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Bits, Bytes and data rates
Confused by bits, bytes, kilobytes and megabytes?
We technical people refer to them all the time, especially when talking about data rates and networking - but what do all the terms actually mean and how do they relate?
Bits & Bytes
The basic unit of information in a digital system is a 'bit' (short for binary digit), and is the smallest unit of data. A bit has a single binary value, which can be in one of two states. The states can be referred to as many things, but are usually described as: "on" and "off", "0" and "1", or "true" and "false".
In most digital systems, there are eight bits in a byte. The value of a bit is usually indicated and stored as either above or below a designated electrical level. Half a byte (four bits) is called a nibble. In some systems, the term octet is used for an eight-bit unit instead of byte. In many systems, four eight-bit bytes (or octets) form a 32-bit word. In such systems, instruction lengths are sometimes expressed as full-word (32 bits in length) or half-word (16 bits in length).
| Bits & Bytes | ||||
| Unit | Symbol | Equivalent to | Or... | |
|---|---|---|---|---|
| bit | b | |||
| Byte | B | 8 bits | ||
| kiloByte | kB | 1024 Bytes | 210 Bytes | |
| MegaByte | MB | 1024 kilobytes | 220 Bytes | |
| GigaByte | GB | 1024 Megabytes | 230 Bytes | |
| TeraByte | TB | 1024 Gigabytes | 240 Bytes | |
| PetaByte | PB | 1024 Terabytes | 250 Bytes | |
| ExaByte | EB | 1024 Petabytes | 260 Bytes | |
Bit rates & Data rates
These definitions are similar - but not identical - to the terms used to describe the rates of data transfer in a digital system. In this case, the bit rate is the number of bits that pass a given point in a network in a given amount of time (usually one second).
The term bit rate is a synonym for data transfer rate (or simply data rate). Bit rate seems to be used more often when discussing transmission technology details and data rate when comparing transmission technologies for the end user.
Data transfer rate is the amount of digital data that is moved from one place to another in a given time (usually in one second). The data transfer rate can be viewed as the speed of travel of a given amount of data from one place to another. In general, the greater the bandwidth of a given path, the higher the data transfer rate.
In systems with larger bandwidth, the data rate is usually measured in some multiple of bits per second - for example, kilobits, or thousands of bits per second (Kbps). On Ethernet local area networks, data transfer can be as fast as 10 megabits per second (Mbps) - this is a million binary pulses per second. When talking about data rates, the multipliers 'k' and 'M', etc, normally refer to powers of 1000, not 1024.
Baud
Data rate is similar to the term baud - but only when the system being described has only two states and no extra framing or stop bits. Baud is the unit in which the information carrying capacity or "signalling rate" of a communication channel is measured. One baud is one symbol (state-transition or level-transition) per second. A symbol is a unique state of the communication channel, distinguishable by the receiver from all other possible states. For example, it may be one of two voltage levels on a wire for a direct digital connection or it might be the phase or frequency of a carrier.
The term "baud" was originally a unit of telegraph signalling speed, set at one Morse code dot per second. Or, more generally, the reciprocal of the duration of the shortest signalling element. It was proposed at the International Telegraph Conference of 1927, and named after J.M.E. Baudot (1845-1903), the French engineer who constructed the first successful teleprinter.
Where data is transmitted as packets, e.g. characters, the actual "data rate" of a channel is given by
R = D / P
where R is the "raw" rate in bits per second, D is the number of data bits in a packet and P is the total number of bits in a packet (including packet overhead). The term "baud" causes much confusion and is usually best avoided. Use "bits per second" instead if that is what you mean, or qualify the term baud.
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