Compaq ProLiant 1000 DDS-3 Enhancing DAT Reliability, Capacity, and Performanc - Page 5

7

%

(cont’d)

ECG036/1297

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

ADVANTAGES OF DDS-3

In comparison to DDS-2, DDS-3 technology provides significant improvements in capacity,

transfer rate, and reliability.

When considering all the new features, the drive price, the media

price, and the overall reliability, it is easy to see DDS-3 also offers a low total cost of

ownership.

Increased Capacity

Although DDS-3 cassettes use almost the same length of tape as existing DDS-2 cassettes,

DDS-3 has three times the native capacity (12 GB vs. 4 GB).

Factors contributing to this

increased capacity include:

•

Increased linear bit density

•

Increased format efficiency

•

Increased use of track space

DDS-3 was originally intended to have a native capacity of 8 GB, twice that of DDS-2.

However, the development team discovered that the linear bit density could be increased more

than they originally thought.

In fact, the density was doubled from 61,000 bits per inch (bpi)

to 122,000 bpi.

While this doubling of the linear bit density does not result in double the

capacity, this increase does significantly impact the capacity of DDS-3.

The capacity of a tape is almost doubled with data compression, depending on the nature of the

data being compressed.

Typically, text can be compressed at a 2:1 ratio, while graphic files

can be compressed at a 5:1 ratio.

However, a file that is already compressed will not compress

further during the backup.

DDS-3 also has increased format efficiency over DDS-2 due to

Partial Response Maximum

Likelihood

(PRML).

PRML is a technique used to differentiate a valid signal from random

noise.

Noise results in data errors.

Because DDS-3 has PRML, it incurs fewer data errors due

to random noise.

In multiple ways, DDS-3 technology increases the amount of track space that can be used to

store data.

First, DDS-3 uses the

timed tracking

servo format instead of the e

mbedded servo

format used in DDS-1 and DDS-2.

Servo tracking is a scheme used to regulate positioning and seeking operations in disk and tape

drives.

This scheme relies on positioning data recorded on the media surface to generate

correction signals for the servomechanism.

A system with an “embedded” servo, as in the

Compaq 2/4 GB DAT and 4/8 GB DAT drives, uses the

automatic track following

(ATF)

method to ensure the tape head is in the center of the track being read.

There are ATF areas

embedded at the beginning and end of each track; user data is recorded between them.

The

read and write heads are wider than a single track, so the head reads the ATF areas from the

two tracks on either side, as well as that of the track the head intends to read.

By comparing

the content and strength of the signals from the ATF areas, the servo system adjusts the drum

and tape and allows the head to pass directly over the centerline of the track being read (Figure

4). The ATF areas of the track cannot be used to store data.

Compaq ProLiant 1000 DDS-3 Enhancing DAT Reliability, Capacity, and Performanc - Page 5

Advantages Of Dds-3

Page 5 highlights