For a unified representation of data in networks with heterogeneous devices and software, the international organization for ISO standards (International Standardization Organization) has developed a basic model for communication of open systems OSI (Open System Interconnection) . This model describes the rules and procedures for transferring data in various network environments when organizing a communication session. The main elements of the model are layers, application processes and physical means of connection. On fig. 1.10 shows the structure of the basic model.
Each layer of the OSI model performs a specific task in the process of transmitting data over the network. The base model is the basis for the development of network protocols. OSI divides communication functions in a network into seven layers, each of which serves a different part of the open systems interoperability process.
The OSI model only describes system-wide means of interaction, not end-user applications. Applications implement their own communication protocols by accessing system facilities.
Rice. 1.10. OSI model
If an application can take over the functions of some of the upper layers of the OSI model, then for communication it accesses directly the system tools that perform the functions of the remaining lower layers of the OSI model.
Interaction of layers of the OSI model
The OSI model can be divided into two different models, as shown in Fig. 1.11:
A horizontal model based on protocols that provides a mechanism for the interaction of programs and processes on different machines;
A vertical model based on services provided by neighboring layers to each other on the same machine.
Each layer of the sending computer interacts with the same layer of the receiving computer as if it were directly connected. Such a connection is called a logical or virtual connection. In fact, the interaction is carried out between adjacent levels of one computer.
So, the information on the sending computer must pass through all levels. Then it is transmitted over the physical medium to the receiving computer and again passes through all the layers until it reaches the same level from which it was sent on the sending computer.
In the horizontal model, two programs need a common protocol to exchange data. In a vertical model, adjacent layers communicate using Application Programming Interfaces (APIs).
Rice. 1.11. Computer Interaction Diagram in the Basic OSI Reference Model
Before being fed into the network, the data is broken into packets. A packet is a unit of information transmitted between stations on a network.
When sending data, the packet passes sequentially through all layers of the software. At each level, control information of this level (header) is added to the packet, which is necessary for successful data transmission over the network, as shown in Fig. 1.12, where Zag is the packet header, End is the end of the packet.
On the receiving side, the packet goes through all the layers in reverse order. At each layer, the protocol at that layer reads the packet's information, then removes the information added to the packet at the same layer by the sender, and passes the packet to the next layer. When the packet reaches the Application layer, all control information will be removed from the packet and the data will return to its original form.
Rice. 1.12. Formation of a package of each level of the seven-level model
Each level of the model has its own function. The higher the level, the more difficult the task it solves.
It is convenient to think of the individual layers of the OSI model as groups of programs designed to perform specific functions. One layer, for example, is responsible for providing data conversion from ASCII to EBCDIC and contains the programs necessary to perform this task.
Each layer provides a service to a higher layer, in turn requesting a service from the lower layer. The upper layers request a service in much the same way: as a rule, it is a requirement to route some data from one network to another. The practical implementation of the principles of data addressing is assigned to the lower levels. On fig. 1.13 provides a brief description of the functions of all levels.
Rice. 1.13. Functions of the OSI Model Layers
The model under consideration determines the interaction of open systems from different manufacturers in the same network. Therefore, it performs coordinating actions for them on:
Interaction of applied processes;
Data presentation forms;
Uniform data storage;
Network resource management;
Data security and information protection;
Diagnostics of programs and hardware.
Application layer
The application layer provides application processes with access to the interaction area, is the upper (seventh) level and is directly adjacent to application processes.
In reality, the application layer is a set of various protocols by which network users access shared resources such as files, printers, or hypertext Web pages, and organize their collaboration, for example, using the email protocol. Special application service elements provide services for specific application programs such as file transfer and terminal emulation programs. If, for example, the program needs to send files, then the FTAM (File Transfer, Access, and Management) file transfer protocol will be used. In the OSI model, an application program that needs to perform a specific task (for example, update a database on a computer) sends specific data in the form of a Datagram to the application layer. One of the main tasks of this layer is to determine how an application request should be processed, in other words, what form the request should take.
The unit of data that the application layer operates on is usually called a message.
The application layer performs the following functions:
1. Performing various types of work.
File transfer;
Job management;
System management, etc;
2. Identification of users by their passwords, addresses, electronic signatures;
3. Determination of functioning subscribers and the possibility of access to new application processes;
4. Determining the sufficiency of available resources;
5. Organization of requests for connection with other application processes;
6. Transfer of applications to the representative level for the necessary methods for describing information;
7. Selection of procedures for the planned process dialogue;
8. Management of data exchanged between application processes and synchronization of interaction between application processes;
9. Determining the quality of service (delivery time of data blocks, acceptable error rate);
10. Agreement on the correction of errors and the determination of the reliability of data;
11. Coordination of restrictions imposed on the syntax (character sets, data structure).
These functions define the kinds of services that the application layer provides to application processes. In addition, the application layer transfers to application processes the service provided by the physical, data link, network, transport, session and presentation layers.
At the application level, it is necessary to provide users with already processed information. This can be handled by system and user software.
The application layer is responsible for accessing applications to the network. The tasks of this level are file transfer, mail exchange and network management.
