Bit rate


In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time.
The bit rate is :wiktionary:quantified|quantified using the bits per second unit, often in conjunction with an SI prefix such as "kilo", "mega", "giga" or "tera". The non-standard abbreviation "bps" 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 environments, one byte per second corresponds to 8 bit/s.

Prefixes

When quantifying large bit rates, SI prefixes are used, thus:
Binary prefixes are sometimes used for bit rates.
The International Standard specifies different abbreviations for binary and decimal prefixes.

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 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
as:
The gross bit rate is related to the symbol rate or modulation rate, which is expressed in bauds 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; for example, this is not the case for modern modulation systems used in modems and LAN equipment.
For most line codes and modulation methods:
More specifically, a line code representing the data using pulse-amplitude modulation with different voltage levels, can transfer. A digital modulation method using different symbols, for example amplitudes, phases or frequencies, can transfer. This results in:
An exception from the above is some self-synchronizing line codes, for example Manchester coding and return-to-zero coding, where each bit is represented by two pulses, resulting in:
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:
In practice this upper bound can only be approached for line coding schemes and for so-called vestigal 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:
In case of parallel communication, the gross bit rate is given by
where n is the number of parallel channels, Mi is the number of symbols or levels of the modulation in the i-th channel, and Ti is the symbol duration time, expressed in seconds, for the i-th channel.

Information rate

The physical layer net bitrate, information rate, useful bit rate, payload rate, net data transfer rate, coded transmission rate, effective data rate or wire speed of a digital communication channel is the capacity excluding the physical layer protocol overhead, for example time division multiplex framing bits, redundant forward error correction codes, equalizer training symbols and other channel 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 the datalink layer and physical layer, and may consequently include data link and higher layer overhead.
In modems and wireless systems, link adaptation is often applied. In that context, the term peak 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. Some operating systems and network equipment may detect the "connection speed" of a network access technology or communication device, implying the current net bit rate. Note that the term line rate in some textbooks is defined as gross bit rate, in others as net bit rate.
The relationship between the gross bit rate and net bit rate is affected by the FEC code rate according to the following.
The connection speed of a technology that involves forward error correction typically refers to the physical layer net bit rate in accordance with the above definition.
For example, the net bitrate of an IEEE 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 ISDN2 Basic Rate Interface 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/second, due to the 4B5B encoding. In this case, the gross bit rate is equal to the symbol rate or pulse rate of 125 megabaud, due to the NRZI line 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 the Manchester line code, each bit is represented by two pulses, resulting in a pulse rate of 20 megabaud.
The "connection speed" of a V.92 voiceband modem typically refers to the gross bit rate, since there is no additional error-correction code. It can be up to 56,000 bit/s downstreams and 48,000 bit/s upstreams. A lower bit rate may be chosen during the connection establishment phase due to adaptive modulationslower but more robust modulation schemes are chosen in case of poor signal-to-noise ratio. Due to data compression, the actual data transmission rate or throughput may be higher.
The channel capacity, also known as the Shannon 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-node communication link.
The channel capacity is proportional to the analog bandwidth in hertz. This proportionality is called Hartley's law. Consequently, the net bit rate is sometimes called digital bandwidth capacity in bit/s.

Network throughput

The term throughput, essentially the same thing as digital 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 datalink 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 also measuring network throughput.

Goodput (data transfer rate)

Goodput or data transfer rate refers to the achieved average net bit rate that is delivered to the application layer, exclusive of all protocol overhead, data packets retransmissions, etc. For example, in the case of file transfer, the goodput corresponds to the achieved file transfer rate. The file transfer rate in bit/s can be calculated as the file size divided by the file transfer time 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 to V.44 data compression, and sometimes lower due to bit-errors and automatic 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.

