Data rates get me confused!

by on 09/11/2009

binary 1s and 0s

Warning – technical blog post ahead!

If you think you are technical (or if you want to test your techie friends and colleagues) ask them how to convert between Kbps and MBps. You may think it’s easy, but do you really know your bits from your bytes and have you spotted the difference between a “k” and a “K” and between a “b” and a “B”?

According to Wikipedia’s article on data rate units, we have k=kilo meaning 1,000 and Ki=kibi meaning 1,024. But it warns that the kibi style prefix is a recent introduction and these “still face low adoption”. Meanwhile, K and k are commonly both used to mean 1,024. So k means both 1,000 and 1,024. Oh dear.

Bit and byte are a little easier. We have b=bit and B=byte, with 1 byte equal to 8 bits. Wikipedia does dryly comment that the abbreviation of bit to b “can lead to confusion”. Indeed. Are you with me so far?

So, the real world example. I’ve just finished writing a consultancy report for a video company, and I’ll admit I was confused. The customer wrote about a streaming rate of 256Kbps and having 110 users viewing a video concurrently. The “K” in Kbps is ambiguous: he could have meant  256 kilobits per second (more properly written 256 kbit/s) or 256 kibibits per second (written 256 Kibit/s). So, with 110 concurrent viewings, that’s either 28,160 kbits or 28,160 Kibits.

He went on to convert this to “very slightly over 3.4 megabytes per second”.  Is this right?

Using kilo we have:

  • 28,160 kilobits = 28,160,000 bits (1,000 bits in a kilobit)
  • = 3,520,000 bytes (8 bits in a byte)
  • = 3.52 megabyte

And with Kibits we have:

  • 28,160 Kibits = 28,835,840 bits (1,024 bits in a Kibit)
  • = 3,604,480 bytes (8 bits in a byte)
  • = 3.60448 megabyte

So yes, we have either 3.5 MB/s or 3.6 MB/s and so he’s perfectly right.

So what, you may say. And largely you would be right, the difference here is negligable.

There might well be cases where the difference was critical. But really, I wrote this as a sort of therapy. I wanted to sort it out in my own mind, and I really can’t believe that we’ve got ourselves in this mess.

I think I understand it now, but I wouldn’t be surprised if I’ve got it wrong, so please do feel free to comment…

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