Each layer in the model defines different protocols using a layered model. Network engineers can define and design protocols that are specific to the task.
Today, competition between different suppliers is growing all the time. The models of technology are diverse, but they are united by common standards, and therefore the superiority of the product is no longer based solely on the use of protocols, since all modern products support them equally.
Seven-level network model
A programming model is a conceptual structure that describes the functions of a network or telecommunication system and uses layers to give a visual description of what is happening with a particular network system. This can help network managers distinguish between physical or software problems. Technology vendors selling new products often refer to the OSI reference model to help customers understand which layer their products work on if they don’t work “through the stack.”
The standard was conceived in the 1970s, in the era of the creation of a computer network. Later, in 1983, two different models were merged into one and officially introduced in 1984 as the new international OSI standard, which most people encounter today. Although some users claim that the OSI model is already theoretically outdated and less necessary than the 4-level TCP / IP model, it is still difficult to analyze the operation of network technologies without seeing links to OSI and its layers. The structure of the model perfectly helps to analyze protocols and compare different technologies.
Benefits of a Layered Scheme
It performs many important operations and goals, reducing the complexity of implementing a large project and breaking it into small tasks. This allows you to standardize the interfaces between devices and facilitates modular engineering, so engineers can work at one level of the network model without worrying about what happens at another level. This modularity accelerates the evolution of technology and, finally, it teaches, dividing the complexity of interworking into discrete, easier-to-learn subsets of operations.
The multilevel model does not define or limit the implementation, but provides their basis. Therefore, the implementation does not comply with the model, but conforms to the standards developed on the basis of the OSI reference model. This can be demonstrated in some context:
- Users are familiar with devices: routers, hubs, and autonomous switches.
- NICs receive data from the upper layer and correctly pack them for transfer to media. In essence, NICs function in the lower 4 layers of OSI.
- Hubs, whether Ethernet or FDDI, on the physical layer only concern the transfer of bits from one station to other connected stations in the network. They do not filter traffic. On the other hand, bridges and switches will filter traffic and build switching tables to keep track of which device is connected to a specific port.
- Routers or routing technology work at the 3rd level.
Understanding Network Protocols
They work at a separate level in the OSI reference model to help a particular level perform related functions. Network protocols allow you to transfer data between computers. When protocols work together to provide OSI model layers, they are called a protocol suite or protocol stack.
When network protocols work together to move data between computers, the following process usually occurs:
- Data is broken down into smaller packets.
- To identify the target computer, address information is added to the data packets. Then they are transferred to the network card for transmission over the network.
- On the receiving computer, data packets are received from the network card.
- Any transmission information that has been added to the packet by the sending computer is deleted.
- The package is reassembled into the original message.
From the above process, you can see that network protocols collect, modify, and disassemble packets as data moves through the protocol stack of the OSI reference network model.
Batch Components
The OSI reference model is useful because it provides process independence between layers. This means that if a change in technology or capabilities is made at one level, it will not affect another level, both above and below it. A layered model is an open standard that does not provide consistency and compatibility between different devices.
The package is divided into segments, which, in turn, include the following aspects:
- Heading - signal, address of sources and destinations.
- The synchronization information needed to synchronize the transmission.
- Data is a packet segment sent to the receiving computer.
- Trailer - includes a cyclic redundancy check (CRC), which checks that the package is not damaged.
Components of the OSI reference seven-level model:
- Source Address - Identifies the PC sending the information.
- Destination Address - Identifies the recipient of the information.
- The way the PC sends information.
- Assembly Information.
- Packet load - information sent to the destination PC.
- Error checking information.
System Level Functions
The services provided by the OSI reference model layer can be characterized as follows:
- Physical level. Provides functional and procedural features for activating, maintaining, and deactivating physical links that transparently send bits, recognizes only individual bits, not characters or multichannel frames.
- Level of information transfer - means of transmitting information from network objects provide maintenance and deactivation of the communication line, grouping bits into characters and message frames, character synchronization, and access control for multimedia.
- Network level. It guarantees data transmission, relay regardless of technology. It masks the features of the environment from high layers for switching, installing, supporting, connecting and transmitting packets by users of the OSI reference network model.
- Transport level. Ensures transparent and reliable packet transfer between layers with the quality of service required by the application. This is the first true through layer.
- Sessional level - mechanisms for organizing and structuring dialogs between application processes. The mechanisms provide two-way simultaneous or two-way alternative work, the creation of primary and secondary synchronization points and methods for structuring the exchange of information.
- The presentation level is the independence of application processes from differences in the presentation of data, that is, in the syntax. Selecting and converting syntax allows you to select a "presentation context" with the conversion between alternative contexts.
- Application level All application processes use service elements provided by the application layer. These elements include library procedures that perform interprocess communication, define common procedures for building protocols provided by servers.
Physical environment
The first level of the OSI reference model, transmitting raw bitstreams through the physical environment, is connected with the establishment of a connection between a PC for communication. It is hardware and deals with the actual connection between the PC and the network media. This includes all devices that operate with physical layer processing alarms.
Details of the actual connection at this level include:
- Physical network topologies.
- Types of network connections and how to connect a cable to an interface card (NIC).
- Data encoding. This applies to analog and digital signaling used to encode information in signals.
- Bit sync and multiplexing.
The information processed on this layer is in bits (1 s and 0 s). 1s and 0s are represented by pulses of light or electricity. The components of this layer include connectors such as terminal and voltage cables, which are defined in the relevant standards organization.
The specifications of the OSI reference model at the physical level include:
- The physical location of the network.
- Voltage variation and process time.
- Packet sending speed.
