Purpose of the transport level of the OSI model. How do network devices work according to the OSI network model

), IPX, IGMP, ICMP, ARP.

It is necessary to understand why it was necessary to build a network layer, why networks built using channel and physical levels could not satisfy user requirements.

Create a complex, structured network with the integration of various basic network technologies, can and means channel level: To do this, some types of bridges and switches can be used. Naturally, the traffic in such a network develops randomly, but on the other hand it is characterized by some laws. As a rule, in such a network, some users working on a common task (for example, employees of the same department) most often contact requests or each other or to a common server, and only sometimes they need access to the resources of the other department of computers. Therefore, depending on network traffic, computers in the network are divided into groups that call network segments. Computers are combined into a group if most of their messages are intended (addressed) to computers of the same group. Separation of the network to segments can carry out bridges and switches. They shielded local traffic inside the segment without passing any frames beyond its limits, except those addressed to computers in other segments. Thus, one network disintegrates into separate subnets. From these subnets in the future, composite networks can be built sufficiently large sizes.

The idea of \u200b\u200bpartitioning on the subnet is the basis for the construction of composite networks.

The network is called composite (Internetwork or Internet) if it can be represented as a set of multiple networks. Networks included in the compound network are called subnets (subnet), which make up networks or simply networks, each of which can work on the basis of its own channel-level technology (although it is not necessary).

But, the embodiment of this idea to life with the help of repeaters, bridges, and switches has very significant limitations and disadvantages.

In the topology of the network built both with the help of repeaters and bridges or switches, there should be no loop. Indeed, a bridge or switch can solve the task of delivering a package only when there is a single path between the sender and the recipient. Although at the same time, the presence of redundant bonds that form loops is often necessary for better load balancing, as well as to increase the reliability of the network by forming backup paths.

Logic network segments located between bridges or switches are weakly isolated from each other. They are not protected from broadcast storms. If any station sends a broadcast message, then this message is transmitted to all stations of all logical network segments. The administrator must manually limit the number of broadcast packets, which is allowed to generate some node per unit of time. In principle, in some way it was possible to eliminate the problem of broadcast storms using the mechanism of virtual networks (setting up VLAN Debian D-Link) implemented in many switches. But in this case, although it is possible to flexibly create isolated on the traffic of a group of stations, but they are completely insulated, that is, the nodes of one virtual network cannot interact with the nodes of another virtual network.

In networks built on the basis of bridges and switches, it is quite difficult to solve the traffic management task based on the data contained in the package. In such networks, this is possible only with the help of custom filters, to task which the administrator has to deal with the binary representation of the contents of the packets.

The implementation of the transport subsystem only with the means of physical and channel levels to which bridges and switches include, leads to a non-sufficiently flexible, single-level addressing system: MAC-address is used as the recipient's station, which is rigidly associated with a network adapter.

All the above disadvantages of bridges and switches are only associated with the fact that they work on channel-level protocols. The thing is that these protocols explicitly do not determine the concept of a part of the network (or subnet, or segment), which could be used when structuring a large network. Therefore, network technologies decided to instruct the task of building a composite network to a new level - network.

Consider in this article appointing the levels of the OSI reference model, with a detailed description of each of the seven model levels.

The process of organizing the principle of network interaction, in computer networks, quite a complex and difficult task, so for the implementation of this task, we decided to use a well-known and universal approach - decomposition.

Decomposition - This is a scientific method using a splitting of one difficult task for a somewhat more simple tasks - Series (modules) interconnected.

Multi-level approach:

• all modules are crushed into separate groups and sorted by levels, thereby creating a hierarchy;
• modules of a single level To perform their tasks, send requests only to the modules of the adjacent underlying level directly;
• the principle of encapsulation is included - the level provides the service, hiding from other levels of its implementation.

The International Standards Organization (International Standards Organization, ISO, created in 1946) laid the task of creating a universal model, which clearly delimit and determine various levels The interaction of systems, with named levels and with the endowment of each level of its specific task. This model was named model of interaction of open systems (OPEN SYSTEM INTERCONNECTION, OSI) or model ISO / OSI .

The reference model of the relationship of open systems (seven-level model OSI) introduced in 1977.

