Study Notes: Chapter 2-Physical Layer

Study Notes: Chapter 2-Physical Layer

(The physical layer only understands a part of the content, so you can write a little bit of what you understand. The part that you don't understand will be understood later)

2.1Basic concepts of the physical layer:

First, we should emphasize that the physical layer should consider how to transmit bit streams over the transmission media connected to various computers, rather than the specific transmission media.

The physical layer also has its own protocols called physical procedures;

You can describe the main tasks of the physical layer to determine some features related to the interface for transmitting media,

(1) mechanical properties; (2) electrical properties; (3) functional characteristics; and (4) process characteristics.

2.2Basic knowledge of data communication:

Data Communication Model:

Source: The source device generates the main transmission data. For example, the input PC.

Sending end: Generally, the digital bit stream generated by the source point can be transmitted in the transmission system only after being encoded by the sending end. It is typical, for example, a modulation; however, many PCs now use built-in modem.

Receiver: receives a signal from the transmission system and converts it to the information processed by the target device. A typical device is a modem. The analog signal from the transmission line is demodulated to extract information, restores the digital bit stream generated by the sender.

End point: the end device obtains the sent digital bit stream from the receiver and then outputs the stream.

Basic terms:

The purpose of communication is to transmit messages;

Data is the entity for transmitting messages;

The signal is electrical or electromagnetic.

Signals can be divided into two types based on the different value ranges of parameters representing information:

(1) analog signal: or continuous signal-indicates that the value of the message parameter is continuous;

(2) digital signal: or discrete signal-representing that the value of the message parameter is discrete;

When a waveform in the time domain (Time Domain) is used to represent a digital signal, the basic waveform representing different discrete values is called a code element.

Several Basic Concepts about channels:

A channel is a media used to transmit information in a certain direction;

There are three basic methods for mutual information interaction:

(1) One-way channel, also known as ticket communication; for example, wide wireless.

(2) bidirectional alternate communication, also known as half-duplex communication, such as walkie talkie.

(3) bidirectional and simultaneous communication, also known as full-duplex communication, such as mobile phones. Obviously, bidirectional and simultaneous communication has the highest transmission efficiency.

Baseband signal: a signal from a source is called a baseband signal.

Modulation can be divided into baseband modulation and band-pass modulation. The most basic methods of band-pass modulation include amplitude modulation, frequency modulation, and phase modulation;

Maximum channel capacity:

The advantage of digital communication is that at the receiving end, as long as we can identify the original signal from the distorted waveform, this distortion will not affect the communication quality.

The higher the transmission rate of the code element, the longer the signal transmission distance, the larger the noise interference, or the worse the quality of the transmission media, the more serious the waveform distortion at the receiving end.

In terms of concept, there are two restrictions on the transfer rate of code elements on the channel.

(1) frequency range that the channel can pass; (2) signal-to-noise ratio (SNR;

The Shannon formula indicates that the higher the signal-to-noise ratio, the higher the transmission rate;

The significance of Shannon formula: as long as the information transmission rate is lower than the maximum transmission rate of the channel, we can find a way to achieve error-free transmission.

2.3Transmission media under the Physical Layer:

Transmission media is also known as transmission media or transmission media. It is a physical path between the transmitter and the receiver in a digital system.

Oriented transmission media:

(1) twisted pair wires: divided into unshielded twisted pair wires (common in LAN Network cables) and shielded twisted pair wires (relatively expensive );

Regardless of the type of wire, the attenuation increases with the increase of frequency. The use of crude wire can reduce the attenuation, But it increases the price and quality of the wire.

(2) coaxial cable: common in cable TV networks;

(3) optical fiber cables: multi-mode optical fiber cables and single-mode optical fiber cables.

Non-oriented transmission media:

The frequency range of wireless transmission is very wide;

Short-Wave Communication (that is, high-frequency communication) relies mainly on the reflection of the electron sphere;

However, the fading phenomenon caused by the instability of the electron and the multi-path effect of the electron reflection make the short-wave communication quality poor;

Radio microwave communication plays an important role in data communication;

Microwave is linearly transmitted in space, so the propagation distance is limited;

Traditional microwave transmission communication: ground microwave connection and satellite communication.

Advantages:

1) high microwave band frequency, wide band range, and large communication channel capacity;

2) Industrial and solar power have relatively low microwave interference and high transmission quality;

3) relatively low investment, fast effect, and easy to span mountains and rivers;

Disadvantages:

1) the adjacent sites must be directly connected;

2) The spread will also be affected by bad weather;

3) Poor concealment and confidentiality;

4) It takes a lot of manpower and material resources to use and manage a large number of trunk stations.

Satellite communication:

Advantages: frequent bandwidth, large communication volume, low signal interference, stable communication, and remote transmission;

Disadvantage: large communication latency.

2.4Communication Multiplexing Technology

2.5Digital Transmission System

2.6Broadband Access Technology

The above is my own summary, pictures from the network, slightly inferior, here is a recommended summary of a great God: http://www.cnblogs.com/kzang/articles/2581289.html

Study Notes: Chapter 2-Physical Layer