OSI has two main components: an abstract network interaction model (seven-layer model) and a set of functional protocols. Part of the seven-level OSI model influenced the promotion of the Internet protocol, as well as the abstract model itself, documented in OSI under the number 7498. In this design, the network system is divided into levels, inside which one or more objects implement their functionality. Each object only contacts the layer below it and provides the means to use it. Protocols allow you to interact in a host with a specific object at the same level.
Introduction to the OSI Model
Computer manufacturers offered promising networking architectures specific to their hardware. For example, IBM introduced SNA, DEC - DNA. However, these architectures had the same drawbacks. Due to the proprietary nature, they could not be combined and the spread of gateway solutions of heterogeneous architectures could not be avoided.
An ISO body of 140 national standardization bodies has developed a seven-level OSI model, which stands for Network Interconnectivity. It describes the concepts used and the approach taken to standardize the relationship between open systems.
When developing this model, the main attention was paid to the heterogeneity of the equipment, as well as the resolution of the relationship with similar systems for historical and economic reasons. The model should not have given preference to a particular supplier, while it should be adapted to the evolution of the information flows to be processed, without prejudice to previous developments. The consideration of heterogeneity required the adoption of general rules for communication between equipment, that is, this development should logically lead to the international standardization of protocols.
The seven-level OSI model is not a network architecture because it does not specify specific layer services and protocols, but only describes the health of the layers.
The first work on the OSI model began in 1977. It was based on experience with large networks. The model has become valid for all types of networks. In 1978, ISO offers it as ISO IS7498. In 1984, 12 European manufacturers that joined large American manufacturers adopted this standard.
7-layer system design
The model is made of 7 layers. Each of them is determined by a narrowly focused task of organizing communication between the two systems. Each level is endowed with functions and protocols that fulfill specific goals. When exchanging data, communication passes through all 7 layers of OSI twice: the first time - at the transmitter, the second - at the receiver.
Logs are a set of rules at a specific level in the seven-level OSI model. Protocols are largely transparent to superhigh and subordinate layers, so their behavior is in direct connection with magazines. Transitions between levels are interfaces that are understood by protocols. Since some protocols are intended only for specific applications, they cover several levels and several tasks. It happens that in some compounds, individual tasks are performed in several shifts and, therefore, several times.
The principles of creating layers
The principles that are used when creating OSI model layers:
- A layer must be created when the system requires a new level.
- The layer has individual functions that comply with the European international standard.
- When choosing the boundaries of the layer, the flow of information arriving at the interfaces should be minimized.
- The number of layers of the OSI model should prevent the emergence of different functions within the same layer.
- Simple and reliable. Architecture does not have to be complicated to manage.
Given these principles, the community has adopted the belief that the lower layers (1, 2, 3, and 4) are necessary for routing information between the respective ends and are dependent on the physical environment. The upper levels (5, 6 and 7) are responsible for processing information related to the management of exchanges between computer systems. In addition, layers 1–3 interact between neighboring machines, and not between end machines, which can be shared by multiple routers. Conversely, layers 4 through 7 only interact between remote hosts.
Types of levels and their purpose
The first physical layer of the 7-level OSI model is associated with the transmission of bits in an unprocessed way over the communication channel and guarantees perfect data transit. A sent bit equal to 1 should be accepted as a bit equal to 1. In a specific plan, this layer standardizes the electrical characteristics, for example, a bit equal to 1 should be represented by a voltage of 5 V.
Mechanical standardization is connected by characteristics: this is the form of connectors, topology, etc. And functional - with the characteristics of data transmission circuits and procedures for establishing, maintaining and releasing a data circuit. A typical information unit of this layer is a bit represented by a specific potential difference.
The data link layer of the 7-level OSI model for the “binder” converts the physical layer into a link that a priori has no transmission errors. The layer splits the input of the transmitter into frames, transmits it, and controls the acknowledgment frames returned by the receiver.
For this level, the data does not have much significance, and the level of the data transmission channel is able to recognize frame boundaries. This can cause some problems, since the bit sequences used for this recognition can be displayed in the data.
The data link layer should be able to return a frame if a problem occurs on the line. In general, an important role of this layer is the identification and correction of errors that occur at the physical level. It also includes a receiver clogging control function.
