Comparing Data Rates and Bandwidth

1/1/98
J. L. Mohler
 
Three common terms that usually come up when discussing the delivery of multimedia and hypermedia are date rate, transfer rate and bandwidth. These terms are often confused and it would do us good to discuss them a little.

Data rate and transfer rate are synonymous and describe the amount of data, in bytes, that can be transferred from one device to another. For example, CD-ROM drives, hard drives, removable drives all have a maximum amount of data the can be read off or written to the device. Generally data rate describes the amount of time required to copy or display a file. Data rate usually refers to the speed of specific hardware devices inside a machine and how long it takes to perform a certain operation, such as displaying a graphic or copying a file. 

Frequently, hypermedia developers speak of data rate in terms of the data being passed over their Internet connection. When you are speaking of data coming from the web it is generically called bandwidth and is measured in bits rather than bytes. Bandwidth refers to the amount of bits being passed over a network cable in a certain number of seconds.

Often times people get confused when talking about bandwidth versus data. To help clear things up, take a look at the table below to see a comparison of common devices and their respective data rates and bandwidth comparisons.
 
Connection  Data Rate  Bandwidth  Time per 100K
14.4 modem  1.8 KB/s  14.4 kb/s  55 seconds
28.8 modem  3.6 KB/s  28.8 kb/s 27 seconds
33.6 modem  4.2 KB/s  33.6 kb/s  23 seconds
ISDN  7-16 KB/s  56-128 kb/s 14-6 seconds
Frame Relay  7-64 KB/s  56-512 kb/s 14-1.5 seconds
T1  32-193 KB/s 256-1,544 kb/s 3.1-.5 seconds
1X CD  150 KB/s 1.2 mb/s .66 seconds
Digital Subscriber Line 188-375 KB/p 1.5- 3 mb/s  .53 seconds
2X CD  200 KB/s  1.6 mb/s  .50 seconds
4X CD  450 KB/s  3.6 mb/s  .02 seconds
10X CD  1.2 MB/s  9.6 mb/s  .08 seconds
Fast Ethernet  1.25 MB/s  10 mb/s  .08 seconds
16X CD  2.4 MB/s  19.2 mb/s  .04 seconds
24X CD  3.6 MB/s  28.8 mb/s  .02 seconds
T3  5.5 MBs  44 mb/s  .018 seconds
ADSL  10 MB/s  80 mb/s  .01 seconds
Universal Serial 
Bus (USB) 
 		 12 MB/s  96 mb/s  .0083 seconds 
IEEE 1394 
(Firewire) 
 		 100-400 MB/s  800-3.2 gbps  .001-.00025 seconds 
Notes:  
Bandwidth - the speed at which data travels or is passed across a network, measured in bits per second 

Data rate - the speed at which data travels or is passed among internal computer components, such as hard drives, CD-ROM, and other devices; measured in bytes per second. 

Remember that 8 bits equals 1 byte. To calculate between date rate and bandwidth you must either divide or multiple by 8, depending which way you are calculating.
 
Converting between Data Rate and Bandwidth

When you are attempting to determine the speed of a particular connection or device, you must ascertain whether the information given you is in terms of data rate or bandwidth. If the figures are given to you as kilobytes per second (KBs) or megabytes per second (MBs), then you are talking about data rate. If the information is given to you in kilobits per second (kbps) or megabits per second (mbps), then you are talking about bandwidth.

Doing the conversion between data rate and bandwidth is really pretty easy as long as you remember that eight bits equal one byte. Eight is the important number. To perform the conversion:

  1. Determine whether you are given a data rate or bandwidth number. If it is KBs then it is data rate. If it is kbps then it is bandwidth.
  2. Next, if you are given a data rate, multiply the number by 8 to obtain the bandwidth in bits per second. If you are given a bandwidth, divide the number by 8 to obtain the data rate.
Finally, if you want to determine the amount of time it will take for a certain amount of data to be downloaded on the device you must know two things: the amount of data you want to push, and the data rate of the device. 

For example, let’s say I want to figure out the minimum amount of time it will take to download 1.35 megabytes of information using a 14.4 kbps modem.  I know that a 14.4 modem has a data rate of 1.8 kilobytes per second (KBs) and that 1.35 megabytes of information is 1382.4 kilobytes. Dividing 1382.4 kilobytes by 1.8 kilobytes per second yields a download time of 768 seconds or 12.8 minutes. Realize that this is the shortest amount of time required – other client or server-side variables may increase this download time.