After approval of this model, the problem of interaction was divided (decomposed) to seven private problems, each of which can be solved regardless of others.

OSI reference model levels represent a vertical structure where all network functions are divided between the seven levels. It should be especially noted that each such level corresponds to strictly described operations, equipment and protocols.

The interaction between levels is organized as follows:

• vertically - inside a separate computer and only with neighboring levels.
• horizontally - organized logical interaction - with the same level of another computer at the other end of the communication channel (that is, the network layer on one computer interacts with the network layer on another computer).

Since the seven-level OSI model consists of a strict integral structure, any higher level uses the underlying level functions, and recognizes in which form and how (i.e., through which interface) you need to transmit a data stream to it.

Consider how the transmission of messages is organized computing network In accordance with the OSI model. The application level is the level of applications, that is, this level is displayed from the user as an used operating system and programs that data is being sent. At the very beginning, it is the application level that generates a message, then it is transmitted to the representative level, that is, descends down the OSI model. The representative level, in turn, analyzes the application header, performs the required actions, and adds its service information to the beginning of the message, as a representative level header, for the representative level of the destination node. Next, the movement of the message continues down, descends to the session level, and it, in turn, also adds its service data, in the form of a header at first messages and the process continues until it reaches the physical level.

It should be noted that in addition to adding service information in the form of a header at the beginning of the messages, levels can add service information and at the end of the message called "trailer".

When the message reached the physical layer, the message is already fully formed to transmit over the communication channel to the destination node, that is, contains all the service information added at the levels of the OSI model.

In addition to the term "data" (DATA), which is used in the OSI model on an application, representative and session levels, other terms are used on other levels of the OSI model so that you can immediately determine at what level the OSI model is processed.

In ISO standards, to designate a particular data portion with which the protocols of different levels of the OSI model are used, the general name is used - the protocol data block (Protocol Data Unit, PDU). To refer to the data blocks of certain levels, special names are often used: frame (frame), package (Packet), segment (segment).

Functions of the physical level

• at this level, the types of connectors and the purpose of contacts are standardized;
• it is determined how "0" and "1" are submitted;
• the interface between the network media and the network device (transmits electrical or optical signals to the cable or radio, receives them and converts to the data bits);
• the functions of the physical layer are implemented in all devices connected to the network;
• physical level equipment: hubs;
• Examples of network interfaces related to the physical level: RS-232C, RJ-11, RJ-45, AUI connectors, VS.

Functions of the channel level

• zero and single bits of the physical layer are organized into frames - "Frame". The frame is a portion of data that has an independent logical value;
• organization of access to the transmission medium;
• processing data errors;
• determines the structure of links between nodes and the methods of their addressing;
• drawing equipment: Switches, bridges;
• examples of protocols related to the channel level: Ethernet, Token Ring, FDDI, Bluetooth, Wi-Fi, Wi-Max, X.25, Framerelay, ATM.

For LAN, the channel level is divided into two sublevels:

• LLC (LogicallinkControl) represents the establishment of the communication channel and for the error-free package and receiving data messages;
• Mac (MediaAccessControl) - provides joint access of network adapters to the physical layer, defining frame boundaries, recognition of destination addresses (for example, access to a common bus).

Network level functions

• Performs features:
• definitions of data transfer paths;
• definitions of the shortest route;
• tracking problems and congestion on the network.
• Solves the problem:
• transmission of communication messages with non-standard structure;
• coordination of different technologies;
• simplifying addressing in large networks;
• creating barriers to unwanted traffic between networks.
• Network level operating equipment: router.
• Types of network-level protocols:
• network protocols (package promotion via the network:, ICMP);
• routing protocols: RIP, OSPF;
• address Resolution Protocols (ARP).

OSI Transport Level Functions

• provides applications (or application and session levels) data transfer with the required degree of reliability compensates for the lack of reliability of lower levels;
• multiplexing and demultiplexing i.e. collecting and disassembling packages;
• protocols are designed to interact the point-to-point type;
• starting from this level, the protocols are implemented by software tools of end nodes of the network - the components of their network OS;
• examples: TCP, UDP protocols.

