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The Linear Advantage.

Linear tape recording technology has been used for computer data storage since the early 1970s. First, the magnetic tape functioned as the primary mass storage device for the computers systems, but as hard disks, with their random access capabilities, became more available, the magnetic tape replaced them as first line storage device. However, for storing huge amounts of sequential data, e.g., seismological test data, the magnetic tape is still the preferred storage medium, unrivalled when it come to cost and capacity.

Another area where the magnetic tape and linear recording has been able to keep pace with the market requirements is for a backup storage device. Measured by capacity, performance, reliability, and price, linear tape recording will, through new technology advances, strengthen its position and continue to be the leading technology for the coming years.


First, why is there even a requirement for backup of data in the first place? While it is true that the basic computer hardware continues to become more reliable, all of the traditional reasons for backing up data continue to be valid into the next century.

The computer in the late 1990s is now used for control/conduct of business at all levels in a corporation, from the receiving dock to the CEO's office. Data today is more valuable than ever and it continues to grow Focus is changing from the computer and its software to the data and how this is processed, stored, and retrieved. Data corruption or loss can occur because of many factors, including hardware and software failures, natural disasters such as floods or earthquakes, fire, theft, vandalism, human error, intentional erasure/corruption, and employee malice. Backing up your data insures that the original data, user applications, and user set-ups can be restored in a timely and efficient manner. This allows businesses that practice good backup procedures to be back online in hours or days, instead of weeks or months. The reason why magnetic tape has come into prominence for computer backup is simple; it provides the lowest cost and highest per unit storage capacity in a removable medium. Tape can also be easily incorporated into robotic handling systems for automated access to large amounts of data. Of course, low cost would not be attractive without reliability. Fortunately, tape storage is very reliable with high levels of ECC (Error Correction Code), robust media, and high Mean Time Between Failure (MTBF) numbers.


In general, data stored on tape and archived under proper environmental storage conditions can be expected to be recovered 20 years or more into the future. Although 20 years does not sound like a long time, consider that 20 years ago very few of us had access to a computer. None of us had a computer on our desktop and 5MB was considered a huge amount of storage

Why is linear tape the correct choice to secure the future? The answer lies in the recording technology itself and how it is applied, compared to its main competition, Helical Scan.

Linear tape recording is a technology that can efficiently cover a broad requirement range. Travan cartridge linear drives are designed for low consumer cost but, with relatively high storage capacity, SLR series drives are designed for broad compatibility and scalability for the entry-level market and upwards. DLT drives have the necessary capacity and performance to make them the defacto tape standard for midrange storage.


* Product range covers growth market

* TRAVAN, SLR, and DLT Product classes yield the right price/performance for each market segment


* In comparison to other tape technologies (helical) linear technology has advantages in large available media areas and simplicity of design, which lead to:

* Lower cost

* Greater data integrity

* Better product futures

As we move on, first, we must investigate the trends in the tape storage market. Table 1 shows the estimated volumes broken down into four capacity segments.

The 4-9GB segment is declining steadily, moving forward; this is due to the general capacity increase of hard disks and increased pressure in the consumer market from removable storage such as Jaz, DVD, etc. Tape drives were never really a mainstream product for the desktop user and the attraction of adding a removable medium device such as DVD that can also play movies, etc. has sealed the fate of general purpose tape drives on the desktop.

Tandberg Data has always focused its business at the professional user segment, which in 1999 starts using tape products in the 10-19GB range. The professional user understands the need for backup and the backup strategy demands a device that can provide a low per GB storage cost in a removable medium. Here, DVD cannot compete.

The 10GB and above segments are all projected to grow, entering the next century. Tandberg Data further believes that the migration to higher per tape cartridge storage capacities will occur faster then currently forecasted by analysts.

With the dramatic growth anticipated in the tape storage market, there are already a number of brands/technologies competing. The fig shows the various available tape technologies and the relative market segments that they address. It also includes some new or planned brands entering the market in 1999. This further illustrates the faith in the growth of the business.

Starting at the top of the fig in the desktop segment is the Travan family of tape drives with capacities up to 10GB (20GB compressed). It should be noted that the tape products in this segment face stiff competition from other removable technologies as previously mentioned. Fortunately, Travan with its NS series of products are positioned towards cost sensitive professionals.

The next product range is the SLR series or Scalable Linear Recording. These products range from the very low end 525MB up to 25GB native. SLR drives are designed for professional backup requirements on entry level and small midrange systems.

Next are the DLT drives, designed to cover the midrange of the market with 35GB and 5MB/sec transfer rate native capability. At the Enterprise storage level, we have product from IBM and StorageTek. These drives tend to be specialized for either access speed or data transfer rate. Below that, we have the DDS range of products with the main products being the DDS2 and DDS3 products, Exabyte 8mm products from 7 to 20GB native, and the Sony AIT range.

