Gross bit rate

In digital communication systems, the physical layer gross bitrate,^[6] raw bitrate,^[7] data signaling rate,^[8] gross data transfer rate^[9] or uncoded transmission rate^[7] (sometimes written as a variable R_b^[6][7] or f_b^[10]) is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead.

In case of serial communications, the gross bit rate is related to the bit transmission time ${\displaystyle T_{b}}$ as:

${\displaystyle R_{b}={1 \over T_{b}},}$

The gross bit rate is related to the symbol rate or modulation rate, which is expressed in bauds or symbols per second. However, the gross bit rate and the baud value are equal only when there are only two levels per symbol, representing 0 and 1, meaning that each symbol of a data transmission system carries exactly one bit of data; for example, this is not the case for modern modulation systems used in modems and LAN equipment.^[11]

For most line codes and modulation methods:

${\displaystyle {\text{Symbol rate}}\leq {\text{Gross bit rate}}}$

More specifically, a line code (or baseband transmission scheme) representing the data using pulse-amplitude modulation with ${\displaystyle 2^{N}}$ different voltage levels, can transfer ${\displaystyle N}$ bits per pulse. A digital modulation method (or passband transmission scheme) using ${\displaystyle 2^{N}}$ different symbols, for example ${\displaystyle 2^{N}}$ amplitudes, phases or frequencies, can transfer ${\displaystyle N}$ bits per symbol. This results in:

${\displaystyle {\text{Gross bit rate}}={\text{Symbol rate}}\times N}$

An 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:

${\displaystyle {\text{Gross bit rate = Symbol rate/2}}}$

A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certain spectral bandwidth in hertz is given by the Nyquist law:

${\displaystyle {\text{Symbol rate}}\leq {\text{Nyquist rate}}=2\times {\text{bandwidth}}}$

In practice this upper bound can only be approached for line coding schemes and for so-called vestigial sideband digital modulation. Most other digital carrier-modulated schemes, for example ASK, PSK, QAM and OFDM, can be characterized as double sideband modulation, resulting in the following relation:

${\displaystyle {\text{Symbol rate}}\leq {\text{Bandwidth}}}$

In case of parallel communication, the gross bit rate is given by

${\displaystyle \sum _{i=1}^{n}{\frac {\log _{2}{M_{i}}}{T_{i}}}}$

where n is the number of parallel channels, M_i is the number of symbols or levels of the modulation in the i-th channel, and T_i is the symbol duration time, expressed in seconds, for the i-th channel.

Information rate

The physical layer net bitrate,^[12] information rate,^[6] useful bit rate,^[13] payload rate,^[14] net data transfer rate,^[9] coded transmission rate,^[7] effective data rate^[7] or wire speed (informal language) of a digital communication channel 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, broadband modem standards and modern copper-based high-speed LANs. The physical layer net bitrate is the datarate measured at a reference point in the interface between the data link layer and physical layer, and may consequently include data link and higher layer overhead.

In modems and wireless systems, link adaptation (automatic adaptation of the data rate and the modulation and/or error coding scheme to the signal quality) is often applied. In that context, the term peak bitrate denotes the net bitrate of the fastest and least robust transmission mode, used for example when the distance is very short between sender and transmitter.^[15] Some operating systems and network equipment may detect the "connection speed"^[16] (informal language) of a network access technology or communication device, implying the current net bit rate. The term line rate in some textbooks is defined as gross bit rate,^[14] in others as net bit rate.

The relationship between the gross bit rate and net bit rate is affected by the FEC code rate according to the following.

Net bit rate ≤ Gross bit rate · code rate

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 bitrate (and thus the "connection speed") of an 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 net bit rate of ISDN2 Basic Rate Interface (2 B-channels + 1 D-channel) of 64+64+16 = 144 kbit/s also refers to the payload data rates, while the D channel signalling rate is 16 kbit/s.

The net bit rate 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 megabaud, 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 as well as 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 megabaud.

The "connection speed" of a V.92 voiceband modem typically refers to the gross bit rate, since there is no additional error-correction code. It can be up to 56,000 bit/s downstream and 48,000 bit/s upstream. 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. Due to data compression, the actual data transmission rate or throughput (see below) may be higher.

The channel capacity, also known as the Shannon 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.

net bit rate ≤ channel capacity

The channel capacity is proportional to the analog bandwidth in hertz. This proportionality is called Hartley's law. Consequently, the net bit rate is sometimes called digital bandwidth capacity in bit/s.

Network throughput

The term throughput, essentially the same thing as 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 above the data link layer. This implies that the throughput often excludes data link 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. See also measuring network throughput.

Goodput (data transfer rate)

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.

As an example, the goodput or 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.

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

goodput ≤ throughput ≤ maximum throughput ≤ net bit rate

for a certain communication path.

Progress trends

These are examples of physical layer net bit rates in proposed communication standard interfaces and devices:

Audio

Video

• 16 kbit/s – videophone quality (minimum necessary for a consumer-acceptable "talking head" picture using various video compression schemes)
• 128–384 kbit/s – business-oriented videoconferencing quality using video compression
• 400 kbit/s YouTube 240p videos (using H.264)^[21]
• 750 kbit/s YouTube 360p videos (using H.264)^[21]
• 1 Mbit/s YouTube 480p videos (using H.264)^[21]
• 1.15 Mbit/s max – VCD quality (using MPEG1 compression)^[22]
• 2.5 Mbit/s YouTube 720p videos (using H.264)^[21]
• 3.5 Mbit/s typ – Standard-definition television quality (with bit-rate reduction from MPEG-2 compression)
• 3.8 Mbit/s YouTube 720p60 (60 FPS) videos (using H.264)^[21]
• 4.5 Mbit/s YouTube 1080p videos (using H.264)^[21]
• 6.8 Mbit/s YouTube 1080p60 (60 FPS) videos (using H.264)^[21]
• 9.8 Mbit/s max – DVD (using MPEG2 compression)^[23]
• 8 to 15 Mbit/s typ – HDTV quality (with bit-rate reduction from MPEG-4 AVC compression)
• 19 Mbit/s approximate – HDV 720p (using MPEG2 compression)^[24]
• 24 Mbit/s max – AVCHD (using MPEG4 AVC compression)^[25]
• 25 Mbit/s approximate – HDV 1080i (using MPEG2 compression)^[24]
• 29.4 Mbit/s max – HD DVD
• 40 Mbit/s max – 1080p Blu-ray Disc (using MPEG2, MPEG4 AVC or VC-1 compression)^[26]
• 250 Mbit/s max – DCP (using JPEG 2000 compression)
• 1.4 Gbit/s – 10-bit 4:4:4 Uncompressed 1080p at 24fps

Notes

For technical reasons (hardware/software protocols, overheads, encoding schemes, etc.) the actual bit rates 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) yield 64 kbit/s.