## Methods of communication

### Multi-level signalling

### Multi-level signalling using a single cable #### There is no reason why data transmission should be limited to a binary (two symbol state) format over a channel, and in theory, it should be possible to use any number of voltage levels or symbol types. |

#### For example, using four voltage levels means that we can uniquely encode two bits into each of the four levels (00 = level A, 01 = level B, 10 = level C, 11 = level D). This means that every time we change the symbol state, two bits of information are conveyed compared with only one for the binary system. Hence, we can send information twice as fast for a given bandwidth of link, or use a link with half the bandwidth and maintain equivalent transmission rate.
#### ### Multi-level signalling using multiple cables #### It is of course possible to use multi-level signalling (often termed M-ary signalling) over parallel channels if so desired, with the consequent increase in throughput or opportunity to reduce the bandwidth on each channel as required. |