Iso/osi seven-layer model structure system

LAN and WAN communications are guided by a network communication model, open Systems Interconnection (OSI). The OSI model is the product of two standardization organizations, ISO, and the American National Standards Association (ANSI). ISO represents the development of more than 100 countries in economic, humanities, scientific and technological standards, and ANSI has established standards for commercial products, including networks and computer products, in conjunction with American business, government agencies and international organizations. The OSI model was developed in 1974 and applied to LAN and WAN communications, which fully embodies the effort to standardize network software and hardware. Over the years, the OSI model has facilitated the development of network communications through the following functions:

. Enables communication between different types of LANs and WANs.

. The provision of network equipment standardization allows a manufacturer's equipment to communicate with another manufacturer's equipment.

. Enables older network devices to communicate with new network devices, so that when new equipment is installed, it is not necessary to replace the original equipment, thereby helping users to invest for a longer period of time.

. For communication within and between networks, it is permissible to develop hardware and software using a common interface.

. The internet is a notable example of making worldwide network communications possible.

Although the OSI model appears earlier than most network devices today, it not only creates a collaborative network of interconnected times, but also constantly accommodates new developments in network interconnection. It is a rigorous theoretical model, not a particular hardware device or a set of software routines, but a set of communication guidelines that vendors must follow in designing hardware and software, as in colloquial grammar. The OSI model guidelines point out:

. How network devices are contacted and how devices that use different protocols communicate.

. How the network device learns when to transfer or not transfer data.

. How to arrange and connect physical network devices.

. A method that ensures that network transmissions are received correctly.

. How network devices maintain a constant rate of data flow.

. How the electronic data is represented on the network media.

The OSI model consists of 7 layers of layers: physical layer, Data link layer, network layer, transport layer, Session layer, presentation layer and application layer. Each layer handles a specific communication task, using protocol based communication to exchange data with the next layer of the protocol stack. Communication between two network devices is accomplished by going up and down through the stack of protocols for each device. For example, there is a workstation that communicates with a server, the task begins at the application level of the workstation, and a class of information is formatted through a lower layer until the data arrives at the physical layer and then transferred over the network to the server. The server obtains information from the physical layer of the protocol stack, sending information to the upper level to interpret the information until it reaches the application tier. Each layer can be named by its name, or it can be indicated in its position in the protocol stack. For example, the lowest level can be called a physical layer or a 1th layer.

The functions performed at the lowest level are related to physical communication, such as building frames, transmitting signals containing packets, and network communication between the middle-tier coordination nodes, such as ensuring uninterrupted communication sessions, and continuing without errors. The highest level of work directly affects software applications and data representation, including data formatting, encryption, and data and file transfer management. To sum up, these layers are called protocol stacks. In the following chapters, we will discuss these 7 layers in detail.

1. Physical Layer

The lowest level of the OSI model is the physical layer. Contains the following:

. Data transmission media (wire and cable, fiber, radio and microwave).

. Network plugs.

. Network topology.

. Signaling and coding methods.

. Data transmission equipment.

. Network interface.

. Signaling error test.

The equipment used by the physical layer is responsible for generating, carrying and checking the voltage to transmit and receive signals containing the data. Network signal transmission has two kinds of analog and digital. Analog transmission can be constantly changing, as with the waveform of positive and negative voltage. Examples of analog transmission applications such as ordinary radio signals and telephone signals, because they can have unlimited range for sound regeneration. Similarly, analog TVs and computer monitors can reproduce millions of colors in any range. Analog traffic is used in a WAN that communicates with an analog modem, for example, through an Internet service provider (ISP) that enables Internet access by the user.

In the signal transmission, the physical layer processes the data transmission rate, monitors the error frequency, and processes the voltage level. Physical network problems, such as communication cable break, electromagnetic interference, etc. will affect the physical layer performance. Nearby electric motors, high-voltage lines, lighting equipment and other electrical equipment can cause interference. Electromagnetic interference (electromagnetic interference,emi) and radio frequency interference (Radio Frequency Interference,rfi) are two major causes of physical layer disturbances. Fans, the magnetic field generated by electric equipment such as elevator motors, portable heaters and air-conditioning facilities generates electromagnetic interference, and the electrical equipment to be used in the transmission of the network signal (such as cable TV components, radio and television stations, amateur radios, town-flow equipment in fluorescent lamps, computer or television, and C-B radio) Radio waves will be released at the same frequency, which is the cause of radio frequency interference.

