Magnetic Tape

Magnetic Tape is a secondary sequential storage media. Magnetic tape has also been used for years as an input or storage mediium and remains a choice today for high speed, large volume storage. Its data density is high, and it transfer rate is comparatively fast.The tape itself may be in a large reel or a smal cartidge or cassette. However packaged , the tape is usually similar to the kind used in tape recoder. It's a plastic ribbon that is coated on one side with iron oxide or some other material that can be magnetised. Tiny invisible spots representing data are recorded by electromagnetic pulses on the coated side of the tape.
Magnetic tape has a read head for reading the information stored on the tape,i.e. for transferring data from the tape into main storage . The tape moves past the read head at a speed of 200 inches per second.It is 0.5"wide and may be 50 ft.,100ft.,200ft.,400ft.,600ft.,1200ft.,2400ft.,long. There are 7 or 9 tracks depending upon the tape unit, which run the length of the tape. The data is coded so that the one character is recorded across the 7 or 9 tracks. The capcity of tapes having 9 tracks is 33% more than 7track system of the same length.Tape recording is sequential recording and if 10000th record is to be read, it has to read all 9999 records unnecessarily which is the one of its disadvantages. In case of floppy disk or hadr disk, the records are randomly acessed and time required to acess is less, compared to the tape unit
ADVANTAGE

• Magnetic tapes can be reused .
• Mass data storage in magnetic tape is cheaper.
• Data can be stored for long term retieval purpose.
• Data can be trasferred directiely into hard disk from magnetic tape.
  DISADVANTAGE
• Data cannot beread or written on magnetic tapes without electronics devices.
• Magnetic tapes are sensitive to dust, smoke, magnetic feild, heat, etc and data are damaged or lost.
• Since they are sequential strorage media, the access os data is relatively slow as the device has to read all the data before a point or access.

CD ROM(Compact Disk Read only Memory)

CDROM is also called optical disk. An optical Disk storage system consist of rotating disk, which is coated with thin metal or other material that is highly reflective. Foccusing a laser beam or the surface of the spining disk which does data recording. The laser bam is turned on and off at a varying rate because of which tiny holes or pits are burnt into the metal burnt coating of the the disk alongthe tracks. In order to read the stored dataa less powerful laser beam is focussed on the surface. This beam isstrongly reflcted by the coated surface and weakly reflected by the pits producing pattern of on/off reflection that can be converted into electronic signals.

The storage density of optical disks is enormous, the storage cost is extermly low and the access time is relatively fast. one small inexpensive disk will able to replace 25 to 30 reels of magnetic tapes and any data on the disk can be accessed within a few milliseconds. Hence it is particularly suitable forb the archival storage of vast amount of data. Typical application are image processing, geological survey data, medical publishing indexes, historicalinformation files, etc. A serious short coming of current aviable optical disk system is thatd they are permananent storage device. Data once recorded cannot be erased and hence cannot be reused. The diameter of CDROM was 12 cm, weight 18gm and 1mm thick which has reduced to 51/4" in diameter. CDROMs are popular to store sound motion , video pictures , three dimensional pictures, graphics etc. New modification of CDs are WORM(write once read many), DVD(digital verstiale disc), RW(Read and write).

ADVANTAGE

• It is mass storage media for sound motion video, graphics, etc
• It is cheap storage media.
• The data can be stored for long time(20years)
• easy to exchange in a CD drive.
• It is reliable and economic media.

DISADVANTAGE

• Data access time is more in CDROM than hard disk
• ONce data is stored, they can't be changed.
• Only final refined data are stored in the CDROM.