These are terms used in computer technology, referring to a number of data transfers (or operations).
They are most commonly used for measuring the number of transfers per second (GT/s or MT/s). 1 GT/s means 109 or one billion transfers per second, while 1 MT/s is 106 or one million transfers per second.
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 200 MT/s, because there are 100 million rising edges per second and 100 million falling edges per second of a clock signal running at 100 MHz.
Megatransfer is most commonly associated with SCSI (Small Computer Systems Interface), whereas Gigatransfer is usually associated with the variable width links PCI Express and HyperTransport.
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