2. Data Link Layer

The role of the data link layer in the LAN is to construct frames. Each frame is formatted in a specific way so that data transfers can be synchronized to reliably transmit the information between nodes. This layer will format the data so that it can be encoded as an electronic signal sent by the transmission node, decoded by the receiving node, and verify the error. The data link layer creates what is called a "data link frame" that contains a domain consisting of address and control information, as follows:

. The starting point of the frame (SOF).

. The address (source address) of the device that sent the frame.

. The address (destination address) of the device receiving the frame.

. Management or communication control information.

. Data.

. Error inspection information.

. An identifier for the end of the report (or the end of the frame).

As long as communication is established between two nodes, their data link layer is connected to the physical (through the physical layer) and the logic (through the Protocol). Communication is first established by the transmission of short signal sets used for data stream timing. Once the link is established, the data link layer at the receiving end decodes the signal into a separate frame. The data link layer checks the received signal to prevent duplicate, incorrect, or incomplete receipt of the received data. If an error is detected, the data is required to be retransmitted from one frame to another from the sending node. The data link error detection process is handled by the cyclic redundancy check (cyclic redundancy CHECK,CRC). Cyclic redundancy check (CRC) is an error detection method that calculates a value for the entire information field contained in the frame (SOF, addressing method, control information, data, CRC, and EOF). This value is inserted by the data link layer to the point where the sending node is near the end of the frame. When the data link layer passes the frame up to the previous layer, the value ensures that the frames are sent in the order in which they were received.

The data link layer contains two important child layers: Logical Link Control (Logic link CONTROL,LLC) and media access control (media access Control,mac). The LLC can initialize communication links between two nodes and prevent links from being interrupted, thus ensuring reliable communication. The Mac is used to verify the address information contained in each frame. For example, the Mac child layer on a workstation verifies every frame received by the workstation, and if the address of the frame matches the address of the workstation, most of the network devices have their own unique addresses, which are permanently on the chip of the device's network interface device.

This address is called a device address or physical address and is encoded in 16, such as 0004AC8428DE. The first half of the address indicates a specific network vendor, and if the device has only one interface, the latter part is unique to the interface or device. Many vendors use an encoding in the latter part to identify the type of device, such as a computer, bridge, router, or gateway.

It is important that both network devices do not have the same physical address. This is a protective measure implemented by network equipment manufacturers. If more than two devices have the same address, it can cause confusion when passing frames over the network.

There are two services for communication between the LLC child layer and the network layer (the high level of the data Link layer in the protocol stack). Type 1 is a connectionless service, and connectionless services do not establish a logical connection between sending and receiving nodes. This does not check that frames are received in the order in which they were sent, nor does it answer that the frames have been accepted, and that there is no error recovery.

Type 2 is a connection-oriented service. In a connection-oriented service, a logical connection is established between the sending node and the receiving node before the full communication begins. The frame contains a sequential number that is checked by the receiving node to ensure that it is processed in the order in which it was sent. Because communication is established, sending a node does not allow the data to be transmitted faster than the receiving node processing data. When the data is transmitted successfully, the receiving node notifies the sending node that the data has been received. If an error is found, the data will be retransmitted.

3. Network layer

The 3rd layer up in the protocol stack is the network layer. This layer along the network controls the path of the packet. All networks are made up of physical routes (cable paths) and logical routes (software paths). The network layer reads the packet protocol address information and forwards each packet along the optimal path (both physical and logical) for efficient transmission. This layer allows packages to be sent from one network to another through routers. Network layer Control Packet path, some like traffic controllers, along several different paths of the most effective one route by the packet. To determine the optimal path, the network layer needs to continuously collect information about each network and node address, a process called discovery. Not all protocols contain information at the network layer, and these protocols are not routable. Two typical network protocols that cannot be routed are the LAT of Dec Company and the NetBEUI of Microsoft Corporation. Both protocols are typically not implemented in midsize and large networks that require routing. Multiple destination addresses can be specified as one group. Packages with group destination addresses are delivered to multiple computers or network devices.

