OSI model. Lower levels

The model consists of 7 levels located one above the other. The layers interact with each other (vertically) through interfaces, and can interact with a parallel layer of another system (horizontally) using protocols. Each level can only interact with its neighbors and perform the functions assigned only to it. More details can be seen in the figure.

Application (Application) level Application layer)

The upper (7th) level of the model ensures interaction between the network and the user. The layer allows user applications to access network services, such as database query handler, file access, email forwarding. It is also responsible for transmitting service information, providing applications with information about errors and generating requests to presentation level. Example: POP3, FTP.

Executive (Presentation Level) Presentation layer)

This layer is responsible for protocol conversion and data encoding/decoding. It converts application requests received from the application layer into a format for transmission over the network, and converts data received from the network into a format application-friendly. This layer can perform compression/decompression or encoding/decoding of data, as well as redirecting requests to another network resource if they cannot be processed locally.

Layer 6 (presentations) of the OSI reference model is typically an intermediate protocol for converting information from neighboring layers. This allows exchange between applications on heterogeneous computer systems in a way that is transparent to applications. The presentation layer provides code formatting and transformation. Code formatting is used to ensure that the application receives information to process that makes sense to it. If necessary, this layer can perform translation from one data format to another. The presentation layer not only deals with the formats and presentation of data, it also deals with the data structures that are used by programs. Thus, layer 6 provides organization of data as it is sent.

To understand how this works, let's imagine that there are two systems. One uses advanced data representation binary code ASCII information exchange (used by most other computer manufacturers). If these two systems need to exchange information, then a presentation layer is needed that will perform the conversion and translate between the two different formats.

Another function performed at the presentation layer is data encryption, which is used in cases where it is necessary to protect transmitted information from being received by unauthorized recipients. To accomplish this task, processes and code in the presentation layer must perform data transformation. There are other routines at this level that compress texts and convert graphics into bitstreams so they can be transmitted over a network.

Presentation layer standards also define how to present graphic images. For these purposes, the PICT format can be used, an image format used to transfer QuickDraw graphics between Macintosh and PowerPC programs. Another presentation format is the tagged JPEG image file format.

There is another group of presentation level standards that define the presentation of audio and film fragments. This includes the electronic interface musical instruments MPEG, used to compress and encode video on CDs, store them in digitized form and transmit at speeds up to 1.5 Mbit/s, and Session layer)

Layer 5 of the model is responsible for maintaining the communication session, allowing applications to interact with each other long time. The layer manages session creation/termination, information exchange, task synchronization, data transfer eligibility determination, and session maintenance during periods of application inactivity. Transmission synchronization is ensured by placing checkpoints in the data stream, starting from which the process is resumed if interaction is disrupted.

Transport layer Transport layer)

The 4th level of the model is designed to deliver data without errors, losses and duplication in the sequence in which they were transmitted. It does not matter what data is transmitted, from where and where, that is, it provides the transmission mechanism itself. It divides data blocks into fragments, the size of which depends on the protocol, combines short ones into one, and splits long ones. Protocols at this level are designed for point-to-point communication. Example: UDP.

There are many classes of transport layer protocols, ranging from protocols that provide only basic transport functions (for example, data transfer functions without acknowledgment), to protocols that ensure that multiple data packets are delivered to the destination in the proper sequence, multiplex multiple data streams, provide data flow control mechanism and guarantee the reliability of the received data.

Some network layer protocols, called connectionless protocols, do not guarantee that data is delivered to its destination in the order in which it was sent by the source device. Some transport layers cope with this by collecting data in the correct sequence before passing it to the session layer. Data multiplexing means that the transport layer is capable of simultaneously processing multiple data streams (streams can also come from various applications) between two systems. A flow control mechanism is a mechanism that allows you to regulate the amount of data transferred from one system to another. Transport layer protocols often have a data delivery control function, forcing the receiving system to send acknowledgments to the sending side that the data has been received.

Network layer Network layer)

3rd level network model OSI is designed to define the path for data transmission. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring problems and congestion in the network. A network device such as a router operates at this level.

Network layer protocols route data from a source to a destination and can be divided into two classes: connection-oriented and connectionless protocols.

You can describe the operation of protocols with connection establishment using the example of work regular phone. Protocols of this class begin data transmission by calling or establishing a route for packets to follow from source to destination. After that, serial data transfer begins and then the connection is terminated upon completion of the transfer.

Connectionless protocols, which send data containing complete address information in each packet, work similarly postal system. Each letter or package contains the address of the sender and recipient. Next, each intermediate post office or network device reads the address information and makes a decision on data routing. A letter or data packet is transmitted from one intermediate device to another until it is delivered to the recipient. Connectionless protocols do not guarantee that information will reach the recipient in the order in which it was sent. For setting the data in the appropriate order when using network protocols transport protocols respond without establishing a connection.

