up vote -6 down vote favorite
share [g+] share [fb]

I heard ISP's use 10 bits instead of 8 for one byte. Why?

link|improve this question
9  
You heard wrong. – Dour High Arch Sep 9 '11 at 10:08
Because it's two more. – Daniel Beck Sep 9 '11 at 10:38
@Dour When an ISP lists it's plans they do it in bps (Mbps for example). At 10Mbps you don't get a download speed of 1.25MBps but of 1MBps. So no, I did not hear it wrong, they do have this convention. – Chris Sep 9 '11 at 12:07
feedback

closed as not a real question by ChrisF, Daniel Beck, David, Journeyman Geek, Nifle Sep 9 '11 at 16:20

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. See the FAQ for guidance on how to improve it.

1 Answer

up vote 9 down vote accepted

Historically, back in the days of analog modems and serial ports, each byte transferred carried along an additional parity bit (for error detection) and from zero to two stop bits (for maintaining synchronization). This typically resulted in transmitting 9 or 10 bits for each byte of actual data.

That no longer applies these days, as modern networking technology doesn't require parity or stop bits on every byte.

Nonetheless, I have heard of people still using 10 bits per byte when estimating data transfer rates. It's usually justified as an attempt to compensate for ethernet packet collisions, header bytes, etc. (all of which are transferred 8-bit bytes, but don't contain actual application data), but I suspect that the real reason has more to do with it being more convenient to divide by 10 in your head than to divide by 8.

link|improve this answer
Thank you, Dave. – Chris Sep 9 '11 at 12:08
10,000,000 bits per second = 1.19MB/s. But the line has to carry address and control information as well as data. Typical line efficiencies for TCP are around 95%. 95% of 1.19MB/s is 1.13MB/s. So a 10Mbps line would generally yield a peak download rate of around 1.1MB/s. – David Schwartz May 3 at 1:48
feedback

Not the answer you're looking for? Browse other questions tagged or ask your own question.