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Baseband and wideband signals
Channel: The medium through which signals are transmitted
Baseband signal: the digital signal 0 and 1 directly with two different voltage representation, and then sent to the digital channel transmission (baseband transmission)—-> to the source of the signal, directly expressed the information to transmit the signal
Wideband signal: frequency division multiplexing analog signal formed by modulation of baseband signal, and then transmitted to analog channel for transmission. After the base band signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band for transmission on the road
When the transmission distance is relatively short, the computer network adopts baseband transmission mode (the attenuation in the near distance is reduced, so that the signal content is not easy to change). When the transmission distance is relatively long, the computer network adopts broadband transmission mode (the attenuation in the long distance is large).
Digital data is encoded into digital signals
Non-return to zero coding high 1 low 0, coding is easy to achieve, but there is no error detection function, and can not determine the beginning and end of a code element, so that the transceiver and sender are difficult to maintain synchronization
Manchester encoding an element can be divided into two equal intervals, an interval before for low level is a space for high level said element 1, element 0, by contrast, the characteristics of the code is in the middle of each element in level jump, a jump in the middle of the both as a clock signal (can be used for synchronization), and a digital signal, But the bandwidth it occupies is twice the width of the original baseband
Differential Mantester encoding is commonly used for LAN transmission. Its rule is: if the source code is 1, the level of the first half of the symbol is the same as the level of the last half of the symbol, if it is 0, the opposite. Features: there is a level jump in the middle of each code element, which can achieve synchronization, and the anti-jamming line is stronger than Manchester code
Return to zero coded signal level is returned to zero within a code element
Reverse non – zero encodingWhen the signal level is flipped, it means 0, and when the signal level is unchanged, it means 1
4B/5B encoding the insertion of additional bits into the stream to break up a series of zeros or ones. This means that five bits are used to encode four bits of data, which is then passed to the receiver. This is called 4B/5B encoding, and the encoding efficiency is 80%
Digital data is modulated into analog signals
Digital data transfer technology converts digital signal to analog signal at sender and analog signal to digital signal at receiver, corresponding to demodulator and mediation process respectively
Analog data is encoded into digital signals
What the computer deals with internally is binary data and data audio, so it is necessary to convert analog audio into discrete sequence of limited digital representation —–> audio digitalization through sampling and quantization
- Sampling:
Sampling theorem: Sampling frequency >=2× maximum frequency of signal
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Quantization: the level amplitude obtained by sampling is converted to the corresponding digital value according to a certain grading scale, and integer, which converts the continuous level amplitude into discrete digital quantity
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coding
Converts the quantized result into the corresponding binary code