Data link layer Data Link layer)

This layer is designed to ensure the interaction of networks at the physical layer and control errors that may occur. It packs the data received from the physical layer into frames, checks it for integrity, corrects errors if necessary (sends a repeated request for a damaged frame) and sends it to the network layer. The data link layer can communicate with one or more physical layers, monitoring and managing this interaction. The IEEE 802 specification divides this layer into 2 sublayers - MAC (Media Access Control) regulates access to the shared physical medium, LLC (Logical Link Control) provides network layer service.

In programming, this level represents the driver network card, operating systems have software interface interaction of the channel and network layers with each other, this is not new level, but simply an implementation of the model for a specific OS. Examples of such interfaces: ODI,

Physical level Physical layer)

The lowest level of the model is intended to directly transmit the data stream. Transmits electrical or optical signals into a cable or radio broadcast and, accordingly, receives them and converts them into data bits in accordance with digital signal coding methods. In other words, it provides an interface between the network media and the network device.

Sources

• Alexander Filimonov Construction of multiservice Ethernet networks, bhv, 2007 ISBN 978-5-9775-0007-4
• Internetworking Technologies Handbook //cisco systems, 4th edition, Williams 2005 ISBN 584590787X

Wikimedia Foundation. 2010.

The concept of a reference model is widely used in communications and computer science.

• Reference model, master model in systemic and program area is a model of something that has a common purpose or idea and can be considered as a standard for various purposes [Wikipedia].

• Reference model - This abstract representation concepts and relationships between them in a certain problem area. Based on the reference model, more specific and detailed models are built, ultimately embodied in real-life objects and mechanisms [Wikipedia-rus].
  

• Reference Model is an abstract structure (framework) for understanding the essential relationships between objects of some environment, which further allows the development of specific architectures using certain standards or specifications supported by this environment. The reference model contains minimum set unified concepts, axioms, and relationships relevant to a specific problem area, and independent of specific standards, technologies, implementations, or other specific details.

  Purpose of introducing a reference model consists in defining the essence of the system architecture and introducing terminology, as well as describing the general principle of the system’s operation. The model defines the connections that are significant for the functioning of the system as an abstract model, independent of the variant technical implementation and from constantly evolving technologies that could affect system implementation. Often, architecture is developed in the context of a predefined configuration that includes protocols, profiles, specifications, and standards.

  There are many uses of the reference model. One use case is to create standards for the objects that are contained in the model and how they interact with each other. When developing specific application communication standards and systems, their architecture is compared with standard model. With this approach, the work of specialists who need to create or analyze communication system objects that behave in accordance with the standard is much easier.
  

• An example of a reference model standard is network interaction reference model open systems (EMVOS) OSI (Open Systems Interconnection Basic Reference Model ) International Organization for Standardization ISO - the basic model of architectures for data communication systems, which is good remedy to analyze and study modern communication standards and technologies.
  

Seven-layer OSI model

The universal nature of the classic seven-layer OSI network reference model makes it possible to create models based on it for specific standards, which are also called reference standards. For example, in Fig.... The DECT reference model is presented, the key functions of which are structured only at the three lower levels of the OSI model: network, data link and physical.

DECT reference model

1. Reference Model for Service Oriented. Architecture 1.0. Committee Specification 1, 2 August 2006. http://www.oasis-open.org/

The OSI reference model is the defining document for the development of open standards for interconnecting communication systems and networks that have varying levels of complexity and use different technologies. In this regard, it is also commonly called open systems architecture or reference model of open systems interaction (OSI)).

The developers of the reference model were guided by the following principles.

· The number of protocol layers should not be too large so that network design and implementation are not overly complex, and at the same time it should not be too small so that the logical modules executed at each level are not overly complex.

· Levels must be clearly distinguished by the functions (objects) and logical modules performed on them.

· Functions and protocols of one layer can be changed if it does not affect other layers.

· The amount of information transferred through interfaces between layers should be minimal.

· Further division of levels into sublevels is allowed if there is a need for local allocation of functions within one level. Division into sublevels is advisable when it is necessary to split a labor-intensive task into separate, less complex ones.

The resulting reference model contains seven levels (Figure 4.24).

The highest, seventh, level of the OSI model is application layer (Application), which manages terminals and application processes in end systems, which are sources and consumers of information in information network. This layer exposes services directly to user applications. To avoid incompatibility between user programs, the application layer defines standard methods representations of services at this level. This frees programmers from having to re-code the same functions in every network application they create. Application layer services themselves are not applications. The application layer provides programmers with a set of open standard interfaces application programming (API - Application Programming Interface), which can be used to perform network application functions such as file transfer, remote registration, etc. As a result, the modules application programs are smaller in size and require less memory.

