[Computer Network] Speak clearly "Physical Layer" in more than 10,000 words

Preface

Because I want to learn the front-end knowledge to understand computer networks, I don’t have high requirements for the physical layer, I just understand some simple skins.

If you want to learn better, you have to look at the principles of communication. I will put aside the study in this area for now.

1. Basic concepts of the physical layer

1.1. Problems to be solved by the physical layer

The physical layer considers how to transmit data bit streams on the transmission media connected to various computers .

The physical layer shields the difference of various transmission media for the data link layer, so that the data link layer only needs to consider how to complete the protocols and services of this layer, instead of considering the specific transmission media of the network.

1.2. The main tasks of the physical layer protocol

1.2.1. Mechanical characteristics

Specify the shape and size of the connector used in the interface, the number and arrangement of pins, and the fixing and locking devices.

1.2.2. Electrical characteristics

Indicate the range of voltages that appear on each line of the interface cable.

1.2.3. Features

Indicate the meaning of a certain level of voltage appearing on a certain line.

1.2.4. Process characteristics

Indicate the order of occurrence of various possible events for different functions.

2. Transmission media below the physical layer

2.1. Guided transmission media

In guided transmission media, electromagnetic waves are guided to propagate along the solid media.

2.1.1. Coaxial cable

Baseband coaxial cable (50 ohms) is used for digital transmission, used for local area network in the past, and now the field of local area network uses twisted pair

Broadband coaxial cable (75 ohms for analog transmission, currently mainly used for cable TV

2.1.2. Twisted pair

The role of twisting

Resist part of electromagnetic interference from the outside world

Reduce electromagnetic interference from adjacent wires

For current household Ethernet, at least Category 5 (5E) twisted pair cable should be used

Shielded twisted pair has better anti-interference performance than unshielded twisted pair, but the price is more expensive.

2.1.3. Optical fiber

2.1.3.1. Multimode fiber

Light is transmitted forward through continuous total reflection in multimode fiber

Due to the dispersion problem, the light in the multimode fiber will cause the pulse broadening problem

Multimode fiber is only suitable for short-distance transmission (in buildings)

Multimode fiber does not require high light source . Inexpensive light-emitting diodes can be used as the light source. Correspondingly, photodiodes can be used to detect light pulses.

2.1.3.2. Single-mode fiber

The diameter of a single-mode fiber is only one wavelength of light. The light travels forward without total reflection .

Single-mode fiber has no pulse broadening problem

Single-mode optical fiber is suitable for long-distance transmission and has low attenuation, but its manufacturing cost is high and the requirements for light source are high .

An expensive laser generator must be used as the light source. Correspondingly, a laser detector is required to detect light pulses.

2.1.4. Power line

Not a new technology (it appeared in the 1920s)

For households or small businesses, it is only used when it is impossible or unwilling to deploy network cables.

2.2. Unguided transmission media

Unguided transmission media refers to free space.

2.2.1. Radio waves

Low frequency LF and intermediate frequency MF frequency bands, using ground wave transmission

High frequency HF and VHF frequency band, rely on ionospheric reflection for transmission

2.2.2. Microwave

Straight line propagation, can penetrate the ionosphere

The 100-meter ground generation tower, the maximum line-of-sight LOS transmission distance is 100 kilometers

Geostationary satellite

Low-orbit satellite

2.2.3. Infrared

Point-to-point transmission

Straight line transmission, no obstacles in the middle, short transmission distance

Low transmission rate (4Mb/s ~ 16Mb/s)

Already obsolete on laptops

2.2.4. Visible light

LiFi has a higher transmission rate than WiFi

Currently still in the experimental research stage

2.3. Radio Spectrum Management Agency

2.3.1. China

Radio Administration of the Ministry of Industry and Information Technology (National Radio Office)

2.3.2. United States

Federal Communications Clerk FCC

2.3.3. ISM frequency band

ISM (Industrial, Scientific, Medical) frequency band

The US ISM frequency bands are 915MHz, 2.4GHz, 5.8GHz

The ISM frequency band of different countries may be slightly different

3. Transmission method

3.1. Serial transmission and parallel transmission

Serial transmission Bits are transmitted one after another on a transmission line. It is suitable for long-distance transmission, and computer networks use this kind of transmission.

