Computer network physical layer summary notes

Physical layer

Basic concepts of the physical layer

• The physical layer is to consider how the various computer reconnection transmission medium to transmit data bit stream , rather than to a specific transmission medium
• The role of the physical layer is to shield as much as possible the differences in different transmission media and communication methods, which can be understood as making the signals transmitted to the physical layer uniform and easy to identify.
• Referred to as physical layer protocol specification

The main tasks of the physical layer

• Mechanical properties
• Electrical characteristics
• Features
• Process characteristics

Basic knowledge of data communication

Simulation of data communication system

• Source system (sender)
• Transmission system (transmission network)
• Destination system (recipient)

Several basic concepts about channels

Channel generally indicates a medium for transmitting information in a certain direction

From the perspective of how the two parties' information in the channel is called tiger, it can be divided into three types:

• One-way communication
• Two-way alternate communication (half-duplex communication, simplex ) means that both parties can send information when they cannot send it at the same time.
• Two-way simultaneous communication (full-duplex communication) means that both parties can communicate at the same time

The signal from the signal source called a baseband signal because it generally contains much low-frequency component, even a DC component, but it is not the transmission of such multi-channel low frequency signal and a DC signal, all only to be modulated .

Modulation classification

• Baseband modulation

  Mode: the waveform of a baseband signal is modulated, so that it can adapt to the characteristics of the channel signal is still converted baseband signal, the digital signal is converted into another digital signal, also called encoding .

• Bandpass modulation

  Mode: required carrier is modulated, the frequency range of the baseband signal moves to high frequency, and converted to an analog signal in analog signal transmission channel into a bandpass signal

Common encoding methods

• Non-return to zero

• Return to zero

• Manchester code

  This means 0

  

（1）

This represents 1

（2）

（3）

• Differential Manchester Coding

  It’s very simple. The boundary changes are 0, and every change is 1.

  

Basic bandpass modulation method

• AM
• FM
• phase modulation

Limit capacity of the channel

If the transmission speed of the digital signal on the channel is too fast, the signal will be distorted. Therefore, the speed cannot be blindly required, and the recognition of the transmission signal must be guaranteed.

There will be an upper limit on the speed channel symbols, to break this limit if the general would be a serious symbol crosstalk phenomenon cause serious distortion.

The frequency range that the channel can pass

Inter-symbol crosstalk

High-frequency signals are transmitted in the channel, and the signal will be attenuated, causing the signal waveform received at the receiving end to lose the clear boundary between the symbols.

Signal to noise ratio

It is the ratio of the average power of the signal to the average power of the noise, denoted as: S/N , and use **decibel (dB)** as the unit:

Shannon Official

C = W /${\log }_2(1+S/N)$ (bit/s)

(C is the transmission rate of the channel, W is the bandwidth of the channel; S is the average power of the signal transmitted in the channel; N is the Gaussian noise of the channel)

The larger the channel ratio , the higher the limit transmission rate of information , but it will cause signal distortion.

The way to increase the transmission rate is to make each symbol carry more bits of information

Guided transmission media

1. Twisted pair

   The twisting of twisted-pair wires will reduce the electromagnetic interference of adjacent wires, and is generally used in telephone systems

2. Shielded twisted pair

3. Same desk cable

4. Optical cable (optical fiber) distinguishes signals by different incident angles of light

   • Multimode fiber optic cable
   • Long transmission distance of single-mode optical cable

Unguided transmission media

• Shortwave communication

  Generally, communication is carried out by the reflection of the ionosphere, and the transmission distance is far, when the distortion is serious.

• Wireless microwave communication

  Microwave straight line, generally two types of terrestrial microwave communications and satellite communications , the ionosphere can penetrate into the universe, all typically used with satellites.

Channel multiplexing technology

Frequency division multiplexing, time division multiplexing and statistical time division multiplexing

As shown in the figure, in a popular sense, multiplexing technology is to combine channels.

1. Frequency division multiplexing

   Frequency division multiplexing is all used at the same time to account for different bandwidth resources, which is equivalent to flying in the sky and walking on the ground.

2. Time division multiplexing

   1. Time division multiplexing means that users use the same bandwidth at different points in time.

      Statistical time division multiplexing (asynchronous time division multiplexing) is an improved version of time division multiplexing that can greatly improve channel utilization

   (Communication channel multiplexer and demultiplexer are used in pairs, share our courier need to send together in one sent in the past, people have a special destination are classified according to different address)

Wavelength division multiplexing

Is optical frequency division multiplexing

Code Division Multiplexing CDMA (Code Division Multiple Access) (emphasis)

Advantages: Since each user uses a specially selected different patterns , so users do not interfere with each other ; strong anti-interference, the spectrum is similar to white noise, the enemy will not be discovered.

CDMA each of m bits into a short time interval, called chips , each station has its own unique m-bit code chip sequence , if a transmitting station is to 1, the transmitter chip sequence their m-bit transmission 0 The inverse code of 1.

Spread spectrum : The data rate of sending information is b bit/s. Since each bit is converted into m-bit chips, the rate of all data sent by a station is increased to mb bit/s.

1. Direct Sequence Spread Spectrum (as mentioned above)
2. Pseudorandom chip sequence

Normalized inner product (emphasis)

Stations of two different orthogonal chip sequences , the vector is S and T normalized inner product is 0 ;

The normalized inner product of the chip vector of the same station and its own chip vector is 1 ;

And the normalized inner product of a chip vector and the vector of the chip's complement is -1 .