[2. Computer Network - Physical Layer] Lots of illustrations

1. Basic concepts of physical layer

• The physical layer considers how toconnect the bit streams transmitted on the transmission media of various computers.
• The physical layer shields the data link layer from the differences in various transmission media, so that the data link layer only needs to consider how to complete the protocols and services of this layer without having to consider the specific transmission media of the network.

2. Baseband signals and broadband signals

Channel: The transmission medium of the signal. Generally used to represent a medium that transmits information in a certain direction, so a communication line often contains a sending channel and a receiving channel.
baseband signal: The digital signals 1 and 0 are directly represented by two different voltages, and then sent to the digital channel for transmission (baseband transmission). Signals from the source, such as computer output data signals representing various text or image files, are all baseband signals. A baseband signal is a signal that directly expresses the information to be transmitted. For example, the sound waves we speak are baseband signals.
wideband signal: The baseband signal is modulated to form a frequency division multiplexed analog signal, and then transmitted to the analog channel for transmission (broadband transmission). After the baseband signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band for transmission in the channel (that is, only within a frequency range able to pass the channel). ● When the transmission distance is short, the computer network uses baseband transmission< /span> mode (long-distance attenuation is large, even if the signal changes greatly, the baseband signal can be finally filtered out)< /span>broadband transmission, the computer network adopts ● When the transmission distance is relatively method (small attenuation at close range, so the signal content is not easy to change)

3. Transmission media below the physical layer

• Guidance media
  • coaxial cable
  • twisted pair
  • optical fiber
  • power line
• non-guided communication media
  • Radio waves: signals propagate in all directions (strong penetration ability, can be transmitted over long distances, and are widely used in the field of communications (such as Mobile communication))
  • Microwave: signal propagates in a fixed direction
  • Infrared ray, visible light: signal propagation in fixed direction

4. Transmission method

Two data transmission methods:
Serial transmission (slow speed, low cost, suitable for long-distance transmission)
Parallel transmission (fast, expensive, suitable for short-distance transmission) used inside computers

Three communication methods:
Simplex communication (broadcast)
Half-duplex communication (telephone)
Full-duplex communication (cannot send and receive at the same time) (walkie-talkie)

Synchronous transmission (Each data bit is sent at the same time interval, and each bit of information must be received at the same time interval as when it is sent. Regardless of whether it is transmitted or not Data, the receiving unit and the sending unit must be synchronized on every binary bit)
Asynchronous transmission (asynchronous between bytes (asynchronous between bytes) The time interval is not fixed)) Each bit in the byte is still synchronized (the duration of each bit is the same)

5. Coding and Modulation

Encoding: Only the digital baseband signal is converted (digital signal → another digital signal analog signal → digital signal) without changing the nature of the signal.

Modulation: Move the frequency range of the digital baseband signal to a higher frequency band and convert it into an analog signal (analog signal → analog signal).

Code element: When using a time domain waveform to represent a digital signal, use a fixed-duration signal waveform (digital pulse), representing basic waveforms of different discrete values. (It is a modulated waveform that can represent bit information)

Rate: Data transmission rate, indicating the amount of data transmitted per unit time. You can use Code element transmission rate< a i=3> (number of pulses) and information transmission rate.

Baud: The unit of code element transmission rate.

The following figure shows several commonly used encodings:

6. The ultimate capacity of the channel

• Distortion: Loss of authenticity, mainly means that the received signal is different from the sent signal, with wear and attenuation.

Factors affecting the degree of distortion:

• code element transmission rate
• Signal transmission distance
• Noise interference
• Transmission media quality

Two important formulas:

Nye's criterion: In a bandwidth-limited and noisy channel, in order to avoid errors, the data transmission rate of information has an upper limit.
             The ultimate transmission rate under an ideal low-pass channel =2 W log2 V (b/s)
W: bandwidth, V: several types of symbols (phases)/number of discrete levels of symbols

Shannon's formula: In any channel, the efficiency of symbol transmission has an upper limit. If the transmission rate exceeds this upper limit, serious inter-code crosstalk problems will occur, causing It becomes impossible for the receiving end to judge (i.e. identify) the code elements.
Signal-to-noise ratio = average power of signal/average power of noise, often recorded as S/N, and decibel (dB) is used as the unit of measurement, that is:
               Signal-to-noise ratio (dB) = 10 log10 (S/N)
            The ultimate transmission rate of the channel =W log2 (1+S/N) (b/s)
W: bandwidth, S/N: signal-to-noise ratio