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The transmission speed generally is measured by transferring a specific block of data of known size across the transmission medium and timing the duration between initiating transfer and successful receipt of the final datum. For instance, to test the speed of a modem connection, you could create a known file of, say, exactly 1,000,000 bytes, then initiate a transfer of that file across the modem link. If the transfer takes 7 minutes 25 seconds, you have a data rate of 1,000,000 bytes per 7:25, or 2,247 bytes per second.

Data transmission speeds are usually given in bits per second, or bytes per second. For fast connections, this is sometimes expressed as kilobits, megabits, or gigabits per second, or as kilobytes, megabytes, or gigabytes per second. One must be very certain to indicate whether a data rate is counting bits or bytes, and one must be aware that marketers will try to conceal that difference; a marketing data rate may be expressed in bits per second but made to look like bytes, or it may be expressed as the theoretical maximum data rate, rather than the actual data rate. An example of the latter is the 802.11g wireless networking standard, which is always shown as 54 megabits per second. That is the raw data rate; once signaling overhead, error correction, link control, and other non-data information is excluded, the actual rate for data transfer in a perfect connection is closer to 33 megabits per second.

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14y ago

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