computer network architecture
Overview of Computer Networks
computer network
composition
From the perspective of components: a complete computer network is mainly composed of three parts: hardware, software, and protocols , all of which are indispensable .
Hardware mainly refers to: host computer, communication link, switching equipment and communication equipment, etc.;
Software mainly refers to: various software used by users;
Protocol refers to the specifications that need to be followed when transmitting data on the network
From the point of view of working method: it can be divided into edge part and core part .
The edge part consists of all hosts connected to the Internet for users;
The core part consists of a large number of networks and routers connecting the networks
From the perspective of functional composition: a computer network is composed of a communication subnet and a resource subnet .
The communication subnet is composed of various transmission media, communication equipment and corresponding network protocols;
A resource network is a collection of devices and software that implement resource sharing functions, providing services to network users
Function
- data communication
- The most basic and important function of the network , used to realize the transmission of information
- Resource Sharing
- Make the computer network division of labor and cooperation , intercommunication
- distributed processing
- Assign a complex task to other computer systems on the network
- Improve reliability
- Each computer in the computer network can replace each other through the network
- load balancing
- Distribute the work evenly among the computers in the computer network
Classification
- distribution area
- Wide area network , the range is usually tens of kilometers - thousands of kilometers
- MAN , a few blocks or an entire city
- LAN , tens of meters to thousands of meters
- transmission technology
- Broadcast network , where all computers share a common communication channel (example: TV station)
- A peer-to-peer network , where each physical line connects a pair of computers (example: telephone calls)
- topology
- bus network
- star network
- ring network
- mesh network
- switching technology
- Circuit switching network , a dedicated channel is established between the sender and receiver for data transmission
- Packet switching network , data plus address and other information are forwarded, and each packet chooses its own route
- Packet switching network , which divides data into smaller data blocks, similar to packet switching, but divides packets into smaller packets
- Transmission medium
- Divided into wired network and wireless network
Performance
- time delay
- Sending delay, calculated from the first bit of the sending packet to the time required for the last bit of the packet to be sent
- Propagation delay, the time it takes for a bit to travel from one end of the link to the other
- Processing latency, time spent analyzing address parts, error checking, etc.
- Queuing delay, the time to wait for processing after entering the router
- round trip delay
- The total elapsed time from when the sender sends data to when the sender receives an acknowledgment from the receiver
- throughput
- The amount of data passing through a network (or interface) per unit time
- Rate/Data Rate/Bit Rate
- Rate: The rate at which a host connected to a computer network transmits data on a digital channel , also known as data rate or bit rate. Usually, the highest data rate is called the bandwidth
- delay-bandwidth product
- It refers to how many bits the sender has sent when the first bit of the sender is about to reach the end.
- Delay-bandwidth product = propagation delay * channel bandwidth
Computer Network Architecture and Reference Model
hierarchical structure
The basic principle:
- Each layer implements a relatively independent function, reducing the complexity of the large system. The interfaces between the layers are natural and clear, easy to understand, and communicate with each other as little as possible.
- The precise definition of the functions of each layer is independent of the specific implementation method and can be implemented with the most appropriate technology
- Maintain the independence of the lower layer from the upper layer, and the upper layer uses the services provided by the lower layer in one direction
When two hosts communicate, the same layer logically has a direct channel, which means that the information is transmitted to the other party without going through the lower layer.
Computer network protocols, interfaces, services
Protocol: A protocol is a collection of rules, consisting of three parts : syntax, semantics, and synchronization . ( Forensic Medicine )
The syntax specifies the format of the transmitted data;
Semantics specify the functions to be accomplished;
Synchronization specifies the timing relationship for performing various operations, etc.
Interface: The interface is the connection point where two adjacent layers exchange information
Service: Service refers to the function call provided by the lower layer for the immediately upper layer
Note: Protocols and services are conceptually different. Only the implementation of this layer protocol can guarantee the provision of services to the upper layer.
Computer services can be divided into three types:
1. Connection-oriented service and connectionless service
In connection-oriented services , both parties must first establish a connection before communicating , allocate resources, and then perform data transmission , and release the connection after the transmission is completed
In a connectionless service , the two communicating parties do not need to establish a connection first , and can send data directly when they need to send data first, which is usually called "best effort delivery"
2. Reliable service and unreliable service
Reliable service means that there is an error correction and error detection mechanism to ensure that the data is correct and reliable
Unreliable service means that the correctness and reliability of the network are guaranteed by applications or users
3. Answered service and unanswered service
Response service means that the receiver gives a corresponding response to the sender after receiving the data
No response service means that the receiver does not automatically give a corresponding response after receiving the data
ISO/OSI Open Systems Interconnection Reference Model
Physical layer: The transmission unit of the physical layer is bit , the task is to transmit the bit stream transparently , and the function is to transparently transmit the original bit stream for the data segment device on the physical medium
Data link layer: The transmission unit of the data link layer is a frame , and the task is to assemble the IP data packets from the network layer into frames . The functions of the data link layer can be summarized as framing, error control, flow control, transmission management, etc.
Network layer: The transmission unit of the network layer is a datagram . The main task is to transmit the packets of the network layer from the source end to the destination end, and provide communication services for different hosts on the packet switching network.
Transport layer: The transport layer is also called the transport layer. The transmission unit is a segment (TCP) or user data packet (UDP). The transport layer is responsible for the communication between two processes in the host
Session layer: The session layer allows individual processes on different hosts to have a session. The session layer uses the end-to-end service provided by the transport layer to establish a connection for the presentation layer entity or user process and transmit data in an orderly manner on the connection . This is also called session establishment synchronization.
Presentation layer: The presentation layer mainly deals with the representation of exchanging information in two communication systems. Data compression, encryption and decryption are also data representation exchange functions that the presentation layer can provide
Presentation layer: is the interface between the user and the network
TCP/IP model
Differences between ISO/OSI and TCP/IP models
The OSI model defines three main concepts: services, protocols, and interfaces
TCP/IP does not clearly distinguish between these three concepts
OSI supports connectionless and connection-oriented communication at the network layer , but only connection-oriented communication at the transport layer
TCP/IP has only one connectionless communication mode at the Internet layer , but supports both connectionless and connection-oriented modes at the transport layer
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