The network layer can be set up by creating virtual (logical) circuits on different paths from the data. A virtual circuit is a logical communication path used to send and receive data. The virtual circuit is for the network layer only. Since the network layer manages data along multiple virtual circuits, the error sequence may occur when the data arrives. The network layer checks the order of the data before it is transmitted to the next layer, and corrects it if necessary. The network layer also addresses and adjusts the size of the frames to meet the needs of the receiving network protocol, and ensures that the frame is not transmitted faster than the receiving layer receives.

4. Transport Layer

As with the data link layer and the network layer, the function of the transport layer is to ensure that the data is reliably sent from the sending node to the target node. For example, the transport layer ensures that the data is sent and received in the same order, and that the receiving node receives a response after transmission. When a virtual circuit is used in a network, the transport layer is also responsible for tracking the unique identification value assigned to each circuit. This i d is called a port, a connection identifier, or a socket, which is specified by the session layer. The transport layer also determines the level of packet error checking, and the highest level ensures that packets are sent from the node to the node without error during acceptable time.

protocols used to communicate between transport layers employ a variety of reliability measures. The 0 class is the simplest protocol to perform these functions on the network layer without performing error checking or flow control. 1 class Protocol monitor packet transmission error, if the error is checked, notify the transport layer sent to send the node to send the packet again. The 2 class protocol monitors transmission errors between the transport layer and session layer and provides flow control. Flow control ensures that the device does not send information at a rate that is higher than the network or receiving device receives information. In addition to providing the functionality of Class 1 and 2 protocols, Class 3 protocols can also recover lost packages in some environments. Finally, the 4-class protocol has an extended error monitoring and recovery capability in addition to the functions of the 3-class protocol.

Another function of the transport layer is to segment messages into smaller units when the network uses different protocols that require varying sizes of packets. The data unit separated by the transport layer on the sending network is again combined in the correct order by the transport layer of the receiving end so that the network layer can explain.

5. Session Layer

The session layer is responsible for establishing and maintaining communication links between the two nodes and determining the correct order for communication between nodes. For example, it can determine which node to transfer first. The session layer can also determine how far a node can transmit and how to recover from a transport error. If the transmission is inadvertently interrupted at the lower level, the session layer attempts to re-establish communication. In some workstation operating systems, you can disconnect workstations from the network and then reconnect them, and you can continue working without logging on. This is because the physical layer is disconnected and reconnected, and the session layer is reconnected.

This layer corresponds to a unique address one by one for each given node, as if the postal code is only associated with a specific postal area. Once the communication session is over, the layer is disconnected from the node.

An example of a communication on this layer occurs when a workstation accesses a server on the Internet. Workstations and servers have unique binary Internet Protocol (IP) addresses, such as 122.72.15.122 and 145.19.20.22, that are separated by a dot number. The session layer uses this address information to help establish the connection between nodes. Once the connection is established, the workstation can log in and the communication session is established through the session layer.

6, the presentation layer

This layer deals with data formatting issues, so data formatting is required because different software applications often use different data format schemes. In a sense, the presentation layer is somewhat like a grammar checker. It ensures that numbers and text are sent in a format that can be read by the presentation layer of the receiving node. For example, data sent from an IBM mainframe may be formatted with EBC DIC characters, and for workstations running Windows 95 or Windows 98 to read information, they must be interpreted as ASCII character format.

The presentation layer is also responsible for encrypting data. Encryption is the process of encoding data so that unauthorized users cannot intercept or read it. For example, the computer's account password can be encrypted on the LAN, or the credit card number can be encrypted on the WAN via the cryptographic sockets Layer (secure Sockets layer,ssl).

Another feature of the presentation layer is data compression. When the data is formatted, spaces may be formatted in the middle of the text and numbers. Data compression deletes these spaces and presses the data to reduce its size for sending. After data transfer, it is decompressed by the presentation layer of the receiving node.

7. Application Layer

The application layer is the highest level of the OSI model, controlling the vast majority of computer users ' direct access to applications and network services. The network services here include file transfer, file management, remote access files and printers, message processing for e-mail, and terminal emulation. Computer programmers use this layer to connect workstations to network services, such as linking applications to e-mail messages, or providing database access on the network.

Microsoft Windows Redirector works across the application tier. A redirector is a network service that makes one computer visible to another so that another computer can access it over the network. When a user shares a folder on a Microsoft network, other computers can find the computer and can access the folder using a redirector.

The OSI model has seven layers of functionality as shown in the table.