Computer network learning notes-Overview of Computer Network

Computer network learning notes-Computer Network Overview 1. Differences between computer networks and distributed systems 1) Computer Networks (autonomous and interconnected) computer networks are connected by many computers, they can ask for help from each other. Three important features: computers connected to each other in computer networks are relatively independent. -- Each computer can provide services to other computers. Other computers send requests to a computer. If the computer is idle, the computer will respond to the requests, if the computer is busy, no response is made. This request method generally uses the C/S and B/S modes. -- These computers use different media links (for example, wireless or optical cables) to communicate with each other. subnet: A computer network interconnected by routers. Each computer connects to the entire network through a device in the Communication Subnet. These equipment rooms are connected to each other in the Communication Subnet. Therefore, computers connected to the Communication Subnet can exchange information. Interconnection: If two computers can exchange information, the two computers are interconnected. Autonomous COMPUTER: A computer that does not belong to any other computer is called an autonomous computer. 2) distributed systems are usually interconnected Based on autonomous computers. However, they are managed by an operating system in a unified manner and have a high degree of integrity and transparency for users. For example, a computer in a distributed system submits a task not to a computer in the system, but to the operating system that manages the distributed system, the operating system allocates tasks for each connected computer according to the situation. After the task is completed, the computer submits the results, and the operating system analyzes the computer's final results and displays them to the end user. (Similar to a project team .) Feature: -- virtual computer, which may be an organic combination of multiple computer networks. -- After you submit a task to the virtual system, the operating system will complete the task. 2. Enterprise Network and Public Network Enterprise Network: purpose (a bit): resource sharing, high reliability (any resource can have multiple copies), saving money. Enterprises use Ethernet and firewalls to separate the Intranet and Internet. Public Network: generally used to access remote information, communication between individuals, and entertainment. 3. Computer Classification 1) a network with a relatively small scale according to technology. A computer publishes information and other computers receive the information. A point-to-point network with a large size. 2) divided by scale LAN man Wan Internet 3) divided by transmission medium wired network wireless network 4) divided by topology structure bus ring mesh Star 5) by scope of use divided into private Network public Network 4, LAN, man and Wan 1) LAN (Local Area Network) region: Coverage of small transmission technology: Bus Type: IEEE 802.3 (Ethernet) CSMA/CD 10 m csma/CD: whether data is transmitted. If data is not transmitted at the same time, data is transmitted after a random delay. Bus Type: IEEE 802.4 (Token Bus) 10 M ring: IEEE 802.5 (IBM licensing ring) 4 M 16 M licensing ring transmits cyclically between interconnected computers, when a computer wants to send a message, it needs to wait for the token to be passed to its own hand, and then get stuck with the token. It will not be passed for now, and then the computer will send the message, the token is passed again. (2) Metropolitan Area Network (man) (3) Generally, broadcast networks are not used, and point-to-point transmission is generally used. A network with a large cross-region. -- End system (host) -- communication subnet (subnet) for short. The network connected by the router. -- All hosts in the resource subnet network. Each host may provide services for other hosts. -- Generally, the Internet consists of a LAN connected by a router. A subnet is usually a point-to-point subnet. Its transmission mechanism is store-and-forward) its transmission mode is packet-switched. 5. Advantages of Computer Network Reference Model layering: -- Implement a relatively independent function at each layer to simplify the problem. The design at each layer is independent, so it does not have to worry about how to implement it next time, you only need to know what services the next layer provides for me and what services I must provide for the previous layer. -- When the implementation of a layer needs to change due to technical changes, other layers are not affected. 1) when the ISO/OSI reference model data is sent, It is uploaded from the seventh layer to the first layer, and the opposite is received. The upper layer is called the application layer, which is used to control software. The lower layer is called the data stream layer to manage hardware. Data is split when it is sent to the data stream layer. The data in the transmission layer is called a segment, the network layer is called a packet, the data link layer is called a frame, and the physical layer is called a bit stream. This is called a PDU (Protocol Data Unit). The application layer is the highest layer in the OSI. Provides access to the OSI environment for specific network applications. The Application Layer determines the nature of communication between processes to meet user needs. The application layer not only provides information exchange and remote operations required by the application process, but also serves as the user agent of the application process to complete some functions necessary for information exchange. It includes: File Transfer access and management FTAM, virtual terminal VT, transaction processing TP, remote database access RDA, manufacturing message specification MMS, directory service DS and other protocols; -- includes all application protocols-for example, full screen function. Different terminals have different control characters and should be converted accordingly. A network terminal is usually defined. -- Different systems have different file transmission modes, but the representation must be consistent. Layer-3 Presentation Layer: it is mainly used to process the expression of information exchanged between two communication systems. Solves the syntax problem of user information for upper-level users. It includes functions such as data format exchange, data encryption and decryption, and data