The content of the notes comes from the video of teacher Han Ligang
2.1 Basic concepts of the physical layer
The physical layer solves how to transmit data bit streams on the transmission media connecting various computers, rather than referring to specific transmission operations.
The main task of the physical layer is described as: determining some characteristics of the interface with the transmission medium, namely:
- Mechanical characteristics: example interface shape, size, number of leads
- Electrical characteristics: the specified voltage range (-5V, +5V)
- Functional characteristics: the example stipulates -5V means 0, +5V means 1
- Process characteristics: also known as procedural characteristics, which stipulate the work steps of each relevant department when establishing a connection
2.2 Basic knowledge of data communication
2.21 Related terms
The purpose of communication is to transmit the message
data (data) ------- the physical
signal that carries the message (signal) ------ the electrical or electromagnetic performance of the data
"analog signal" --——parameter representing the message The value of is continuous.
"Digital signal" ————The value of the parameter representing the message is a discrete
code element (code)————When the waveform in the time domain is used to represent the digital signal, the basic waveform representing different discrete values is called a code yuan
2.22 Several basic concepts about channels
Channel: A channel generally represents a medium that transmits information in one direction. So we say that ordinary communication lines often include a channel for sending information and a channel for receiving information.
One-way channel (simplex channel)-There can only be one direction of communication and no interaction in the opposite direction. (Radio)
Two-way alternate communication (half-duplex communication) --- Both parties in the communication can send messages, but they cannot send or receive at the same time. (Previously walkie-talkie)
Two-way simultaneous communication (full-duplex communication) --- both parties can send and receive information at the same time.
2.3 Transmission media under the physical layer
Oriented transmission media
Twisted pair
coaxial cable
optical fiber
Undirected transmission media
Shortwave communication mainly relies on ionospheric reflection, but the quality of communication is very poor.
Microwave communication Passing through the transmission tower, a certain height of the transmission tower is required (microwaves are mainly transmitted in a straight line in space) Ground microwave communication, satellite communication
Physical layer equipment
The working characteristics of the hub : it plays a role in signal amplification and retransmission in the network, and its purpose is to expand the transmission range of the network, but does not have the ability of directional transmission of signals
2.4 Channel multiplexing technology
Reuse is the basic concept of communication technology.
Frequency division multiplexing,
time division multiplexing
statistics, time division multiplexing,
wavelength division multiplexing,
code division multiplexing, making a chip that is the same as others, you can answer others' calls
2.5 Digital Transmission System
2.6 Broadband access calculation
ADSL Baidu Encyclopedia :
ADSL (Asymmetric Digital Subscriber Line), an asymmetric digital subscriber line, generates three information channels on the telephone line: a high-speed downlink channel with a rate of 1.5Mbps-9Mbps for users to download information; one with a rate of 16Kbps -1Mbps medium-speed duplex channel; an ordinary old telephone service channel; and these three channels can work at the same time.
Fiber coaxial hybrid network HFC
FFTx
