2.2.1 Model of data communication system
A data communication system includes three parts: source system (or sender, sender), transmission system (or transmission network) and destination system (or receiver, receiver).
Some common terms:
- data - the entity that carries the message.
- signal - The electrical or electromagnetic representation of data.
- Analogous signal - The value of the parameter representing the message is continuous .
- digital signal - The value of the parameter representing the message is discrete .
- Code - The fundamental waveform that represents different discrete values when using a waveform in the time domain (or simply time domain) to represent a digital signal .
The relationship between symbols and data rate: log2 base N * symbol rate = data rate
2.2.2 Several basic concepts about channels
- Channel - Generally used to denote a medium that transmits information in a certain direction .
- Unidirectional Communication (Simplex Communication) - There can be only one direction of communication and no interaction in the opposite direction.
- Two-way alternating communication (half-duplex communication) - both parties in the communication can send information, but not both at the same time (and certainly not at the same time).
- Two-way simultaneous communication (full-duplex communication) - two parties in the communication can send and receive information at the same time.
- Baseband signal (ie, baseband signal) - The signal from the source. The data signals that represent various text or image files output by a computer are all baseband signals.
- Baseband signals often contain more low-frequency components, even DC components, and many channels cannot transmit such low-frequency components or DC components. Therefore, the baseband signal must be modulated.
Modulations fall into two broad categories: - Baseband modulation: Only the waveform of the baseband signal is transformed so that it can be adapted to the channel characteristics. The transformed signal is still a baseband signal. This process is called encoding .
- Band-pass modulation: Use a carrier for modulation to move the frequency range of the baseband signal to a higher frequency band and convert it to an analog signal, so that it can be better transmitted in an analog channel (that is, only within a certain frequency range able to pass the channel).
- Bandpass signal: The signal modulated by the carrier.
(1) Commonly used coding methods
- Non-return-to-zero system: positive level represents 1, negative level represents 0.
- Return-to-zero system: positive pulses represent 1, and negative pulses represent 0.
- Manchester encoding : An upward transition in the center of the bit cycle represents a 0, and a down transition in the center of the bit cycle represents a 1 . But it can also be defined in reverse.
- Differential Manchester encoding : there is always a transition at the center of each bit. A transition on the bit start boundary represents a 0 , and a bit start boundary without a transition represents a 1 .
- As can be seen from the signal waveforms, Manchester encoding and differential Manchester encoding produce a higher frequency signal than the non-return-to-zero system.
- From the point of view of self-synchronization ability , the non-return-to-zero system cannot extract the signal clock frequency from the signal waveform itself (this is called no self-synchronization ability), while Manchester encoding and differential Manchester encoding have self-synchronization ability .
(2) Basic bandpass modulation method
Baseband signals often contain more low-frequency components, even DC components, and many channels cannot transmit such low-frequency components or DC components. To solve this problem, the baseband signal must be modulated.
The most basic binary modulation methods are as follows:
- Amplitude Modulation (AM) : The amplitude of the carrier wave varies with the baseband digital signal.
- Frequency Modulation (FM) : The frequency of the carrier varies with the baseband digital signal.
- Phase Modulation (PM) : The initial phase of the carrier varies with the baseband digital signal.
In order to achieve a higher information transmission rate, a more complex multivariate amplitude-phase hybrid modulation method must be adopted. For example: Quadrature Amplitude Modulation QAM (Quadrature Amplitude Modulation).
It is not that the more symbols, the better. If the number of bits that can be represented by each symbol is more, it will be more difficult to correctly identify each state during demodulation at the receiving end, and the error rate will increase.