Final net full performance plan textbooks: Computer networks (7th edition) Xie Xiren edition

The physical layer

Characteristics of 1.

  1. mechanical
  2. electrical
  3. function
  4. discipline

2. Messages, data and signals

  • Message: a specific object transmitted in a communication system. (e.g. symbols, words, speech, images, etc.)
  • Data: is the entity that ships the message.
  • Signal: An electrical or electromagnetic representation of data

3. Time domain analysis, frequency domain analysis, Fourier analysis and cutoff frequency

4. Bits and symbols, bit rates and baud rates

  • Bit: a measure of the amount of information. It is the smallest unit of information
  • Symbol: a signal waveform (digital pulse) of a fixed duration representing the basic waveform of different discrete values.
  • Bit rate: the number of bits transmitted per second. The unit is bit per second (BPS), which can also be expressed as B /s. The higher the bit rate is, the greater the data amount (bits) transmitted per unit time is.
  • Baud rate: The number of symbol symbols transmitted per second. It is a measure of the rate at which symbols are transmitted.

5. Transmission mode

A medium that transmits information in one direction or another

5.1 channel

  1. Unidirectional communication (simplex communication) communication in only one direction and no interaction in the opposite direction.
  2. Two-way alternating communication :(half-duplex communication) two parties can send messages, but not at the same time. Only one side can accept the message. After a period of time, the other side can reverse.
  3. Bidirectional simultaneous communication (full-duplex communication) Both sides of a communication can send and receive information at the same time.

5.2

  1. serial

Data is transmitted bit by bit in chronological order. Good for long distances.

  1. parallel

During data transmission, n parallel channels are adopted, and each channel is transmitted in serial, with n bits transmitted at a time. Suitable for short distances.

6. Naismith’s criterion

In any channel, there is an upper limit to the rate at which the symbol can be transmitted. If the transmission rate exceeds the upper limit, serious inter-code crosstalk will occur, making it impossible for the receiver to determine the symbol (i.e., identify it).

Shannon’s Law

7.1 signal-to-noise ratio

The ratio of the average power of the signal to the average power of the noise is often denoted as S/N, and the decibel is used as the unit of measurement.

SNR (db) = 10 log10(S/N) (db)Copy the code

When S/N=10, the SNR is 10 dB, and when S/N=1000, the SNR is 30dB

7.2 Shannon formula

Channel limit information transmission rate C

C = W*log2(1 + S/N) (bit/s)
Copy the code

The higher the bandwidth of the channel or the SNR of the channel, the higher the limit transmission rate of information.

8. Basic concepts of transmission

  • Source point: The source point device generates data to be transmitted, such as Chinese characters typed by a computer keyboard
  • Baseband signal: the signal that comes to the source (source point)
  • Baseband modulation: The waveform of the baseband signal is transformed to make it adapt to the signal characteristics.

Reuse technology

Basic concepts in communications technology

  • Time division multiplexing (TDM) : The same bandwidth resources are occupied at different times.
  • Frequency division multiplexing (FDM) : Occupies different bandwidth resources simultaneously.

  • Wavelength division multiplexing (WDM) : Optical frequency division multiplexing
  • Code Division multiplexing (CDM) (Code division multiple Access (CDMA)) : can use the same frequency band for communication at the same time, has a strong anti-interference ability, the spectrum is similar to white noise, not easy to be detected by the enemy.
  • Chip: Each bit time is subdivided into m short intervals, usually m with a value of 64 or 128.
  • Chip sequence: Each station is assigned a unique M bit.

10. Exercises after class

Question 1 :(2-04)

  • Baseband signal: the signal that comes to the source (source point)
  • Symbol: a signal waveform (digital pulse) of a fixed duration representing the basic waveform of different discrete values.
  • Unidirectional communication (simplex communication) communication in only one direction and no interaction in the opposite direction.
  • Two-way alternating communication :(half-duplex communication) two parties can send messages, but not at the same time. Only one side can accept the message. After a period of time, the other side can reverse.
  • Bidirectional simultaneous communication (full-duplex communication) Both sides of a communication can send and receive information at the same time.
  • Serial transmission: Data is transmitted bit by bit in chronological order
  • Parallel transmission: data transmission, using n parallel channels, in each channel serial transmission, each transmission of N bits.

Question 2: The meaning of fragrance formula: (2-06)

As long as the information transmission rate is below the limit of the information transmission rate of the channel, there must be some way to achieve error-free transmission.

The difference between bits/SEC and symbols/SEC: the two are not exactly the same.

  1. When binary encoding is used, a symbol corresponds to a bit. In this case the code element and the bit are numerically identical.
  2. In other encodings, however, a symbol does not always equal a bit. It may be several bits equal to a symbol, or several codes equal to a bit.

Problem 3 :(2-07) assume that a channel has a maximum code rate of 20,000 codes per second, which is limited by the Fourier criterion. What data rate (b/s) can be obtained if the amplitude of the symbol is divided into 16 different levels for transmission using amplitude modulation

If we use binary numbers to represent the 16 different levels, there will be 2 to the fourth, or 4 binary bits. So it’s a symbol for 4 bits. So the symbol rate is 20,000 bits per second, and we’re going to get four times the symbol rate which is 80,000 bits per second

Question 4 (2-13) Commonly used channel multiplexing technology:

Frequency division multiplexing, time division multiplexing, wavelength division multiplexing and code division multiplexing.

There are four stations for code division Multiple Access CDMA communication. Four station of code sequence is: A: (- 1-1-1 + 1 + 1 1 + 1 + 1). B: (1-1 + 1 + 1 + 1 + 1-1-1). C: (1 + 1 + 1 + 1 + 1-1-1-1). D: (1 +1 -1 -1 -1 -1 +1 -1). The received chip sequence is (-1 + 1-3 + 1-1-3 +1 +1). Which station sent the data? Is the station sending the data sending a 1 or a 0? Write down the detailed steps.