Functions of session level

• maintaining a communication session, allowing applications to interact with each other for a long time;
• creating / completing a session;
• information exchange;
• synchronization of tasks;
• definition of data transfer rights;
• maintaining a session during application inactive periods.
• the transmission synchronization is provided by the placement in the data flow of the control points, starting with which the process is resumed during failures.

Functions of representative level

• responsible for the protocol conversion and encoding / decoding data. Application requests received from the application level converts to a transmission format over a network, and the data obtained from the network converts to the format, understandable applications;
• perhaps implementation:
• compression / unpacking or encoding / decoding data;
• redirects requests to another network resource if they cannot be processed locally.
• example: sSL protocol (Provides secret messaging for TCP / IP application level).

OSI application level functions

• is a set of diverse protocols by which network users get access to shared resources, organize collaboration;
• provides network interaction and user;
• allows user applications to have access to network services such as requests for database requests, file access, email forwarding;
• responsible for transferring official information;
• provides applications for errors;
• example: HTTP, POP3, SNMP, FTP.

SET-DESIGN AND DESIGNED LEVELS OF THE OSI seven-level

According to mine functional features Seven levels of the OSI model can be attributed to one of the two groups:

• the group in which the levels depend on the specific technical implementation of the computer network. Physical, channel and network levels - are simulated, in other words, these levels are inextricably linked with specific network equipment used.
• the group in which the levels are mainly focused on working with applications. Session, representative and application levels are focused on applications used and practically do not depend on what network equipment is used in a computer network, that is, network-independent.

I will start with the definition, as is customary. The OSI model is the theoretical ideal data transmission model over the network. This means that in practice you will never meet the exact coincidence with this model, this is a reference to network program developers and network equipment manufacturers in order to support their products compatibility. You can compare this with the idea of \u200b\u200bpeople about the perfect person - you will not meet anywhere, but everyone knows what to strive for.

Immediately I want to designate one nuance - what is transmitted over the network within the OSI model, I will call the data that is not entirely correct, but in order not to confuse the novice reader by the terms, I went on a compromise with conscience.

Below is the most well-known and most understandable scheme of the OSI model. The article will still have drawings, but first I propose to consider the main one:

The table consists of two columns, at the initial stage only the right interests us. We will read the table from the bottom up (and how otherwise :)). In fact, this is not my whim, but I do it for the convenience of learning information - from simple to complex. Go!

On the right side of the above table from the bottom up, the data transmitted path transmitted over the network (for example, from your home router to your comer). Refinement - OSI levels from the bottom up, then it will be the path of data on the receiving side, if on top down, then on the contrary - the sending. I hope it is understandable. To dispel the finally doubt, here's another scheme for clarity:

To track the path of the data and the changes to them from the levels, it is enough to imagine how they move along the blue line in the diagram, first moving down the OSI levels from the first computer, then from the bottom up to the second. Now we will analyze each of the levels in more detail.

1) physical (Phisical) - It includes the so-called "data environment", i.e. Wires, optical cable, radio wave (in case of wireless connections) and the like. For example, if your computer is connected to the Internet on the cable, then the quality of data transmission on the first, physical level, the wires, contacts at the end of the wire, contacts the network card connector of your computer, as well as internal electrical circuits on the computer boards. Network engineers have the concept of "problem with physics" - this means that the specialist saw the culprit of the "indescribing" of the data device of the physical layer, for example, somewhere network cable, or low signal level.

2) Channel (Datalink) - It's much more interesting here. To understand the channel level, we will have to first learn the concept of the MAC address, since it is he will be the main acting person in this chapter :). The MAC address is also called the "physical address", "hardware address". Represents a set of 12 characters in hexadecimalcalculus system separated by 6 ocetova dash or colon, for example 08: 00: 27: B4: 88: C1. It is necessary for the unambiguous identification of the network device on the network. Theoritically, the MAC address is globally unique, i.e. Nowhere in the world of such an address can not be "sewn" into the network device at the production stage. However, there are simple ways to change it to arbitrarily, and, moreover, some unscrupulous and little-known manufacturers are not bent the fact that it can be riveted for example, a batch of 5,000 network cards with exactly the same Mac. Accordingly, if at least two such "acrobat brothers" appear in one local network, conflicts and problems will begin.