- Transmission Limit
Binding process
If a computer has several interface adapters, it may have an identical protocol associated with a number of network cards. A driver operating at the information channel level is connected to a network adapter. TCP / IP and the NWLINK session can be associated with the driver.
The binding procedure is used through the layers to link the protocols, for which it will be necessary to establish the correspondence of the levels of the OSI reference model. PCs exchange information using protocols, with direct connection and without (UDP). UDP does not provide verification that data has been delivered. These protocols work well on low-load networks and fail on high-load networks.
The TCP / IP packet is a protocol with a connection that guarantees verification of the delivery of information to the end user. The reference model for the interaction of open OSI systems has:
- TCP / IP
- AppleTalk
- NetWare
- NetBIOS.
Firewalls and Reference Model
You can ask five different people what a firewall is and get at least four different answers. However, there are only a few types of firewalls, and the rest are just their variations. Batch filtering, channel-level gateways, status checking, application-level gateways, in-depth checking - all these conditions are widely distributed by various companies trying to sell their new offer.
Typically, the higher the firewall technology, the higher the performance of the network layer features of the OSI reference model. The first and most basic type of firewall is simply called a packet filter. These firewalls worked at the 3rd level of the OSI model as well as at the network level. Packet filters functioned mainly from two parameters in packets - source and destination IP addresses, but they could also view (and filter) the protocol field in the IP headers. At the same time, very few checks were performed and only at the network level. As a result, it became quite trivial for cybercriminals to deceive these types of filters using various tricks.
Spoofing, fragmentation and various other methods of interception allow them to receive traffic through simple packet filters that have been configured to block. However, one of the advantages of packet filters was (and is) their speed. Because they perform so few checks, they can do it quite efficiently.
Communication Operation Example
The concept of multi-level communication is essential to ensure the interconnection of all parts of the network. You can imagine the work of the basic reference model for the interaction of open OSI systems with a simple example.
In this example, the goal is to obtain information from location A to location B. The sender does not know what language the receiver speaks, so the sender transmits information to the translator. Without worrying about the contents of the package, it will translate it into a language that is understandable in most cases, so it does not matter what language the final recipient speaks.
The translator indicates the type of language, and then passes the message to the administrative assistant. The administrative assistant, not caring for the language or message, will work to ensure reliable delivery of the message to the destination. In this example, she will attach the fax number, and then fax the document to destination B. Location
The document was received by the administrative assistant at location B. The assistant at location B can even call the assistant at location A to inform him that the fax was received. Then the assistant at location B will send a message to the translator in his office. The translator will see that the message is written, for example, in Dutch. The translator, knowing that the person to whom the message is addressed, speaks, for example, only in French, will translate the message so that the recipient can read the message correctly. This completes the process of moving information from one place to another.
A closer examination of the process used for communication, you can notice that communication took place at different levels. At the first level, administrative assistants interacted together. At level 2, translators exchanged. And at the third level, the sender was able to contact the recipient.
Troubleshooting
When troubleshooting network problems, it is always wise to approach the problem using the network layer of the osi reference model. Its beauty lies in the fact that it is possible to individually eliminate a separate layer using simple methods. It is recommended that you start work from level 1 until a problem is found.
The first layer provides hardware media, including the definition of cables, cards, and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components. If there are no indicators on the network card, the cable may be broken, there is a hardware malfunction on the network card itself. You can use testing to check the cables to isolate the cause of the problem until the user makes sure that the operating system sees all the devices and shows that they are functional.
The second layer of the OSI Open Systems Interconnection Reference Model handles physical failures through streams and frame synchronization. The layer is divided into two sublayers:
- MAC environment
- Logical Link LLC.
The MAC sublayer determines how a PC on a network accesses packets and allows them to be transmitted. LLC level controls synchronization. Most of the OSI reference model compatibility issues at this level can be resolved with the arp command, in any case in windows. MAC addresses are unique to the device, but some users like to play around with the settings, which can cause problems and spoof arp. Using arp will show which MAC addresses are mapped to IP addresses.
The third layer provides routing technologies by creating logical paths, known as virtual paths, for transmitting a packet from node to node. Routing and forwarding are the functions of this layer, as well as addressing, interworking, error handling, congestion control and packet sequestration. This is a vast area for a quick guide. It covers the protocols rip1 and 2, ospf, igrp and some others, as well as routed IP. You can troubleshoot IP problems with icmp packets. Utilities such as ping and tracert use icmp packets to receive responses from network hosts. Utilities for packets can be configured to view IP packets moving through hubs (switches) in the same way as if the user were viewing frame headers. The print print command will show the routing table in the windows. Each operating system has commands for displaying the route table (in IOS it will be “sho ip route”). The usual crashes on this layer are duplicate IP addresses.
The fourth layer guarantees transparent packet transfer, as well as control over end-to-end fault recovery and flow control. Most of the troubleshooting here will be done using the packet sniffer. TCP is used with IP as a means of ensuring that data in packets is sent and received without loss. If an error occurs, packets are resubmitted (this would be appropriate for the tgp packet header structure for googling) with the correct sequence number so that data is not lost (this ensures complete data transfer).
The fifth layer establishes, manages and completes the interconnection of applications, exchanges and dialogs between them. It focuses on session coordination. It is most likely that the user will troubleshoot tcpip on this netbios layer. Windows has some very useful utilities, such as nbtstat and a group of “net” commands, that will help it. Manifestation of common mistakes is that people forget to install “file and printer sharing” and “client for Microsoft” in Win9x OS. Protocols such as DNS, LDAP (for most active directory replication), NFS, SQL, RPC, and XWindows are eliminated on this layer.
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If all other layers work and have been tested, then usually failures relate to the issue of applying patches, software, or the quality of software reinstallation.