Below the dotted line are planned new product families from various companies: the Onstream (8 channel linear serpentine), Benchmark, Ultrium, and Accelis (LTO)--all being based on linear technology. The Ecrix drive is based on Helical recording with overscan technology.

There are two things from the Fig that should be noted:

1. Travan, SLR, and DLT can cover all needs in the professional segments.

2. Of all the new tape storage product families being developed, only one is based on helical scan technology. At least four new product families are being developed around linear recording.


Both linear and helical product families have announced future capacity and performance extensions. Table 2 notes how both capacity and performance can be increased in magnetic tape recording products. The left-hand column shows the technology approach and the right hand side shows the benefits derived from the technology extension. In a removable media device, both the media and the drive can be changed to provide positive benefits.

Adding additional surface area by extending the length or making the tape wider or a combination of both can enhance the media's storage capability. Here, Linear tape has an advantage in that the media carriers for Travan, SLR, and DLT media are larger than DAT, 8mm, or AIT products.

Of course, additional performance and capacity can be gained by:

* Increasing bit density and decreasing track width

* Increasing linear density by advanced encoding, allowing increased bit density without increasing the magnetic recorded density on the tape

* Increasing the number of data channels to provide higher transfer rates for the same tape speed and linear density

* Increasing tape speed

* Adding a tracking servo system, allowing reduced track spacing and width

* Adding data compression

Helical and Linear tape technologies can benefit equally from the various technology advancements, but Helical has already had to develop each technology further to provide today's capacities due to its limited tape area. Besides relative recording density; another consideration is recording technique. Here is where the real differences lie between Travan, SLR, DLT linear products, and DAT, 8mm, AIT helical products.

The rotating head used to record helical tracks requires the tape to be pulled out and guided around the head. This requires additional guides and motors compared to linear devices. The lower mechanical parts count and the fixed tape guides in the drive (DLT) or cartridge (Travan and SLR) contribute to the lower drive cost and higher field reliability of linear technology.

The linear technology cartridges themselves have been designed and built specifically for the purpose of storing large amounts of data and they are built from high durability polycarbonate plastic with a solid aluminium base plate. This compared to helical scan technology cartridges, which are based on cartridges originally developed for use in mass consumer applications such as DAT audio recorders and video cameras.

The topic of areal density needs to be examined closely. The important thing with Table 3 is comparative relationships. You can easily see that the linear devices have more area per cartridge compared to helical. In fact linear devices generally have a 2:1 to 4:1 advantage in media area. This is important; the tighter the data is packed, the tougher it is to read (less future also). The capacity points shown are for the flagship capacities in each technology class.

Probably more important is. the areal density or the ratio of how tightly the data is packed on the media to achieve a particular storage capacity--this is the number in the bottom row.You can see that the DDS-3 drive with its small media surface area and 12GB capacity is packing the data at 16MB per square inch. Compare that to linear DLT with over 10,000 square inches of media area. Even at 35GB, the DLT is only packing the data at 3.27MB per square inch.

The next item we need to examine is the actual track density and track width. Remember, narrower tracks and more of them lead to additional storage capacity.

Here again, Linear has room to grow. Table 4 shows the number of tracks expressed in TPI or tracks per Inch--this is done to compare relative track pitch densities. All technologies record data at different track widths. By comparing the number of tracks that would be realized at one inch of width for each technology, we can compare the relative density. The mid row data points correspond to the tracks per inch. You will note that Linear track densities are all below 700TPI, while Helical track densities are all above 2000TPI.

If we move to the bottom row and examine the data points, we can see the actual written track width. Here, you can see that the helical devices all have track widths of 18 microns or less, while the helical devices all have track widths of 40 microns or more. The top row shows the native storage capacity at the various TPI and track width for each technology.

In talking about the technology, we also need to address the linear or data density recorded along the track in the direction of recording. For linear, this is lengthwise on the tape. For Helical, this is diagonal across the tape. Table 5 shows the linear density and type of data encoding utilized (which also has an effect on density).The tape speed noted has an effect on transfer rate, as well. What you can note from this table is, although helical drives move the tape in the cartridge relatively slowly, the magnetic head is spinning at 2000rpm (revolutions per minute) or more. This makes the relative head to tape speed (the apparent speed between the head contact and tape) much higher.

Also of note is that all of the linear drives, even with their high capacity and performance, have not yet implemented advanced encoding (PRML = Partial Response Maximum Likelihood). In fact, Travan and DLT drives do not even require a full time servo in today's flagship products.

Finally, the number of parallel data channels has an effect on transfer rate. Here, linear recording has a big advantage because the linear scans much easier, allowing parallel data channels compared to helical technology. Linear can have 2, 4, and even 8 channels without major technical obstacles. It is very difficult to implement parallel channel recording on helical devices and high parallel channel counts are impossible in helical devices.


So having examined today's situation, how will the next step products make it? In Table 6, you can see the AIT going from 25 to 50GB, the 8mm going from 20 to 60GB, and the DDS going from 12 to 20GB.

