Near-line Storage
Near-line storage refers to material that is stored in a manner that is more cost effective than on-line storage, but at the expense of a time delay required to retrieve it. This means, for a Video On Demand system for example, that if a user requests a video title that is not stored within the on-line storage facility the data is transferred from the much larger near-line storage unit to the on-line storage unit, and is then delivered to the user. The delay depends on the type of system used, and can be as little as a matter of minutes for a robotic system.

The near-line storage facility must be significantly cheaper than the on-line storage unit as it is used to archive much more material. Generally less than 10% of material would be stored on-line and the remaining 90% of the collection would be stored near-line.

For such huge storage requirements a jukebox style setup or robotic tape library is usually employed with the data stored on CD ROM, DAT tape, Digital Linear Tape medium (DLT) or other tape format. In the future, the Digital Versatile Disk (DVD) will also be used.

The disadvantages of magnetic tape are

The advantages of magnetic tape are that CD ROMs have the advantage that they are random access devices but the disadvantage is that each CD cannot hold enough material to make it viable. A CD ROM can hold about 640 Mbytes of data, a one hour movie encoded at 2Mbit/s requires 900Mbytes of storage. CDs are also write-once devices which makes them considerably less flexible than tape. Newer technologies will address this problem, however, and read-write DVD-RAMs are supposed to become available in 1998 with capacity of about 3 Gigabytes. Read only DVDs with capacity up to 15.9G are becoming available now.

Table 1 summarises the speed and capacity specifications for various offline data archiving media.

DeviceCapacityData Access SpeedMedia LifetimeWrite once or Write many
DAT DDS24-8 Gbyte510 Kbyte/s10-25 YrsWM
DAT DDS312-24 Gbyte1 Mbyte/s10-25 YrsWM
CD-ROM640 MbyteX times 1.5 Mbits/s to Read10 Yrs PlusWO
CD-RW640 MbyteX times 1.5 Mbits/s to Read10 Yrs PlusWM
Exabyte20-40 Gbyte3-6 Mbyte/s10-25 YrsWM
DLT35 Gbyte5 MByte/s30 YrsWM
DVDup to 15GbyteNot KnownNot KnownWO
DTF42Gbyte12 Mbyte/s10-25 YrsWM
Data D350 Gbyte12 Mbyte/s10-25 YrsWM
DVD-RAMup to 3 GbyteNot KnownNot KnownWM
Magneto-optical2.6-5.6 GbyteNot KnownNot KnownWM
Table 1 Device Parameters