Bit rate

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Bit rates
Decimal prefixes (SI)
Name Symbol Multiple
kilobit per second kbit/s 103
megabit per second Mbit/s 106
gigabit per second Gbit/s 109
terabit per second Tbit/s 1012
Binary prefixes
(IEC 60027-2)
kibibit per second Kibit/s 210
mebibit per second Mibit/s 220
gibibit per second Gibit/s 230
tebibit per second Tibit/s 240

In telecommunications and computing, bitrate (sometimes written bit rate, data rate or as a variable R or fb) is the number of bits that are conveyed or processed per unit of time.

The bit rate is quantified using the bits per second (bit/s or bps) unit, often in conjunction with an SI prefix such as kilo- (kbit/s or kbps), mega- (Mbit/s or Mbps), giga- (Gbit/s or Gbps) or tera- (Tbit/s or Tbps). 1 kbit/s has almost always meant 1,000 bit/s, not 1,024 bit/s, also before 1999 when SI prefixes were defined for units of information in an IEC standard.

The formal abbreviation for "bits per second" is "bit/s" (not "bits/s"). In less formal contexts the abbreviations "b/s" or "bps" are often used, though this risks confusion with "bytes per second" ("B/s", "Bps").


[edit] Bit rates at various protocol layers

[edit] Physical layer gross bit rate

In digital communication systems, the gross bitrate, raw bitrate, line rate or data signaling rate is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead. The gross bit rate is related to, but should not be confused with, the baud rate in symbols/s or pulses/s. Gross bit rate can be used interchangeably with "baud rate" only when each modulation transition of a data transmission system carries exactly one bit of data; something not true for modern modem modulation systems, for example.

For most line codes and modulation methods:

Gross bit rate ≥ Baud rate

More specifically, a line code representing the data using pulse-amplitude modulation with 2N different voltage levels, or a digital modulation method using 2N different symbols, for example 2N amplitudes, phases or frequencies, can transfer N bit/symbol, or N bit/pulse. This results in:

Gross bit rate = Baud rate · N

The 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 = Baud rate / 2

A theoretical upper bound for the baud rate in symbols/s or pulses/s for a certain analog bandwidth in Hertz is given by the Nyquist law:

Baud rateNyquist rate = 2 · bandwidth

[edit] Physical layer net bit rate

The net bitrate, useful bit rate or information rate of a digital communication link 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 and broadband modem standards. The relationship between the gross bit rate and net bit rate is affected by the FEC code rate according to the following.

Gross bit rate · code rate ≥ Net bit rate

The connection speed of a network access technology or communication device is indicated by some operational systems. 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 bit rate of a 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 connection speed of ISDN Basic Rate Interface (2 B-channels + 1 D-channel) of 64+64+16 = 144 kbit/s also refers to the user data rates, while the line rate is 160 kbit/s.

The net bitrate of the Ethernet 100Base-TX physical layer standard is 100 Mbit/s, while the gross bitrate is 125 Mbit/second, 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 Mbaud, 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 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 Mbaud.

The connection speed of a V.92 voiceband modem 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 modulation - slower but more robust modulation schemes are chosen in case of poor signal-to-noise ratio.

The channel 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.

Channel capacity ≥ Net bit rate

The channel capacity is proportional to the analog bandwidth in Hertz. Consequently the net bit rate is sometimes called digital bandwidth capacity in bit/s.

[edit] Network throughput (data transfer rate) and goodput

The term throughput, essentially the same thing as data transfer rate or 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 below the network layer and above the physical layer. This implies that the throughput often excludes data link layer protocol overhead and sometimes network 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.

As an example, the 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.

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.

If no data compression is provided by the network equipment or protocols, we have the following relation:

Net bit rate ≥ Maximum throughput ≥ Throughput ≥ Goodput

for a certain communication path.

[edit] Multimedia bit rate

In digital multimedia, bit rate often refers to the number of bits used per unit of playback time to represent a continuous medium such as audio or video after source coding (data compression). The size of a multimedia file in bytes is the product of the bit rate (in bit/s) and the length of the recording (in seconds), divided by eight.

