6.2 Working with bits

You may have met the term bit, perhaps in connection with computers. The term 'bit' is also important in communication systems. It is an abbreviation for 'binary digit'. A binary digit can have just one of two values: it can be either 1 or 0. Pulses can be represented by 1s and 0s, that is, as bits, and so it is convenient to think of streams of 1s and 0s being conveyed along the communications link.

The rate at which the 1s and 0s are conveyed is known as the data rate or bit rate. Every communication link has a maximum data rate it can support, and that's what we mean by the link's bandwidth. (You may possibly have met another meaning of the term 'bandwidth': a frequency range. That meaning is different from the one we are discussing here.) Data rate and bandwidth are both measured as a number of bits per second. For convenience, 'bits per second' is often abbreviated to bps. For instance the data rate might be 100 000 bps (i.e. 100 000 bits per second), 250 000 000 bps or much more.

Clearly we are going to have to deal with large numbers when talking about data rates, so I'm going to introduce a way of making these large numbers more manageable.

You will be familiar with the prefix 'kilo' in words such as kilogram, which is 1000 grams, or kilometre, which is 1000 metres. So it will come as no surprise that 1000 bits per second can also be described as a kilobit per second. The prefix 'mega' is similarly used for a million, so 1 000 000 bits per second is a megabit per second The prefix 'giga' is used for a billion (that is, a thousand million).

All of these prefixes have standard abbreviations. For instance, instead of writing 'kilobits per second' we can write kbps – that is, we write k for kilo. Similarly, M is used for 'mega' and G for 'giga'. (Notice that by convention the k is lower-case but the M and G are upper-case.) Table 1 summarises all this information.

You may have come across examples of data rates and bandwidths in connection with modems and broadband. At the time of writing (early 2005) a dialup modem typically provides a data rate of 56 kbps, which is 56 000 bps, and Internet Service Providers (ISPs) advertise broadband connections with bandwidths such as 1 megabit per second (which is 1 Mbps or 1 000 000 bps).