compression and recovery. The syntax and semantics of the presentation layer are concerned. The description of relevant data is defined in an abstract way, for example, floating point numbers all use scientific notation-Representation Transformation of relevant data -- Transformation of abstract data structures Layer 2 Session Layer: Establish an end connection between two nodes. Provides a dialog control mechanism between applications in the end system. This service includes setting the connection in full or half duplex mode, although the duplex mode can be processed in Layer 4. -- for example, one party can listen to the response of the other party when talking about it, because the line may be disconnected. Layer 2 Transport Layer: conventional data delivery-connection oriented or connectionless. It provides an end-to-end reliable, transparent, and optimized data transmission service mechanism for session-layer users. Includes full or half duplex, flow control, and error recovery services. -- The transport layer divides the data that needs to be transmitted at the higher level into several packets. -- A message is different from a frame. A frame has only a frame sign (start sign and end sign), but the address and port of the message, the sequence number of the message, and the confirmation number of the message. -- The Lower Layer 3 Communication objects are generally vrouters and the transmission layer is end-to-end. Therefore, we must consider how packets can be correctly transmitted to the destination, while the source and destination are usually hosts. Layer 2 network layer: This layer establishes a connection between two nodes through addressing, and selects appropriate routes and exchange nodes for the Group sent by the transport layer of the source end, it is correctly transmitted to the destination transport layer according to the address. It includes routing and relay data through an interconnected network. -- Select routing-congestion control-protocol conversion-segmentation and reorganization-data link layer 2nd for user grouping, character, and other records: At this layer, data is divided into frames and traffic control is processed. Shield the physical layer and provide a data link connection for the network layer to transmit data with almost no errors on a physical connection that may be faulty. This layer specifies the topology and provides hardware addressing. -- identifies frames. -- Frame receiving requires verification and confirmation. -- After the sender times out or receives a negative message, the sender must resend the message. -- Duplicate frames must be discarded. -- Solve the Problem of channel sharing in the shared network. Layer 2 Physical Layer: the bottom layer of the OSI reference model. The main function of the physical layer is to use physical transmission media to provide physical connections to the data link layer for transparent transmission of bit streams. Converts the signal and binary data on the transmission media. -- A string of binary data that is sent or received on a physical interface in the format of a rule. -- The physical layer defines the mechanical, electrical, functional, and process features of interfaces. -- For example, the geometric size of the plug and socket. Transmission data of each pin. OSI data transmission: 2) TCP/IP Reference Model TCP/IP is a group of communication protocols used to achieve network interconnection. The Internet architecture is centered on TCP/IP. Based on the TCP/IP reference model, the protocols are divided into four layers: network access layer, Internet connection layer, transmission layer (host-to-host), and application layer. 1. the application layer corresponds to the top layer of the OSI reference model and provides users with various services, such as FTP, Telnet, DNS, and SMTP. 2. the transport layer corresponds to the transport layer of the OSI reference model and provides end-to-end communication for the application layer entities, ensuring the ordered transmission of data packets and data integrity. This layer defines two main Protocols: Transmission Control Protocol (TCP, providing reliable services) and User Datagram Protocol (UDP, providing unreliable services ). TCP provides a reliable connection-oriented data transmission service, while UDP provides unreliable and connectionless data transmission services. 3. the Internet layer corresponds to the network layer of the OSI reference model. It is based on a connectionless group switching network, which mainly solves the communication problem between the host and the host. It includes protocol design data packets for logical transmission across the network. Focus on re-assigning a Host IP address to address the host. It is also responsible for routing data packets in multiple networks. The layer has four main Protocols: Internet Protocol (IP), Address Resolution Protocol (ARP), Internet Group Management Protocol (IGMP) and Internet Control Packet Protocol (ICMP ). The IP protocol is the most important protocol in the Internet layer. It provides an unreliable, connectionless datagram transmission service. 4. The network access layer (host-network layer) corresponds to the physical layer and data link layer in the OSI reference model. It monitors data exchange between the host and the network. In fact, TCP/IP itself does not define the protocol for this layer, and the networks involved in the interconnection use their own physical layer and data link layer protocols, and then connect with the TCP/IP network access layer. 5. Network Protocol: The network protocol is a certain agreement and rule that computers follow when communicating with each other. 1) three elements of network protocols: (1) Syntax: used to determine the format of protocol elements, that is, the structure and format of data and control information. (2) semantics: used to determine the type of protocol elements and specify the control information, actions, and responses required by both parties. (3) Timing: used to determine the matching and timing of the communication speed, that is, a detailed description of the event Implementation sequence. 6. Network Architecture: 1) the advantage of hierarchical model introduction is that problems can be solved in different layers. Each layer solves a small problem and finally solves the entire problem. TCP/IP protocol cluster TCP/IP protocol is a general term for a group of protocols, including: IP layer (that is, for the network layer): IP, ICMP, ARP, RARP, OSPF and other TCP (Transport Layer): TCP, UDP