So, on the channel level, the data is processed with a network device, which is only one thing - our notorious MAC address, i.e. He is interested in the delivery addressee. The channel layer devices include for example, switches (they are the same switches) - they keep the MAC address of the network devices with which they have direct, direct communication and when receiving data to their receiving port, are checked by MAC addresses in data from Mac -presses available in memory. If there are coincidences, the data is transmitted to the addressee, the rest are simply ignored.

3) Network (Network) - "Sacred" level, understanding the principle of functioning of which is mostly and makes a network engineer as such. Here I am already iron "IP address", here it is the basis of the foundation. Thanks to the impression of the IP address, it becomes possible to transfer data between computers that are not included in one local network. Transferring data between different local networks is called routing, and devices allowing to do - routers (they are routers, although in recent years the concept of the router was greatly distinguished).

So, the IP address - if you do not go into details, then this is a set of 12 digits in a tenolic ("normal") calculus system, separated by 4 octets separated by a point that is assigned to a network device when connected to the network. Here you need to deepen a little: for example, many are known address from a number of 192.168.1.23. It is quite obvious that there are no 12 digits here. However, if you write the address in full format, everything becomes in its place - 192.168.001.023. You will not dig further deeper at this stage, as IP addressing is a separate topic for the story and display.

4) transport level (Transport) - as follows from the name, you need to deliver and send data to the addressee. After conducting an analogy with our long-suffering mail, the IP address is the actual delivery address or receipt, and the transport protocol is a postman who knows how to read and knows how to deliver the letter. Protocols are different, for different purposes, but they have one - delivery.

The transport level is the latter, which by and large interests network engineers, system administrators. If all 4 lower levels worked as it should, but the data did not reach the destination, it means that the problem must be searched already in the software specific computer. The protocols of the so-called upper levels are strongly worried about programmers and sometimes still system administrators (if it is engaged in servicing servers, for example). Therefore, further I will describe the purpose of these levels casual. In addition, if you look at the situation objectively, most often in practice the function at once several top levels of the OSI model takes on one application or service, and it is impossible to unambiguously say where to attribute it.

5) Session (Session) - manages the discovery, closing the data transmission session, checks the access rights, controls the synchronization of the beginning and end of the transmission. For example, if you swing some file from the Internet, then your browser (or what you download there) sends a request to the server on which the file is located. At this point, session protocols are included, which provide successfully downloading the file, after which the idea is automatically turned off, although there are options.

6) representative (Presentation) - prepares the data to the processing end application. For example, if this is a text file, then you need to check the encoding (so that it does not work out "krajabyabrov"), it is possible to unpack from the archive .... But here it is clearly traced, what I wrote earlier - it is very difficult to separate where the representative end Level and where the following begins:

7) Applied(Application) - As can be seen from the name, the level of applications that enjoy the data received and we see the result of the works of all levels of the OSI model. For example, you read this text, because it opened it in the right encoding, the desired font, etc. Your browser.

And now, when we have at least a general understanding of the process technology, I consider it necessary to tell you about the bits, frames, packages, blocks and data. If you remember, at the beginning of the article I asked you to not pay attention to the left column in the main table. So, it has come her time! Now we run again across all the levels of the OSI model and usuries, as simple bits (zeros and units) are transformed into data. We will also go upward upwards, in order not to disturb the sequence of mastering the material.

At the physical level, we have a signal. It can be electric, optical, radio wave, etc. So far, it is not even bits, but the network device analyzes the resulting signal and converts it to zeros of units. This process is called "hardware transformation". Further, already inside the network device, the bits are combined into (in one byte eight bits) are processed and transmitted to the channel level.

On the channel level we have the so-called frame.If rudely, this is a bundle byte, from 64 to 1518 in one pack, of which the switch reader heading, in which the recipient's MAC addresses and the sender are recorded as well technical information. Seeing the coincidence of the MAC address in the title and in its switching table(Memory), the switch transmits frames with such coincidences appointment

On the networkthe level of this good is added to the recipient and sender's IP addresses, which are extracted from the same header and is called this package.

On the transport level, the package is addressed to the corresponding protocol, the code which is specified in the header service information and is given to the service of the upper levels, for which it is already full data, i.e. Information in a respondent, suitable for use by the form.

