Downloads

For some reason I keep getting these download questions wrong... can some one help explain to me how to solve the following question:

How long would it take to download a 200KB file using a 128kbps line?

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Webmaster

Since the first is in Kilobytes and the second in kilobits(or baud if you like) you first have to change one of them to match the other. I.e. in your example, 200KB is 1600kb (8 bits in a byte). 1600kb / 128kb is 12.5. So it would theoretically take 12.5 seconds. In reality there will be some overhead.

FijianTribe

I think I see where my math was wrong now. Some one had told me basically to take 200KB and multiply by 1024 then divide by the speed 128kb, which was not making sense nor getting the correct answer.

Webmaster

FijianTribe wrote:

I think I see where my math was wrong now. Some one had told me basically to take 200KB and multiply by 1024 then divide by the speed 128kb, which was not making sense nor getting the correct answer.

Although Kilo usually means 1000, in bits and bytes it's 1024. So multiplying the file size in KB by 1024 will get you the corresponding bytes value. I.e. 200KB = 204800 bytes. If you do that, you have to do the same for the 128kbs (multiply it by 1024) and still multiply the 200KB with 8 to turn it into bits (or divide the 128 by 8 to turn the speed into bytes).

FijianTribe

Makes more sense. Thanks you guys make this site awesome.

FijianTribe

200KB file with a 128kbps ISDN line:
128 Kbps / 8 = 16 KBps
200 KB / 16 KBps = 12.5 seconds

500 MB file with a 1.54Mbps T-1 line:
1.54Mbps / 8 = 0.1925MBps
500 MB / 0.1925MBps = 2597.4 seconds or 43.29 minutes

100 MB file with a 128kbps ISDN line:
128Kbps /8 = 16 KBps #figure out bits to byte converstion
100MB * 1024 = 102400 KB #Convert MBytes to KBytes
102400KB / 16KBps = 6400 seconds #Calculate downlad in seconds
6400 / 60 = 106.66 #Convert Seconds to Minutes and round time.
= 1 hour 46 minutes #convert minutes to minutes and hours

My math isnt exactly like it shows up on Intel's calculator but it is really close. I am not sure if I am doing something wrong or if there are vairables either myself or Intel's calculator differ on...

http://www.intel.com/personal/resources/broadband/calculator.htm

If I am way off please let me know. I also know these are the theories and not the exacts since there are network issues such as lag and overhead, etc.

Webmaster

I don't know why Intel's calc is slightly off from your calculations but your calculation are theoretically correct. Maybe Intel includes some overhead or other vars, but if you encounter an exam question (or need to make an estimate in real life) this will do.