Channel characteristics of the 2016.7.18 Network engineer Tutorial

1. Basic concepts of data communication:

(1) As a general communication system, the information generated by the source may be analog data or digital data;

(2) The analog data takes the continuous value, and the digital data takes the discrete value;

(3) before the data into the channel to become suitable for transmission of electromagnetic signals, these signals can also be analog or digital;

(4) The analog signal is a continuous change in time signal, the signal of a certain parameter (amplitude, phase, frequency) can represent the information to be transmitted, the digital signal only takes a limited number of discrete values, and the conversion between the digital signal is almost instantaneous, the digital signal in a moment of state to indicate the information they transmit;

(5) Data communication is a reference to the source and the information in the form of the signal is digital, in the channel can be transmitted in accordance with the need for analog transmission or digital transmission mode;

(6) The channel utilization of analog transmission is high, but it is easy to distort; The digital transmission channel utilization is low, but it is not easy to distort;

2. Channel Characteristics:

(1) Channel bandwidth: W=F1-F2,F1 is the lowest frequency that the channel can pass, F2 is the highest frequency that the channel can pass, when the circuit composing the channel is made, the channel bandwidth is decided;

(2) digital channel is a discrete channel, it can only transmit the digital signal of discrete value;

(3) The bandwidth of the channel determines the maximum rate of the pulse sequence that can be transmitted in the channel without losing fidelity;

(4) A digital pulse is called a code element, using the code element rate to represent the number of times of the signal waveform in unit time, that is, the number of code elements transmitted through the channel in unit time; The unit of the code element rate is called Porter;

(5) Nyquist theorem indicates that the maximum code element rate is: b=2w (Baud); W is bandwidth

(6) The amount of information carried by the code element is determined by the number of discrete values taken by the code element: n=log2n; If the code element is 4 kinds of discrete values, a code element carries two bits of information;

(7) The amount of information (number of bits) transmitted over a channel per unit of time is called the data rate; In a certain baud, the way to increase the rate is to use a code element to represent more digits: r=blog2n=2wlog2n (bps)

**R: Data rate; B: The code element rate (baud rate); N: Number of types of code elements; W: Channel bandwidth;

* * Data rate and baud rate are two different concepts, only when the code element takes two discrete values, the two are equal;

(8) The actual channel will be disturbed by a variety of noise, Shannon formula shows that there is a noise channel limit data rate: C=WLOG2 (1+s/n) unit bps

**W: Channel bandwidth; S: Average rate of the signal; N: Noise average power; n/A: Snr Db=10log10 (N/a): signal-to-noise ratio commonly used decibel number representation;

* * This formula is independent of the number of discrete values taken by the signal, that is, regardless of the way the modulation, as long as the given signal-to-noise ratio, the maximum amount of information in the unit time is determined;

(9) in the analog channel, the bandwidth is w=f1-f2 by the formula; the bandwidth of the digital channel is the maximum data rate that the channel can reach;

3. BER: In noisy channels, the increase of data rate means that the concept of error in transmission increases; the probability of error in transmitting bits is expressed by bit error rate:

**P=N1 (number of faulted digits)/n2 (number of bits transmitted)

* * In the computer communication network, the bit error rate generally requires less than 10-6 times, that is, the average 1 gigabit per transmission to allow the wrong one;

Channel characteristics of the 2016.7.18 Network engineer Tutorial