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We know that the purpose of communication is to deliver information quickly and efficiently. Let's start with a brief introduction to the basics of communication:
Modern information is expressed in a variety of forms, such as text, symbols, sounds, and image data. Millimeter wave spectrum 5g In order to realize the information transmission, the signal of light and electricity is usually used as the carrier of information.
信号：Forms of Information expression
信道：The signal transmission channel, is the signal transmission medium general name.
信源：Where the message is sent
信宿：End of message delivery
Depending on the transmitting information信道(signal transmission medium), the communication can be divided into wired communication and wireless communication.
有线通信的信道：including cable, optical cable, wire and other visible physical media.
无线通信的信道：There are long wave, medium wave, short wave, ultrashort wave, microwave, millimeter wave, infrared, visible light, X-ray, cosmic ray and other different频率radio waves, radio waves, also known as electromagnetic waves, are transmitted in the form of sky waves, ground waves, space waves, scattering waves.
In addition, meteor trails can also be used as a channel for transmitting information, which is called Meteor Trace communication (long-distance communication using meteor trails to reflect radio waves).
Millimeter wave 5g speed
(ii) Computer network communication:Millimeter wave health risks
Computer network has a wide area network, metropolitan area network, WAN, Bluetooth and other types, including both wired channels and wireless channels. Through these wired or wireless channels, the user's various fixed terminal or mobile terminal, you can access a variety of communication networks or computer networks, to obtain or exchange information, to achieve the purpose of communication.
网络传输介质：Refers to the transmission of information in the network carrier, the same as the above channel meaning. Wired transmission media refers to the physical connection between two communication devices, which transmits the signal from one party to the other, and the cable transmission media mainly have twisted pair, coaxial cable and optical fiber. Twisted-pair and coaxial cables transmit electrical signals, and optical fibers transmit light.
Different transmission media, the characteristics of different, their characteristics of the network data communication quality and communication speed has a greater impact.
LAN (local area Network,lan) is in a local geographical scope (such as a school, factories and institutions), generally within a radius of thousands of meters, will be a variety of computers, external equipment and databases connected to each other to make up a computer communication network.
LAN consists of network hardware (including network server, network workstation, network printer,
网卡, network interconnection devices, etc.) and network transmission media, as well as network software components.
5g and millimeter wave
The local area network is strictly the closed, can be composed of two computers in the office, can also be composed of thousands of computers within a company.
If the classification according to network use传输介质, LAN can be divided into wired network and wireless network.
1.星状结构：A star-shaped network is made中心节点up of (also known as a central docking station, typically a hub or switch) and other nodes (from nodes). Each of these devices is centered从节点on the central node and is connected to the central node through the connection line. The central node can communicate directly, and the从节点slave node and the slave node must pass through the central node to communicate. In a star-like network system, the communication between any two workstations is only two steps, so the transmission speed is fast. In addition, its network configuration is simple, easy to build network, easy to control and management. But this network system of network reliability is low, network共享能力差.
So why is the sharing ability poor? Because the sharing here refers to the mutual exchange of resources (from the node), and because each resource must pass through the central node in order to exchange visits, so certainly will be much slower, sharing ability is poor.
Let's take a look at the reliability, obviously, in this structure, as long as the central node one hangs the whole network is paralyzed, big and unreliable wow!
2。总线结构：is to connect all the devices in the network directly to the common transmission media (a common channel that is accessible by all PCs) via the corresponding hardware interface and cable.
The data transmission of the total linear topology is a broadcast transmission structure, and the node device can send data to all the computers on the network through the bus. Here, although the connection on the bus任何一个节点can receive data from the bus, but only the computer address and the destination address in the signal to match the computer is actually received.
The bus has a certain load capacity, therefore, the bus length has a certain limit, a bus can only connect a certain number of nodes.
Bus structure network is simple, flexible, expandable performance is good, so it is very convenient to insert and disassemble the junction device. In addition, the bus structure network reliability is high, the network node responds quickly, the sharing resource is strong, the equipment input small cost is low, the installation is easy to use, when a workstation node fails, the whole network system has little impact. Therefore, the bus structure network is one of the most widely used networks. But because all workstations use one bus, the real-time is poor.
