HP Q153090901 - Manual (Page 187)

Glossary
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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 LTO-DC algorithm can typically achieve a compression ratio of
between 2:1 and 4:1 depending on the nature of the data.
data set A fixed-size block of compressed host information.
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 asynchronoussss, 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 eight-bit bytes
(“narrow”) or sixteen-bit words (“wide”). Paced transfers are always wide.
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.