The most common top three layer protocols are:
FTP (File Transfer Protocol) file transfer protocol;
TFTP (Trivial File Transfer Protocol) is the simplest file transfer protocol;
X.400 email;
Telnet work with a remote terminal;
SMTP (Simple Mail Transfer Protocol) is a simple mail exchange protocol;
CMIP (Common Management Information Protocol) common information management protocol;
SLIP (Serial Line IP) IP for serial lines. Protocol for serial character-by-character data transfer;
SNMP (Simple Network Management Protocol) simple network management protocol;
FTAM (File Transfer, Access, and Management) is a protocol for transferring, accessing and managing files.
Presentation layer
The functions of this level are the presentation of data transmitted between application processes in the desired form.
This layer ensures that the information passed by the application layer will be understood by the application layer in another system. If necessary, the presentation layer at the time of information transfer performs the conversion of data formats into some common presentation format, and at the time of reception, respectively, performs the reverse conversion. Thus, application layers can overcome, for example, syntactical differences in data representation. This situation can occur in a LAN with computers of different types (IBM PC and Macintosh) that need to exchange data. So, in the fields of databases, information should be presented in the form of letters and numbers, and often in the form of a graphic image. You need to process this data, for example, as floating point numbers.
The common data representation is based on the ASN.1 system, which is common for all levels of the model. This system serves to describe the structure of files, and also solves the problem of data encryption. At this level, data encryption and decryption can be performed, thanks to which the secrecy of data exchange is ensured immediately for all application services. An example of such a protocol is the Secure Socket Layer (SSL) protocol, which provides secure messaging for the application layer protocols of the TCP/IP stack. This layer provides data transformation (encoding, compression, etc.) of the application layer into an information stream for the transport layer.
The representative layer performs the following main functions:
1. Generation of requests to establish interaction sessions between application processes.
2. Coordination of data presentation between application processes.
3. Implementation of data presentation forms.
4. Presentation of graphic material (drawings, drawings, diagrams).
5. Classification of data.
6. Sending requests to terminate sessions.
Presentation layer protocols are usually part of the protocols of the top three layers of the model.
Session layer
The session layer is the layer that defines the procedure for conducting sessions between users or application processes.
The session layer provides conversation control to keep track of which side is currently active and also provides a means of synchronization. The latter allow you to insert checkpoints into long transfers so that in case of a failure, you can go back to the last checkpoint, instead of starting all over again. In practice, few applications use the session layer, and it is rarely implemented.
The session layer controls the transfer of information between application processes, coordinates the reception, transmission and issuance of one communication session. In addition, the session layer additionally contains the functions of password management, conversation control, synchronization and cancellation of communication in a transmission session after a failure due to errors in the lower layers. The functions of this layer are to coordinate communication between two application programs running on different workstations. It comes in the form of a well-structured dialogue. These functions include creating a session, managing the transmission and reception of message packets during a session, and terminating a session.
At the session level, it is determined what the transfer between two application processes will be:
Half duplex (processes will send and receive data in turn);
Duplex (processes will send data and receive them at the same time).
In half-duplex mode, the session layer issues a data token to the process that initiates the transfer. When the time comes for the second process to respond, the data token is passed to it. The session layer allows transmission only to the party that possesses the data token.
The session layer provides the following functions:
1. Establishment and completion at the session level of a connection between interacting systems.
2. Performing normal and urgent data exchange between application processes.
3. Managing the interaction of applied processes.
4. Synchronization of session connections.
5. Notification of application processes about exceptional situations.
6. Establishment of labels in the applied process, allowing, after a failure or error, to restore its execution from the nearest label.
7. Interruption in the necessary cases of the application process and its correct resumption.
8. Termination of the session without data loss.
9. Transmission of special messages about the progress of the session.
The session layer is responsible for organizing data exchange sessions between end machines. Session layer protocols are usually a component of the protocols of the top three layers of the model.
Transport Layer
The transport layer is designed to transfer packets through a communication network. At the transport layer, packets are divided into blocks.
On the way from the sender to the recipient, packets can be corrupted or lost. While some applications have their own error handling, there are some that prefer to deal with a reliable connection right away. The job of the transport layer is to ensure that applications or upper layers of the model (application and session) transfer data with the degree of reliability that they require. The OSI model defines five classes of service provided by the transport layer. These types of services differ in the quality of the services provided: urgency, the ability to restore interrupted communications, the availability of multiplexing facilities for multiple connections between different application protocols through a common transport protocol, and most importantly, the ability to detect and correct transmission errors such as distortion, loss and duplication of packets.
The transport layer determines the addressing of physical devices (systems, their parts) in the network. This layer guarantees the delivery of blocks of information to recipients and manages this delivery. Its main task is to provide efficient, convenient and reliable forms of information transfer between systems. When more than one packet is in processing, the transport layer controls the order in which the packets pass through. If a duplicate of a previously received message passes, then this layer recognizes this and ignores the message.
The functions of the transport layer include:
1. Network transmission control and ensuring the integrity of data blocks.
2. Detection of errors, their partial elimination and reporting of uncorrected errors.
3. Recovery of transmission after failures and malfunctions.
4. Consolidation or division of data blocks.
5. Granting of priorities at transfer of blocks (normal or urgent).
6. Transfer confirmation.
7. Elimination of blocks in deadlock situations in the network.
Starting from the transport layer, all higher protocols are implemented in software, usually included in the network operating system.
