Glossary
algorithm | rigorous set of rules for a procedure. In the context of data compression, the rules |
| are for transforming the way data is represented. |
ANSI | American National Standards Institute, which sets standards for, amongst other |
| things, SCSI and the safety of electrical devices. |
asynchronous | see data transfer phase |
BOM | Beginning Of Media. The first point on the tape that can be accessed by the drive. |
checksum | The sum of a series of bytes written to the tape, which can be checked against the |
| sum of the same series of bytes when the tape is read in order to identify errors. |
compression | A procedure in which data is transformed by the removal of redundant information |
| in order to reduce the number of bits required to represent the data. |
compression ratio | A measure of how much compression has occurred, defined as the ratio of the |
| amount of uncompressed data to the amount of compressed data into which it is |
| transformed. The |
| between 2:1 and 4:1 depending on the nature of the data. |
data set | A |
data transfer phase | After accepting a command from the host (initiator), the tape drive (target) sets the |
| SCSI bus to transfer the data associated with the command. Information is |
| transferred in one of three ways, depending on the negotiated settings between |
| initiator and target. The simplest (and slowest) is asynchronous, where the target |
| requests the transfer of each byte or word, then must wait for the initiator to signal |
| that the byte or word has been transferred before it starts the next byte transfer. |
| Synchronous transfers allow the target to request many transfers at once, up to a |
| limit agreed between initiator and target at startup. Only once the “credit” has |
| been used up, does the target have to pause and wait for the initiator to signal |
| successful transfer of the words. Ultra320 SCSI introduces a faster still method |
| called paced transfers; these are similar in concept to synchronous transfers. |
| Asynchronous and synchronous transfers may take place either with |
| (“narrow”) or |
decompression | A procedure in which the original data is generated from compressed data. |
DSIT | Data Set Information Table. Part of the data set that describes its contents. |
Glossary
Glossary 187