In the scheme below, it will be visible more clearly:

Network model OSI. (Open Systems InterConnection Basic Reference Model is a basic reference model of interaction of open systems, POCK. EMVOS.; 1978) - Network models of the OSI / ISO network protocols (GOST R ISO / IEC 7498-1-99).

OSI General Characteristics

Due to the prolonged development of OSI protocols, currently the main used protocol stack is TCP / IP, designed before adopting the OSI model and out of connection with it.

By the end of the 70s, the world already existed a large number of branded stacks of communication protocols, among which can be called, for example, such popular stacks such as DECNET, TCP / IP and SNA. Such a variety of means of firewall brought to the fore the problem of incompatibility of devices using different protocols. One of the ways to resolve this problem at that time was seen a universal transition to a single, common protocol stack for all systems, created with the deficiencies of already existing stacks. Such an academic approach to creating a new stack began with the development of the OSI model and took seven years (from 1977 to 1984). The appointment of the OSI model consists in a generalized submission of network interaction. It was designed as a kind of universal language of network specialists, which is why it is called the reference model. In the OSI model, the interaction means are divided into seven Levels: Applied, View, Session, Transport, Network, Channel and Physical. Each level deals with a completely defined aspect of interaction of network devices.

Applications can implement their own interaction protocols using a multi-level set of system tools for these purposes. It is for this that the programmers are provided with an application program interface (Application Program Interface, API). In accordance with the ideal scheme of the OSI model, the application can handle requests only to the top level - application, however, in practice, many stacks of communication protocols provide the ability to program the programmers directly access the services, or services below the levels. For example, some DBMSs have built-in tools for remote access to files. In this case, the application, performing access to remote resources, does not use the system file service; It bypasses the upper levels of the OSI model and refers directly to the responsible for transporting messages on the network by system tools that are located at the lower levels of the OSI model. So, let the application of the node A wants to interact with the application of the Node V. For this, Appendix A refers to the request to the application layer, such as the file service. Based on this query, the application-level software generates a standard format message. But in order to deliver this information on purpose, many more tasks have to be solved, the responsibility for which the underlying levels are carried out. After the message is generated, the application level sends it down the stack of the view level. The presentation level protocol based on the information received from the application-level message header performs the required actions and adds to the message its own service information - the view level header in which the guidelines are contained for the address protocol of the address of the addressee. The resulting message is transmitted down by a session level, which, in turn, adds its heading, etc. (Some protocol implementations place the service information not only at the beginning of the message in the form of a header, but at the end in the form of the so-called confection.) Finally, the message reaches the lower, physical, level, which, in fact, and transmits it over the communication lines to the machine-addressee. By this time the message "Faces" the headlines of all levels.

The physical layer places a message on the physical output interface of the computer 1, and it starts its "travel" over the network (up to this point the message was transmitted from one level to another within the computer 1). When a network message arrives at the input interface of the computer 2, it is taken by its physical level and sequentially moves up from the level to the level. Each level analyzes and processes the title of its level by performing the appropriate functions, and then deletes this title and transmits the message by the above level. As can be seen from the description, the protocol entities of one level do not communicate directly, intermediaries are always involved in this communication - media users are always involved - means of underlying level protocols. And only the physical levels of different nodes interact directly.

OSI model levels

In the literature, it is most often customary to start a description of the levels of the OSI model from a 7th level, called application, on which user applications turn to the network. The OSI model ends with a 1st level - physical, on which standards are defined by independent manufacturers to data transmission environments:

• type of transmitting medium (copper cable, fiber optic, radio ester, etc.),
• signal modulation type
• signal levels of logical discrete states (zero and units).

Any OSI model protocol must interact either with its level protocols or protocols per unit higher and / or below its level. Interactions with the protocols of their level are called horizontal, and with levels per unit higher or lower - vertical. Any OSI model protocol can only execute the functions of its level and cannot perform the functions of another level, which is not performed in the protocols of alternative models.

Each level with a certain part of the convention corresponds to its operand - a logically indivisible data element, which can be operated on the model and used protocols at a particular level: at the physical level, the smallest unit - bit, on the channel level the information is combined into frames, on the network - in packets ( Datagram), on transport - to segments. Any data fragment, logically combined - frame, package, datagram - is considered to be a message. It is messages generally that are operands of session, representational and application levels.