The most common transport layer protocols include:
TCP (Transmission Control Protocol) TCP/IP stack transmission control protocol;
UDP (User Datagram Protocol) is the user datagram protocol of the TCP/IP stack;
NCP (NetWare Core Protocol) basic protocol for NetWare networks;
SPX (Sequenced Packet eXchange) Novell Stack Sequenced Packet Exchange;
TP4 (Transmission Protocol) - class 4 transmission protocol.
Network Layer
The network layer provides for the laying of channels connecting subscriber and administrative systems through a communication network, choosing the route of the fastest and most reliable way.
The network layer establishes communication in a computer network between two systems and provides the laying of virtual channels between them. A virtual or logical channel is such a functioning of network components that creates the illusion of laying the necessary path between the interacting components. In addition, the network layer informs the transport layer about errors that occur. Network layer messages are commonly referred to as packets. They contain pieces of data. The network layer is responsible for their addressing and delivery.
Laying the best path for data transmission is called routing, and its solution is the main task of the network layer. This problem is compounded by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transfer along this route; it depends on the bandwidth of communication channels and traffic intensity, which can change over time. Some routing algorithms try to adapt to load changes, while others make decisions based on long-term averages. Route selection can also be based on other criteria, such as transmission reliability.
The link layer protocol provides data delivery between any nodes only in a network with an appropriate typical topology. This is a very strict limitation that does not allow building networks with a developed structure, for example, networks that combine several enterprise networks into a single network, or highly reliable networks in which there are redundant links between nodes.
Thus, within the network, data delivery is regulated by the link layer, but data delivery between networks is handled by the network layer. When organizing the delivery of packets at the network level, the concept of a network number is used. In this case, the recipient's address consists of a network number and a computer number on that network.
Networks are interconnected by special devices called routers. A router is a device that collects information about the topology of interconnections and, based on it, forwards network layer packets to the destination network. In order to transfer a message from a sender located in one network to a recipient located in another network, it is necessary to make a certain number of transit transmissions (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers that a packet traverses.
The network layer is responsible for dividing users into groups and routing packets based on the translation of MAC addresses into network addresses. The network layer also provides transparent transmission of packets to the transport layer.
The network layer performs the following functions:
1. Creation of network connections and identification of their ports.
2. Detection and correction of errors that occur during transmission through a communication network.
3. Packet flow control.
4. Organization (ordering) of sequences of packages.
5. Routing and switching.
6. Segmentation and consolidation of packages.
The network layer defines two kinds of protocols. The first type refers to the definition of rules for the transmission of packets with data of end nodes from a node to a router and between routers. It is these protocols that are usually referred to when talking about network layer protocols. However, another type of protocol, called routing information exchange protocols, is often referred to as the network layer. Routers use these protocols to collect information about the topology of interconnections.
Network layer protocols are implemented by software modules of the operating system, as well as software and hardware of routers.
The most commonly used protocols at the network layer are:
IP (Internet Protocol) Internet protocol, a network protocol of the TCP/IP stack that provides address and routing information;
IPX (Internetwork Packet Exchange) is an Internet packet exchange protocol designed for addressing and routing packets in Novell networks;
X.25 international standard for global packet-switched communications (this protocol is partially implemented at layer 2);
CLNP (Connection Less Network Protocol) is a network protocol without organizing connections.
Link layer (Data Link)
The information unit of the link layer are frames (frame). Frames are a logically organized structure into which data can be placed. The task of the link layer is to transfer frames from the network layer to the physical layer.
At the physical layer, bits are simply sent. This does not take into account that in some networks, in which communication lines are used alternately by several pairs of interacting computers, the physical transmission medium may be busy. Therefore, one of the tasks of the link layer is to check the availability of the transmission medium. Another task of the link layer is to implement error detection and correction mechanisms.
The link layer ensures that each frame is transmitted correctly by placing a special bit sequence at the beginning and end of each frame to mark it, and also calculates a checksum by summing all the bytes of the frame in a certain way and adding a checksum to the frame. When a frame arrives, the receiver again calculates the checksum of the received data and compares the result with the checksum from the frame. If they match, the frame is considered valid and accepted. If the checksums do not match, then an error is generated.
The task of the link layer is to take packets coming from the network layer and prepare them for transmission by fitting them into a frame of the appropriate size. This layer is required to determine where the block starts and ends, and to detect transmission errors.
At the same level, the rules for using the physical layer by network nodes are defined. The electrical representation of data in the LAN (data bits, data encoding methods, and markers) is recognized at this and only at this level. Here, errors are detected and corrected (by requesting data retransmission).
The link layer provides the creation, transmission and reception of data frames. This layer services network layer requests and uses the physical layer service to receive and transmit packets. The IEEE 802.X specifications divide the link layer into two sublayers:
LLC (Logical Link Control) logical link control provides logical link control. The LLC sublayer provides services to the network layer and is concerned with the transmission and reception of user messages.
