Communication methods

Overview

Recall questions on serial against parallel transmission, why serial wins over distance, and synchronous against asynchronous transmission and their overheads.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define serial transmission.

   Model answer

   Serial transmission sends data one bit at a time, in sequence, down a single wire or channel. The bits arrive one after another and are reassembled into bytes at the receiving end.

2. 2

   Define parallel transmission.

   Model answer

   Parallel transmission sends several bits at the same time, each bit travelling down its own separate wire in a multi-wire link. A group of bits (for example eight) is therefore transmitted simultaneously rather than one after another.

3. 3

   Why is serial transmission preferred over parallel for anything but very short distances?

   Model answer

   Three reasons. First, skew: the wires in a parallel link are never electrically identical, so bits sent together arrive at slightly different times, and over distance the skew grows until the bits of one group overlap those of the next. Second, crosstalk: current in one wire induces interference in its neighbours, and this worsens as the wires get longer and as the data rate rises. Third, cost and bulk: a single-conductor cable is cheaper, thinner and more flexible than a multi-wire ribbon. Serial links can therefore be clocked much faster over distance, which more than offsets sending only one bit at a time.

4. 4

   Parallel transmission sends eight bits at once and serial only one. So why is a serial link often faster in practice?

   Model answer

   Because the achievable clock rate matters more than the number of wires. A parallel link is limited by skew and crosstalk, so its clock must be kept low enough that all eight bits still arrive within the same window. A serial link has neither problem, so it can be clocked at a far higher frequency. A single wire running many times faster beats eight wires running slowly.

5. 5

   Where is parallel transmission still a sensible choice?

   Model answer

   Over very short distances inside a computer, where skew and crosstalk have too little cable length to build up — for example the internal buses of a processor, or the connections between a processor and memory on a motherboard.

6. 6

   Define synchronous data transmission.

   Model answer

   In synchronous transmission the sender and receiver are kept in step by a shared clock signal, usually carried on a separate line or recovered from the data stream itself. Data is sent as a continuous stream of bits timed against that clock, with no gaps between characters, and the receiver samples the line at each clock tick.

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