Basic network technologies include physical and channel levels.

Applied level

Application Level (Application Level; Application Layer) - the top level of the model, which ensures the interaction of custom applications with the network:

• allows applications to use network services:
  • remote access to files and databases,
  • email shipment;
• responsible for transferring official information;
• provides applications for errors;
• generates requests to the view level.

Applied level protocols: RDP, HTTP, SMTP, SNMP, POP3, FTP, XMPP, OSCAR, MODBUS, SIP, Telnet and others.

Presentation level

Executive Level (Presentation Level; Presentation Layer) Provides protocol transformation and encoding / decoding data. Application requests received from the application level are converted to the format for transmission over the network, and the data obtained from the network are converted to application format. At this level, compression / unpacking or encryption / decryption, as well as reciprocating requests to another network resource, if they cannot be processed locally.

The level of representations is usually an interim protocol to convert information from adjacent levels. This allows you to exchange between applications on heterogeneous computer systems Transparent for applications manner. Presentation level provides code formatting and conversion. Code formatting is used to guarantee the application for processing information that would have meaning for it. If necessary, this level can translate from one data format to another.

The view level is dealt not only with data formats and presentation, it is also engaged in data structures that are used by programs. Thus, level 6 provides the organization of data when sending them.

To understand how it works, imagine that there are two systems. One uses advanced data binary code EBCDIC information exchange, for example, it may be the IBM mainframe, and the other is the American standard ASCII information exchange code (it uses most of other manufacturers of computers). If these two systems need to exchange information, then the level of representations, which performs the transformation and translates between two different formats.

Another function performed at the presentation level is the data encryption that is used in cases where it is necessary to protect the transmitted information from access by unauthorized recipients. To solve this task, processes and codes that are at the view level must perform data conversion. At this level, there are other subprograms that compress texts and convert graphic images into bit streams, so that they can be transmitted over the network.

Presentation level standards also define ways of presenting graphic images. For these purposes, PICT format can be used - image format used to transmit QuickDraw graphics between programs.

Another representation format is a tagged file format images TIFF.which is usually used for high-resolution raster images. The following standards of representations that can be used for graphic images is the standard developed by the United Expert Group on the photograph (Joint photographic Expert Group); In everyday use, this standard is just called JPEG.

There is another group of standards levels of representations, which determines the presentation of sound and film films. Musical Instrument Digital Interface, MIDI (Musical Instrument Digital Interface, MIDI) includes a digital music presentation, developed by the expert group of cinematography MPEG standard, used to compress and encode video on CDs, storage in digitized and transmission with speeds up to 1.5 Mbit / s, and QuickTime is a standard that describes audio and video items for programs performed on Macintosh and PowerPC computers.

Presentation Level Protocols: AFP - Apple Filing Protocol, ICA - Independent Computing Architecture, LPP - Lightweight Presentation Protocol, NCP - NetWare Core Protocol, NDR - Network Data Representation, XDR - External Data Representation, X.25 Pad - Packet Assembler / Disassembler Protocol .

Session Level

Session Layer model ensures that the communication session is maintained, allowing applications to interact with each other for a long time. The level controls the creation / completion of the session, the exchange of information, task synchronization, the definition of data transfer and maintaining a session during applications' inactivity periods.

Session Log Protocols: ADSP (Appletalk Data Stream Protocol), ASP (Appletalk Session Protocol for Multimedia Communication), ISO-SP (OSI Session Layer Protocol (X.225, ISO 8327)), ISNS Internet Storage Name Service), L2F (Layer 2 Forwarding Protocol), L2TP (Layer 2 Tunneling Protocol), NetBiOS (Network Basic Input Authentication Protocol), PPTP (Point-to-Point Tunneling Protocol), RPC (Remote Procedure Call Protocol), RTCP (Real-Time Transport Control Protocol), SCP (Session Control Protocol), ZIP (SOCKETS Direct Protoco]), SCP (SOCKETS DIRECT PROTOCO]) .