MAC (Media Assess Control) media access control. The MAC sublayer regulates access to the shared physical medium (token passing or collision or collision detection) and controls access to the communication channel. The LLC sublayer is above the MAC sublayer.
The data link layer defines media access and transmission control through a data transfer procedure over a link.
With large sizes of transmitted data blocks, the link layer divides them into frames and transmits frames as sequences.
Upon receipt of frames, the layer forms transmitted data blocks from them. The size of a data block depends on the transmission method, the quality of the channel through which it is transmitted.
In LANs, link-layer protocols are used by computers, bridges, switches, and routers. In computers, the functions of the link layer are implemented by the joint efforts of network adapters and their drivers.
The link layer can perform the following types of functions:
1. Organization (establishment, management, termination) of channel connections and identification of their ports.
2. Organization and transfer of personnel.
3. Detection and correction of errors.
4. Data flow management.
5. Ensuring the transparency of logical channels (transfer of data encoded in any way over them).
The most commonly used protocols at the link layer include:
HDLC (High Level Data Link Control) high-level data link control protocol for serial connections;
IEEE 802.2 LLC (Type I and Type II) provide MAC for 802.x environments;
Ethernet network technology according to the IEEE 802.3 standard for networks using bus topology and multiple access with carrier sniffing and collision detection;
Token ring network technology according to the IEEE 802.5 standard, using a ring topology and a token passing ring access method;
FDDI (Fiber Distributed Date Interface Station) IEEE 802.6 network technology using fiber optic media;
X.25 is an international standard for global packet-switched communications;
Frame relay network organized from X25 and ISDN technologies.
Physical Layer
The physical layer is designed to interface with the physical means of connection. Physical connectivity is the combination of physical media, hardware and software that enables signaling between systems.
The physical medium is a material substance through which signals are transmitted. The physical medium is the foundation upon which the physical means of connection are built. Ether, metals, optical glass and quartz are widely used as physical media.
The Physical Layer consists of a Media Interface Sublayer and a Transmission Transformation Sublayer.
The first of them provides pairing of the data flow with the used physical communication channel. The second performs transformations related to the applied protocols. The physical layer provides the physical interface to the data channel and also describes the procedures for transmitting signals to and from the channel. At this level, the electrical, mechanical, functional and procedural parameters for physical communication in systems are defined. The physical layer receives data packets from the overlying link layer and converts them into optical or electrical signals corresponding to 0 and 1 of the binary stream. These signals are sent through the transmission medium to the receiving node. The mechanical and electrical/optical properties of the transmission medium are defined at the physical layer and include:
Type of cables and connectors;
Pin assignment in connectors;
Signal coding scheme for values 0 and 1.
The physical layer performs the following functions:
1. Establishment and disconnection of physical connections.
2. Transmission of signals in serial code and reception.
3. Listening, if necessary, channels.
4. Identification of channels.
5. Notification of the occurrence of faults and failures.
Notification about the occurrence of malfunctions and failures is due to the fact that a certain class of events is detected at the physical layer that interferes with the normal operation of the network (collision of frames sent by several systems at once, channel break, power failure, loss of mechanical contact, etc.). The types of service provided to the data link layer are defined by the physical layer protocols. Listening to the channel is necessary in cases where a group of systems is connected to one channel, but only one of them is allowed to transmit signals at the same time. Therefore, listening to the channel allows you to determine whether it is free to transmit. In some cases, for a clearer definition of the structure, the physical layer is divided into several sublevels. For example, the physical layer of a wireless network is divided into three sublayers (Figure 1.14).
Rice. 1.14. Wireless LAN physical layer
Physical layer functions are implemented in all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter. Repeaters are the only type of equipment that only works at the physical layer.
The physical layer can provide both asynchronous (serial) and synchronous (parallel) transmission, which is used for some mainframes and minicomputers. At the Physical layer, an encoding scheme must be defined to represent binary values for transmission over a communication channel. Many local area networks use Manchester encoding.
An example of a physical layer protocol is the specification of 10Base-T Ethernet technology, which defines a category 3 unshielded twisted pair with a characteristic impedance of 100 ohms, an RJ-45 connector, a maximum length of a physical segment of 100 meters, a Manchester code for data representation, and other characteristics as the cable used. environment and electrical signals.
The most common physical layer specifications include:
EIA-RS-232-C, CCITT V.24/V.28 - Mechanical/Electrical Unbalanced Serial Interface;
EIA-RS-422/449, CCITT V.10 - mechanical, electrical and optical characteristics of a balanced serial interface;
Ethernet is an IEEE 802.3 network technology for networks using bus topology and multiple access with carrier sniffing and collision detection;
Token ring is an IEEE 802.5 network technology that uses a ring topology and a token passing ring access method.
The development of which was not related to the OSI model.