Compare the advances for helical scan with DLT going from 35 to 100GB, SLR going from 25 to 50GB, and Travan 10 to 18GB and the technology necessary to implement that and you will find that the linear products are pushing the technology less. That is to say, the next generation linear products at higher capacity and transfer rates are implementing areal densities and technologies already in today's helical products.

To state it simply, Linear has more leverage to provide increased transfer rates and higher capacity than helical technologies. System level products such as autoloaders and libraries are available for each Linear technology discussed, further extending the flexibility.


* Most new tape entries are linear based.

* Existing linear products today have a solid foundation to build on.

* All linear product families can be extended by systems products--automation, tape arrays, Fibre Channel to SCSI bridge, NAS, SAN, etc.

* Further advancements will bring:

* Higher transfer rates (72 - 144GB per hour)

* Higher capacities per cartridges (800 + GB)

* Significantly faster access time ([less than]10s)

* Low audible noise

Runar Angelsen is the product manager of media at Tandberg Data ASA (Oslo, Norway).

This article is based on the 1999 ICC-show presentation from Greg Ormsbee, Tandberg Data ASA.
 Market Trends - Tape Drives. Unit sales forecast
 per capacity segments. Units volume in thousands.
Capacity segment 1997 1998 1999 2000 2001 2002
30GB+ 10 196 323 442 584 713
20GB-29GB 250 287 370 401 420 440
10GB-19GB 250 662 1288 1629 1872 1930
4GB-9GB 2003 1496 1365 1064 724 544
Source: Dataquest and Tandberg Data, June 1998
 Density and Migration - Technology Enablers for increased
 capacity and performance - related to tape drives
 Implementation Product enhancements
 Technology Approach Higher Faster Higher
 Storage Backup/ Reliability
 Capacity Restore
 Media Drive
 Longer Tape *
 Wider Tape *
 Higher Coercivity * * *
 Higher Linear Bit Density * *
 Narrower Tracks/Stripes *
 Advanced Encoding /PRML/VR2 * *
 Increased Number of Data * *
Increased Relative Head to Tape *
 Track positioning Servo * *
 Data Compression * *
 Technology Approach Lower
 Cost Per
 Media Drive
 Longer Tape *
 Wider Tape *
 Higher Coercivity *
 Higher Linear Bit Density *
 Narrower Tracks/Stripes *
 Advanced Encoding /PRML/VR2 *
 Increased Number of Data
Increased Relative Head to Tape
 Track positioning Servo *
 Data Compression *
 Recording area and areal density
 Linear Helical
[In.sup.2] of Media Area 2797 5670 10680 744 2108 2108
Storage Capacity in GB 10 25 35 12 20 25
Areal Density in MB/[In.sup.2] 3.57 4.5 3.27 16 9.84 11.8
 Track density and track width
 Linear Helical
Storage Capacity in GB 10 25 35 12 20 25
Tracks/Stripes per inch 343 576 416 2804 2208 2310
Written Track Width in 70 40 43 10 16 14
 Linear density and no. of channels by technology
 Linear Helical
Capacity in GB
Native 10 25 35 12 20 25
Transfer Rate GB per
Hour Native 3.6 7.2 18 4.3 10.8 10.8
Linear Density BPI 102,000 101,600 86,000 122,000 77,000 116,000
# of Write Channels 1 2 4 2 2 2
Dynamic Tracking
Servo System No Yes No Yes Yes Yes
Relative Head to
Tape speed IPS 110 120 120 237 150 ?
 Next step tape backup products
Technology Model, capacity, transfer rate
 Current Next model
Helical Mammoth EXB-M, 20 GB, EXB-M2, 60 GB,
 9.9 GB/hr. 43.4 GB/hr.
 AIT AIT1, 25 GB, AIT2, 50 GB,
 10.8 GB/hr. 306 GB/hr.
 DDS DDS3, 12 GB, DDS4, 20 GB,
 4.32 GB/hr. 5.4 GB/hr.
Linear DLT DLT7000, 35GB, SDLT, 100 GB,
 15.4 GB/hr. 36.1 GB/hr.
 SLR SLR50, 25GB, SLR100, 50 GB,
 6.6 GB/hr. 18.1 GB/hr
 Travan NS20, 10 GB, NS36, 18GB,
 3.6 GB/hr. 6 GB/hr.
Technology Performance Gain Technology
 Capacity Transfer
 GB rate
Helical 40 33.5 Advanced encoding, more
 channals, more tape area
 25 19.8 Higher linear density,
 more tape area
 8 1.1 More tracks, more tape
Linear 65 20.7 Advanced encoding, more
 channels, servo
 25 11.5 Advanced encoding, more
 8 2.4 Advanced
COPYRIGHT 1999 West World Productions, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1999, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Technology Information
Publication:Computer Technology Review
Date:Oct 1, 1999
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