Bit rate

Key takeaways

Bit rate belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Bit rate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Bit rate from memory before moving on to harder problems.

Reference excerpt

In telecommunications and computing, bit rate (bitrate or as a variable R) is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed as bits per second (symbol: bit/s), often with an SI prefix such as kilo (1 kbit/s = 1,000 bit/s), mega (1 Mbit/s = 1,000 kbit/s), giga (1 Gbit/s = 1,000 Mbit/s) or tera (1 Tbit/s = 1,000 Gbit/s). The non-standard abbreviation bps is often used: 1 Mbps is 1 Mbit/s, that is, one million bits per second. The bit rate is different from the transfer rate, measured in transfers per second, when the channel is parallel and thus transfers multiple bits per transfer. In most computing and digital communication environments, one byte per second (symbol: B/s) corresponds to 8 bit/s (1 byte = 8 bits). However if stop bits, start bits, and parity bits need to be factored in, a higher number of bits per second will be required to achieve a throughput of the same number of bytes.

Prefixes For large or small bit rates, SI prefixes (also known as metric prefixes or decimal prefixes) are used:

The binary prefixes defined by International Standard IEC 80000-13 are sometimes used: e.g., 1 KiB/s = 1024 B/s = 8192 bit/s, and 1 MiB/s = 1024 KiB/s.

In data communications

Gross bit rate

In digital communication systems, the physical layer gross bitrate, raw bitrate, data signaling rate, gross data transfer rate or uncoded transmission rate (sometimes written as a variable Rb or fb) is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead. In case of serial communications, the gross bit rate is related to the bit transmission time T b {\displaystyle T_{\text{b}}}

as:

R b = 1 T b , {\displaystyle R_{\text{b}}={1 \over T_{\text{b}}},}

The gross bit rate is related to the symbol rate or modulation rate, which is expressed in baud or symbols per second. However, the gross bit rate and the baud value are equal only when there are only two levels per symbol, representing 0 and 1, meaning that each symbol of a data transmission system carries exactly one bit of data; this is not the case for modern modulation systems used in modems and LAN equipment. For most line codes and modulation methods:

symbol rate ≤ gross bit rate {\displaystyle {\text{symbol rate}}\leq {\text{gross bit rate}}}

More specifically, a line code (or baseband transmission scheme) representing the data using pulse-amplitude modulation with 2 N {\displaystyle 2^{N}} different voltage levels, can transfer N {\displaystyle N} bits per pulse. A digital modulation method (or passband transmission scheme) using 2 N {\displaystyle 2^{N}} different symbols, for example 2 N {\displaystyle 2^{N}} amplitudes, phases or frequencies, can transfer N {\displaystyle N} bits per symbol. This results in:

gross bit rate = symbol rate × N {\displaystyle {\text{gross bit rate}}={\text{symbol rate}}\times N}

An exception from the above is some self-synchronizing line codes, for example Manchester coding and return-to-zero (RTZ) coding, where each bit is represented by two pulses (signal states), resulting in:

gross bit rate = symbol rate/2 {\displaystyle {\text{gross bit rate = symbol rate/2}}}

A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certain spectral bandwidth in hertz is given by the Nyquist law:

symbol rate ≤ Nyquist rate = 2 × bandwidth {\displaystyle {\text{symbol rate}}\leq {\text{Nyquist rate}}=2\times {\text{bandwidth}}}

In practice this upper bound can only be approached for line coding schemes and for so-called vestigial sideband digital modulation. Most other digital carrier-modulated schemes, for example ASK, PSK, QAM and OFDM, can be characterized as double sideband modulation, resulting in the following relation:

symbol rate ≤ bandwidth {\displaystyle {\text{symbol rate}}\leq {\text{bandwidth}}}

In case of parallel communication, the gross bit rate is given by

∑ i = 1 n log 2 ⁡ M i T i {\displaystyle \sum _{i=1}^{n}{\frac {\log _{2}{M_{i}}}{T_{i}}}}

where n is the number of parallel channels, Mi is the number of symbols or levels of the modulation in the ith channel, and Ti is the symbol duration time, expressed in seconds, for the ith channel.

Information rate

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Bit rate

Start with the simplest possible case. Write down what Bit rate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Bit rate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Bit rate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Bit rate

In research: Bit rate appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Bit rate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Bit rate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Audio engineering, Data compression, Data transmission, so understanding it makes those chapters shorter.
In everyday life: Look for Bit rate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Bit rate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Bit rate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Bit rate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Bit rate in simple terms?

In telecommunications and computing, bit rate (bitrate or as a variable R) is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed as bits per second (symbol: bit/s), often with an SI prefix such as kilo (1 kbit/s = 1,000 bit/s), mega (1 Mbit/s = 1,000 kbit/…

Why does Bit rate matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Bit rate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Bit rate.