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).
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.
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.
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.
for a certain communication path.
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).
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.
Proposed standards and first devices :
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: