In the Iso/osi reference Model, the main function of the network layer is ()----Baidu 2016 research and development engineer Pen question (vi)

In the Iso/osi reference Model, the main function of the network layer is () the correct answer: a your answer: a(correct)

Route selection, congestion control and network interconnection

Provides a reliable end-one service that transparently transmits messages

Data format transformation, data encryption and decryption, data compression and recovery

• Add notes
• Collection
• Error correcting

OSI (open System interconnect), which is open systems interconnection. Commonly called the OSI Reference Model, is the network interconnection model that the ISO (International Organization for Standardization) organization studied in the 1985. The architecture standard defines a seven-layer framework for Network interconnection (physical layer, Data link layer, network layer, transport layer, Session layer, presentation layer, and Application layer), the ISO Open System Interconnect Reference Model. In this framework, the functionality of each layer is further elaborated to achieve interoperability, interoperability, and portability of applications in an open system environment.

In order to make the network application more popular, ISO has introduced the OSI Reference model. The implication is that all companies are recommended to use this specification to control the network. So all companies have the same specifications, they can be connected. Computer network systems that provide a variety of Web services functions are very complex. According to the principle of divide and conquer, ISO divides the entire communication function into seven levels, the principle of division is:

(1) Each node in the network has the same level;

(2) The same layer of different nodes has the same function;

(3) communication between adjacent layers in the same node through interface;

(4) Each tier uses services provided on the lower level and provides services to the upper layers;

(5) The same layer of different nodes realizes the communication between peers according to the protocol.

The advantage of layering is that the information exchange problem of open system can be decomposed into a series of easy-to-control software and hardware modules-layer, and each layer can modify or expand the function independently according to the need, meanwhile, it is advantageous to the interconnection of equipment of different manufacturers, and also helps us to learn and understand the data communication network.

Different layers in the OSI reference model perform different functions, and each layer communicates with each other through a standard interface.

 7    osi     provides a means of access  OSI  environment for specific types of network applications. The application layer determines the nature of the communication between processes to meet the needs of the user. The application layer not only provides the information exchange and remote operation required by the application process, but also acts as a user agent for the application process to accomplish some functions necessary for the exchange of information. It includes: File transfer access and management  FTAM , virtual terminal  VT , transaction processing  TP , remote database access  RDA , manufacturing message Specification  mms  , directory services  DS  and other protocols, the application layer can communicate with the application interface to achieve the purpose of display to the user.   This common protocol has  :HTTP , HTTPS , FTP , TELNET ,SSH , SMTP  , POP3  and so on.

 6     to resolve the syntax of user information for upper-level users. It includes data format Exchange, data encryption and decryption, data compression and terminal type conversion.

 5   layer Session Layer: Establish an end connection between two nodes.   provides a dialog control mechanism between applications that are end-systems. This service includes setting up a connection in full-duplex or half-duplex mode, although you can manage the login and logoff processes in layer  4  on the duplex  ; session layer. It specifically manages the dialogue between two users and processes. If only one user is allowed to perform a specific action at a time, the session layer protocol manages these operations, such as blocking two users from updating the same set of data in the database at the same time.

 4   Layer Transport Layer:-General data Delivery-connection-oriented or no connection.   provides an end-to-end, reliable, transparent, and optimized data transfer service mechanism for session-level users. Includes full-or half-duplex, flow control, and error recovery services; The transport layer divides the messages into several groupings and reorganizes them at the receiving end. Different groupings can be transferred to the host through different connections. This allows for high bandwidth without affecting the session layer. When a connection is established, the Transport layer can request quality of service, which specifies acceptable ber, latency, security and other parameters, as well as an end-to-end traffic control capability.

 3   Layer network layer: This layer establishes a connection between two nodes through addressing, For packets sent from the transport layer at the source end, select the appropriate routing and switching nodes, which are correctly routed to the destination's transport layer according to the address.   It includes routing and relaying data through an interconnected network  ; in addition to selecting routes, the network layer is responsible for establishing and maintaining connections, controlling congestion on the network, and generating billing information when necessary. Common devices have switches;

 2   layer data Link layer: This layer will frame the data and process flow control.   shields the physical layer, providing a data link connection to the network layer, and making almost error-free data transfer (error control) on a physical connection that has the potential to make a mistake. This layer specifies the topology and provides hardware addressing. Common equipment has network card, bridge, switch;

Layer 1 Physical layer: at The very bottom of the OSI reference Model. The primary function of the physical layer is to use physical transport media to provide a physical connection to the data link layer for transparent transfer of bitstream. Common equipment has (a variety of physical equipment) hub, repeater, modem, network cable, twisted pair, coaxial cable.

When data is sent, it is transmitted from the seventh layer to the first layer, while receiving data is reversed.

The upper three layer is the general application layer, used to control software aspects. The next four layers are collectively the data flow layer used to manage hardware. Other layers, except the physical layer, are implemented with software.

Data is split when it is sent to the data flow layer.

