KiloBytes-per-sec or kilobits-per-sec?

Posted on January 29, 2010

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What Dhiragu and other service providers didn’t explain to there customers.

Modem speeds are rated in kilobits per second. The best dial-up modems have download speeds of 56 kbps (kilobits per second). A typical cable modem has a download speed of 512 kbps. It takes 8 bits to code one byte (character), so one might expect 512 kbps to be equivalent to 64 kBytes per second. Things are not that simple.

First, when used in measuring communication speeds in bits per second, the prefix kilo- means 1000. For instance, the maximum download speed of a 56 kbps dial-up modem is 56000 bps. A cable modem download cap of 512 kbps means 512000 bps, not 512*1024 bps. In contrast, when used in measuring computer file sizes in bytes, the prefix kilo- is commonly taken to mean 1024 = 210. So you cannot translate from kbps to kBytes per second just by dividing by 8.

Next, the maximum amount of real user data that can be accommodated in one data packet is 1460 bytes. Then there are 20 bytes of TCP overhead, plus 20 bytes of IP overhead, plus 18 bytes of MAC overhead, making 1518 bytes to be transmitted to carry 1460 bytes, a 4% overhead. Then between each packet there will be an inter-packet gap of indeterminate size.

So at a true transmission rate of 512 kbps, the apparent user data rate will be significantly below 64 kBytes per sec, because both the above effects will reduce the equivalent rate in kBytes per sec. The maximum sustained data download rate you can expect is 512 * 1000/1024 * 1460/1518 * 1/8 = 60 kBytes/sec, and not 64 as a simple division by 8 might suggest.

With NTL stand-alone cable modems, the download cap for the silver service is in fact 600 kbps. The maximum data rate you can expect from this is 600 * 1000/1024 * 1460/1518 * 1/8 = 70 kBytes/sec, and not 75 as a simple division by 8 might suggest.

Speed tests measure real data rates in kBytes per second, because that is all they can do. Any figure they give for equivalent transmission speed in kbits per second will be an estimate calculated from the measured data rate in kBytes per second.

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