These terms alone do not specify the
bit rate at which binary data are being transferred because they do not specify the number of bits transferred in each transfer operation (known as the channel width or
word length). In order to calculate the data transmission rate, one must multiply the transfer rate by the information channel width. For example, a data bus eight-bytes wide (64 bits) by definition transfers eight bytes in each transfer operation; at a transfer rate of , the data rate would be 8 billion bytes per second, i.e. , or approximately . The
bit rate for this example is (8 × 8 billion bits per second). The formula for a data transfer rate is:
Channel width (bits/transfer) × transfers/second = bits/second. Expanding the width of a channel, for example that between a CPU and a
northbridge, increases data
throughput without requiring an increase in the channel's operating frequency (measured in transfers per second). This is analogous to increasing throughput by increasing bandwidth but leaving
latency unchanged. The units usually refer to the "effective" number of transfers, or transfers perceived from "outside" of a system or component, as opposed to the internal speed or rate of the clock of the system. One example is a
computer bus running at
double data rate where data is transferred on both the rising and falling edge of the clock signal. If its internal clock runs at 100 MHz, then the effective rate is , because there are 100 million rising edges per second and 100 million falling edges per second of a clock signal running at 100 MHz. Buses like
SCSI and
PCI fall in the megatransfer range of data transfer rate, while newer bus architectures like the
PCI-X,
PCI Express,
Ultra Path, and
HyperTransport operate at gigatransfer rates. The choice of the symbol
T for
transfer conflicts with the
International System of Units, in which T is the symbol for the
tesla, a unit of
magnetic flux density, so
megatesla per second would be a reasonable unit to describe the rate of a rapidly changing magnetic field, such as in a
pulsed field magnet or
kicker magnet. ==See also==