The information block of the 7-level OSI model is a frame that includes from several hundred to thousands of bytes. The network layer manages the subnet. During its development, it is necessary to determine the routing mechanism and the calculation of routing tables, static or dynamic. The network layer also controls subnet congestion. Its information block is a package.
The transport layer is responsible for correctly routing complete messages to the recipient. It receives messages from the session layer, if necessary, reduces them to smaller ones and transfers them to the network layer with a guarantee of quality. This layer also reassembles the message when the blocks are received. He is responsible for optimizing network resources, creating a network connection on demand of the session, and is able to provide many network connections for the process. Conversely, it uses one connection to carry multiple messages simultaneously through multiplexing. This level is responsible for establishing and releasing connections. Therefore, it is one of the most important, because it provides the basic service on the network, and also manages all the connection processes with all restrictions, being an information device.
The session level organizes and synchronizes exchanges between remote tasks. It implements the relationship between the logical and physical addresses of distributed tasks, and also establishes the relationship between applications that must collaborate and command the dialogue. In the latter case, this service is called token management.
The presentation layer is responsible for the syntax and semantics of the data and processes the information to make it compatible between the transmitted streams. This will ensure the independence of the user from the transport of information. Typically, this layer converts data, reformatts, encrypts and compresses them.
The application layer is the point of contact between the user and the network. That is why it will provide the user with the basic services offered by the network: file transfer and messaging.
Inter-layer data transfer
The seven-level OSI model for dummies is an instruction for organizing data transfer. The sender process delivers data that moves to the receiver process at the application level with a single AH header. Then the result is passed to the level that converts this message, and adds a new header. The presentation layer does not and should not know about the possible existence of AH. For the presentation layer, AH is actually part of user data. Upon completion of processing, the presentation layer sends a new “message” to the session level, and the same process starts again.
Then the data reaches the physical layer, which actually transfers it to the recipient. At the reception, the message will rise through the levels, and the headers will be gradually deleted until the reception process is reached. An important concept of the seven-level OSI model for dummies is as follows:
- Each layer is programmed as if it were truly horizontal.
- When interacting, each layer adds a title and sends it thanks to the underlying layer.
Instructions for "dummies"
Even after reading the descriptions of each of the seven levels, the user may still not be fully aware of what features of the OSI model are used in a real network. Usually he misunderstands how a model can be used to solve real problems (and in some cases, it may not be the best approach for this). However, in many aspects of information and communication technology (ICT), the model can be extremely useful, which directly relates to the OSI model.
When a novice user first encounters a network problem, this can be misleading, and they don’t know where to start. The OSI model for dummies helps to begin to find out the source of the problem and, therefore, helps to solve it. For example, the client computer cannot connect to the website. The problem is isolated from one device on the network, so we can assume that it is probably created at the application level.
However, after testing, it is discovered that the problem still exists when another web browser is used, so the application error level is eliminated. The user assumes that the problem is at the presentation level, and begins to look for the wrong settings. After some analysis, he discovers that the client has mistakenly entered the DNS settings.
Perhaps a person is already using the OSI model to easily solve problems, such as the example above, without even realizing it. However, when working with large corporate networks, the targeted use of the model makes this process much easier and more enjoyable.
Connection Setup Description
The functions of the individual layers provide the parent layer with a specific service. The tasks of the individual layers are defined in the OSI layer model. Presented and described here, it is greatly simplified and adapted to network technologies. This layer model is not complete or final. From a user perspective, a simplified view of OSI is explained below.
Application levels 5, 6, and 7 define all the protocols that programs directly access.
In the Windows network world, SMB uses NetBIOS to connect to the transfer layer. When Unix comes in contact with Windows, the service is used to provide them resources on a Windows network on Unix. Communication between layers 7 of the OSI application and transfer models is established through TCP ports. Applications and services identify their data through these ports. The data stream is packed using a connection-oriented TCP or UDP connectionless protocol.
Internet Protocol (IP) handles packet addressing. The bitstream is sent to NDIS, which contacts the network card driver. The driver sends data to the network interface card (NIC), from where it enters the network.
For incoming data, they are returned in the opposite way:
- NetBIOS - Network Base I / O.
- TCP is a network management protocol.
- UDP is a user datagram protocol.
- IPv4 is the Internet Protocol version 4URL address (universal resource locator) in Windows, the NetBIOS name for the computer. It is used to identify the computer and the services that run on it.
