Definitions

Bit_rate

Bit rate

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).

Bit rates at various protocol layers

Gross bit rate and net bit rate at the physical layer

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.

The net bitrate or useful bit 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) and other channel coding. 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 connection speed 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 speeds 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.

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 connection speed of the Ethernet 100Base-TX physical layer standard is 100 Mbit/s, which also is its gross bit rate.

The connection speed of maximum 56000 bit/s in the downlink of a V.92 modem refers to the gross bit rate, while the modem data transfer rate (the throughput or average useful bit rate, also affected by the modem data link layer protocol) sometimes can be higher higher due to data compression, and sometimes lower due to bit-errors and retransmissions.

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 point-to-point communication channel.

Channel capacity ≥ Net bit rate

Network throughput and goodput

The term throughput or digital bandwidth consumption denotes the achieved average 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.

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 byte), divided by the file transfer time (in seconds), and multiplied by eight.

Net bit rate ≥ Maximum throughput ≥ Throughput ≥ Goodput

for a certain communication path.

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 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

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).

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.

Progress trends

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 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
  • 1997: 802.11 2 Mbit/s
  • 1999: 802.11b 11 Mbit/s
  • 1999: 802.11a 54 Mbit/s
  • 2003: 802.11g 54 Mbit/s
  • 2005: 802.11g (proprietary) 108 Mbit/s
  • 2007: 802.11n 540 Mbit/s
  • 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:

    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.
    • 224–320 kbit/s — Near CD quality.

    Other audio

    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

    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:

    See also

    References

    Maximum PC - Do Higher MP3 Bit Rates Pay Off?

    External links

    Bandwidth conversion

    Bandwidth calculator online

    Bitrates of DVB-S TV and radio channels

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

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