Transport level

Transport Layer Model is designed to ensure reliable data transfer from the sender to the recipient. At the same time, the level of reliability may vary widely. There are many classes of transport level protocols, ranging from protocols that provide only the main transport functions (for example, data transmission functions without confirmation of reception), and ending with protocols that guarantee delivery to the destination of several data packets in the proper sequence, multiplexing multiplexes of data streams, provide The data flow control mechanism and ensure the accuracy of the received data. For example, UDP is limited to the control of the integrity of the data within one datagram, and does not exclude the possibility of losing the entire package, or duplicating packets, violating the procedure for obtaining data packets; TCP provides reliable continuous data transfer that eliminates data loss or violation of their arrival or duplication, can redistribute data, breaking large data portions into fragments and, on the contrary, gluing fragments in one package.

Transport level protocols: ATP (Appletalk Transaction Protocol), CUDP (CYCLIC UDP), DCCP (Datagram Congestion Control Protocol), FCP (Fiber Channel | Fiber Channel Protocol), IL (IL Protocol), NBF (Netbios Frame Protocol), NCP ( Netware Core Protocol), SCTP (Stream Control Transmission Protocol), SST (Structured Stream Transport), TCP (TRANSMISSION CONTOCOL), UDP (User Datagram Protocol).

Network level

Network Level (Lang-en | Network Layer) The model is designed to determine the data transfer path. Responsible for the broadcast of logical addresses and names in the physical, determination of shortest routes, switching and routing, tracking problems and "congestion" on the network.

Network level protocols route data from the source to the recipient. The device operating at this level (routers) is conventionally called third-level devices (by level number in the OSI model).

Network Level Protocols: IP / IPv4 / IPv6 (Internet Protocol), IPX (Internetwork Packet Exchange, Fireless Exchange Protocol), X.25 (Partially This Protocol is implemented at 2), CLNP (Network Protocol without Connections), IPsec (Internet Protocol Security). Routing Protocols - RIP (Routing Information Protocol), OSPF (Open Shortest Path First).

Channel Level

Data Link Layer is designed to ensure network interaction on the physical level and control over errors that may occur. The data obtained from the physical layer presented in the bits, it packs into frames, checks them for integrity and, if necessary, corrects errors (forms a repeated request of the damaged frame) and sends to a network level. The channel level can interact with one or more physical levels, controlling and managing this interaction.

The IEEE 802 specification shares this level into two supremes: Mac (Media Access Control) adjusts access to a shared physical environment, LLC (Logical Link Control) provides network level maintenance.

At this level, switches, bridges and other devices work. It is said that these devices use a second level addressing (by level number in the OSI model).

Channel Level Protocols: ArcNet, ATM (Asynchronous Transfer Mode), Controller Area Network (CAN), ECONET, IEEE 802.3 (Ethernet), Ethernet Automatic Protection Switching (EAPS), Fiber Distributed Data Interface (FDDI), Frame RELAY, HIGH-LEVEL Data Link Control (HDLC), IEEE 802.2 (Provides LLC Functions to IEEE 802 Mac Layers), Link Access Procedures, D Channel (LAPD), IEEE 802.11 Wireless Lan, Localtalk, MultiproTocol Label Switching (MPLS), Point-to-Point Protocol (PPP), Point-to-Point Protocol Over Ethernet (PPPOE), Starlan, Token Ring, Unidirectional Link Detection (UDLD), x.25]], ARP.

In programming, this level represents the network card driver, in operating systems there is a software interface of interaction between the channel and network layer. This is not a new level, but simply the implementation of the model for a specific OS. Examples of such interfaces: ODI, NDIS, UDI.

Physical level

Physical Level (Physical Layer) is the lower level of the model that defines the data transfer method presented in binary form from one device (computer) to another. Different organizations are engaged in the preparation of such methods, including: Institute of Electrical Engineering Engineers and Electronics, Electronic Industry Alliance, European Telecommunications Standards Institute and others. We transmit electrical or optical signals to the cable or in the radio and, accordingly, their reception and conversion into data bits in accordance with the methods of coding digital signals.

At this level also work hubs]], signal repeaters and media converters.

The functions of the physical layer are implemented on all devices connected to the network. On the computer, the physical layer function is performed by a network adapter or serial port. The physical level includes physical, electrical and mechanical interfaces between two systems. The physical layer defines such types of data environments such as fiber, twisted pair, coaxial cable, satellite data transmission, etc. The standard types of network interfaces relating to the physical level are :)