Block diagram of digital communication system

A digital communication system moves information from a source to a destination as a stream of bits. In full-duplex mode each end has both a transmitter and a receiver, so both sides send and receive at the same time.

 Side A                                       Side B
+--------+  +-------+  +-------+  +-------+  +-------+
|Source/ |->|Source |->|Channel|->|Modula-|->|       |
|  Info  |  |encoder|  |encoder|  | tor   |  |       |
+--------+  +-------+  +-------+  +-------+  | Chan- |
+--------+  +-------+  +-------+  +-------+  |  nel  |
|Destin. |<-|Source |<-|Channel|<-|Demodu-|<-|       |
|        |  |decoder|  |decoder|  | lator |  | (+    |
+--------+  +-------+  +-------+  +-------+  | noise)|
                                             +-------+
 (Side B has the same chain in the opposite direction;
  two separate paths/frequencies give full duplex.)

Function of each block

1. Information source / input transducer: produces the message (speech, video, text) and converts it to an electrical signal.
2. Source encoder: digitises the signal (sampling, quantization, PCM) and removes redundancy (compression) so fewer bits are sent.
3. Channel encoder: adds controlled redundancy (parity, CRC, Hamming, convolutional codes) so the receiver can detect and correct errors.
4. Modulator (line coder): converts the bits into a signal suited to the channel: baseband pulses, or a carrier modulated by ASK/FSK/PSK/QAM.
5. Channel: the physical medium (twisted pair, coaxial, fibre, air). It adds attenuation, distortion and noise.
6. Demodulator: recovers the bit stream from the received, distorted signal.
7. Channel decoder: uses the redundant bits to detect/correct errors and removes them.
8. Source decoder: rebuilds the original message (decompression, D/A conversion).
9. Destination / output transducer: delivers the message to the user (speaker, screen, computer).

In full duplex, a hybrid or separate paths (two wire pairs, or two frequency bands) keep the two directions from interfering. A telephone call is a common example.