The composition of computer networks and the functions of each component

In the early days of network history, the International Organization for Standardization (ISO) and the International Telegraph Telephone Advisory Committee
CCITT has jointly published a layer-7 Reference Model for Open System Interconnection. Network processes in a computer operating system include application requests (at the top of the protocol stack) to network media (bottom)
The OSI reference model divides functions into seven discrete layers. Figure 2.1 shows the OSI layered model.

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│ Application Layer │ Seventh Layer

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│ Presentation layer │

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│ Session Layer │

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│ Transport layer │

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│ Network layer │

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│ Data link layer │

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│ Physical layer │ second layer

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Figure 2.1 OSI Layer-7 Reference Model

Layer 7 of the OSI model performs the following operations:

The first layer ?? Physical Layer

The first layer is responsible for final encoding of information into current pulses or other signals for online transmission. It consists of the actual interface between the computer and the network medium. It can define the electrical signal, symbol, line status and clock requirements.
Connector for data encoding and data transmission. Such as the most commonly used RS-232 specification, 10base-t Manchester code and RJ-45 belongs to the first layer. All layers higher than the physical layer pass
Call an interface defined in advance. For example, the Ethernet ancillary unit interface (AUI), a DB-15 connector can be used to connect Layer 1 and Layer 2.
Layer 2 ?? Data Link Layer

The data link layer provides reliable data transmission through physical network links. Different data link layers define different network and Protocol features, including physical addressing, network topology, error verification, and frame sequence.
Column and traffic control. The physical addressing (corresponding to the network addressing) defines the addressing method of the device at the data link layer. The network topology defines the physical connection mode of the device, such as the bus topology and the ring topology; error
An alarm is triggered by Incorrect verification to the upper-layer protocol when a transmission error occurs. The data frame sequence is reorganized and frames other than the sequence are transmitted. throttling may delay data transmission, so that the receiving device does not receive more
The information flow of management capability crashes. The data link layer is actually composed of two independent parts: Media Access
Control, Mac) and Logical Link
Control, LLC ). Mac describes how to schedule, generate, and receive data on a site in a shared media environment. Mac ensures reliable cross-link information transmission, synchronizes data transmission, and identifies errors.
Mistaken and controlling data flow direction. Generally, Mac is only important in the shared media environment. Only nodes in the shared media environment can be connected to the same transmission media. IEEE
Mac rules define addresses to identify multiple devices in the data link layer. The sub-layer of the Logical Link Control manages the communication between devices on a single network link.
802.2 the standard defines LLC. LLC supports connectionless services and connection-oriented services. Many domains are defined in the Information frames at the data link layer. These domains allow multiple high-level protocols to share one physical data.
Link.
Layer 3 ?? Network Layer

The Network Layer establishes a connection between the source and the end. It generally includes network path finding, and may also include traffic control and error checks. Data transmission between different network segments of the same mac standard generally only involves the data link layer, while data transmission between different MAC standards involves the network layer. For example, an IP router works at the network layer, and thus multiple networks can be interconnected.

Layer 4 ?? Transport Layer

The transport layer provides a reliable end-to-end network data stream service to the high level. The functions of the transport layer generally include flow control, multi-channel transmission, virtual circuit management, error checksum and recovery. Data transfer between traffic control management devices
To ensure that the transmission device does not send data that is larger than the processing capability of the receiving device. Multi-Channel Transmission allows the data of multiple applications to be transmitted to one physical link; virtual circuits are established, maintained, and terminated by the transport layer; Error Correction
Verification includes the various structures established to detect transmission errors, and error recovery includes the actions taken (such as request data re-transmission) to resolve any errors. The Transmission Control Protocol (TCP) is reliable
The transport layer protocol in the TCP/IP protocol family for data transmission.
Layer 5 ?? Session Layer

The Session Layer establishes, manages, and terminates communication sessions between the presentation layer and the entity. Communication sessions include service requests and service responses that occur between different network application layers. These requests and responses are implemented through the Session Layer Protocol. It also includes creating checkpoints so that a previous status can be returned when communication is interrupted.

Layer 6 ?? Presentation Layer

The presentation layer provides multiple functions for data encoding and conversion at the application layer to ensure that information sent at the application layer of one system can be recognized by the application layer of another system. The encoding and conversion modes of the presentation layer include public data representation format, performance conversion representation format, public data compression mode, and public data encryption mode.

