When purchasing or recommending a CD-ROM drive, you need to consider two values. The first is data transfer rate. The long-time standard for transfer rate has been 150 KB per second, and this is the basis for measuring CD-ROM drives today. A 2X CD-ROM drive operates at 300 KB per second, a 4X at 600 KB per second, and so on. A typical CD-ROM drive today will operate at 24X or 32X (4.8 MB per second) or faster. A hard disk drive typically operates between 800 KB and 1.8 MB per second.
The second value to look at is the drive's mean access time. This is the time it takes the head to move over half the tracks. Typical access time is 200 to 400 milliseconds (ms). Today's CD-ROM drives can have faster data transfer speeds than many hard drives, but their mean access time is 20 or so times slower. This means that while a CD-ROM drive will outperform the hard disk drive for copying or loading a large chunk of contiguous data, it will be beaten by the hard drive on random access tasks.
Although the transfer rate increases in multiples, the mean access time does not. The following table lists transfer rates and access speeds for some common CD-ROM drives.
|CD-ROM Speed||Transfer Rate||Access Speed|
|4X||600 KB per second||220 ms|
|6X||900 KB per second||145 ms|
|8X||1200 KB per second||100 ms|
|12X||1800 KB per second||125 ms|
|16X||2.4 MB per second||100 ms|
|24X||3.6 MB per second||95 ms|