Computer Network Learning Summary 1

1. Bandwidth of the channel and the bandwidth of the signal

Channel bandwidth: refers to the frequency range that the channel can transmit.

For digital channels, bandwidth is often represented by the number of bits that can be transmitted per second.

Bandwidth of a signal: the band width of a signal.

According to the Fourier series, any signal (periodic signal) can be decomposed into a plurality of different amplitude, frequency, phase of the sine wave combination.
The signal bandwidth is the value of the maximum frequency of these signals minus the lowest frequency (many times the highest frequency).

2. Low-pass filtering and bandpass filtering

Low-pass filtering: The rules for low-frequency signals can pass normally, and the high-frequency signal exceeding the set threshold is blocked.

Bandpass filtering: Allows a signal in a frequency band to pass.

3. Bit rate and baud rate

Bit rate refers to the rate at which data is transmitted.

Baud rate refers to the transmission rate of the signal.

Bit rate = Baud Rate * code element length

Refer to the example of Baidu Encyclopedia:

if in the digital transmission process, uses the 0V to represent the numeral 0,5v to represent the number 1, then each code element has two states 0 and 1. Each code element represents a binary number. at this point the number of yards per second and the number of binary code per second is the same, which is called two-phase modulation, the baud rate is equal to the bit rate. if 0V, 2V, 4V, and 6V represent 00, 01, 10, and 11 respectively in the digital transmission, then each code element has four states 00, 01, 10, and 11. Each code element represents two binary digits. at this point the number of yards per second is half the number of binary code per second, which is called four-phase modulation, the baud rate is equal to half the bit rate. 4. Correlation theorem of channel bandwidthThe Nigerian theorem:Ideal low communication channel maximum code element transfer rate: r=2w (unit baud Baud)W refers to the bandwidth of the ideal low communication channel. Ideal low communication channel data transfer rate:

R= 2wlog2v (BITS/S)

V refers to the number of binary bits that a code element carries.

Shannon formula:

– The maximum data transfer rate for a channel with limited bandwidth and noise interference. –

C = W log2 (1+s/n) (BITS/S)

• W is the channel bandwidth (in Hz);• S is the average power of the signal transmitted within the channel;• N is the Gaussian noise power inside the channel. s/n commonly used decibels, 1 decibels =10lgs/n5. Data encoding Technology(1) Pulse code modulation pcm--analog data to digital signaloperation: Sampling, quantization, encodingApplication: Voice digitization(2) Analog digital coding Method-digital data to analog signalapplication: Using telephone exchange network for computer communication– Analog data encoding by changing 3 electrical parameters• Amplitude um--amplitude keying ask• Angular frequency ω--shift keying FSK• Phase φ-- Phaser keying PSK(3) Digital data encoding Method-digital data to digital signalkey: How to stay in sync–NRZ Encoding–nrzi Encoding– Manchester Code– Differential Manchester code6. Multiplexing TechnologyFrequency Division multiplexingin a communication line design multi-channel communication channels;the signals of each channel are modulated by different carrier frequencies;Each carrier frequency is non-overlapping, so a communication line can simultaneously transmit multi-channel signals independently. Time Division Multiplexing: Digital signal only time-division multiplexingWavelength Division multiplexing is used for optical fiber communication, and also belongs to frequency division multiplexing.

Computer Network Learning Summary 1