Parallel transmission Multiple bits are simultaneously transmitted on multiple transmission lines. It is not suitable for long-distance transmission and the cost is too high. This kind of transmission is used inside the computer.

3.2. Synchronous transmission and asynchronous transmission

Synchronous transmission Bits are transmitted one after another with no gap in between, and the duration of each bit is equal. There are two ways to synchronize the clocks of the sender and receiver.

External synchronization: Add a separate clock signal line between the sender and receiver.

Internal synchronization: The transmitter encodes the clock synchronization signal into the transmitted data and transmits it together (for example, Manchester encoding).

Asynchronous transmission is transmitted in bytes. The interval between bytes is not fixed, but the bit duration in each byte is the same. In other words, the bytes are asynchronous, but the bits are still synchronous. To this end, a start bit and an end bit need to be added to each byte.

3.3. Simplex, half-duplex, and full-duplex transmission

Simplex One-way communication, such as broadcast.

Half - duplex Two-way alternate communication (not at the same time), such as walkie-talkie.

Full duplex Two-way simultaneous communication, such as a telephone.

4. Coding and Modulation

4.1. Commonly used terms in data communication

4.1.1. Message

The text, pictures, audio, and video that require computer processing are collectively referred to as messages.

4.1.2. Data

Data is the entity that carries the message. The computer can only process binary data.

4.1.3. Signal

Signal is the electromagnetic representation of data

4.1.3.1. Baseband signal

The original electrical signal from the source is called the baseband signal

4.1.3.1.1. Digital baseband signal

For example, in the computer, the signal transmitted between the CPU and the memory.

4.1.3.1.2. Analog baseband signal

For example, the audio signal generated after the microphone collects the sound.

4.2. Encoding

4.2.1. The digital signal is converted into another digital signal and transmitted in the digital channel

For example, Ethernet uses Manchester encoding, 4B/5B, 8B/10B and other encodings.

4.2.2. Convert analog signal to digital signal and transmit in digital channel

For example, pulse code modulation PCM, which encodes audio signals.

4.2.3. Commonly used codes

4.2.3.1. Non-return to zero coding

There will be no zero level during the entire symbol time

There is a synchronization problem, and an additional transmission line is needed to transmit the clock signal to synchronize the sender and receiver.

For computer networks, I would rather use this transmission line to transmit data signals than to transmit clock signals.

4.2.3.2. Zeroing code

The signal must be "returned to zero" after the transmission of each symbol, so the receiver only needs to sample after the signal is returned to zero, without a separate clock signal.

In fact, return-to-zero encoding is equivalent to encoding the clock signal in the data in a "return-to-zero" manner, which is called a "self-synchronization" signal.

However, most of the data bandwidth in the return-to-zero coding is used to transmit the "return-to-zero" and wasted.

4.2.3.3. Manchester encoding

The level jump occurs at the middle of the symbol, which represents both the clock and the data

Positive transition means 1 or 0, negative transition means 0 or 1, which can be customized

Traditional Ethernet (10Mb/s) uses this code

4.2.3.4. Differential Manchester encoding

The level jump occurs at the middle of the symbol, and the jump only represents the clock.

Whether the level changes at the beginning of the symbol indicates data.

Compared with Manchester encoding, it has less changes and is more suitable for higher transmission rates.

4.3. Modulation

4.3.1. Convert digital signal to analog signal and transmit in analog channel

For example, WiFi uses complementary code keying CCK/direct sequence spread spectrum DSSS/orthogonal frequency division multiplexing OFDM and other modulation methods.

4.3.2. Convert the analog signal to another analog signal and transmit it in the analog channel

For example, voice data is loaded into an analog carrier signal for transmission.

Frequency division multiplexing FDM technology makes full use of bandwidth resources.

4.3.3. Basic modulation (binary system)

4.3.3.1. AM

The modulated signal consists of two basic waveforms with different amplitudes.

Each basic waveform can only represent 1 bit of information.

4.3.3.2. FM

The modulated signal consists of two basic waveforms with different frequencies.

