Bit rate
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 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 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).
In digital communication systems, the gross bitrate, raw bitrate, data signaling rate or line 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.
The net bitrate, useful bit rate or data transfer rate of a digital communication link is the capacity excluding the physical layer protocol overhead, typically redundant forward error correction and other channel coding. The relationship between the gross bit rate and net bit rate is affected by the forward error correction 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 typically refers to the physical layer net bit rate in accordance with the above definition. For example, the bit rate of 100 Mbit/s of an Ethernet 100Base-TX physical layer, the downlink bit rate of 56000 bit/s of a V.92 modem, the bit rate 64000 bit/s of one ISDN B channel, and the bit rate of between 6 and 54 Mbit/s of a 802.11a wireless network, all refer to the net bit rate.
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 point-to-point communication channel.
- Channel capacity ≥ Net bit rate
The term throughput or digital bandwidth consumption denotes the achieved 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.
Goodput refers to the achieved 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 byte), divided by the file transfer time (in seconds), and multiplied by eight.
- Net bit rate ≥ Maximum throughput ≥ Throughput ≥ Goodput
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 byte 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.
- Required goodput ≥ Goodput
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[edit] Usage notes
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"). 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".
Gross bit rate 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).
While often referred to as "speed", bitrate does not measure distance/time but quantity/time, and should be distinguished from the "propagation speed" (which depends on the transmission medium and has the usual physical meaning).
[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 :
| WAN | LAN | WLAN |
|---|---|---|
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[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)
- 192 kbit/s — DAB (Digital Audio Broadcasting) quality. Quickly becoming the new 'standard' bitrate for MP3 music; difference can be heard by few people.
- 224–320 kbit/s — Near CD quality. Sound is nearly indistinguishable from most CDs.
[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 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
- 1.25 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:
- Telephone circuits using µlaw or A-law companding (pulse code modulation) — 64 kbit/s
- CDs using CDDA — 1.4 Mbit/s
[edit] See also
- average bitrate
- available bitrate
- bandwidth
- Baud (symbol rate)
- clock rate
- code rate
- constant bitrate
- data signaling rate
- data transfer rate
- goodput
- line rate
- list of device bandwidths
- measuring network throughput
- spectral efficiency
- throughput
- variable bitrate
[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.
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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2008) |
[edit] External links
[edit] Bandwidth conversion
- CaMoPyRo's Experiments, easy conversion from kbit/s to MB/h to GB/day to TB/month, etc.
[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.
- DVD-HQ bitrate calculator Calculate bitrate for various types of digital video media.
[edit] Bitrates of DVB-S TV and radio channels
- Linowsat - daily updated audio and video bitrates of European satellites.
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