The common data representation format is the standard image, sound, and video format. By using these standard formats, different types of computer systems can exchange data. The conversion mode uses different text and
Data Representation, information exchange between systems, such as ASCII (American Standard Code for information)
Interchange, American Standard Information Exchange Code); standard data compression mode ensures that compressed data on the original device can be properly extracted on the target device; encryption mode ensures that the original device is encrypted
Data can be correctly decrypted on the target device.
The presentation layer protocol is generally not associated with a special protocol stack. For example, QuickTime is the video and audio standard of the applet computer, and MPEG is the ISO video compression and encoding standard. Common image formats such as PCX, GIF, and JPEG are different static image compression and encoding standards.

Layer 7 ?? Application Layer

The application layer is the closest to the end user's OSI layer, which means that the OSI Application Layer Directly Interacts with users through application software. Note that the application layer is not a practical application running on a computer
The Application Program
Application interfaces), which are beyond the scope of the OSI model. The functions of the application layer generally include identifying communication partners, defining resource availability and synchronous communication. Because
In order to possibly lose the communication partner, the application layer must define the identification and availability of the communication partner for the Application subroutine that transmits data. When defining resource availability, the application layer must determine whether there is sufficient network resources for communication requests.
Source. In synchronous communication, communications between all applications require collaborative operation at the application layer.
OSI application layer protocols include file transmission, access and management protocols (FTAM), file virtual terminal protocol (VIP), and public management system information (CMIP.

2.2 TCP/IP layered model

The TCP/IP layening model is called the Internet layering model and the Internet Reference Model ). Figure 2.2 shows the layer-4 layer of the TCP/IP layered model.

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│ D │ F │ W │ F │ H │ g │ t │ I │ s │ U │

│ N │ I │ H │ t │ o │ e │ R │ M │ s │ its │

│ Layer 4, application layer │ s │ n │ o │ P │ t │ P │ l │ C │ t │ e │

│ G │ I │ P │ H │ n │ P │ n │

│ E │ s │ e │ it │

│ R │ t │

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│ Layer 3, transmission layer │ TCP │ UDP │

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│ ICMP │

│ Second layer, inter-network layer │ sampled ---- sampled │

│ IP │

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│ Layer 1, network interface │ arp/RARP │ others │

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Figure 2.2 layer-4 TCP/IP Reference Model

The TCP/IP protocol is organized into four conceptual layers, with three layers corresponding to the corresponding layers in the ISO Reference Model. The ICP/IP protocol family does not contain the physical layer and data link layer. Therefore, it cannot independently complete the functions of the entire computer network system and must work with many other protocols.

The four protocol layers of the TCP/IP layered model provide the following functions:

The first layer ?? Network Interface Layer

The network interface layer includes the protocols used to transmit the IP address data on the existing network media. In fact, TCP/IP standards do not define functions that correspond to the ISO data link layer and physical layer. Instead, it
Address Resolution
Protocol (ARP) provides an interface between the data structure of the TCP/IP protocol and the actual physical hardware.
Layer 2 ?? Inter-network layer

The internetlayer corresponds to the network layer of the OSI Layer-7 Reference Model. This layer includes IP protocol and RIP Protocol (routing information ).
Is responsible for data packaging, addressing, and routing. It also includes the inter-network Control Message Protocol (Internet Control Message ).
Protocol, ICMP) is used to provide network diagnostic information.
Layer 3 ?? Transport Layer

The transport layer corresponds to the transport layer of the OSI Layer 7 reference model. It provides two end-to-end communication services. Among them, the TCP protocol (Transmission Control Protocol) provides reliable data stream transportation services, and the UDP protocol (use datainprotocol) provides unreliable User Datagram services.

Layer 4 ?? Application Layer

The application layer corresponds to the application layer and expression layer of the OSI Layer-7 Reference Model. Internet application layer protocols include finger, Whois, FTP (file transfer protocol), Gopher,
HTTP (Hypertext Transfer Protocol), Telent (Remote Terminal protocol), SMTP (Simple Mail Transfer Protocol), IRC (Internet Relay Session), NNTP (network news transmission protocol)