| Name | Symbol | Multiple | |
|---|---|---|---|
| bit per second | bit/s | 1 | 1 |
| Metric prefixes (SI) | |||
| kilobit per second | kbit/s | 103 | 10001 |
| megabit per second | Mbit/s | 106 | 10002 |
| gigabit per second | Gbit/s | 109 | 10003 |
| terabit per second | Tbit/s | 1012 | 10004 |
| Binary prefixes (IEC 80000-13) | |||
| kibibit per second | Kibit/s | 210 | 10241 |
| mebibit per second | Mibit/s | 220 | 10242 |
| gibibit per second | Gibit/s | 230 | 10243 |
| tebibit per second | Tibit/s | 240 | 10244 |
Intelecommunications andcomputing,bit rate (bitrate or as a variableR) is the number ofbits that are conveyed or processed per unit of time.[1]
The bit rate is expressed in the unitbit per second (symbol:bit/s), often in conjunction with anSI prefix such askilo (1 kbit/s = 1,000 bit/s),mega (1 Mbit/s = 1,000 kbit/s),giga (1 Gbit/s = 1,000 Mbit/s) ortera (1 Tbit/s = 1,000 Gbit/s).[2] The non-standard abbreviationbps is often used to replace the standard symbol bit/s, so that, for example, 1 Mbps is used to mean one million bits per second.
In most computing and digital communication environments, onebyte per second (symbol:B/s) corresponds to 8 bit/s.
When quantifying large or small bit rates,SI prefixes (also known asmetric prefixes or decimal prefixes) are used, thus:[3]
| 0.001 bit/s | = 1 mbit/s (one millibit per second, i.e., one bit per thousand seconds) |
| 1 bit/s | = 1 bit/s (one bit per second) |
| 1,000 bit/s | = 1 kbit/s (one kilobit per second, i.e., one thousand bits per second) |
| 1,000,000 bit/s | = 1 Mbit/s (one megabit per second, i.e., one million bits per second) |
| 1,000,000,000 bit/s | = 1 Gbit/s (one gigabit per second, i.e., onebillion bits per second) |
| 1,000,000,000,000 bit/s | = 1 Tbit/s (one terabit per second, i.e., onetrillion bits per second) |
Binary prefixes are sometimes used for bit rates.[4][5]The International Standard (IEC 80000-13) specifies different symbols for binary and decimal (SI) prefixes (e.g., 1KiB/s = 1024 B/s = 8192 bit/s, and 1MiB/s = 1024 KiB/s).
In digital communication systems, thephysical layergross bitrate,[6]raw bitrate,[7]data signaling rate,[8]gross data transfer rate[9] oruncoded transmission rate[7] (sometimes written as a variableRb[6][7] orfb[10]) is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead.
In case ofserial communications, the gross bit rate is related to the bit transmission timeas:
The gross bit rate is related to thesymbol rate or modulation rate, which is expressed inbauds or symbols per second. However, the gross bit rate and the baud value are equalonly when there are only two levels per symbol, representing 0 and 1, meaning that each symbol of adata transmission system carries exactly one bit of data; for example, this is not the case for modern modulation systems used inmodems and LAN equipment.[11]
For mostline codes andmodulation methods:
More specifically, a line code (orbaseband transmission scheme) representing the data usingpulse-amplitude modulation with different voltage levels, can transfer bits per pulse. Adigital modulation method (orpassband transmission scheme) using different symbols, for example amplitudes, phases or frequencies, can transfer bits per symbol. This results in:
An exception from the above is some self-synchronizing line codes, for exampleManchester coding andreturn-to-zero (RTZ) coding, where each bit is represented by two pulses (signal states), resulting in:
A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certainspectral bandwidth in hertz is given by theNyquist law:
In practice this upper bound can only be approached forline coding schemes and for so-calledvestigial sideband digital modulation. Most other digital carrier-modulated schemes, for exampleASK,PSK,QAM andOFDM, can be characterized asdouble sideband modulation, resulting in the following relation:
In case ofparallel communication, the gross bit rate is given by
wheren is the number of parallel channels,Mi is the number of symbols or levels of themodulation in theithchannel, andTi is thesymbol duration time, expressed in seconds, for theith channel.
Thephysical layernet bitrate,[12]information rate,[6]useful bit rate,[13]payload rate,[14]net data transfer rate,[9]coded transmission rate,[7]effective data rate[7] orwire speed (informal language) of a digitalcommunication channel is the capacity excluding thephysical layer protocol overhead, for exampletime division multiplex (TDM)framing bits, redundantforward error correction (FEC) codes, equalizer training symbols and otherchannel coding. Error-correcting codes are common especially in wireless communication systems, broadband modem standards and modern copper-based high-speed LANs. The physical layer net bitrate is the datarate measured at a reference point in the interface between thedata link layer and physical layer, and may consequently include data link and higher layer overhead.
In modems and wireless systems,link adaptation (automatic adaptation of the data rate and the modulation and/or error coding scheme to the signal quality) is often applied. In that context, the termpeak bitrate denotes the net bitrate of the fastest and least robust transmission mode, used for example when the distance is very short between sender and transmitter.[15] Some operating systems and network equipment may detect the "connection speed"[16] (informal language) of a network access technology or communication device, implying the current net bit rate. The termline rate in some textbooks is defined as gross bit rate,[14] in others as net bit rate.
The relationship between the gross bit rate and net bit rate is affected by the FECcode rate according to the following.
The connection speed of a technology that involves forward error correction typically refers to the physical layernet bit rate in accordance with the above definition.
For example, the net bitrate (and thus the "connection speed") of anIEEE 802.11a wireless network is the net bit rate of between 6 and 54 Mbit/s, while the gross bit rate is between 12 and 72 Mbit/s inclusive of error-correcting codes.
