Read "The World is Digital" ④

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The seventh Chapter study programming

Advantages of javascript:

    1. It is supported by all browsers. Its code is also easy to show to others.
    2. The language itself is relatively simple and requires a low demand for learners. But it's very powerful and can perform extremely complex computational tasks.
    3. Many world-class big Web sites provide JavaScript APIs.

7.1 Basic concepts of programming languages

① any programming language will provide some means to get lazy to complete the calculated input data, perform arithmetic calculations, store and get intermediate values during calculation, display results, determine the next calculation step based on previous calculations, and save the results when the calculation is complete.

② language has grammar, and grammar is a series of rules. Language also has semantics, which specifies the meaning of all elements in a language.

The ③javascript consists of three areas:

The language itself includes statements/test conditions that allow the computer to perform arithmetic calculations, and rules for repeating calculations.

The second is the JavaScript code base.

The third is to access the browser and Web page interface, the JavaScript program through these interfaces can get user input in their web page, in response to user action/Let the browser display different content or other Web pages.

7.2 First JavaScript program

When writing a JavaScript program, you must use standard double quotes, using Notepad or a textedit-like text editor. When you save the program file, you save it as a plain text file with an. html extension.

7.3 A second JavaScript program

An equal sign is an example of semantics. The action performed by the equals sign is called an assignment.

7.4 Cycles

The statement of the while loop, which is more regular and more organized when repeatedly executing a series of instructions.

7.5 Items

Both while and if statements are indented as a standard practice.

Another important aspect of programming: testing.

7.6 Libraries and interfaces

As an extension mechanism, JavaScript plays a very important role in advanced Web applications.

7.7 JavaScript How to work

When a browser encounters JavaScript code in a Web page, it hands over the code text to the JavaScript compiler-usually a standalone program or a library of browsers. The compiler handlers, detects errors, and compiles them into assembly language instructions with an imaginary machine.

Part III: Communication

Most systems now combine hardware, software, and Communications in a Trinity.

Eighth Chapter

① all communication systems have some commonality. is to convert information into physical form, so that it can be transformed back into a form that people understand by means of media transmission.

② bandwidth is a fundamental feature that describes the speed at which a system transmits data.

③ wait or delay measures the time required for a particular piece of information to pass through the system.

④ jitter, which is the variability of latency.

⑤ refers to the way in which a technology can be networked across a wide geographic range.

8.1 Telephones with modems

The telephone system transmits analog sound signals and does not enter data. The conversion between digitized information and analog sound is realized by demodulation, thus transmitting data using the telephone network. A device capable of completing modulation and demodulation functions is called a modem.

8.2 Wired and DSL

① The device that converts the wired signal and the bit data back and forth is called a cable demodulator.

Wired or DSL (Digital subscriber loop) is the applicable networking technology in the ② family

③DSL does not interfere with voice signals when sending data using a telephone line, but the DSL has a distance limit. DSL is non-shared.

8.3 LAN and Ethernet

① Ethernet is the most commonly used system for several days. But the distance of transmission is also limited.

The information in the ② Ethernet is transmitted in the form of a packet. A package is a container that wraps bits or bytes of information.

8.4 Wireless Network

① wireless networks transmit data over radio waves. Use electromagnetic waves to transmit signals. The electromagnetic wave is a specific frequency of the radio wave, its vibrational frequency measured in Hz.

② wireless networking to the frequency range that can be used-band.

③ wireless devices can encode digitized information into a form suitable for transmission via radio waves.

④ uses the widest range of wireless networking technologies: Bluetooth, RFID (radio frequency identification), GPS (Global Positioning System)

8.5 Phones

① Mobile phones use a narrow band and have limited ability to transmit information. Mobile phones use different bands in different parts of the world.

② each phone has an Ethernet-equivalent identification code. The cell phone will broadcast its own identification code, and the nearest base station will receive a signal from the cell phone, the verification code is verified by the backend system.

③ mobile phone and base station communication signal strength is very high.

8.6 Summary

① frequency band is the key resource of wireless networking system.

② Wireless network is broadcast media, anyone can listen. Encryption is the only way to protect wireless information and control access.

Nineth Chapter-Internet

The internet is a loosely, unstructured, chaotic, self-organizing set of networks that is connected by a standard that defines the network and the rules of communication between computers.

9.1 Internet Overview

① the internet is made up of thousands of loosely connected independent networks, each of which is connected to another network or networks. The neighboring computer is connected by an Ethernet-based LAN, and then the network and the network are linked together. A device used by a network connection is called a gateway or router.

② on the Internet, the packet that transports the data is called the IP packet. The larger IP packets are split into multiple smaller Ethernet packets for transmission. Each IP packet passes through multiple gateways, each of which passes the package to the next gateway closer to the destination of the packet.

The core protocol of the ③ Internet is called IP, which is an information transmission mechanism and a common format. Different types of physical networks use their underlying protocols to deliver IP packets.