Layers of the OSI model
The model consists of 7 levels located one above the other. Layers interact with each other (vertically) through interfaces, and can interact with a parallel layer of another system (horizontally) through protocols. Each level can interact only with its neighbors and perform functions assigned only to it. More details can be seen in the figure.
| OSI model | ||
|---|---|---|
| Data type | Level | Functions |
| Data | 7. Application layer | Access to online services |
| 6. Presentation Layer | Representation and encoding of data | |
| 5. Session layer | Session management | |
| Segments | 4. Transport | Direct communication between endpoints and reliability |
| Packages | 3. Networked | Route determination and logical addressing |
| Personnel | 2. Channel | Physical addressing |
| bits | 1. Physical layer | Working with media, signals and binary data |
Application (Application) level (eng. application layer)
The top level of the model provides the interaction of user applications with the network. This layer allows applications to use network services such as remote access to files and databases, e-mail forwarding. It is also responsible for the transfer of service information, provides applications with information about errors and generates requests to presentation layer. Example: HTTP , POP3 , SMTP , FTP , XMPP , OSCAR , BitTorrent , MODBUS, SIP
Executive (Presentation layer) 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 understandable by applications. At this level, compression/decompression or encoding/decoding of data can be performed, as well as redirecting requests to another network resource if they cannot be processed locally.
Layer 6 (representations) of the OSI reference model is usually an intermediate protocol for converting information from neighboring layers. This allows communication between applications on dissimilar computer systems in a manner that is transparent to the applications. The presentation layer provides formatting and code transformation. Code formatting is used to ensure that the application receives information for processing that makes sense to it. If necessary, this layer can translate from one data format to another. The presentation layer deals not only with the formats and presentation of data, it also deals with the data structures that are used by programs. Thus, layer 6 provides for the organization of data during its transfer.
To understand how this works, imagine that there are two systems. One uses the EBCDIC Extended Binary Information Interchange Code, such as the IBM mainframe, for data representation, and the other uses the American Standard ASCII Information Interchange Code (used by most other computer manufacturers). If these two systems need to exchange information, then a presentation layer is needed to perform the transformation 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, the processes and code at the view level must perform data transformations. At this level, there are other subroutines that compress texts and convert graphic images into bitstreams so that they can be transmitted over the network.
Presentation-level standards also define how graphics are presented. For this purpose, the PICT format, an image format used to transfer QuickDraw graphics between programs for Macintosh and PowerPC computers, can be used. Another representation format is the tagged TIFF image file format, which is commonly used for high resolution bitmaps. The next presentation level standard that can be used for graphics is that developed by the Joint Photographic Expert Group; in everyday usage, this standard is simply referred to as JPEG.
There is another group of presentation level standards that define the presentation of sound and movies. This includes the Musical Instrument Digital Interface (MIDI) for the digital representation of music, developed by the Cinematography Expert Group, the MPEG standard, used to compress and encode videos on CD, store them digitally, and transfer at speeds up to 1.5 Mbps. /s, and QuickTime, a standard that describes audio and video elements for programs running on Macintosh and PowerPC computers.
The session layer session layer)
The 5th level of the model is responsible for maintaining the communication session, allowing applications to interact with each other for a long time. The layer manages session creation/termination, information exchange, task synchronization, determination of the right to transfer data, and session maintenance during periods of application inactivity. Transmission synchronization is ensured by placing checkpoints in the data stream, starting from which the process resumes if the interaction is broken.
The 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. At the same time, it does not matter what data is transferred, 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. Example: TCP , 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 correct sequence, multiplex multiple data streams, provide data flow control mechanism and guarantee the validity 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 deal with this by collecting data in the right order before passing it to the session layer. Multiplexing (multiplexing) data means that the transport layer is able to simultaneously process multiple data streams (streams may come from different 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 the function of data delivery control, forcing the system receiving data to send acknowledgments to the transmitting side that data has been received.
You can describe the operation of protocols with the establishment of a connection using the example of a conventional telephone. Protocols of this class begin data transmission by invoking or setting the path of packets from source to destination. After that, the serial data transfer is started and then, at the end of the transfer, the connection is disconnected.
Connectionless protocols that send data containing full address information in each packet work similarly to the mail system. Each letter or package contains the address of the sender and the recipient. Next, each intermediate post office or network device reads the address information and makes a decision about 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 arrive to the recipient in the order in which it was sent. The transport protocols are responsible for setting up the data in the appropriate order when using connectionless network protocols.
The network layer network layer)
The 3rd layer of the OSI network model is designed to determine the data transfer path. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring network problems and congestion. A network device such as a router operates at this level.
Network layer protocols route data from a source to a destination.
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 for integrity, corrects errors if necessary (sends a repeated request for a damaged frame) and sends it to the network layer. The link layer can interact with one or more physical layers, controlling and managing this interaction. The IEEE 802 specification divides this level into 2 sublevels - MAC (Media Access Control) regulates access to the shared physical medium, LLC (Logical Link Control) provides network level service.
In programming, this level represents the network card driver, in operating systems there is a programming interface for the interaction of the channel and network levels with each other, this is not a new level, but simply an implementation of a model for a specific OS. Examples of such interfaces: ODI , NDIS
The physical layer physical layer)
The lowest level of the model is intended directly for the transfer of data flow. Carries out the transmission of electrical or optical signals to a cable or radio air and, accordingly, their reception and conversion into data bits in accordance with the methods of encoding digital signals. In other words, it provides an interface between a network carrier and a network device.