The application layer for users is the most visible part of the OSI model, since it is responsible for launching programs, their execution, data input and output, administrative management network. The protocols for interaction between objects of the seventh level are called applied.

Presentation layer (Presentation) interprets and transforms data transmitted over the network into a form understandable for application processes. Provides data presentation in consistent formats and syntax, translation and interpretation of programs from various languages, data encryption and compression. Thanks to this, the network does not impose any restrictions on the use of various types of computers as end systems. In practice, many of the functions of this layer are grouped with functions of the application layer, so the presentation layer protocols have not received adequate development and are not used in many networks.

Session layer (Session) provides the performance of functions for managing a communication session (session) focused on end-to-end transmission of messages, such as, for example: establishing and terminating a session; control of queue and data transmission mode (simplex, half-duplex, full-duplex); synchronization; session activity management; preparation of reports on exceptional situations.

Figure 4. OSI reference model

In logical connection sessions, connection establishment and connection termination requests, as well as data transfer requests, are forwarded to the underlying transport layer. At the end of a session, the session level carries out a gradual, rather than sudden, end of the session, and performs a handshake procedure (sending a service message about the end of the communication session), which helps prevent data loss in the case when one of the parties wants to interrupt the dialogue, but the other does not. Sessions are extremely useful when there is a logical connection between a client and a server on a network. It should be noted that without establishing a logical connection, a session, as a rule, is not possible. However, there is an exception to this rule and some networks support connectionless file transfer. Even so, the session layer provides some useful functions to manage the dialogue. Session layer services are optional and useful only for certain applications; for many applications they provide only limited benefit. Often the functions of this layer are implemented at the transport layer, so session layer protocols have limited use.

Transport layer (Transport) performs message segmentation and manages end-to-end, error-free transport of data from source to consumer. The complexity of transport layer protocols is inversely proportional to the reliability of services at lower layers (network, data link and physical).

Segmentation function consists of splitting long information messages into transport level data blocks - segments. In the case of a small message, the segment is associated with its size. When managing end-to-end data transportation, the transport layer supports functions such as: addressing, connection establishment and termination, data flow control, data prioritization, error detection and correction, failure recovery, multiplexing. Transport layer protocols are divided into two types: connection-oriented protocols and protocols that provide for higher layers reliable service without establishing connections. With the increasing number of applications that do not require guaranteed message delivery or do not allow message retransmission as a method of error control (real-time applications such as video streaming or IP telephony), non-deliverable transport layer protocols are gaining popularity.

Addressing function at the transport layer, in contrast to addressing at the network and data link layers, consists of attaching an additional unique address that identifies the application process running in the end system. Most computers are capable of running multiple processes simultaneously, supporting multiple applications running simultaneously. However, at the network level, each of them, as a rule, is associated with one address - this is the hardware address of the port of the destination computer. When a packet (a block of network layer data) reaches a port on a destination computer, the latter must know which running process it is destined for. It is this information that the unique transport layer address provides.

So the transport layer address is logical(corresponds to the software port associated with a specific application). It is the only one that addresses a process, not a machine (unlike addresses at the link and network layers).

Connection establishment and disconnection function upon request of the session layer between peer objects of the transport layer is implemented through the procedure three-way handshake.

This procedure minimizes the chance of accidentally establishing an erroneous connection by requiring two acknowledgments in response to a single connection request. A connection is established only when all three events (request, confirmation of request receipt, confirmation of receipt of acknowledgment) occur within a specified time period. This allows us to judge that both transport layer objects are ready for the communication session. If the procedure's actions do not fit within the specified period of time, for example, due to delays or damage to service packets, it is initiated again.

The release of the transport layer connection is also controlled by a three-way handshake, which ensures its correctness. The connection is terminated separately in the forward and reverse directions, which eliminates the possibility of loss of user data in the case when one of the parties has completed data transmission, and the other is still active.

Flow control function consists of agreeing on transmission parameters during the three-way handshake procedure. These parameters include: the maximum size of the data segment for the established connection; the size of the free space of the receiver buffer where incoming segments will be placed; the size of a group of segments upon receipt of which the receiver must send an acknowledgment of receipt to the transmitter. Confirmations serve not only as evidence of correctly received data, but also indicate what next number of segments can be received taking into account the current load of the receiving buffer.

Data Prioritization Function is the exclusive prerogative of the transport level. The lower network layer has no idea about the existence of priority traffic and perceives all packets (network layer data blocks) as the same.

Many transport layer protocols support two priorities: ordinary data and urgent. The priority assignment request comes from the session layer. The assigned priority identifier is placed in the transport layer overhead field appended to the segment.

Separate buffer pools can be organized for each priority. In this case, the transportation algorithm provides for priority servicing of the urgent data buffer and only after its emptying - the regular data buffer.