The net bit rate of ISDN2Basic Rate Interface (2 B-channels + 1 D-channel) of 64+64+16 = 144 kbit/s also refers to the payload data rates, while the D channel signalling rate is 16 kbit/s.
The net bit rate of the Ethernet 100BASE-TX physical layer standard is 100 Mbit/s, while the gross bitrate is 125 Mbit/s, due to the4B5B (four bit over five bit) encoding. In this case, the gross bit rate is equal to the symbol rate or pulse rate of 125 megabaud, due to theNRZIline code.
In communications technologies without forward error correction and other physical layer protocol overhead, there is no distinction between gross bit rate and physical layer net bit rate. For example, the net as well as gross bit rate of Ethernet 10BASE-T is 10 Mbit/s. Due to theManchester line code, each bit is represented by two pulses, resulting in a pulse rate of 20 megabaud.
The "connection speed" of aV.92voicebandmodem typically refers to the gross bit rate, since there is no additional error-correction code. It can be up to 56,000 bit/sdownstream and 48,000 bit/supstream. A lower bit rate may be chosen during the connection establishment phase due toadaptive modulation – slower but more robust modulation schemes are chosen in case of poorsignal-to-noise ratio. Due to data compression, the actual data transmission rate or throughput (see below) may be higher.
Thechannel capacity, also known as theShannon capacity, is a theoretical upper bound for the maximum net bitrate, exclusive of forward error correction coding, that is possible without bit errors for a certain physical analog node-to-nodecommunication link.
The channel capacity is proportional to theanalog bandwidth in hertz. This proportionality is calledHartley's law. Consequently, the net bit rate is sometimes calleddigital bandwidth capacity in bit/s.
The termthroughput, essentially the same thing asdigital bandwidth consumption, denotes the achieved average useful bit rate in a computer network over a logical or physical communication link or through a network node, typically measured at a reference point above the data link layer. This implies that the throughput often excludes data link layer protocol overhead. The throughput is affected by the traffic load from the data source in question, as well as from other sources sharing the same network resources. See alsomeasuring network throughput.
Goodput ordata transfer rate refers to the achieved average net bit rate that is delivered to theapplication layer, exclusive of all protocol overhead, data packets retransmissions, etc. For example, in the case of file transfer, the goodput corresponds to the achievedfile transfer rate. The file transfer rate in bit/s can be calculated as the file size (in bytes) divided by the file transfer time (in seconds) and multiplied by eight.
As an example, the goodput or data transfer rate of a V.92 voiceband modem is affected by the modem physical layer and data link layer protocols. It is sometimes higher than the physical layer data rate due toV.44data compression, and sometimes lower due to bit-errors andautomatic repeat request retransmissions.
If no data compression is provided by the network equipment or protocols, we have the following relation:
for a certain communication path.
These are examples of physical layer net bit rates in proposed communication standard interfaces and devices:
| WANmodems | Ethernet LAN | WiFiWLAN | Mobile data |
|---|---|---|---|
|
|
See also:comparison of mobile phone standards |
In digital multimedia, bit rate represents the amount of information, or detail, that is stored per unit of time of a recording. The bitrate depends on several factors:
Generally, choices are made about the above factors in order to achieve the desired trade-off between minimizing the bitrate and maximizing the quality of the material when it is played.
Iflossy data compression is used on audio or visual data, differences from the original signal will be introduced; if the compression is substantial, or lossy data is decompressed and recompressed, this may become noticeable in the form ofcompression artifacts. Whether these affect the perceived quality, and if so how much, depends on the compression scheme, encoder power, the characteristics of the input data, the listener's perceptions, the listener's familiarity with artifacts, and the listening or viewing environment.
The encoding bit rate of a multimedia file is its size inbytes divided by the playback time of the recording (in seconds), multiplied by eight.
For real-timestreaming multimedia, the encoding bit rate is thegoodput that is required to avoid playback interruption.
The termaverage bitrate is used in case ofvariable bitrate multimedia source coding schemes. In this context, thepeak bit rate is the maximum number of bits required for any short-term block of compressed data.[17]
A theoretical lower bound for the encoding bit rate forlossless data compression is thesource information rate, also known as theentropy rate.
The bitrates in this section are approximately theminimum that theaverage listener in a typical listening or viewing environment, when using the best available compression, would perceive as not significantly worse than the reference standard.
Compact Disc Digital Audio (CD-DA) uses 44,100 samples per second, each with a bit depth of 16, a format sometimes abbreviated like "16bit / 44.1kHz". CD-DA is alsostereo, using a left and rightchannel, so the amount of audio data per second is double that of mono, where only a single channel is used.
The bit rate of PCM audio data can be calculated with the following formula:
For example, the bit rate of a CD-DA recording (44.1 kHz sampling rate, 16 bits per sample and two channels) can be calculated as follows:
The cumulative size of a length of PCM audio data (excluding a fileheader or othermetadata) can be calculated using the following formula:
The cumulative size in bytes can be found by dividing the file size in bits by the number of bits in a byte, which is eight:
Therefore, 80 minutes (4,800 seconds) of CD-DA data requires 846,720,000 bytes of storage:
whereMiB is mebibytes withbinary prefix Mi, meaning 220 = 1,048,576.
TheMP3 audio format provideslossy data compression. Audio quality improves with increasing bitrate:
For technical reasons (hardware/software protocols, overheads, encoding schemes, etc.) theactual bit rates used by some of the compared-to devices may be significantly higher than what is listed above. For example, telephone circuits usingμlaw orA-lawcompanding (pulse code modulation) yield 64 kbit/s.
