Function of basic components of a digital communication system

A digital communication system (DCS) sends information as a sequence of discrete symbols. Its basic components and their functions are:

 Source->[Source enc]->[Channel enc]->[Modulator]
                                          |
                                       Channel <- noise
                                          |
 User <-[Source dec]<-[Channel dec]<-[Demodulator]

1. Information source: produces the message, analog (speech) or digital (text, computer data). Analog messages are converted to digital by sampling, quantizing and coding (A/D or PCM).
2. Source encoder: converts the source output into an efficient binary sequence by removing redundancy (data compression), e.g. Huffman coding, DPCM. It reduces the bit rate and hence bandwidth.
3. Channel encoder: adds controlled redundant bits in a known way (e.g. parity, Hamming, convolutional codes) so the receiver can detect and correct transmission errors. Code rate $r=k/n$.
4. Digital modulator: maps the coded bits to analog waveforms suitable for the channel, such as ASK, FSK, PSK or QAM for band-pass channels, or line codes for baseband.
5. Channel: the physical medium (cable, fibre, radio link) that carries the signal. It introduces attenuation, distortion (ISI), noise and interference.
6. Digital demodulator: processes the received noisy waveform (filtering, matched filter, sampling, decision) and gives an estimate of the transmitted bits.
7. Channel decoder: uses the redundancy to detect and correct errors, recovering the information bits.
8. Source decoder: reverses the source coding (decompression), and for analog messages converts back by D/A conversion and filtering, delivering the message to the user (destination).

Optional blocks are encryption/decryption for security and multiplexing/multiple access for sharing the channel; synchronization is needed at the receiver for carrier and bit timing.