1. Data communication system model
A data communication system can be divided into three parts, namely source system (or sender, sender), transmission system (or transmission network) and destination system (or receiver, receiver).
Ⅰ. The source system generally includes the following two parts:
① Source point: The source point device generates the data to be transmitted.
②Transmitter: Usually, the digital bit stream generated by the source point can be transmitted in the transmission system only after being encoded by the transmitter. A typical transmitter is a modulator.
Ⅱ. The target system generally includes the following two parts:
① Receiver, which receives the signal transmitted by the transmission system and converts it into information that can be processed by the destination device. A typical receiver is a demodulator.
② End point: The end point device obtains the transmitted digital bit stream from the receiver, and then outputs the information.
Ⅲ. Transmission system: The transmission system can be a simple transmission line or a complex network system connected between the source system and the destination system.
2. Channel
A channel is generally used to represent a medium that transmits information in a certain direction. From the point of view of the way of information exchange between the two parties in the communication, there are three basic ways:
①One-way communication: also known as simplex communication, that is, there can only be communication in one direction, and there is no interaction in the opposite direction.
②Two-way alternate communication: also known as half-duplex communication, that is, both parties to the communication can send information, but they cannot send information at the same time (of course they cannot receive at the same time).
③ Two-way simultaneous communication: also known as full-duplex communication, that is, both parties to the communication can send and receive information at the same time.
3. Signal
The signal from the source is often called the baseband signal, and the data signal representing various text or image files output by the computer belongs to the baseband signal. Baseband signals often contain more low-frequency components, even DC components, and many channels cannot transmit such low-frequency components or DC components.
4. Coding and modulation
Modulation can be divided into two categories, one is to exchange only the waveform of the baseband signal so that it can adapt to the channel characteristics. The exchanged signal is still a baseband signal, and this type of modulation is called baseband modulation. Since this baseband modulation converts a digital signal into another form of digital signal, this process is called encoding. Another type of modulation needs to be modulated with a carrier to move the frequency range of the baseband signal to a higher frequency band and convert it into an analog signal, so that it can be better transmitted in the analog channel. The signal after carrier modulation is called a band-pass signal (that is, it can only pass through the channel within a certain frequency range), and the modulation using the carrier is called band-pass modulation.
Ⅰ. Commonly used encoding methods
①Non-return-to-zero system: Positive level means 1, negative level means 0.
②Return to zero system: Positive pulses represent 1, and negative pulses represent 0.
③Manchester encoding: The upward transition of the center of the bit period represents 0, and the downward transition of the center of the bit period represents 1, and it can also be defined in reverse.
④ Differential Manchester encoding: There is always a jump at the center of each bit. A transition at the bit start boundary represents 0, and a transition without a bit start boundary represents 1.
It can be seen from the signal waveform that the signal frequency generated by Manchester encoding is higher than that of non-return-to-zero. From the perspective of self-synchronization capability, the non-return-to-zero system cannot extract the signal clock frequency from the signal waveform itself (this is called no self-synchronization capability), while Manchester encoding has self-synchronization capability.
Ⅱ. Basic bandpass modulation method
① Amplitude modulation AM: That is, the amplitude of the carrier wave changes with the baseband digital signal. For example, 0 or 1 corresponds to no carrier or carrier output, respectively.
②Frequency modulation FM: That is, the frequency of the carrier changes with the baseband digital signal. For example, 0 or 1 corresponds to frequency f1 or f2, respectively.
③Phase modulation PM: That is, the initial phase of the carrier wave changes with the baseband digital signal. For example, 0 or 1 corresponds to a phase of 0 degrees or 180 degrees, respectively.
5. Nye's criterion
In a low-communication channel with a bandwidth of W (Hz), if the influence of noise is not considered, the highest rate of symbol transmission is 2W (symbol/second). If the transmission rate exceeds this upper limit, serious intersymbol interference will occur, making it impossible for the receiving end to judge (that is, identify) the symbols.
6. Shannon formula
The signal-to-noise ratio is the ratio of the average power of the signal to the average power of the noise, often recorded as S/N.
Signal-to-noise ratio (dB) = 10 log₁₀(S/N)(dB)
The limit information transmission rate of the channel C=W log₂(1+S/N) (bit/s)
W is the bandwidth of the channel (in Hz), S is the average power of the signal transmitted in the channel, and N is the Gaussian noise power inside the channel.
The larger the bandwidth of the channel or the signal-to-noise ratio in the channel, the higher the limit transmission rate of information.
Shannon's formula points out that no matter how complicated the encoding technology is, it is impossible to break through the absolute limit of the information transmission rate on a noisy channel.
7. Transmission media
Transmission media is also known as transmission medium or transfer medium. Transmission media can be divided into two categories, guided transmission media (twisted pair, coaxial cable or optical fiber) and non-guided transmission media (wireless, infrared or atmospheric laser).