Another approach is to group urgent and regular data segments into one transmitted block and place a boundary indicator of their location in the service information field.

Error detection and correction function performed by many link layer protocols, but the transport layer does not duplicate it at all. The difference is that the data link layer detects and corrects bit errors that occur at the physical layer when transmitting bits, and the transport layer eliminates errors that arise as a result of incorrect operation of the network layer (packet loss, late delivery of packets, etc.). In addition, in networks where the data link layer is not responsible for detecting and correcting errors in binary bits or where this layer is absent altogether, the transport layer takes on these functions.

The transport layer's function of identifying erroneous packets is based on the ordering of segments. To do this, each segment is assigned a serial number and its own timer starts at the moment of sending. The timer runs until an acknowledgment (positive or negative) of packet reception is received at the receiving end. In case of negative confirmation, the transmitter repeats the transmission of the segment.

In some simpler implementations of transport layer protocols, a positive acknowledgment of receipt of the last segment of a message is perceived as the error-free receipt of all its segments. Receiving a negative acknowledgment means that the transmitter must retransmit segments from the point (segment) where the error occurred (this mechanism is called back-to-N transmission). If the segment timer expires, an error detection procedure is initiated.

Crash recovery feature provides the ability to recover lost data in the event of network malfunctions. Failures include: failure of the communication line (and, as a result, loss of the virtual connection), failure of the network node equipment (and, as a result, loss of packets in a connectionless environment), and, finally, failure of the computer to which the data is addressed. If failure individual components network is short-lived and it is quickly possible to establish a new virtual channel or find a route that bypasses the faulty node, the transport layer, analyzing serial numbers segments specifies which segments have already been received and which should be retransmitted. In case of long-term network damage, the transport layer can establish a transport connection in a backup network (if one is provided).

If the transmitting or receiving computer fails, the operation of the transport layer is suspended, since it operates under the control of the operating systems installed in them. After the machine is restored to working capacity, the transport layer begins to initiate broadcasting broadcast messages all computers operating on the network, in order to identify the one that had an active transport connection with the failed one. In this way, the restored computer is able to restore the interrupted connection by relying on information stored in healthy machines.

Multiplexing function allows in one network connection organize several transport layer connections. The transport layer address, discussed earlier, allows the transport layer to distinguish between segments addressed to different application processes. The advantage of such multiplexing is that it reduces the cost of transporting data in the network. However, it makes sense only when the network operation mode is connection-oriented (virtual channel).

In conclusion, let us dwell once again on the features of the transport layer in connectionless mode. As noted above, it is used when guaranteed end-to-end data delivery is not required. These are primarily processes that exchange data in real time (audio or video processes), for which delivery without delay is much more important than the reliability achieved through repeated transmissions of segments. In addition, connectionless mode allows you to use your network more efficiently without occupying it. throughput a fair amount of proprietary information. The doubt may arise: “Is the transport layer even necessary when running real-time applications?” And here we should once again emphasize the relevance of the transport layer addressing function, which provides support for several simultaneously running application processes on one machine, which is not possible without transport layer services.

Network layer (Network) performs the main telecommunications function - ensuring communication between end systems of the network. This communication can be implemented by providing an end-to-end channel switched from individual sections in accordance with the optimally selected route, a logical virtual channel, or by direct routing of a data block during its delivery. In this case, the network layer frees higher levels from knowing through which sections of the network or through which networks the information transmission route passes. While higher layers (application, presentation, session and transport) are typically present in end systems communicating over a network, the lower three layers (network, link and physical) are also required for all intermediate network devices located in transit points data transmission route.

The main function of the network layer is routing It consists in deciding through which specific intermediate points the data transmission route sent from one end system to another should pass and how switching should be performed between the inputs and outputs of network devices located in intermediate points network corresponding to a specific route.

The blocks of data that the network layer operates on are called packages. A packet is formed by adding to the segment transmitted from the transport layer a header including network layer address. It consists of two parts and identifies as the network address end user, and the user himself in it.

Networks with different network addresses connect with each other routers(see section “physical structure of the network”). In order to transmit a packet from a sender located on one network to a recipient located on another network, it is necessary to make several transit “hops” - hops(hops) between networks, each time choosing the best (in terms of travel time or reliability) route. The network layer also solves problems of interaction between networks and various technologies and creating protective barriers to prevent unwanted traffic between networks.

At the network level, two types of protocols are used. These are the actual network protocols that ensure the movement of packets through the network. They are usually associated with network-level punctures. Another type of network protocol is routing protocols, which deal with the exchange of routing information. Using these protocols, routers collect information about the topology of internetwork connections. Network layer protocols are executed by modules operating system, as well as software and hardware of routers.

