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 (). 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: 0.001 bit/s = 1 mbit/s (one millibit per second, i.e., one bit per thousand seconds) = 1 bit/ks 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., one billion bits per second) 1,000,000,000,000 bit/s = 1 Tbit/s (one terabit per second, i.e., one trillion bits per second)
The binary prefixes defined by International Standard IEC 80000-13 are sometimes used:Schlosser, S. W., Griffin, J. L., Nagle, D. F., & Ganger, G. R. (1999). Filling the memory access gap: A case for on-chip magnetic storage (No. CMU-CS-99-174). CARNEGIE-MELLON UNIV PITTSBURGH PA SCHOOL OF COMPUTER SCIENCE. 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 as:
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.Lou Frenzel. 27 April 2012, "What's The Difference Between Bit Rate And Baud Rate?". Electronic Design. 2012.
For most line codes and modulation methods:
More specifically, a line code (or baseband transmission scheme) representing the data using pulse-amplitude modulation with different voltage levels, can transfer bits per pulse. A digital modulation method (or passband 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 example Manchester coding and return-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 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 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:
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 ith channel, and Ti is the symbol duration time, expressed in seconds, for the ith channel.
Information rate
The physical layer net bitrate,Theodory S. Rappaport, Wireless communications: principles and practice, Prentice Hall PTR, 2002 information rate, useful bit rate,Lajos Hanzo, Peter J. Cherriman, Jürgen Streit, Video compression and communications: from basics to H.261, H.263, H.264, MPEG4 for DVB and HSDPA-style adaptive turbo-transceivers, Wiley-IEEE, 2007. payload rate,V.S. Bagad, I.A. Dhotre, Data Communication Systems, Technical Publications, 2009. net data transfer rate, coded transmission rate, effective data rate or wire speed (informal language) of a digital communication channel is the capacity excluding the physical layer protocol overhead, for example time division multiplex (TDM) framing bits, redundant forward error correction (FEC) 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 data 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 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.Sudhir Dixit, Ramjee Prasad Wireless IP and Building the Mobile Internet, Artech House Some operating systems and network equipment may detect the "connection speed"Guy Hart-Davis,Mastering Microsoft Windows Vista home: premium and basic, John Wiley and Sons, 2007 (informal language) of a network access technology or communication device, implying the current net bit rate. 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. net bit rate ≤ gross bit rate × code rate
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 (and thus the "connection speed") 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 (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 the 4B5B (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 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 downstream and 48,000 bit/s upstream. 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 (see below) 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. net bit rate ≤ channel capacity
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 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 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 (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 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: goodput ≤ throughput ≤ maximum throughput ≤ net bit rate for a certain communication path.
Progress trends These are examples of physical layer net bit rates in proposed communication standard interfaces and devices:
WAN modems Ethernet LAN WiFi WLAN Mobile data 1972: Acoustic coupler 300 baud 1977: 1200 baud Vadic and Bell 212A 1986: ISDN introduced with two 64 kbit/s channels (144 kbit/s gross bit rate) 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 (ITU G.992.1) up to 10 Mbit/s 2003: ADSL2 (ITU G.992.3) up to 12 Mbit/s 2003: GPON up to 1 Gbit/s 2005: ADSL2+ (ITU G.992.5) up to 26 Mbit/s 2005: VDSL2 (ITU G.993.2) up to 200 Mbit/s 2010: 10G-PON up to 10 Gbit/s 2014: G.fast (ITU G.9701) up to 1 Gbit/s 1975: Experimental 2.94 Mbit/s 1981: 10 Mbit/s 10BASE5 (coaxial cable) 1990: 10 Mbit/s 10BASE-T (twisted pair) 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 (WCDMA) 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+ (Without MIMO) 28 Mbit/s downstreams (56 Mbit/s with 2×2 MIMO), 22 Mbit/s upstreams 2010: CDMA2000 EV-DO Rev. B 14.7 Mbit/s downstreams 2011: HSPA+ accelerated (With MIMO) 42 Mbit/s downstreams