Communication revision notes

Serial and parallel transmission

Serial transmission sends data one bit at a time, sequentially, along a communication channel. Parallel transmission sends several bits at the same time, using multiple channels or wires. Serial transmission has important advantages over parallel transmission: it requires fewer wires or channels, which can reduce the complexity of the connection, and it avoids problems caused by separate signals becoming misaligned. Parallel transmission can send multiple bits simultaneously, but the individual bits may not arrive at exactly the same time, making the received group harder to interpret.

Synchronous and asynchronous transmission

In synchronous transmission, the sender and receiver use a shared timing method. Data is sent in a stream according to this timing, so the receiver knows when to sample or interpret the incoming bits. In asynchronous transmission, data is not sent as one continuously timed stream. Each unit of data is identified using control bits. A start bit signals that a unit of data is beginning, allowing the receiver to prepare to read it. One or more stop bits signal that the unit has ended, allowing the receiver to recognise the boundary before the next unit.

Synchronous transmission can avoid the repeated start and stop information required by asynchronous transmission. Asynchronous transmission is useful when data is sent intermittently because each unit can identify its own beginning and end.

Communication terminology

• Baud rate is the number of signal changes, or symbols, transmitted per second.
• Bit rate is the number of bits transmitted per second.
• Bandwidth is the capacity of a communication channel to carry data.
• Latency is the delay involved in communication.
• A protocol is a set of rules that controls how data is communicated between devices.

Baud rate and bit rate are not necessarily equal. If each signal change represents one bit, the values are equal. If each signal change encodes more than one bit, the bit rate is higher than the baud rate. For example, if every signal change represents two bits and there are 1000 signal changes per second, the bit rate is 2000 bits per second.

Bit rate is directly proportional to bandwidth: increasing the available bandwidth allows a higher bit rate, while reducing bandwidth reduces the possible bit rate, assuming the other relevant conditions remain the same.

Common errors

Do not use *baud rate* and *bit rate* as interchangeable terms. Do not describe parallel transmission as sending one bit at a time. Do not omit the purpose of start and stop bits when explaining asynchronous transmission. Also, do not confuse latency with bandwidth: latency concerns delay, whereas bandwidth concerns the channel's data-carrying capacity.