Each basic waveform can only represent 1 bit of information.

4.3.3.3. Phase Modulation PM

The modulated signal consists of two basic waveforms with different initial phases.

Each basic waveform can only represent 1 bit of information.

4.3.4. Mixed modulation (multiple system)

For example, a quadrature amplitude modulation QAM in which phase and amplitude are mixed and modulated.

QAM16 can modulate 12 kinds of phases, each phase has 1 or 2 kinds of amplitudes to choose from.

16 basic waveforms can be modulated, and each waveform can correspond to 4 bits.

4.4. Symbols

When using time-domain waveforms to represent digital signals, they represent basic waveforms of different discrete values.

Simply put, a symbol is a basic modulated waveform that can represent bit information.

5. The limit capacity of the channel

5.1. Factors that cause signal distortion

5.1.1. Symbol transmission rate

5.1.2. Signal transmission distance

5.1.3. Noise interference

5.1.4. Transmission media quality

5.2. Nyquist criterion

Under the assumed ideal conditions , in order to avoid inter-symbol interference , the symbol transmission rate has an upper limit.

The highest symbol transmission rate of the ideal low communication channel = 2W Baud = 2W symbols/sec

Ideal maximum symbol transmission rate with communication channel = W Baud = W symbols/sec

5.2.1. The highest symbol transmission rate of an ideal low communication channel

2W Baud = 2W Baud = 2W symbols/sec

Among them, W is the channel bandwidth, and the unit is Hz.

5.2.2. The highest symbol transmission rate with an ideal channel

W Baud = W baud = W symbols per second

Among them, W is the channel bandwidth, and the unit is Hz.

5.2.3. The relationship between baud rate and bit rate

The symbol transmission rate is also called baud rate, modulation rate, waveform rate or symbol rate. It has a certain relationship with the bit rate.

When 1 symbol only carries 1 bit of information, the baud rate (symbols/sec) and the bit rate (bits/sec) are equal in value;

When 1 symbol carries n bits of information, when the baud rate is converted to bit rate, the value must be multiplied by n.

5.2.4. Matters needing attention

The highest symbol rate that the actual channel can transmit is significantly lower than the upper limit given by the Nyquist criterion. This is because the actual channel will also be interfered by many other factors (such as noise interference, signal attenuation, transmission media quality, etc.).

To increase the information transmission rate (bit rate), it is necessary to try to make each symbol carry more bits of information. This requires a pluralistic system.

It is not that an unlimited increase in the number of bits carried by each symbol can increase the transmission rate of information without limitation. Because the limit information transmission rate of the channel is also limited by the actual signal-to-noise ratio when the signal is transmitted in the channel.

5.3. Claude Shannon Official

The limit information transmission rate of a channel with limited bandwidth and interference from Gaussian white noise.

5.3.1. The limit information transmission rate of the channel

c = W x log2 (1 + S/N)

c: The limit information transmission rate of the channel (unit: b/s)

W: channel bandwidth (unit: Hz)

S: The average power of the signal transmitted in the channel

N: Gaussian noise power in the channel

S/N: Signal-to-noise ratio, using decibels (dB) as the unit of measurement. Signal to noise ratio (dB) = 10 x log1(S/N)(dB)

5.3.2. Matters needing attention

The information transmission rate that can be achieved on the actual channel is much lower than the limit transmission rate of this formula. This is because in the actual channel, the signal has to receive other damages, such as various impulse interference, signal attenuation and distortion in transmission, etc. These factors are not considered in the Shannon formula.

The greater the channel bandwidth or the signal-to-noise ratio in the channel, the higher the limit transmission rate of information.

5.4. The significance of the Nyquist criterion and Shannon's formula

In the case of a certain channel bandwidth , according to the Nyquist criterion and Shannon’s formula, if you want to increase the transmission rate of information, you must adopt a multiplex system (a better modulation method) and strive to improve the signal-to-noise ratio in the channel .

Since the publication of Shannon’s formula, various new signal processing and modulation methods have continued to appear, all of which aim to get as close as possible to the transmission rate limit given by Shannon’s formula.