④ is TCP on the IP protocol, which uses the IP protocol to provide a reliable transmission mechanism so that a sequence of bytes of any length can be sent from the source address to the destination address.


Most of the core technologies of ① Internet are developed by the Internet Engineering Task Force. The IETF has designed the way in which the elements of the Internet are run and writes them into canonical documents.

② is a nonprofit organization called the Internet name and digital address allocation Agency (ICANN), which manages the rest of the internet business. It is responsible for the technical coordination of the Internet and is also responsible for authorizing domain name registrars.

9.2.1 Domain Name System (DNS)

The ① domain name represents only the logical structure and is not subject to any geographic limitations.

② in the domain Name System, the. com,. edu, and other organization codes and two-letter country codes such as. US,. CA, are called top-level domains. Top-level domains delegate management responsibility and more names to subordinate domains.

9.2.2IP Address

① each network and each networked host must have an IP address, ICANN assigns the address in blocks, and the organization that gets the address block then divides it into sub-blocks assigned to subordinate institutions or individuals.

② Home wireless routers generally provide network address translation services, with a single external IP address for multiple internal IP addresses to provide networking services.

9.2.3 root servers

The key function of DNS is to convert the name to an IP address. The conversion of a top-level domain is handled by a set of root domain servers.

9.3 Routing

In any network, routing is the Core. The internet uses a hierarchical structure to manage routing information. On the top level of the routing system, tens of thousands of autonomous systems provide routing information for the networks they contain. A meticulous system also usually corresponds to a large Internet service provider.

9.4 protocol

IP and TCP define how IP packets are combined into a data stream and how to connect to the service.

9.4.1 Internet Protocol

The packet delivery service provided by IP is unreliable and non-connected. Each IP packet is independent. IP has no status or memory, once the packet is passed to the next gateway, it is no longer necessary to save any information about the package.


①TCP can provide users with reliable bi-directional data: The data placed at one end flows out from the other end, the delay is small, the error rate is low.

②TCP also includes mechanisms for improving transmission efficiency.

9.5 High-level protocols

TCP provides two-way communication that allows data to be reliably transmitted back and forth between two computers.


Tools used to obtain information, experimental data and test results, or to generate and analyze these data are called file transfer protocols.

9.7 compression

Data compression is a good way to make more efficient use of existing memory and bandwidth. The purpose of compression is to encode the same information into fewer bits or bits.

9.8 Error detection and correction

① error detection and correction is the process of adding carefully controlled redundancy information to detect errors or even correct errors.

② the simplest error detection algorithm applied to binary is the parity check code. This algorithm appends a parity bit to the Meizu bits.

Tenth World Wide Web

The ① World Wide Web connects computers that provide information and request information, connects and transmits information over the Internet, and provides human-machine interfaces for other services supported by the Internet.

② Four components of the World Wide Web: URL (Uniform Resource Locator), HTTP (Hypertext Transfer Protocol), HTML (Hypertext Markup Language), browser.


Most of the files returned by the ① server are in HTML format, which contains text content and formatting information.

The ②html file uses the label pull to represent the format information, and the label can not only embed the file, but also represent the starting and ending positions of the page area.

10.3 form

In the ①http protocol, there is a mechanism for clients to pass information to the server, called Customs Management Interface (CGI).

The ② form does not verify the correctness of the form data and does not provide any security protection for the password input fields. But it is still an important part of the World Wide Web.

10.7 Viruses and worms

① viruses and worms are usually malicious code that spreads between systems. The transmission of the virus requires human intervention, and the transmission of worms is non-artificial.

② Trojan Horse is a program disguised as beneficial or harmless, but actually harmful.

108,000-dimensional network security

The security threats encountered by the world Wide Web fall into three categories: attacks on clients, attacks on servers, and attacks on information in transit.

10.9 Cryptographic Technique

10.9.1 Key Encryption

① in the early 1976-2000, the most common key encryption algorithm was DES (data Encryption Standard). Later the most used is AES (Advanced Encryption Standard)

The problem with ②aes and other key cryptography systems is key distribution.

10.9.2 Public Key Cryptography

① in a public key cryptography system, everyone has a key pair that contains a public key and a private key.

② Public Key cryptography is a key element for secure communication on the Internet.

③ Public key cryptography can be used as a digital signature scheme.

Chapter 11th data, information and privacy

11.1 Search

Search engine, fill in the query conditions in the Web form, send the conditions to the server, return a set of connection and text summary, the server generates a set of pages containing query keywords, according to the degree of relevance of the order, and then attach the HTML page summary, and then send to the user.

11.2 Tracking

Some information will be sent to the server every time the browser requests it, including your IP address, the page you are browsing, the type and version of the browser, the operating system, and so on.

11.5 Cloud Computing

① cloud computing relies on client-side fast processing and large amounts of memory, as well as high-bandwidth servers.

② Cloud Computing can transfer the calculated load and transfer the location of the data store during processing.

Read "The World is Digital" ④

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