Protocols: IRDA , USB , EIA RS-232 , EIA-422 , EIA-423 , RS-449 , RS-485 , Ethernet (including 10BASE-T , 10BASE2 ,
The main defect of OSI is an ill-conceived transport layer. On it, OSI allows data to be exchanged between applications (introducing the concept port- application identifier), however, the possibility of exchanging simple datagrams (of the UDP type) is not provided in OSI - the transport layer must form connections, provide delivery, manage the flow, etc. (of the TCP type). Real protocols implement this possibility.
TCP/IP family
The TCP / IP family has three transport protocols: TCP, which is fully OSI-compliant, providing verification of receipt of data, UDP, which corresponds to the transport layer only by the presence of a port, provides datagram exchange between applications, does not guarantee receipt of data, and SCTP, designed to eliminate some of the shortcomings of TCP and in which added some innovations. (There are about two hundred other protocols in the TCP/IP family, the best known of which is the service protocol ICMP , which is used internally to ensure operation; the rest are also not transport protocols.)
IPX/SPX family
In the IPX/SPX family, ports (called "sockets" or "sockets") appear in the IPX network layer protocol, enabling the exchange of datagrams between applications (the operating system reserves some of the sockets for itself). The SPX protocol, in turn, complements IPX with all other transport layer capabilities in full compliance with OSI.
For the host address, IPX uses an identifier formed from a four-byte network number (assigned by routers) and the MAC address of the network adapter.
DOD Model
A TCP/IP protocol stack using a simplified four-layer OSI model.
Addressing in IPv6
Destination and source addresses in IPv6 are 128 bits or 16 bytes long. Version 6 generalizes the special address types of version 4 into the following address types:
- Unicast is an individual address. Specifies a single node - computer or router port. The packet must be delivered to the node via the shortest route.
- Cluster is the address of the cluster. Denotes a group of hosts that share a common address prefix (for example, attached to the same physical network). The packet must be routed to a group of nodes along the shortest path, and then delivered to only one of the members of the group (for example, the nearest node).
- Multicast is the address of a set of hosts, possibly on different physical networks. Copies of the packet must be delivered to each node in the set using hardware multicast or broadcast capabilities, if possible.
As with IPv4, IPv6 addresses are divided into classes based on the value of the most significant few bits of the address.
Most of the classes are reserved for future use. The most interesting for practical use is the class intended for Internet service providers, called Provider-Assigned Unicast.
The address of this class has the following structure:
Each ISP is assigned a unique identifier that tags all networks it supports. Next, the provider assigns unique identifiers to its subscribers, and uses both identifiers when assigning a block of subscriber addresses. The subscriber himself assigns unique identifiers to his subnets and nodes of these networks.
A subscriber can use the subnetting technique used in IPv4 to further subdivide the subnet ID field into smaller fields.
The described scheme approximates the IPv6 addressing scheme to those used in territorial networks such as telephone networks or X.25 networks. The hierarchy of address fields will allow backbone routers to work only with the higher parts of the address, leaving processing of less significant fields to subscriber routers.
A minimum of 6 bytes must be allocated under the host ID field in order to be able to use LAN MAC addresses directly in IP addresses.
For compatibility with the IPv4 version of the addressing scheme, IPv6 has a class of addresses that have 0000 0000 in the high-order bits of the address. The lower 4 bytes of this class address must contain an IPv4 address. Routers that support both versions of addresses must provide translation when passing a packet from a network that supports IPv4 addressing to a network that supports IPv6 addressing, and vice versa.
Criticism
The seven-layer OSI model has been criticized by some experts. In particular, in the classic book UNIX. System Administrator's Guide" by Evi Nemeth and others write:
access to the network environment. In the same time, link layer manages the process of placing transmitted data in the physical environment. So link layer divided into 2 sublevels (Fig. 5.1): upper sublevel logical link control(Logical Link Control - LLC), which is common to all technologies, and the lower sublevel media access control(Media Access Control - MAC). In addition, the link layer tools allow you to detect errors in the transmitted data.... While the ISO committees were arguing about their standards, the whole concept of networking was changing behind their backs and the TCP / IP protocol was being introduced around the world. …
…
And so, when the ISO protocols were finally implemented, a number of problems emerged:
These protocols were based on concepts that make no sense in today's networks.
Their specifications were in some cases incomplete.
In terms of their functionality, they were inferior to other protocols.
The presence of multiple layers has made these protocols slow and difficult to implement.
…
… Now even the most zealous supporters of these protocols admit that OSI is gradually moving towards becoming a small footnote in the pages of computer history.
Rice. 5.1.
The interaction of local network nodes occurs on the basis of link layer protocols. Data transmission in local networks occurs over relatively short distances (inside buildings or between closely spaced buildings), but at a high speed (10 Mbps - 100 Gbps). distance and transmission speed data is determined by the hardware of the relevant standards.
International Institute of Electrical and Electronics Engineers (Institute of Electrical and Electronics Engineers - IEEE) a family of 802.x standards was developed, which regulates the functioning of the data link and physical layers of the seven-layer ISO / OSI model. A number of these protocols are common to all technologies, for example, the 802.2 standard, other protocols (for example, 802.3, 802.3u, 802.5) define the features of LAN technologies.