The network layer can also operate protocols to map the destination address of the network layer to the link layer address of the network where the end user is located.

Link layer (Data-link) is responsible for high-quality data transmission between two points connected by a physical channel, taking into account the characteristics of the transmitting environment. The term " data transfer" in contrast to the term " transfer of information"emphasizes precisely this aspect of the activity of the link layer. If a connection is established between two end systems that are not directly connected, it will involve as many independently operating physical data links as possible. However, their physical transmission media may differ (copper, optical fiber). The requirements for the format of data presentation in each channel, which is called linear coding. In this situation, the data link layer takes over the functions of adapting the data to the type of physical communication channel, providing the upper layers with a “transparent connection”.

A block of data at the link layer is called frame or frame. Network layer packets combined into a frame are framed by separating flags (special sequences of bits placed at the beginning and end of a block of packets). In addition, a checksum is added to the frame, which is used to check the accuracy of the frame transmitted over the channel. If an uncorrectable error is detected, the receiver requests the transmitter to retransmit the frame. Data transmission theory and coding theory are quite well developed, which makes it possible to ensure high efficiency of link layer protocols. It should be noted that the bit error correction function is not always mandatory for the link layer, so it is absent in some link layer protocols (Ethernet, Frame relay). Sometimes in global networks It is generally difficult to isolate the functions of the link layer in their pure form, since in the same protocol they are combined with functions of the network layer (ATM, Frame relay).

To the number important functions The link layer also includes: access control to the communication channel, frame synchronization, data flow control, addressing, connection establishment and disconnection.

Channel access control determined by the type of physical channel connecting the stations and the number of stations connected to it. The type of channel is determined by its operating mode (duplex, half-duplex) and configuration (two-point - only two stations, multipoint - more than two stations). Access control is relevant in half-duplex mode of operation of a channel with a multipoint configuration, when stations must wait for the moment to begin their data transmission.

Frame synchronization provides the receiver with the opportunity precise definition the beginning and end of the received frame. Two methods are defined for data transmission: character-oriented (usually 8-bit character) asynchronous transmission, where the transmission of each character is preceded by a start bit and terminated by a stop bit, and frame-oriented synchronous transmission, where the start and flag flags are used as synchronization sequences. end of frame.

Data Flow Control is to provide the receiver with the opportunity to inform the transmitter about its readiness or unreadiness to receive frames. The effect is that it prevents a situation where the transmitter floods the receiver with frames that it is unable to process.

Addressing required in the case of a multipoint channel configuration with more than two stations to identify the recipient. Link layer addresses are called hardware. The address field contains the destination address and the source address.

Establishing and releasing a connection is a procedure for activating and deactivating a link layer connection that is performed by software. In this case, the transmitting station initiates the connection by sending to the recipient special team“start”, and the receiver sends a connection confirmation, after which data transmission begins. This procedure is also performed after crashes and restarts. software link level. There is also a "stop" command that stops the software from running.

Physical layer (Physical) is responsible for placing bits of information into the physical medium. The following types of media can be used at the physical layer: cable " twisted pair", coaxial cable, fiber optic cable, local digital channel and airwaves. The main characteristics of physical transmission media are parameters such as bandwidth, noise immunity, characteristic impedance, etc. Here, the physical interfaces of devices with the transmission medium and between devices between which bits are transmitted are implemented.

The main characteristics of the physical layer can be grouped into the following groups.

Mechanical. These are characteristics that relate to the physical properties of the interface with the transmission medium, i.e. connectors that provide connection of a device to one or more conductors. The types of connectors and the purpose of each pin are usually standardized.

Electrical. Determine the requirements for the representation of bits transmitted to the physical medium, for example, current or voltage levels of transmitted signals, pulse slopes, types of linear codes, signal transmission rates.

Functional. Determine the functions of individual channels of physical interfaces of devices interacting through the transmission medium. The main schemes for interaction of devices at the physical layer are: simplex communication (one-way), half-duplex communication (alternate) and full-duplex communication (two-way, simultaneous), sometimes called full-duplex. In this case, two options for organizing communication can be implemented: “ point-to-point" And " dot-many dots" In the first option, two devices share one link, which in turn can be simplex, half-duplex or full-duplex. The second option assumes that data transmitted by one device is received by many devices. As a rule, such connections are simplex (cable television) or half-duplex (local network based on Ethernet standard). In some cases, they can also be used duplex communications(network based on SONET technology). Other physical layer topologies can be used, such as tire, star, ring, however, they are all variations of the point-to-point and point-to-multipoint communication options. Thus, the bus topology is a typical “point - many points” option, the star topology is a set of point-to-point connections, the ring is a set of circular point-to-point connections.