In case of streaming multimedia, this bit rate measure is the goodput that is required to avoid interrupts. For streaming multimedia without interrupts, we have the following relationship:

Multimedia bit rate = Required goodput

The term average bitrate (ABR) is used in case of variable bitrate multimedia source coding schemes.

A theoretical lower bound for the multimedia bit rate for lossless data compression is the source information rate, also known as the entropy rate.

[edit] Prefixes


For large bitrates, SI prefixes are used:

1,000 bit/s date=1 kbit/s (one kilobit or one thousand bits per second)
1,000,000 bit/s date=1 Mbit/s (one megabit or one million bits per second)
1,000,000,000 bit/s date=1 Gbit/s (one gigabit or one billion bits per second)

When describing bitrates, binary prefixes have almost never been used and SI prefixes are almost always used with the standard, decimal meanings, not the old computer-oriented binary meanings. Binary usage may occasionally be seen when the unit is the byte/s, and is not typical for telecommunication links. Sometimes it is necessary to seek clarification of the units used in a particular context.

[edit] Progress trends

Proposed standards and first devices :

  • 1972: Acoustic coupler 300 baud
  • 1985: 1200 baud
  • 1990: increasing Modem bandwidth: 2400 / 4800 / 9600 / 19200 bit/s
  • 1995: v.34 modems with 28.8 kbit/s, v.90 modems with 56 kbit/s
  • 1996: ISDN with two 64 kbit/s channels
  • 1998: ADSL from 128 kbit/s to 8 Mbit/s, ADSL2 up to 12 Mbit/s, ADSL2+ up to 24 Mbit/s
  • 1972: IEEE 802.3 Ethernet 2.94 Mbit/s
  • 1985: 10b2 10 Mbit/s coax thinwire
  • 1990: 10bT 10 Mbit/s
  • 1995: 100bT 100 Mbit/s
  • 1999: 1000bT (Gigabit) 1 Gbit/s
  • 2003: 10GBASE 10 Gbit/s

[edit] Bitrates in 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:

  • The original material may be sampled at different frequencies
  • The samples may use different numbers of bits
  • The data may be encoded by different schemes
  • The information may be digitally compressed by different algorithms or to different degrees

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:

[edit] Audio (MP3)

  • 32 kbit/s – MW (AM) quality
  • 96 kbit/s – FM quality
  • 128–160 kbit/s – Standard Bitrate quality; difference can sometimes be obvious (e.g. bass quality)[citation needed]
  • 192 kbit/s – DAB (Digital Audio Broadcasting) quality.[citation needed]
  • 224–320 kbit/s – Near CD quality. The average person cannot tell the difference between a bitrate above 192 kbit/s and the original CD/WAV.

[edit] Other audio

[edit] Video (MPEG2)

  • 16 kbit/s – videophone quality (minimum necessary for a consumer-acceptable "talking head" picture)
  • 128 – 384 kbit/s – business-oriented videoconferencing system quality
  • 1.25 Mbit/s – VCD quality
  • 5 Mbit/s – DVD quality
  • 15 Mbit/s – HDTV quality
  • 36 Mbit/s – HD DVD quality
  • 54 Mbit/s – Blu-ray Disc quality

[edit] Notes

For technical reasons (hardware/software protocols, overheads, encoding schemes, etc.) the actual bitrates used by some of the compared-to devices may be significantly higher than what is listed above. For example:

[edit] See also

[edit] References

Maximum PC - Do Higher MP3 Bit Rates Pay Off? This article contains material from the Federal Standard 1037C (in support of MIL-STD-188), which, as a work of the United States Government, is in the public domain.

[edit] External links

[edit] Bandwidth conversion

[edit] Bandwidth calculator online

[edit] Bitrates of DVB-S TV and radio channels

  • Linowsat - daily updated audio and video bitrates of European satellites.
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