DAT Technology

Within the tape market there is a wide range of tape technologies; the question for SMB customers is which one to choose. The answer is clear - DDS/DAT, a technology that continues to flourish because it meets the needs of the SMB market for cost-effective, reliable backup better than any other technology available.

DAT was first introduced in 1989 and today it continues to be the unit volume leader in the low-end tape drive market for smaller businesses. Since then, we have seen the DDS/DAT format go through six generations of product, from DDS-1 to DDS-4, DAT 72 and more recently DAT 160. This has created a massive world-wide installed base of over 6 million drives in current operation, and a grand-total of over 18 million DAT drives shipped since its introduction in 1989. With the seventh generation DAT 320 format announced and DAT 320 products under development, the technology continues to keep pace with the performance and capacity needs of smaller business environments.

Unlike AIT, VXA, ADR, SLR, DLT and SDLT, DDS/DAT is based on an open standard format for data interchange. The influential DAT manufacturers group presides over this open standard with verification processes that ensure compliance. With multiple manufacturers developing and shipping DDS/DAT products, customers benefit from increased competition, which leads to better prices, faster development and more choice.

The DAT Manufacturers Group is continuing to invest in DAT technology; the development of the 7th Generation of DAT drive, the DAT 320, is underway with a published roadmap of a further generation. Not only does an investment in DAT technology provide the security of a format with a history of outstanding reliability, the legacy of backward compatibility also provides the potential for future expansion without compromising your investment in the technology.

How DAT Works

DDS format for DAT is a helical scan format where data is written diagonally across the width of the tape. The advantage is that, by allowing subsequent tracks to have different writing angles and to overlap, a much greater capacity can be achieved for a given length of tape because no guard bands are necessary. To achieve this, the rotating drum is tilted at an angle from the vertical.


Tape Azimuth Recording Pattern

The drum has four heads (two write heads and two read heads), located at 90° intervals. For each rotation of the drum, two tracks are written. The read heads verify the data that has been written (recorded) and the drive rewrites if necessary.

Transfer rate on DAT technology is determined partly by the wrap angle (amount of tape wrapped around the rotating helical scan drum) and the speed of rotation of the drum. All generations of DAT drive up to and including DAT72 featured a 90 degree wrap angle, so that the tape was in contact with a quarter of the drum. However the wrap angle of the DAT160 drive is 180 degrees to make more efficient use of the time tape is in contact with the head to increase transfer rate.

On DAT72 drives, write head A is in contact with the drum for 90 degrees, followed by read head A to verify the data is written correctly. The same process is repeated for write head B and read head B. While this is a secure and reliable process it leaves an “idle” time where data is not being written, and in turn this limits the transfer rate. With DAT160 drives, the writing and read verification is taking place for a full 360-degree revolution of the drum, making the whole write and verify process more efficient because there is no idle time.

Regardless of this difference in operation between DAT72 and DAT160, the open standard format continues to ensure backward compatibility between DAT160 and DAT72 and also interchangeability between different manufacturers drives.

Tape media

At the heart of DAT technology is a compact helical scan data cartridge. The media improvements over successive product generations form the basis of the continued capacity improvements for DAT technology with the evolution of capacity and speed closely linked to the continuous improvements in metal particle (MP) coating technology. Prior to MP media most computer tape products were based on standard ferric oxide media which limited recording densities. Today, MP media dominates the digital data storage market with LTO Ultrium, DLT-VS, Super DLT and other popular media formats also relying on MP formulations for reliable, high-density data recording.

In addition to the evolution of MP media, the media used in successive generations of DAT drive has varied in length, and more recently with DAT160, in the width of tape used to deliver the 36x capacity increase over the last decade.

Technology advances between DAT generations

DAT 160 (Sixth generation of DDS)
DAT160 doubles the capacity and performance of DAT72, with a native capacity of 80GB and a native transfer rate of 6.9MB/second. The DAT160 employs the same media as DDS-4, with the higher capacity and transfer rate achieved by using wider tape and a larger media wrap angle around the drum. However, it still retains two generation backward compatibility with DAT72 and DDS-4 media.

DAT 72 (fifth generation of DDS)
DAT 72 was built on the technology foundation of the previous DDS media format to achieve native capacity of 36GB using a new MP++++ formulation. DAT 72's 80% capacity boost over DDS-4 is the result of longer tape (170m rather than 150m), increased MP formulation, greater recording density (from 122 to 162 kbpi) and a higher track density.

DDS-4
DAT technology reached the 20GB per cartridge threshold with DDS-4. Following a familiar model, DDS-4 media included an improved MP media formulation and longer length tape. A higher-output MP formulation, designated MP+++, was developed to handle the higher track densities of the DDS-4 format.

DDS-3
The transition from DDS-2 to DDS-3 brought the largest capacity jump in the history of DAT technology, a 3x improvement over the previous generation. The 12GB native capacity of the DDS-3 format was derived from two key media improvements: a higher-coercivity (1700 oersted vs. 1530 oe) version of the MP+ formulation used in DDS-2 media, and a nominal increase in tape length to 125m. The enhanced magnetic properties of the higher-coercivity MP++ media supported a doubling of the linear bit density to 122 kbpi. DDS-3 also benefited from a significant non-media technology enhancement: the debut of highly efficient Partial Response Maximum Likelihood (PRML) read channel technology, contributing further to increased capacity.

DDS-2
The first major upgrade to DAT technology doubled the capacity from 2GB to 4GB. Media length was increased to 120m and a new higher-coercivity MP+ formulation was introduced. To reach the 120-meter target, a thinner tape was required. DDS-2 media was reduced to 6.9 um thickness, compared to 9.0 um for DDS-1.

DDS (or DDS-1)
The first implementation of the DDS-1 format used 60-meter tapes for a 1.3GB capacity. Capacity was upgraded to 2GB by extending the tape length to 90m, with no other changes in the tape formulation or the recording format.