Procedural. They define the rules by which bit streams are exchanged across the physical medium. These are sequential and parallel interfaces. In the first case, there is only one communication channel between interacting devices, through which bits are transmitted one after another. This results in a limited transfer rate and therefore a slow interface. In the second case, several bits are transmitted between communicating devices simultaneously over several channels. At the same time, the transmission speed increases.

One of the important functions of the physical layer is multiplexing, providing the combination of many narrowband (low-speed) channels into one broadband (high-speed) channel. As is known, according to the technological principle, a distinction is made between frequency division multiplexing (FDM) and time division multiplexing (TDM). FDM and TDM technologies can be combined in such a way that a subchannel in a frequency division multiplexing system is divided into several channels by time division multiplexing. This technique is used in the operation of digital cellular networks.

Just started working as a network administrator? Don't want to get confused? Our article will be useful to you. Have you heard a time-tested administrator talk about network problems and mention some levels? Have you ever been asked at work which levels are protected and work if you use old firewall? To understand the basics of information security, you need to understand the hierarchy of the OSI model. Let's try to see the capabilities of this model.

A self-respecting system administrator should be well versed in network terms

Translated from English - the basic reference model for the interaction of open systems. More precisely, the network model of the OSI/ISO network protocol stack. Introduced in 1984 as a conceptual framework that separated the process of sending data to world wide web in seven easy steps. It is not the most popular, since the development of the OSI specification has been delayed. The TCP/IP protocol stack is more advantageous and is considered the main model used. However, you have a huge chance to encounter the OSI model in your position system administrator or in the IT field.

Many specifications and technologies have been created for network devices. It's easy to get confused in such diversity. It is the open systems interaction model that helps network devices using each other to understand each other. various methods communication. Note that OSI is most useful for software and hardware manufacturers involved in the design of compatible products.

Ask, what benefit does this have for you? Knowledge multi-level model will give you the opportunity to freely communicate with employees of IT companies; discussing network problems will no longer be oppressive boredom. And when you learn to understand at what stage the failure occurred, you can easily find the reasons and significantly reduce the range of your work.

OSI levels

The model contains seven simplified steps:

• Physical.
• Duct.
• Network.
• Transport.
• Sessional.
• Executive.
• Applied.

Why does breaking it down into steps make life easier? Each level corresponds to a specific stage of sending a network message. All steps are sequential, which means that the functions are performed independently, there is no need for information about the work at the previous level. The only necessary components are how the data from the previous step is received, and how the information is sent to the subsequent step.

Let's move on to a direct acquaintance with the levels.

Physical layer

The main task of the first stage is sending bits through physical communication channels. Physical communication channels are devices created for transmitting and receiving information signals. For example, fiber optic, coaxial cable or twisted pair. Transfer can also take place via wireless communication. The first stage is characterized by the data transmission medium: protection from interference, bandwidth, characteristic impedance. The qualities of the electrical final signals (type of encoding, voltage levels and signal transmission speed) are also set and brought to standard types connectors, contact connections are assigned.

The functions of the physical stage are performed on absolutely every device connected to the network. For example, a network adapter implements these functions on the computer side. You may have already encountered the first step protocols: RS-232, DSL and 10Base-T, which define the physical characteristics of the communication channel.

Data Link Layer

At the second stage, the abstract address of the device is associated with the physical device, and the availability of the transmission medium is checked. Bits are formed into sets - frames. The main task of the link layer is to identify and correct errors. For correct transmission, specialized bit sequences are inserted before and after the frame and a calculated checksum is added. When the frame reaches the destination, the checksum of the already arrived data is calculated again if it matches checksum in the frame, the frame is recognized as correct. Otherwise, an error appears that can be corrected by retransmitting information.

Channel stage does possible transfer information, thanks to a special structure of connections. In particular, buses, bridges, and switches operate through link layer protocols. Step two specifications include: Ethernet, Token Ring and PPP. The functions of the channel stage in a computer are performed by network adapters and drivers for them.

Network layer

In standard situations, the functions of the channel stage are not enough for high-quality information transfer. Second step specifications can only transfer data between nodes with the same topology, for example, a tree. There is a need for a third stage. It is necessary to form a unified transport system with a branched structure for several networks that have an arbitrary structure and differ in the method of data transfer.

To explain it differently, the third step processes the Internet protocol and performs the function of the router: search the best way for information. A router is a device that collects data about the structure of internetwork connections and transmits packets to the destination network (transit transfers - hops). If you encounter an error in the IP address, then it is a problem originating at the network level. The third stage protocols are broken down into networking, routing or address resolution protocols: ICMP, IPSec, ARP and BGP.

Transport layer

In order for the data to reach applications and the upper layers of the stack, a fourth stage is required. It provides the required degree of reliability of information transmission. There are five classes of transport stage services. Their difference lies in the urgency, feasibility of restoring interrupted communication, and the ability to detect and correct transmission errors. For example, packet loss or duplication.

