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1.1 The role of computer networks in the information age
Connectivity: To communicate with each other and exchange information
Sharing: Information sharing, software and hardware sharing (e.g., remotely connecting to a computer to use its software, printer)
1.2 Overview of the Internet
Network: Consists of nodes (computers) and links connecting these nodes.
Internet: Networks connected by routers form a larger network called the Internet
Internet: The largest Internet in the world
ISP (Internet Service Provider) : With the development of the network, the Internet has gradually formed a multi-level ISP structure.
Isps are also divided into different tiers: backbone ISPs, regional ISPs, and local ISPs, depending on the area of service they provide and the number of IP addresses they have.
1.3 Composition of the Internet
(1) Edge part: it consists of all hosts connected to the Internet. This part is directly used by the user for communication (data, audio, or video) and resource sharing.
(2) Core part: it consists of a large number of networks and routers connected to these networks. This part serves the edge part (providing connectivity and exchange).
At the heart of the network is the router, a special computer. Router is the key component of packet switching. Its task is to forward received packets.
3.1 Communication mode of edge part
Communication mode between hosts: client server mode, peer connection mode
3.1.1Client server mode
The client-server approach describes the relationship between processes that serve and are served. The client is the service requester and the server is the service provider.
3.1.2Peer connection mode
A peer-to-peer connection is one in which two hosts communicate without distinguishing which one is the service requester or provider. As long as both hosts are running peer-to-peer (P2P) software, they can communicate on an equal, peer-to-peer basis. Each party can then download the shared document that the other party has stored on its hard drive.
3.2 Data exchange mode of core parts
3.2.1 Circuit Switching
The bitstream of the entire message is continuously transmitted from the source point to the destination point, as if in a pipe; Suitable for real-time transmission with large amount of data, circuit switching can be used between core routers.
3.2.2 Packet Switching
The whole message is first transmitted to the adjacent node, all stored and then searched for the forwarding table (it takes time), forward to the next node.
3.2.3 Packet Switching
A single packet (which is only a portion of the packet) is sent to a neighboring node, stored, searched for a forwarding table, and forwarded to the next node.
1.4 Categories of computer networks
The simplest definition of a computer network is a collection of interconnected, autonomous computers.
1.4.1 Classification by scope of network
Wide Area Network (WAN) : You need to pay for services and bandwidth. Wans typically have a range of tens to thousands of kilometers, and their task is to transport data sent by hosts over long distances. The links connecting the switches of wan nodes are generally high-speed links with large communication capacity.
MAN (Metropolitan Area Network) : Ranges from 5 to 50 km.
Local Area Network (LAN) : The device is purchased and maintained by the device itself, with fixed bandwidth. LAN is generally connected with a microcomputer or workstation through a communication line, the rate is usually above 10Mb/s, the range is relatively small, such as within 1km.
PAN (Personal Area Network) : it is a Network that connects Personal electronic devices with wireless technology at the workplace. The range is about 10 mbit/s.
A new understanding that distinguishes Lans from Wans in terms of network coverage alone; Wan technology is applied, even if the distance is close to the WAN, LAN technology is applied, even if the distance is far.
1.4.2 Network classification by user
Public network: a large network financed by a telecommunications company. Public means that it is available to all who are willing to pay a fee set by the telecommunications company.
Private network: a network built by a department or industry to serve its own needs and not to serve outsiders.
1.4.3 Topology structure
Bus, ring, star, tree, mesh
1.4.4 Classification by switching mode
Circuit switching network, message switching network, packet switching network
1.4.5 Classification of working methods
Resource subnet, communication network, access network
1.6 Computer network performance
1.6.1 Performance Characteristics
1. Rate = Total bits/time (bit/s)
2. Bandwidth: Used to describe the range of signals allowed to pass through a communication line, it is now used to describe the ability of a network communication line to transmit data, i.e. the “maximum data rate” that can pass from one point of the network to another in unit time.
3. Throughput: Indicates the amount of data passing through a network (channel or interface) in a unit time. (bit/s)
4. Delay: The time it takes for data (a message or packet, or even bits) to travel from one end of a network (or link) to the other. The delay in the network is composed of the following different parts:
4.1. Transmission delay: refers to the time required by the host or router to send data frames.
4.2 propagation delay: refers to the time it takes for electromagnetic wave to propagate a certain distance in the channel.
4.3. Processing delay: it takes a certain amount of time for the host or router to process the packet when it receives it, such as analyzing the header of the packet, extracting the data part from the packet, checking for errors or finding the appropriate route, etc.
4.4. Queuing delay: Packets need to pass through many routers when they are transmitted through the network. However, after entering the router, the packet must queue up in the input queue for processing. After the router determines the forwarding interface, it has to queue up in the output queue to wait for forwarding, resulting in queuing delay.
