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 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 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.

Glossary

Glossary 187