How to choose a transport stage service class? When the quality of communication channels is high, a lightweight service is an adequate choice. If communication channels are not secure at the very beginning, it is advisable to resort to a developed service that will provide maximum possibilities to find and solve problems (data delivery control, delivery timeouts). Stage 4 specifications: TCP and UDP of the TCP/IP stack, SPX of the Novell stack.

Association first four levels is called the transport subsystem. It fully provides the selected level of quality.

Session layer

The fifth stage helps in regulating dialogues. It is impossible for interlocutors to interrupt each other or speak synchronously. The session layer remembers the active party at a particular moment and synchronizes information, coordinating and maintaining connections between devices. Its functions allow you to return to control point during a long shipment and do not start all over again. Also at the fifth stage, you can terminate the connection when the exchange of information is completed. Session layer specifications: NetBIOS.

Executive level

The sixth stage is involved in the transformation of data into a universal recognizable format without changing the content. Since in different devices various formats are utilized, information processed at the representational level makes it possible for systems to understand each other, overcoming syntactic and coding differences. In addition, at the sixth stage, it becomes possible to encrypt and decrypt data, which ensures secrecy. Examples of protocols: ASCII and MIDI, SSL.

Application layer

The seventh stage on our list and the first if the program sends data over the network. Consists of sets of specifications through which the user, Web pages. For example, when sending messages by mail, it is at the application level that you select convenient protocol. The composition of the seventh stage specifications is very diverse. For example, SMTP and HTTP, FTP, TFTP or SMB.

You may have heard somewhere about the eighth level of the ISO model. Officially, it does not exist, but a comic eighth stage has appeared among IT workers. This is all due to the fact that problems can arise due to the fault of the user, and as you know, a person is at the pinnacle of evolution, so the eighth level appeared.

Having considered the OSI model, you were able to understand complex structure network operation and now you understand the essence of your work. Things get pretty simple when you break the process down!

It is definitely better to start with theory, and then gradually move on to practice. Therefore, first we will consider the network model (theoretical model), and then we will lift the curtain on how the theoretical network model fits into the network infrastructure (network equipment, user computers, cables, radio waves, etc.).

So, network model is a model of interaction between network protocols. And protocols, in turn, are standards that determine how different programs will exchange data.

Let me explain with an example: when opening any page on the Internet, the server (where the page being opened is located) sends data to your browser ( hypertext document) By HTTP protocol. Thanks to the HTTP protocol, your browser, receiving data from the server, knows how it needs to be processed, and successfully processes it, showing you the requested page.

If you don’t yet know what a page on the Internet is, then I’ll explain in a nutshell: any text on a web page is enclosed in special tags that tell the browser what text size to use, its color, location on the page (left, right, or in the center). This applies not only to text, but also to pictures, forms, active elements and generally all content, i.e. what is on the page. The browser, detecting the tags, acts according to their instructions, and shows you the processed data that is enclosed in these tags. You yourself can see the tags of this page (and this text between the tags), to do this, go to the menu of your browser and select - view source code.

Let's not get too distracted, "Network Model" the right topic for those who want to become a specialist. This article consists of 3 parts and for you, I tried to write it not boringly, clearly and briefly. For details, or for additional clarification, write in the comments at the bottom of the page, and I will certainly help you.

We, as in the Cisco Networking Academy, will consider two network models: the OSI model and the TCP/IP model (sometimes called DOD), and at the same time compare them.

OSI stands for Open System Interconnection. In Russian it sounds like this: Network model of interaction of open systems (reference model). This model can be safely called a standard. This is the model that network device manufacturers follow when developing new products.

The OSI network model consists of 7 layers, and it is customary to start counting from the bottom.

Let's list them:

• 7. Application layer
• 6. Presentation layer
• 5. Session layer
• 4. Transport layer
• 3. Network layer
• 2. Data link layer
• 1. Physical layer

As mentioned above, the network model is a model of interaction between network protocols (standards), and at each level there are its own protocols. It’s a boring process to list them (and there’s no point), so it’s better to look at everything using an example, because the digestibility of the material is much higher with examples;)

Application layer

The application layer or application layer is the most top level models. He communicates custom applications with the network. We are all familiar with these applications: web browsing (HTTP), sending and receiving mail (SMTP, POP3), receiving and receiving files (FTP, TFTP), remote access(Telnet), etc.

Executive level

Presentation layer or presentation layer – it converts data into the appropriate format. It’s easier to understand with an example: those pictures (all images) that you see on the screen are transmitted when sending a file in the form of small portions of ones and zeroes (bits). So, when you send your friend a photo via email, the SMTP Application Layer protocol sends the photo to the lower layer, i.e. to the Presentation level. Where your photo is converted into a convenient form of data for more low levels, for example into bits (ones and zeros).