Delay = transmission delay + propagation delay + processing delay + queuing delay
5. Latency bandwidth product = Propagation latency x bandwidth. Indicates the number of bits that a link can hold.
6. Round trip time RTT: indicates the total time from the time when the sender sends data to the time when the sender receives the confirmation from the receiver (the receiver sends the confirmation immediately after receiving the data).
7. Utilization rate: channel utilization rate and network utilization rate. Channel utilization indicates what percentage of a channel’s time is used (with data passing); Network utilization is the weighted average of channel utilization of the whole network.
High channel or network utilization will result in very large delay. As a result, some ISPs with larger backbone networks typically control their channel utilization to no more than 50%. If it does, prepare to expand and increase the bandwidth of the line.
1.6.2 Non-performance Characteristics
Cost: includes design and implementation costs
Quality: depends on the reliability of the network, the ease of management, and some performance of the network.
Standardization: It is better to adopt international standard design, which can achieve better interoperability, easier upgrade and maintenance, and easier technical support.
Reliability: Is closely related to the quality and performance of the network.
Scalability and upgradeability: Increased scale and improved performance and version.
Easy to maintain and use: Without good management and maintenance, a network is difficult to achieve and maintain the performance it is designed for.
1.7 Computer network architecture
1.7.1 Protocol and Hierarchy
In order to exchange data methodically in a computer network, there are some agreed rules that must be followed. These rules specify the format of the data to be exchanged and the related issues of synchronization, where synchronization refers to what should happen under certain conditions (such as sending a reply message) and thus implies timing. These rules, standards, or conventions established for the exchange of data over a network are called network protocols.
The architecture of a computer network is the precise definition of what the network and its components should do.
Network protocol consists of the following three elements:
(1) Grammar, that is, the structure or format of data and control information;
(2) semantics, that is, what kind of control information should be sent, what kind of action should be completed and what kind of response should be made;
(3) synchronization, that is, a detailed description of the sequence of events.
1. Benefits of layering
(1) Independent between the layers. A layer does not need to know how its next layer is implemented, but only the services it provides through interlayer interface locks. The overall complexity is reduced.
(2) Good flexibility. When any layer changes (for example due to a change in technology), the layers above or below are not affected as long as the layer interface relationship remains unchanged.
(3) Structurally separable. Each layer can be implemented using the most appropriate technology.
(4) Easy to implement and maintain. Because the whole system is decomposed into a number of relatively independent subsystems.
(5) Can promote standardization work. Because the functionality of each layer and the services it provides are well defined.
2. How to layer
When layering, care should be taken to make the function of each layer very clear. Too few layers make the protocols at each layer too complex; However, too many layers will encounter more difficulties in describing and integrating the functions of each layer in the system engineering task. In general, each layer performs the following functions (can include only one or more) :
(1) Error control: make the communication with the corresponding level of the network peer more reliable.
(2) Flow control: make the sending end send rate is not too fast, to make the receiving end receive in time.
(3) Segmentation and reinstallation: the sending end divides the data to be sent into smaller units and restores it at the receiving end.
(4) Reuse and reuse: several high-level sessions of the sender reuse a low-level connection, and then reuse it at the receiver.
(5) Connection establishment and release: establish a logical connection before exchanging data. Release the connection at the end of data transfer.
There are some disadvantages to layering, for example, some functions can be repeated in different layers, resulting in additional overhead.
1.7.2 Architecture with five layers of protocol
Application processes interact to complete a specific network application. The data unit of the interaction is called a packet.
(2) Transportation layer
Responsible for providing a common data transfer service for communication between processes in two hosts. Application processes use the service to transmit application-layer packets.
The transport layer mainly uses the following two protocols:
- Transmission control protocol TCP – provides connection-oriented, reliable data transfer services in the form of message segments.
- User datagram protocol UDP – provides a connectionless, best-effort data transfer service (without guarantee of reliable transmission) in the unit of user datagram.
(3) Network layer
Responsible for providing communication services to different hosts on the packet switched network. When sending data, the network layer encapsulates the packet segments or user datagrams generated by the transport layer into packets or packets for transmission. Called datagrams for short.
(4) Data link layer
When transmitting data between two adjacent nodes, the data link layer assembles IP datagrams handed over by the network layer into frames, and transmits frames on the link between the two adjacent nodes. Each frame contains data and necessary control information (such as synchronization information, address information, error control, etc.).
When receiving data, control information enables the receiver to know which bits a frame begins and ends in. In this way, the data link layer receives a frame and extracts the data portion from it and presents it to the network layer.
(5) Physical layer
The units of data transmitted at the physical layer are bits. When the sender sends a 1 (or 0), the receiver shall receive a 1 (or 0) instead of a 0 (or 1). Therefore, the physical layer considers how much voltage is used to represent “1” or “0” and how the receiver identifies the bits sent by the originating sender.