LLC sublevel implemented software tools. At the LLC sublayer, there are several procedures that allow you to establish or not establish a connection before the transmission of frames containing data, to restore or not to restore frames when they are lost or errors are detected. sublevel LLC implements communication with network layer protocols, usually with the IP protocol. Communication with the network layer and the definition of logical procedures for the transmission of frames over the network implements the 802.2 protocol. The 802.1 protocol provides general definitions for local area networks, linking to the ISO/OSI model. There are also modifications of this protocol.
The MAC sublayer defines the features of access to the physical medium using various local area network technologies. Each MAC layer technology (each protocol: 802.3, 802.3u, 802.3z, etc.) corresponds to several variants of physical layer specifications (protocols) (Fig. 5.1). Specification MAC-layer technologies - defines the physical layer environment and the main parameters of data transfer ( transmission speed, medium type, narrowband or wideband).
At the link layer of the transmitting side, frame, in which the package is encapsulated. In the process of encapsulation, a frame header and trailer (trailer) are added to a network protocol packet, such as IP . Thus, the frame of any network technology consists of three parts:
- header,
- data fields where the package is placed,
- trailer.
On the receiving side, the reverse decapsulation process is implemented when a packet is extracted from the frame.
header includes frame delimiters, address and control fields. Separators frames allow you to determine the beginning of the frame and provide synchronization between the transmitter and receiver. Addresses link layer are physical addresses. When using Ethernet-compatible technologies, data addressing in local networks is carried out by MAC addresses, which ensure the delivery of the frame to the destination node.
trailer contains a checksum field ( Frame Check Sequence - FCS), which is calculated when transmitting a frame using a cyclic code CRC. On the receiving side check sum frame is calculated again and compared with the received one. If they match, then consider that the frame was transmitted without errors. If the FCS values differ, the frame is discarded and retransmission is required.
When transmitted over a network, a frame sequentially passes through a number of connections characterized by different physical environments. For example, when data is transmitted from Node A to Node B (Figure 5.2), the data passes in sequence through: the Ethernet connection between Node A and router A (copper, unshielded twisted pair), the connection between routers A and B (fiber optic cable), point-to-point serial copper cable between router B and WAP wireless access point, wireless connection (radio link) between WAP and end Node B. Therefore a separate frame is formed for each connection specific format.
Rice. 5.2.
The packet prepared by Node A is encapsulated in a LAN frame, which is transmitted to router A. The router decapsulates the packet from the received frame, determines which egress interface to send the packet to, then forms a new frame for transmission over the optical medium. Router B decapsulates the packet from the received frame, determines which egress interface to send the packet to, then constructs a new frame for transmission over the peer-to-peer copper media. The WAP wireless access point, in turn, forms its own frame to transmit data over the air to the end Node B.
When creating networks, various logical topologies are used that determine how nodes communicate through the medium, how access control medium. The most well-known logical topologies are point-to-point, multiaccess, broadcast, and token passing.
Sharing the environment between multiple devices is implemented based on two main methods:
- method competitive (non-deterministic) access(Contention-based Access), when all network nodes are equal, the order of data transfer is not organized. To transmit, this node must listen to the medium, if it is free, then information can be transmitted. This may cause conflicts collisions) when two (or more) nodes start transmitting data at the same time;
- method controlled (deterministic) access(Controlled Access), which provides the nodes with the order of access to the medium for data transfer.
In the early stages of the creation of Ethernet networks, the "bus" topology was used, the shared data transmission medium was common to all users. At the same time, the method multiple access to the common transmission medium (802.3 protocol). This required carrier sensing, the presence of which indicated that some node was already transmitting data over a common medium. Therefore, a node wishing to transfer data had to wait until the end of the transfer and, when the medium was released, try to transfer the data.
Information transmitted to the network can be received by any computer whose network adapter NIC address matches the destination MAC address of the transmitted frame, or by all computers on the network when broadcasting. However, only one node can transmit information at any given time. Before starting a transmission, the node must make sure that the public bus is free, for which the node listens on the medium.
When two or more computers transmit data at the same time, a conflict occurs ( collision), when the data of the transmitting nodes are superimposed on each other, distortion occurs and loss of information. Therefore, collision processing and retransmission of the frames involved in the collision are required.
Similar Method non-deterministic(associative) access named by Wednesday Carrier Sense Media Access with Collision Detection( Carrier Sense Multiply Access
open systems interactions. In other words, it is a certain standard by which network technologies operate.This system consists of seven layers of the OSI model. Each protocol works with protocols of its level, either a level lower or higher from itself.
Each level operates on a specific data type:
- Physical - bit;
- Channel - frame;
- Network - package;
- Transport - segments/datagrams;
- Session - session;
- Executive - flow;
- Applied - data
Layers of the OSI model
Application layer ( application layer)
This is the top one OSI network model layer. It is also called the application layer. Designed for user interaction with the network. The layer provides applications with the ability to use various network services.
Functions:
- remote access;
- Post service;
- formation of requests to the next level ( presentation layer)
Network layer protocols:
- bittorrent
- http
- SMTP
- SNMP
- TELNET
presentation layer ( presentation layer)
This is the second level. Otherwise known as the representative level. Designed for protocol conversion, as well as for encoding and decoding data. At this stage, requests delivered from the application layer are formed into data for transmission over the network and vice versa.