Ⅰ. Guided transmission media
① Twisted pair: No matter what kind of twisted pair attenuation is, it increases with the increase of frequency. Using thicker wire reduces attenuation, but increases the weight and price of the wire.
② Coaxial cable: Coaxial cable consists of inner conductor copper core wire (single-strand solid wire or multi-strand wire), insulation layer, mesh braided outer conductor shielding layer (can also be single-strand) and insulation protection Composed of layers. Due to the effect of the shielding layer of the outer conductor, the coaxial cable has good anti-interference characteristics and is widely used to transmit data at a higher rate.
③Optical cable: Optical fiber communication is to use optical fiber to transmit optical pulses for communication. The optical pulse is equivalent to 1, and the absence of optical pulse is equivalent to 0. The transmission bandwidth of an optical fiber communication system is far greater than the bandwidth of other various transmission media at present. Optical fiber is the transmission medium of optical fiber communication. There can be multiple light rays incident at different angles that are transmitted in one light, and this kind of fiber is called a multimode fiber. Optical pulses gradually broaden as they travel through multimode fibers, causing distortion. Therefore, multimode fiber is only suitable for short-distance transmission.
Ⅱ. Non-guided transmission media
For a given modulation method and data rate, the greater the signal-to-noise ratio, the lower the bit error rate.
For the same signal-to-noise ratio, modulation techniques with higher data rates have higher bit error rates.
Constantly changing one's geographical location during communication will cause changes in wireless channel characteristics, so the signal-to-noise ratio and bit error rate will change.
Microwave relay communication: The capacity of the communication channel is large, the quality of microwave transmission is high, and the investment in communication construction is small, but it will also be affected by bad weather, the concealment and confidentiality are poor, and the use and maintenance are more expensive.
The biggest feature of satellite communication is that the communication distance is long, the communication cost has nothing to do with the communication distance, the communication is stable, the confidentiality is relatively poor, the cost is high, and it has a large propagation delay.
8. Characteristics of the physical layer interface
①Mechanical characteristics: specify the shape and size of the connector used for the interface, the number and arrangement of pins, fixing and locking devices, etc.
②Electrical characteristics: Indicate the range of voltage appearing on each line of the interface cable.
③Functional characteristics: Indicate the meaning of a certain level of voltage appearing on a certain line.
④ Process characteristics: Indicate the sequence of occurrence of various possible events for different functions.
9. Physical layer equipment
①Repeater (repeater): The signal is reshaped and forwarded (signal regeneration rather than simple amplification).
②5-4-3 rule: Only 3 of the 5 segments of communication media connected in series with 4 repeaters can be connected to a computer.
③ Hub (Hub): multi-port repeater.
10. Channel multiplexing technology
①Frequency division multiplexing FDM: Each signal of frequency division multiplexing occupies different bandwidth resources at the same time (the bandwidth here is the frequency bandwidth rather than the data transmission rate). When communicating, multiplexers are always used in pairs with demultiplexers. Between the multiplexer and the demultiplexer is a high-speed channel shared by users.
② Time-division multiplexing TDM: All users of time-division multiplexing occupy the same frequency band bandwidth at different times. Compared with frequency division multiplexing, time division multiplexing is more conducive to the transmission of digital signals.
③ Statistical time division multiplexing STDM: It can obviously improve the utilization rate of the channel. Concentrators often use this statistical time-division multiplexing. Statistical time-division multiplexing is also called asynchronous time-division multiplexing, and ordinary time-division multiplexing is called synchronous time-division multiplexing. The concentrator using statistical time division multiplexing is also called an intelligent multiplexer, which can provide the ability to store and forward the entire message (but most multiplexers can only store one character or one bit at a time), and make each user More reasonable shared channel.
④Wavelength division multiplexing WDM: It is the frequency division multiplexing of light. Use one optical fiber to simultaneously transmit multiple optical carrier signals with very close frequencies.
⑤Code Division Multiplexing CDM: Each user can use the same frequency band for communication at the same time. Since each user uses a specially selected different code pattern, there is no interference between users. The signal sent by this system has a strong anti-interference ability, and its spectrum is similar to white noise, which is not easy to be found.
11. Broadband access technology
①Asymmetric digital subscriber line technology ADSL: Use digital technology to transform the subscriber line of the existing analog telephone so that it can carry broadband digital services. ADSL does not guarantee a fixed data rate.
② Optical fiber coaxial hybrid network HFC: developed on the basis of cable TV network.
③FTTx technology: that is, fiber to...
In order to effectively utilize optical fiber resources, a passive optical network (PON) is widely used between optical fiber trunk lines and users. Passive optical network does not need to be equipped with power supply, and its long-term operation cost and management cost are very low. The most popular passive optical networks are Ethernet passive optical network EPON and gigabit passive optical network GPON.