3.环状结构：The ring structure network is a closed loop structure network which is connected by a communication link which is connected with each node.
Any node on the loop can request a message to be sent. Once the request is approved, the message can be sent to the loop. Information in the network has a fixed direction one-way flow, because there is only one path between the two workstation nodes, so there is no channel selection problem in the system.
As the loop is common, the information sent by a node must pass through all the loop interfaces in the loop, and when the destination address in the information flow matches a node address on the ring, the message is received by the loop interface of the node, and the information continues to flow to the next loop interface until it flows back to the Loop interface node where the message is sent. As the information source passes through the nodes serially in the loop, when there are too many nodes in the ring, it is bound to affect the information transmission rate, so that the response time of the network is prolonged; the loop is closed and inconvenient for expansion.
Simple ring topology with low reliability and one node fault will cause the whole network to be paralyzed, maintenance is difficult, and the branch node fault location is difficult. The current local area network almost does not use the simple ring topological structure. A form of change in ring structure, also known as Star ring topology, is prevalent in some types of networks.
4.树状结构：Tree-like structure is a natural hierarchical structure, also known as hierarchical centralized network.
1.以太网： 是Today's existing LAN is the most common通信协议标准, it is not a specific network, but a technical specification. Ethernet Specifies the content of the physical layer, the electrical signal and the media Access layer protocol, which largely replaces the other local area network standards.
IEEE 802.3是一种基带总线局域网, where a passive cable is used as the bus for data transmission.
In a bus-structured LAN, the signals sent by any one terminal can be received by all other terminals. Since multiple terminals share a single transmission channel, the media access control protocol must be followed to determine which terminal on the transmission channel can transmit data at any one time.
The media access control method used in Ethernet network is CSMA/CD (carrier monitoring multiplexing and collision detection) technology. Before introducing how CSMA/CD works, let's introduce the concept of conflict:
If there are two or more workstations on the network sending data at the same time, the signal will be mixed on the bus, which workstation can not tell what the real data is, which is called data conflict or collision.
CSMA/CD工作原理：The terminal device listens to the bus whether it is idle (conflict) before sending data to the bus. If the bus is idle, the prepared data is sent to the bus, and if the bus is busy (conflicting), it is not sent. But send a jam signal that strengthens the conflict so that all workstations on the network know that there is a conflict on the web. Then, wait for a random time for a subscription, and then resend the data that is not finished when the bus is idle.
2.IEEE 802.4令牌总线：Token bus是A "token" is used in a bus topology to access public transport media as a control node确定型介质访问控制方法.
令牌is a special kind of frame, in all node devices on a LAN network that needs to use the token concept, only the node that gets the token can send the frame.
The idea of tokens is used in the token bus policy to avoid采用令牌总线方法的局域网frame collisions that are caused by simultaneous access to multiple nodes (in the token bus LAN).
The main feature of the token bus LAN is that it is physically a bus network, which is logically a token net.
There is only one token on the entire network, and all logically nodes form a ring, and tokens are passed along the logical loop, regardless of the physical location of the station.
3.令牌环：Because the idea of token ring is mainly used in Token ring network, the Token Ring network is now almost completely replaced by Ethernet, which is no longer explained in detail.
4.无线局域网：Wireless LAN is a local network environment that uses electromagnetic waves to transmit data in the air without the need for cable media.
Hardware devices in a wireless LAN:
A. Wireless network card: Wireless network card is the interface of wireless LAN, it is not through the wired connection, the use of wireless signal for data transmission terminal. It can realize the connection and communication between each client of WLAN.
B. Wireless AP: The wireless AP is the access point of the WLAN, which is equivalent to a bridge connecting the wired network and the wireless network. Its main role is similar to the hub in the wired network, is to connect each wireless network client together, and then connect the wireless network to the Ethernet.