Progress trends

These are examples of physical layer net bit rates in proposed communication standard interfaces and devices:
WAN modemsEthernet LANWiFi WLANMobile data

  • 1972: Acoustic coupler 300 baud
  • 1977: 1200 baud Vadic and Bell 212A
  • 1986: ISDN introduced with two 64 kbit/s channels
  • 1990: V.32bis modems: 2400 / 4800 / 9600 / 19200 bit/s
  • 1994: V.34 modems with 28.8 kbit/s
  • 1995: V.90 modems with 56 kbit/s downstreams, 33.6 kbit/s upstreams
  • 1999: V.92 modems with 56 kbit/s downstreams, 48 kbit/s upstreams
  • 1998: ADSL up to 10 Mbit/s
  • 2003: ADSL2 up to 12 Mbit/s
  • 2005: ADSL2+ up to 26 Mbit/s
  • 2005: VDSL2 up to 200 Mbit/s
  • 2014: G.fast up to 1000 Mbit/s
  • 1975: Experimental 2.94 Mbit/s
  • 1981: 10 Mbit/s 10BASE5
  • 1990: 10 Mbit/s 10BASE-T
  • 1995: 100 Mbit/s Fast Ethernet
  • 1999: Gigabit Ethernet
  • 2003: 10 Gigabit Ethernet
  • 2010: 100 Gigabit Ethernet
  • 2017: 200/400 Gigabit Ethernet
  • 1997: 802.11 2 Mbit/s
  • 1999: 802.11b 11 Mbit/s
  • 1999: 802.11a 54 Mbit/s
  • 2003: 802.11g 54 Mbit/s
  • 2007: 802.11n 600 Mbit/s
  • 2012: 802.11ac ~1000 Mbit/s
  • 1G:
  • * 1981: NMT 1200 bit/s
  • 2G:
  • * 1991: GSM CSD and D-AMPS 14.4 kbit/s
  • * 2003: GSM EDGE 296 kbit/s down, 118.4 kbit/s up
  • 3G:
  • * 2001: UMTS-FDD 384 kbit/s
  • * 2007: UMTS HSDPA 14.4 Mbit/s
  • * 2008: UMTS HSPA 14.4 Mbit/s down, 5.76 Mbit/s up
  • * 2009: HSPA+ 28 Mbit/s downstreams, 22 Mbit/s upstreams
  • * 2010: CDMA2000 EV-DO Rev. B 14.7 Mbit/s downstreams
  • * 2011: HSPA+ accelerated 42 Mbit/s downstreams
  • Pre-4G:
  • * 2007: Mobile WiMAX 144 Mbit/s down, 35 Mbit/s up
  • * 2009: LTE 100 Mbit/s downstreams, 50 Mbit/s upstreams
    • See also comparison of mobile phone standards

    For more examples, see list of device bit rates, spectral efficiency comparison table and OFDM system comparison table.

    Multimedia

    In digital multimedia, bitrate 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.
    If lossy 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 of compression 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 bitrates in this section are approximately the minimum that the average listener in a typical listening or viewing environment, when using the best available compression, would perceive as not significantly worse than the reference standard:

    Encoding bit rate

    In digital multimedia, bit rate refers to the number of bits used per second to represent a continuous medium such as audio or video after source coding. The encoding bit rate of a multimedia file is the size of a multimedia file in bytes divided by the playback time of the recording, multiplied by eight.
    For realtime streaming multimedia, the encoding bit rate is the goodput that is required to avoid interrupt:
    The term average bitrate is used in case of variable bitrate multimedia source coding schemes. In this context, the peak bit rate is the maximum number of bits required for any short-term block of compressed data.
    A theoretical lower bound for the encoding bit rate for lossless data compression is the source information rate, also known as the entropy rate.

    Audio

    CD-DA

    , the standard audio CD, is said to have a data rate of 44.1 kHz/16, meaning that the audio data was sampled 44,100 times per second and with a bit depth of 16. CD-DA is also stereo, using a left and right channel, 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 can be calculated as follows:
    The cumulative size of a length of PCM audio data 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 of CD-DA data requires 846,720,000 bytes of storage:

    MP3

    The MP3 audio format provides lossy data compression. Audio quality improves with increasing bitrate: