HP Ultrium Tape Drive - Manual (Page 35)

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Glossary
AT&T mode Berkeley and AT&T functional modes differ in “read-only” close functionality. In
AT&T mode, a device close operation will cause the tape to be repositioned just
after next filemark on the tape (the start of the next file).
Berkeley mode Berkeley and AT&T functional modes differ in “read-only” close functionality. In
Berkeley mode the tape position will remain unchanged by a device close
operation.
BOT Beginning Of Tape. The first point on the tape that can be accessed by the drive.
buffered mode A mode of data transfer in write operations that facilitates tape streaming. It is
selected by setting the Buffered Mode Field to 1 in the SCSI MODE SELECT
Parameter List header.
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. This is
basically done by representing strings of bytes with codewords.
In Ultrium drives, the data is compressed using the LTO-DC compression format
which is based on ALDC (licensed from Stac/IBM) with two enhancements. One
limits the increase in size of data that cannot be compressed that ALDC produces.
The other is the use of embedded codewords.
data transfer phase On a SCSI bus, devices put in requests to be able to transfer information. Once a
device is granted its request, it and the target to which it wants to send information
can transfer the data using one of three protocols (assuming both devices support
them): asynchronous, synchronous, and wide.
In asynchronous transfers, the target controls the flow of data. The initiator can only
send data when the target has acknowledged receipt of the previous packet. All
SCSI devices must support asynchronous transfer.
In synchronous data transfer, the initiator and target work in synchronization,
allowing transmission of a packet of data to start before acknowledgment of the
previous transmission.
In wide (16-bit) data transfer, two bytes are transferred at the same time instead of
a single byte.
HP Ultrium drives support asynchronous, synchronous and narrow (8-bit) wide
transfers.