Composing the communication system of
a general model of the source -> the transmission device -> channel (noise sources) -> receiving apparatus -> sink
communications system refers generally required to transmit the information to all the equipment and channels.
Source: Change the original message electrical signal (baseband signal), non-power into electricity.
For example: a microphone (sound -> audio signal) analog sources, analog signal transmission.
Teletypewriter (keyboard character -> digital signals) a digital signal source, a digital transmission signal.
Transmitting device: the original electrical signal generating source is converted into a form suitable for transmission channel. Converting the form of modulation, amplification, filtering, coding, multiplexing and the like.
Channels: channel transmission signal transmission medium. While administration of a channel, the channel will signal loss and interference generated.
Concentrated expression of the entire communication system noise: noise source.
Reception apparatus: on the contrary, the purpose of restoring the original signal from the received electrical signal with the transmitting device impaired.
Sink: the destination of the message. In contrast with the source function, the original message is converted to an electrical signal.
2 analog communication system model (the analog signals to convey information using a communication system)
analog sources -> modulator -> Channel -> Demodulator -> sink
^
noise source
of the system, two important changes in
the message to the baseband signal, to accomplish this transformation and inverse transformation are the source and sink.
Baseband signal and the modulated signal, to accomplish this transformation and inverse transformation are usually modulator and demodulator.
3 model of a digital communication system
using the communication system to transmit digital information signals for digital communications systems. (The following is a general digital communication model)
Source -> Source Coding -> Encryption -> channel coding -> modulator -> channel (noise sources) -> demodulator -> channel decoder - >
Source and sink may be may be analog or digital.
Source coding: First, complete A / D converter (analog to digital signal); second compressed digital signal processing to reduce redundancy and to improve the effectiveness of information transfer. Source decoder the reverse process.
Encryption and decryption of action is to improve the security of information.
Modulation effect: adapting the signal to channel characteristics.
Synchronous digital communication system is necessary, according to a synchronization function can be divided into: carrier synchronization, symbol synchronization, group (frame) synchronization, and network synchronization.
Advantages and disadvantages of digital communications (as compared with an analog communication)
the advantage of
strong anti-interference ability of 1, and no noise accumulation. (Using a relay means)
2 controllable transmission errors (the use of channel coding techniques)
3 facilitates signal processing, conversion and storage, it may be the signals from different sources with integrated transmission.
4 ease of integration, miniaturization of the communication device.
5 easy encryption, security and good.
Disadvantage of
a take up more bandwidth.
High 2 synchronization requirements, thus the system more complex.
