In a LAN with a star topology, there is a passive connector or an active repeater in the center - quite simple and reliable devices. To protect against violations in the cable, a central relay is used, which turns off failed cable beams.
Rice. 11.1. Broadcast network configurations: A - common bus; b- tree; V - star with passive center
Rice. 11.2. Sequential network configurations: a - arbitrary (mesh); b- hierarchical; V - ring; G - chain; e - a star with an intellectual center; e - snowflake
In sequential configurations, typical for networks with information routing, data transmission is carried out sequentially from one PC to a neighboring one, and different types of physical transmission medium can be used in different parts of the network.
The requirements for transmitters and receivers are lower than in broadcast configurations. Sequential configurations include: arbitrary (cellular), hierarchical, ring, chain, star with an intelligent center, snowflake. In the LAN, the ring and the star are most widely used, as well as mixed configurations - star-ring, star-bus.
On a LAN with a ring topology, signals travel in only one direction, usually counterclockwise. Each PC has a memory of up to a whole frame. When a frame moves around the ring, each PC receives the frame, analyzes its address field, makes a copy of the frame if it is addressed to this PC, and retransmits the frame. Naturally, all this slows down the data transfer in the ring, and the duration of the delay is determined by the number of PCs. Removing a frame from the ring is usually done by the sending station. In this case, the frame makes a full circle around the ring and returns to the sending station, which perceives it as a receipt - confirmation of the receipt of the frame by the addressee. Removing a frame from the ring can also be carried out by the receiving station, then the frame does not complete a full circle, and the sending station does not receive confirmation receipts.
Ring.structure provides quite wide functionality LAN with high efficiency of using a monochannel, low cost, simplicity of management methods, the possibility of monitoring the performance of a monochannel.
In broadcast and most serial configurations (with the exception of the ring), each cable segment must provide signal transmission in both directions, which is achieved: in half-duplex communication networks - using one cable for alternate transmission in two directions; in duplex networks - using two unidirectional cables; in broadband systems - the use of different carrier frequencies for simultaneous transmission of signals in two directions.
Global and regional networks, as well as local ones, in principle can be homogeneous (homogeneous), in which software-compatible computers are used, and heterogeneous (heterogeneous), including software-incompatible computers. However, given the length of the DHW and RVS and a large number of computers used in them, such networks are more often heterogeneous.