C. Wireless antennas: When a computer is far away from a wireless AP or other computer, or is unable to communicate with the AP or other computers at all, it is necessary to gain (amplify) the signal received or sent with the aid of a wireless antenna.
Each wireless workstation network card is identified by a unique physical address that is encoded like an Ethernet physical address and is 48 bits. Network administrators can manually maintain a set of MAC address lists that allow access or not to be accessed in a Wireless LAN access point (AP) to enable access filtering for physical addresses.
A WAN is a network of computers connected to a wider area (such as a city, a country, or even the world), usually a remote public information communication network, sometimes referred to as a remote network, that is operated and managed by the posts and telecommunications department, which goes beyond departments and areas to provide public access.
The communication subnets of wide area network mainly use packet switching technology to interconnect local area networks or computer systems in different regions to achieve the purpose of resource sharing.
The Internet (Internet) is a global network (logical network) connected by routers that communicate with each other and the public Internet (the public Internet is connected by many computers), which is a collection of information resources and resource sharing.
1.E-mail服务：
E-mail is similar to ordinary correspondence, and its sending and receiving processes are very similar to ordinary correspondence. However, the e-mail message sent by the email service is not a physical object, but an electrical signal, so it can transmit many things that ordinary letters cannot transmit, such as animations.
常用的电子邮件协议：
SMTP (Simple Mail Transfer Protocol): a set of rules for sending mail that controls the way letters are relayed, which helps each computer find the next destination when sending or relaying letters.
POP: The Post Office Protocol that is used for e-mail reception. Primarily used to support the use of clients to remotely administer messages on the server. For example, it supports the user agent on the local computer to connect to the mail server and take the user's messages back to local reading.
IMAP (Internet Message Access Protocol): a protocol that is more powerful and more complex to receive messages.
2.共享远程的资源（远程登录服务Telnet）:Telnet refers to a terminal that allows a local user to log on to another host on the network (a remote system) to become that host. After the login is successful, the local user can manipulate the resources of the network host as a user of the remote system.
With Telnet, the local computer can get "contacted" and interact with another remote computer on the network程序.
3.FTP服务：
FTP is both a protocol for controlling file transfers and an application. As an application, it is the most important tool for file transfer and can transmit data in any format. FTP applications perform file transfers between client/server based on the FTP protocol.
4.WWW:
WWW (World Wide Web or short web) is an information retrieval service system, which provides a means for people to find and share information through Internet access.
Divided into Web clients and Web server programs. WWW allows Web clients (popular browsers) to access pages on a Web server. The Web server是can provide documentation to the requesting browser程序.
It is“文档”或“网页”displayed by the Web browser program that is taken back from the Web server.
The web connects all the existing resources on the Internet, enabling users to provide Hypertext Media Resource documentation on all sites on the Internet that have established a WWW server.
A network server is a dedicated computer that provides a service to a customer under a network environment.
本地地址：An IP address that is valid only within the native structure.
全球地址：A globally unique IP address applied to the governing body of the Internet.
专用地址：Can only be used as a local address and not as a global address.
Due to the shortage of IP address, the number of IP addresses an organization can request is often less than the number of hosts owned by the institution. In many cases, many hosts communicate primarily with other hosts within the organization, and it is clear that these computers do not all need to be connected to the Internet. To conserve valuable global IP resources, you can use local addresses for computers within the organization.
In order to avoid host and Internet connections within the organization, the local address and an IP address in the Internet coincide, RFC1918 indicates a number of private addresses that can only be used for intra-agency communications and not for communication with hosts on the Internet. All routers in the Internet are not forwarded for datagrams where the destination address is a private address.
The private address indicated by RFC1918 is:
10.0.0.0-10.255.255.255 (10/8-bit prefix)
172.16.0.0-172.31.255.255 (172.16/12 bit prefix)
192.168.0.0-192.168.255.255 (192.168/16 bit prefix)
The use of such a dedicated IP address of the internet is called the private Internet or the local Internet, referred to as private network. There may be many private networks with the same private IP address, but because these private addresses are used only within the organization, they do not cause trouble. A private IP address is also called a reusable address.
