CSE 228  Additional Note on CD Drive

Multimedia Servers

CD

The CD or Compact Disc was originally invented by Philips, and several different standards have been established along with Sony and others. Some of these include

 

CD has become popular because it is a robust media for distribution. CDs are much more durable and hold a great deal more than traditional floppy disks, yet they are quite inexpensive to make.

How is data stored on a CD? If you look at a CD, data is stored in a spiral pattern that originates from the center of the disk and spirals out toward the outer edge. This is a contrast to magnetic disks that store their information in concentric circles (called tracks). Why are CDs stored this way? CDs are primarily designed to hold continuous media like audio, rather than random access data.

This brings up another interesting difference between CD and magnetic disk. Magnetic disks spin at a Constant Angular Velocity (CAV), which is about 5500 rpm on average. Because the disk is always spinning at the same speed, data read on outer tracks will be read faster than data read on inner tracks. This is because there can be more information in one track near the edge than near the center, but it takes the same amount of time for one rotation to complete for either. CD's need to be continuous, so they use a Constant Linear Velocity (CLV). A 1x CD-ROM drive transfers data at 150 KB/sec. Faster speed CD-ROM drives are merely a multiple of this speed, so 10x = 1.5MB/sec. This means that as the head moves toward the outer edge of the CD, the rotation speed must slow down to keep the transfer at a constant speed. The fact that the drive needs to be constantly changing speeds is what makes CD-ROM drives slower than traditional disks.

Data stored on a CD is on a metallic layer and it looks like this:

This media is read is by reflecting a light beam off this metallic layer. If the light beam is reflected (no transition), the bit is a zero. If the light beam is not directly reflected (a transition), the bit is a 1.

DVD

DVD, Digital Versatile Disc (Originally Digital Video Disc), is basically an improvement on the technology of CDs. How did they improve the technology?

A CD-ROM has a capacity of about 800 MB, of which about 650 is usable for data. In order to increase the capacity, DVD does the following things:

  1. Increases the density of bits on the metallic layer
  2. It uses multiple layers, one on top of another. Using optical tricks, light can penetrate one layer and read data on another rather than just having a single layer.
  3. Double-Sided discs. CDs only write on one side of the disc. DVD can write to both, doubling the capacity of a single-sided disc.

 

DVD does (or will do) all of the above. Currently, DVDs can hold about 4.7GB, and transfer the data at about 10 Mb/sec. It is expected that the next 3 releases of DVD will have a capacity of 8.5GB, 10GB, and 17GB, respectively.

DVD is not an international standard like CD. Instead, there is a DVD consortium consisting of Sony, Philips, and Toshiba. You have to pay them in order to use the DVD standard in your hardware.

Storage Hierarchy

If we consider the idea of a video server once more, how do we arrange different storage technologies in order to provide for the most cost-efficient video storage system? Once again, we can arrange the technologies in a hierarchy as follows:

RAM has the highest bandwidth but also the highest cost per unit of storage. Disk Arrays are in the middle for both cost and performance, and DVD is the slowest but also the least expensive medium.

Optimization

If we want to optimize our storage hierarchy to provide the necessary bandwidth at the least cost, how do we determine the optimal strategy? Just as we did when we considered an optimal arrangement of video storage servers, we will need to assign costs to each of the various media and find the optimal solution based on the number of users our system will need to support.

Let's take an example:

First, assume that a digital MPEG-2 movie will take about 2GB to store and requires 5Mb/s bandwidth to transmit. If we assume that a DVD drive capable of storing 4.7GB and having a bandwidth of 10Mb/s costs about $200, a Magnetic Disk capable of storing 1 GB and delivering a bandwidth of 40 Mb/s costs about $300, and RAM capable of storing 1GB and delivering a bandwidth of 1Gb/s costs about $7,000, we can find our optimal solution by charting the cost function.

The mathematical function that determines the cost, c, for a device to support a certain number of users, n, can be written as follows:

If we plot c vs. n for each of the three devices, we can see the optimal cost:

As you can see, for a small number of users DVD will be the cheapest storage device. For most other cases, disk will be the cheapest. For specific numbers such as about 175-200 users, RAM will be the cheapest.