Functions:
- data compression/decompression;
- data encoding/decoding;
- redirect requests
Network layer protocols:
- LPP
- NDR
session layer ( session layer)
This OSI network model layer responsible for maintaining the session. Thanks to this layer, applications can interact with each other for a long time.
Functions:
- granting rights
- creating/suspending/restoring/ending communication
Network layer protocols:
- ISO-SP
- L2TP
- NetBIOS
- PPTP
- SMPP
transport layer ( transport layer)
This is the fourth level, if you count from above. Designed for reliable data transfer. However, transmission may not always be reliable. Duplication and non-delivery of the data package are possible.
Network layer protocols:
- UDP
- SST
- RTP
network layer ( network layer)
The OSI network model layer is responsible for determining the best and shortest route for data transfer.
Functions:
- address assignment
- collision tracking
- route definition
- switching
Network layer protocols:
- IPv4/IPv6
- CLNP
- IPsec
- RIP
- OSPF
Link layer ( data link layer)
This is the sixth layer, which is responsible for the delivery of data between devices that are in the same network area.
Functions:
- addressing at the hardware level
- error control
- error correction
Network layer protocols:
- SLIP
- LAPD
- IEEE 802.11 wireless LAN,
- FDDI
- ARCnet
Physical layer ( physical layer)
The lowest and most recent OSI network model layer. Serves to define the method of data transmission in the physical/electrical environment. Let's say any site, for example " play online casino http://bestforplay.net ", is located on some kind of server, the interfaces of which also transmit some kind of electrical signal through cables and wires.
Functions:
- determination of the type of data transfer
- data transfer
Network layer protocols:
- IEEE 802.15 (Bluetooth)
- 802.11 WiFi
- GSMUm radio interface
- ITU and ITU-T
- EIARS-232
Table of the 7 layer OSI model
| OSI model | ||
| Data type | Level | Functions |
| Data | Applied | Access to online services |
| Flow | Executive | Representation and encryption of data |
| Sessions | session | Session management |
| Segments/Datagrams | Transport | Direct communication between endpoints and reliability |
| Packages | network | Route determination and logical addressing |
| Personnel | ducted | Physical addressing |
| bits | Physical | Working with media, signals and binary data |
The OSI Reference Model is a 7-level network hierarchy created by the International Standards Organization (ISO). The presented model in Fig.1 has 2 different models:
- a horizontal protocol-based model that implements the interaction of processes and software on different machines
- a vertical model based on services implemented by neighboring layers to each other on the same machine
In the vertical - neighboring levels change information using APIs. The horizontal model requires a common protocol for exchanging information at the same level.
Picture 1
The OSI model describes only system interaction methods implemented by the OS, software, etc. The model does not include end user interaction methods. Ideally, applications should access the upper layer of the OSI model, but in practice, many protocols and programs have methods for accessing lower layers.
Physical layer
At the physical level, data is represented as electrical or optical signals corresponding to 1s and 0s of the binary stream. The transmission medium parameters are defined at the physical layer:
- type of connectors and cables
- pin assignment in connectors
- signal coding scheme 0 and 1
The most common types of specifications at this level are:
- - unbalanced serial interface parameters
- — balanced serial interface parameters
- IEEE 802.3 -
- IEEE 802.5 -
At the physical level, you can not delve into the meaning of the data, since it is represented in the form of bits.
Link layer
This channel implements the transport and reception of data frames. The layer implements the network layer requests and uses the physical layer for receiving and transmitting. The IEEE 802.x specifications divide this layer into two sublayers: logical link control (LLC) and medium access control (MAC). The most common protocols at this level are:
- IEEE 802.2 LLC and MAC
- ethernet
- token ring
Also at this level, detection and correction of transmission errors is implemented. At the link layer, the packet is placed in the data field of the frame - encapsulation. Error detection is possible using different methods. For example, the implementation of fixed frame boundaries, or a checksum.
network layer
At this level, network users are divided into groups. It implements packet routing based on MAC addresses. The network layer implements the transparent transmission of packets to the transport layer. At this level, the boundaries of networks of different technologies are erased. work at this level. An example of the network layer is shown in Fig. 2. The most common protocols:
Drawing - 2
transport layer
At this level, information flows are divided into packets for transmission at the network level. The most common protocols of this layer are:
- TCP - Transmission Control Protocol
session layer
At this level, the organization of information exchange sessions between terminal machines takes place. At this level, the active side is determined and session synchronization is implemented. In practice, many other layer protocols include session layer functionality.
Presentation Layer
At this level, data is exchanged between software on different operating systems. At this level, information transformation (compression, etc.) is implemented to transfer the information flow to the transport level. Layer protocols are used and those that use the higher layers of the OSI model.
Application layer
The application layer implements the application's access to the network. The layer manages file transfer and network management. Protocols used:
- FTP/TFTP - File Transfer Protocol
- X 400 - email
- telnet
- CMIP - information management
- SNMP - Network Management
- NFS - network file system
- FTAM - file transfer access method