The data in the transport layer is called a segment, the network layer is called the packet, the data link layer is called the frame, the physical layer is called the bitstream, so calledPDU(Protocol data Unit)Detailed features of each layer

(1) Physical layers (physical layer)

The physical layer is the lowest layer of the OSI Reference Model, which uses transport media to provide physical connectivity to the data link layer. It is primarily concerned with transmitting bitstream from one node to another through physical links, which may be copper wire, satellite, microwave, or other communication medium. It is concerned with: how much volt voltage represents 1? How many volts do you mean 0? What is the clock rate? Full-duplex or half-duplex transmission? In general the physical layer is concerned with the mechanical, electrical, functional and procedural characteristics of the link.

(2) Data link layer

The data link layer serves the network layer, solves the communication problem between two neighboring nodes, and transmits the protocol data unit called the data frame.

The data frame contains the physical address (also known as MAC address), control code, data and check code and other information. The main function of this layer is to transform the unreliable physical link into a data link that is error-free to the network layer by means of checking, confirming and feedback re-sending.

In addition, the data link layer also coordinates the transmission rate of the two sides, that is, traffic control, to prevent the receiver from processing the sender's high-speed data caused by buffer overflow and line blocking.

(3) Network layer

The network layer serves the transport layer, and the transmitted protocol data units are called packets or groupings. The primary role of this layer is to solve the problem of how the packets are routed through the nodes, that is, routing the packets to the destination through the path selection algorithm (route). In addition, in order to avoid the network congestion caused by excessive packets in the communication subnet, it is necessary to control the number of incoming packets (congestion control). When a packet crosses multiple communication subnets to reach its destination, it also solves the problem of Internet interconnection.

(4) The transmission layer (Transport layer)

The role of the transport layer is to provide end-to-end reliable and transparent data transfer services for the upper layer protocol, including handling error control and flow control issues. This layer shields the details of the lower data communication to the high level, so that the high-level users can see only a host-to-host, user-controlled and reliable data path between the two transport entities.

The Protocol data Unit transmitted by the transport layer is called a segment or message.

(5) Session Layer

The primary function of the session layer is to manage and coordinate communication (dialogs) between various processes on different hosts, which is responsible for establishing, managing, and terminating the sessions between applications. The reason the session layer is named is that it is very similar to the concept of a conversation between two entities. For example, an interactive user session begins by logging on to the computer, ending with logoff.

(6) Presentation layers (Presentation layer)

The presentation layer handles the representation of the data encoding that flows through the nodes to ensure that the information emitted by one system application layer is read out by the application layer of the other system. If necessary, the layer provides a standard representation for converting multiple data representation formats inside a computer into a standard representation used in network traffic. Data compression and encryption are also one of the transformations that the presentation layer can provide.

(7) Application tier (Application layer)

The application layer is the highest layer of the OSI Reference model and is the interface between the user and the network. This layer uses the application to complete the application needs of network users, such as file transfer, e-mail and so on. Data encapsulation Process: each layer of the package after the name of the data unit is different, in the application layer, the presentation layer, the session layer of Protocol Data unit is referred to as data, in the Transport Layer Protocol data unit called segment (data segment), in the network layer called Packet (packet), The Data Link Layer Protocol data unit is called a frame (data frame), which is called bits (bitstream) in the physical layer.data encapsulation for OSIWhen the data arrives at the receiving end, each layer reads the corresponding control information according to the content of the control information to pass the data unit upward layer, and then remove the control header and tail information (if any) of this layer before passing up the layer. This process is called unpacking. This process is performed layer by level until the data generated by the peer application layer is sent to the corresponding application process on the local side. Take the user to browse the website as an example to explain the data encapsulation, unpacking process.Data EncapsulationWhen the user enters the web site information to be browsed, the relevant data is generated by the application layer, which is transformed into a computer-readable ASCII code by the presentation layer, and then the corresponding host process is transmitted to the transport layer by the session layer. The transport layer will use the above information as data and the corresponding port number information for the purpose of the host to identify the message, know the specific task to be handled by the machine, in the network layer with the IP address so that the message can be confirmed to a specific host, and then the data link layer with the MAC address, transferred to the bit stream information, thereby transmitting on the Messages are received on the network by each host, by checking the destination MAC address of the message to determine whether it is the message they need to process, if the MAC address is found to be inconsistent with their own, then discard the message, unanimously remove the Mac information sent to the network layer to determine its IP address And then according to the destination port number of the message to determine which process is the local processing, which is the message of the package process.

7 Application Layer: Boss

6 Presentation tier: the equivalent of the company's demo boss, a letter to the boss's assistant

Work metaphor

5 Session Layer: The Secretary who is the equivalent of the company receiving letters, writing letters and unpacking envelopes.

4 Transport Layer: the equivalent of the company running the post office of the Messenger clerk

3 Network layer: equivalent to the sorting workers in the post office

2 Data Link layer: equivalent to a loading and unpacking worker in a post office

1 Physical layer: equivalent to a porter in the post officeThe OSI is a well-defined set of protocol specifications and has many optional parts to accomplish similar tasks.
it defines the hierarchy of open systems, the interrelationships between hierarchies, and the possible tasks that are included in each layer. is to coordinate and organize the services provided by each layer as a framework.
The OSI Reference Model does not provide an achievable approach, but rather describes the concepts used to coordinate the development of inter-process communication standards.  that is, the OSI Reference Model is not a standard, but a conceptual framework used in the development of standards.

In the Iso/osi reference Model, the main function of the network layer is ()----Baidu 2016 research and development engineer Pen question (vi)