专用网举例：For colleges and universities, campus network or library internal network and communication network is a private network.
Well, the computer network communication system is large and complex, I briefly introduced some basic concepts, I hope to understand I will introduce the following content to help.
In the basic concept of computer network, the hierarchical architecture is the most basic.
网络协议：The computer network is composed of multiple interconnected nodes, and the data and control information are exchanged continuously between nodes. To be able to exchange data methodically, each node must adhere to a set of pre-agreed rules. These rules are called Network Protocols (protocol). The best way to organize complex computer network protocols is to use hierarchical models, which logically divide different protocols at different network levels, and each layer can have one or more protocols.
网络体系结构：The various layers of a computer network and the set of protocols, called the network architecture.
网络服务：The use of a service-oriented, distributed program-based software module that runs on a network服务方式.
开放系统互连参考模型（OSI/RM）：The Open Systems Interconnection Reference Model is a hierarchical model of communication function established for the interconnection of open systems.
OSI/RM is divided into seven levels based on the entire function of the network. The connection between layers is carried out through the interfaces between the layers, and the upper layer provides service requests through the interface, while the lower level provides services to the upper layers through the interface.
From bottom to top Osi/rm's layers and their main functions are as follows:
物理层：The primary function of the physical layer is to use physical transmission media (wired or wireless media) to form a pathway between the two data terminals of the communication in order to transparently transmit the bitstream.
数据链路层：On the basis of providing the bit-stream service in the physical layer, the data link connection is established between the communication entities, and the data of the unit is transferred, and the帧error control and flow control method are used to make the error-free physical line become the data link without error.
When two hosts are communicating, both host 1 and host 2 have a complete 5-layer protocol (assuming there is no session layer and presentation layer), but the router uses the same protocol stack when forwarding the packet with only the following three layers. The data enters the router first from the physical layer to the layer, in the forwarding post to find the next hop address, and then down to the physical layer forwarding out. So the data that is transferred from host 1 to host 2 needs to flow up and down multiple times in the path of each node in the protocol stack.
网络层：The network layer is between the transport layer and the data link layer, and it further manages the data communication in the network on the transfer function between the two adjacent endpoints provided by the data link layer. The purpose of the network layer is to achieve a two-terminal system数据透明传送(the physical layer is a bit transparent transmission).
传输层：传输层为应用进程之间提供端到端的逻辑通信。
会话层：Establish, organize, and coordinate interactions between two applications that communicate with each other.
表示层：It is mainly used to deal with the representation of exchanging information in two communication systems. An abstract method is used to define a data type or structure and to transform data types and encodings between the various end systems by using this abstract data structure.
应用层：The application layer is the interface between the computer network and the end user, and is the only layer that provides service directly to the application using network resources.
a.When two computers communicate over the network, only the physical layer is directly connected. The remaining layers are communicated through the protocols of each peer layer, such as two peer network layers using Network layer protocol communication. Only two physical layers can communicate real data through the media.
b.The flow of communication information between systems in OSI/RM is as follows: '
Each layer of the sending end is gradually added to the control information of each layer from top to bottom, and the stream of bits is transmitted to the physical channel, through the physical channel to the receiving end of the physical layer, the receiving end from the bottom to the upper layer to remove the corresponding layer of control information, the resulting data flow eventually transferred to the application layer process.
c.链路is a physical line between a node and its neighboring nodes (no other nodes in the middle), which is just part of a path. The communication path between the two computers often goes through many of these links.
In addition to the physical circuitry required to transmit data on a single line, communication protocols are required to control the transmission of data. The link that adds to the communication protocol is what we call the data link.
d.From the IP layer (network layer), the two ends of the communication are two hosts. But the real communication entity is the process of two hosts, two hosts to communicate is two host applications to communicate with each other, the specific implementation method is controlled by the transport layer.
In short, if you want to understand the computer network deeply, it must be mastered for its architecture and simple communication process. I have outlined some basic concepts, if you want to understand deeply, it is recommended to focus on keywords reading data.
Computer network architecture and its simple communication
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