Shulou(Shulou.com)06/01 Report--

One: data and data communication

data

In the computer system, the combination of various letters and numeric symbols, voice, graphics, images and so on are collectively referred to as data.

Data communication

The process of exchanging data between two devices through a transmission medium such as a cable.

Second: network definition and simple network

Computer network is a system that uses communication lines and communication equipment to connect multiple autonomous computer systems distributed in different locations, share hardware and software, and finally realize resource sharing according to the common network protocol.

Two terminals, connected by a physical medium that can carry data transmission, form the simplest network.

Simple network

The need for terminals to transmit information and share resources is the main reason for the emergence of the network.

The terminal can generate, send and receive data. The network is the medium for the terminal to establish communication, and the terminal establishes the connection through the network. The carrier used to transmit data becomes a medium, and the network can use a variety of media for data transmission, including physical cables, radio waves and so on.

Network is a large-scale and powerful system formed by the interconnection of media bar terminals, so that many terminals can easily transmit information to each other and share information resources.

Three: physical media

Coaxial cable is a kind of transmission medium used in the early days. There are two standards for coaxial cable. As shown in the figure above, both standards support the transmission rate of 10Mbps. In general, 10BASE2 coaxial cable uses BNC connector and 10BASE5 coaxial cable uses N-type connector. Nowadays, coaxial cable is rarely used in current enterprise networks.

Twisted pair has lower manufacturing and deployment costs than coaxial cable, so it is widely used in enterprise networks. Twisted pair can be divided into shielded twisted pair and unshielded twisted pair. The shielded twisted pair has a metal shielding layer between the twisted pair and the outer insulation envelope, which can shield electromagnetic interference. There are many types of twisted pair, as shown in the figure above.

Twisted pair and coaxial cable use electrical signals when transmitting data, while optical fiber uses optical signals to transmit data. Optical fiber is divided into single-mode optical fiber and multimode optical fiber, the former has a long transmission distance and the latter has a short transmission distance.

All kinds of serial port cables are used in network communication. The serial cable is shown in the image above.

IV: conflict domain

Definition: a collision domain is a collection of all nodes that transmit bidirectionally through a shared physical medium. When hosts in the same collision domain send data at the same time, conflicts may occur before the data reaches its destination.

As shown in the figure, each host uses a coaxial cable to communicate with other hosts, so the coaxial cable here is also called shared media, and the corresponding network is called shared media network, or shared network for short. The problem of signal conflict will occur when different hosts send data at the same time in the shared network. the method to solve this problem is to use carrier sensing multiple access / conflict detection technology.

The working process of this technology is:

1) the terminal device constantly detects the status of the shared line. If the line is idle, this can send data; if the line is not idle, wait.

2) if another device sends data at the same time, the data sent by the two devices will inevitably conflict and the signal on the straight line will be confused.

3) after the terminal device detects this conflict, it immediately stops sending its own data.

4) after sending a series of interference pulses, the terminal equipment waits for a period of time before sending data.

To sum up, it is: first listen and then send, listen while sending, conflict stop, random retransmission.

Five: duplex mode

Half-duplex: in half-duplex mode, both sides of the communication can send and receive data, but not at the same time. When one device sends it, the other can only accept it, and vice versa. Like walkie-talkies.

Half-duplex: in full-duplex mode, both sides of the communication can send and receive data at the same time, such as a telephone.

Both modes can support two-way data transmission.

The data communication modes on Ethernet include half-duplex and full-duplex. In half-duplex mode, both parties sharing physical media must adopt CSMA/CD mechanism to avoid conflicts. In full-duplex mode, two-way communication can be realized at the same time, and there is no conflict in this mode, so it is not necessary to use CSMA/CD mechanism. But the two devices connected to the same link must be consistent.

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