Gross bit rate In digital communication systems, the
physical layer gross bitrate,
raw bitrate,
data signaling rate,
gross data transfer rate or
uncoded transmission rate) 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 T_\text{b} as: : R_\text{b} = {1 \over T_\text{b}}, The gross bit rate is related to the
symbol rate or modulation rate, which is expressed in
baud 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; this is not the case for modern modulation systems used in
modems and LAN equipment. For most
line codes and
modulation methods: : \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 2^N different voltage levels, can transfer N bits per pulse. A
digital modulation method (or
passband transmission scheme) using 2^N different symbols, for example 2^N amplitudes, phases or frequencies, can transfer N bits per symbol. This results in: : \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: : \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: : \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: : \text{symbol rate} \leq \text{bandwidth} In case of
parallel communication, the gross bit rate is given by : \sum_{i = 1}^{n} \frac{\log_2 {M_i} }{T_i} where
n is the number of parallel channels,
Mi is the number of symbols or levels of the
modulation in the
ith
channel, and
Ti is the
symbol duration time, expressed in seconds, for the
ith channel.
Information rate The
physical layer net bitrate,
information rate,
payload rate,
net data transfer rate, Some operating systems and network equipment may detect the "
connection speed" (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, 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/s, 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 modulationslower 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: == Multimedia ==