Bitrate

From Wikipedia, the free encyclopedia

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_bit) is the number of bits that are conveyed or processed per unit of time. Bit rate is often used as synonym to the terms connection speed, transfer rate, channel capacity, maximum throughput and digital bandwidth capacity of a communication system.

In digital multimedia, bitrate is the number of bits used per unit of time to represent a continuous medium such as audio or video after source coding (data compression). In this sense it corresponds to the term digital bandwidth consumption, or goodput.

The bit rate is quantified using the 'bit per second' (bit/s or bps) unit, often in conjunction with a 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).

While often referred to as "speed", bitrate does not measure distance/time but quantity/time, and thus should be distinguished from the "propagation speed" (which depends on the transmission medium and has the usual physical meaning).

Gross bitrate or raw bitrate is the total number of physically transferred bits per second, including both useful payload data and protocol overhead. The net bitrate or useful bit rate is measured at some reference point above the physical layer, and excludes lower layer protocol overhead, for example redundant channel coding (forward error correction).

1 Usage notes
2 Progress
3 Bitrates in multimedia
4 References
5 See also
6 External links

[edit] Usage notes

The formal abbreviation for "bit 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"). Even less formally, it is common to drop the "per second", and simply refer to "a 128 kilobit audio stream" or "a 100 megabit network".

"Bitrate" is sometimes used interchangeably with "baud rate", which is correct 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). Similarly, hertz, the SI unit of frequency, is not precise without some context, such as the number of bits carried per cycle.

For large bitrates, SI prefixes are used:

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

When describing bitrates, binary prefixes are almost never used and SI prefixes are almost always used with the standard, decimal meanings, not the old computer-oriented binary meanings. Binary usage is more often 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

Looking at the development of transmission speeds, Moore's Law may be applied not only to transistor densities, but as well as to transmission speeds: bitrates doubled about every 18 months.

Improvement in applied bitrates :

year	WAN	LAN	WLAN
2005	16 M	1 G	100 M
2000	2 M	100 M	10 M
1995	128 k	10 M	1 M
1990	19 k	1 M
1985	1 k
1970		?

Proposed standards and first devices :

WAN	LAN	WLAN
1972: Acoustic_coupler 300 baud 1985: 1200 baud 1990: increasing Modem speed: 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_802.3 802.3 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	1999: 802.11b 11 Mbit/s 2003: 802.11g 54 Mbit/s 2005: 108 Mbit/s 2007: 802.11n 540 Mbit/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.

Experts and audiophiles may detect artifacts in many cases in which the average listener would not. Some musicians enjoy the distinct artifacts of low bitrate (sub-FM quality) encoding and there is a growing scene of net labels distributing stylized low bitrate music.

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 - Decent quality, difference can sometimes be obvious
192 kbit/s — Good quality; difference can be heard by only a few
224 - 320 kbit/s — High quality, nearly lossless quality

[edit] Other audio

800 bit/s — minimum necessary for recognizable speech (using special-purpose FS-1015 speech codecs)
8 kbit/s — telephone quality (using speech codecs)
500 kbit/s–1 Mbit/s — lossless audio as used in formats such as FLAC, WavPack or Monkey's Audio
1411 kbit/s — PCM (WAV) sound format of Compact Disc Digital 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 Mbit/s — VHS quality
5 Mbit/s — DVD quality
15 Mbit/s — HDTV 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:

Telephone circuits using µlaw or A-law companding (pulse code modulation) — 64 kbit/s
CDs using CDDA — 1.4 Mbit/s

[edit] References

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] See also

[edit] External links

[edit] Bandwidth conversion

Allow easy conversion from kbit/s to MB/h to GB/day to TB/month to ...

[edit] Bandwidth calculator online

VoIP Bandwidth Calculator - Given a codec type and sample period calculate the actual IP and Ethernet bandwidth.
VoIP Bandwidth Calculation White Paper - Companion paper to the above calculator explaining how Voice becomes Voice over IP.
StreamingMarketplace.com( Calculate streaming bandwidth and storage)