In exactly the same way, when your friend starts receiving your photo, it will come to him in the form of the same ones and zeros, and it is the Presentation layer that converts the bits into a full-fledged photo, for example, a JPEG.

This is how this level works with protocols (standards) for images (JPEG, GIF, PNG, TIFF), encodings (ASCII, EBDIC), music and video (MPEG), etc.

Session layer

Session layer or session layer - as the name suggests, it organizes a communication session between computers. A good example will serve as audio and video conferences, at this level it is established which codec the signal will be encoded with, and this codec must be present on both machines. Another example is the SMPP protocol (Short message peer-to-peer protocol), which is used to send well-known SMS and USSD requests. One last example: PAP (Password Authentication Protocol) is an old protocol for sending a username and password to a server without encryption.

I won’t say anything more about the session level, otherwise we’ll delve into the boring features of the protocols. And if they (features) interest you, write letters to me or leave a message in the comments asking me to expand on the topic in more detail, and a new article will not be long in coming;)

Transport layer

Transport layer – this layer ensures the reliability of data transmission from sender to recipient. In fact, everything is very simple, for example, you communicate using a webcam with your friend or teacher. Is there a need for reliable delivery of every bit of the transmitted image? Of course not, if a few bits are lost from streaming video You won’t even notice this, even the picture will not change (maybe the color of one pixel out of 900,000 pixels will change, which will flash at a speed of 24 frames per second).

Now let’s give this example: A friend forwards it to you (for example, via mail) in an archive important information or program. You download this archive to your computer. This is where 100% reliability is needed, because... if a couple of bits are lost when downloading the archive, you will not be able to unzip it, i.e. extract the necessary data. Or imagine sending a password to a server, and one bit gets lost along the way - the password will already lose its appearance and the meaning will change.

So, when we watch videos on the Internet, sometimes we see some artifacts, delays, noise, etc. And when we read text from a web page, the loss (or distortion) of letters is not acceptable, and when we download programs, everything also goes without errors.

At this level I will highlight two protocols: UDP and TCP. The UDP protocol (User Datagram Protocol) transfers data without establishing a connection, does not confirm the delivery of data and does not make repetitions. TCP protocol(Transmission Control Protocol), which before transmission establishes a connection, confirms the delivery of data, repeats it if necessary, and guarantees the integrity and correct sequence of the downloaded data.

Therefore, for music, video, video conferencing and calls we use UDP (we transfer data without verification and without delays), and for text, programs, passwords, archives, etc. – TCP (data transmission with confirmation of receipt takes more time).

Network layer

Network layer - this layer determines the path along which data will be transmitted. And, by the way, this is the third level of the OSI Network Model, and there are devices that are called third-level devices - routers.

We have all heard about the IP address, this is what the IP (Internet Protocol) protocol does. An IP address is a logical address on a network.

There are quite a lot of protocols at this level, and we will examine all of these protocols in more detail later, in separate articles and with examples. Now I’ll just list a few popular ones.

How has everyone heard about the IP address? ping command– this is how the ICMP protocol works.

The same routers (with which we will work in the future) use protocols of this level to route packets (RIP, EIGRP, OSPF).

Data Link Layer

Data link layer – we need it for the interaction of networks at the physical level. Probably everyone has heard about the MAC address, here it is physical address. Link layer devices - switches, hubs, etc.

IEEE (Institute of Electrical and Electronics Engineers) defines the data link layer as two sublayers: LLC and MAC.

LLC – Logical Link Control, created to interact with the upper level.

MAC – Media Access Control, created to interact with the lower level.

I’ll explain with an example: your computer (laptop, communicator) has a network card (or some other adapter), and so there is a driver to interact with it (with the card). A driver is some program- the upper sublayer of the channel layer, through which it is possible to communicate with the lower levels, or rather with the microprocessor ( iron) – lower sublayer of the data link layer.

There are many typical representatives at this level. PPP (Point-to-Point) is a protocol for connecting two computers directly. FDDI (Fiber Distributed Data Interface) - the standard transmits data over a distance of up to 200 kilometers. CDP (Cisco Discovery Protocol) is a proprietary (own) protocol company-owned Cisco Systems, it can be used to detect neighboring devices and obtain information about these devices.

Physical layer

Physical layer is the lowest level that directly transfers the data stream. The protocols are well known to us all: Bluetooth, IRDA (Infrared communication), copper wires (twisted pair, telephone line), Wi-Fi, etc.

Conclusion

So we have analyzed the OSI network model. In the next part, we will move on to the TCP/IP Network model, it is smaller and the protocols are the same. To successfully pass the CCNA tests, you need to make a comparison and identify the differences, which will be done.