- To resolve the IP address of the IP address, use the hosts file, which lists all the URL and IP addresses. Since there are many URLs, DNS (Domain Name System), which is hierarchical, has been introduced. So-called DNS servers can query for unknown DNS names from a higher level DNS server.
- On a Windows network, the lmhosts or WINS (server) file is used to resolve NetBIOS names to IP addresses. If the IP address of the transfer layer is resolved, then ARP ( Address Resolution Protocol) is used to resolve the IP address to the MAC address (Media Access Control) of the NIC (physical layer).
- The MAC address is the only final address that can be used to reliably identify a computer on the network. It is fixed on the network card,
Where:
- URL - a single resource index.
- DNS is a domain name system.
- WINS is a Windows domain name service.
- ARP is an address resolution protocol.
- MAC Address
Network criticism
The most striking thing about the OSI model is that it is the most studied and globally recognized network structure, and yet, in the full network sense, it is not a model. The specialists who analyzed this failure identified the main causes of the failure:
- The OSI interoperability model was ideally positioned for research, but TCP / IP was already in a vibrant investment phase when the OSI model was released. US universities have already successfully used TCP / IP, and industrialists did not feel the need to invest in it.
- Sophisticated technology. OSI is actually too complex for a correct and efficient implementation. The standard development committee even had to leave aside certain technical issues, such as security and coding, since it was difficult to maintain a specific role for each completed layer. Flow and error management appears at almost every level, which complicates the use of the 7-level OSI model for dummies. At the implementation level, TCP / IP is much more optimized and efficient.
- The harshest criticism that sounds to the model is that it is not at all suitable for computer telecommunication applications. Some choices made are not consistent with how computers communicate with software. The standard actually made the choice of an “interrupt system” for signaling events, but this is not possible in high-level programming languages.
- Incorrect implementation of the seven-level OSI model. Briefly, the reasons can be explained as follows: this is because it is relatively complex, and as a result, the first implementations were relatively bulky and slow. In contrast, the pilot implementation of TCP / IP at Unix Berkeley (BSD) was free and relatively efficient. Historically, there has been a natural tendency to use TCP / IP.
- The OSI model actually suffers from too much standardization. The efforts to introduce the model were largely bureaucratic. Conversely, TCP / IP comes from Unix and is immediately used. The lack of standardization of TCP / IP is balanced by a fast and efficient implementation and use in an environment that promotes its distribution.
- The basic OSI reference model is much more recognized than the OSI protocols themselves. There are several reasons for this. A process based on the OSI committee has spawned unimaginable inefficient protocols.
- The biggest problem with OSI is that it really doesn't offer anything new. The strongest example for implementation is its status as an “international standard”, but people already have a de facto international standard - the Internet, around which OSI protocols exist.
The advantages of a multi-layer platform
OSI's most significant contribution is the network philosophy, represented by a layered model. It is most often seen as a model that explains network capabilities. The levels of the OSI model are defined in web development, sometimes called the "stack". , , , . LAMP.
:
- , , . .
- .
- .
- , , , .
- OSI .
OSI , . DoD (TCP/IP) .
The problem with the OSI layer model is the standardization of the ISO organization, which was simply too cumbersome to quickly establish a structure for the tasks of protocols and transmission systems in network technologies. TCP / IP was freely available, it worked and quickly spread with other OSI model layer protocols. Therefore, ISO had no choice. It was necessary to consider TCP / IP in the OSI layer model.
In addition to TCP / IP, other network protocols have been developed. However, in the end they were replaced by him. Almost all networks today operate on the basis of TCP / IP.
The future of network world standardization
Despite being updated in 1994, the seven-tier OSI open system interconnection model clearly lost the battle against TCP / IP. Only some large dominant manufacturers retain the model, but many experts believe that it will disappear faster and faster as the Internet explodes TCP / IP. Nevertheless, the OSI model will remain in use, as this is one of the first major efforts to standardize the network world.
OSI will also exist for another reason: even if TCP / IP is used in practice, OSI is used as the current reference network model. In fact, TCP / IP and OSI have very similar structures, and basically it is OSI standardization, which caused confusion between the two models. TCP / IP is usually considered an actual OS implementation.
If the user is learning a new specialty to become a network engineer, or simply interested in high-level concepts, the OSI model is a great model to learn. No matter what sub-area of IT he plans to work in, the system will help him to solve even the most complex tasks.