HP Ultrium tape drives Q1538-90925 - Manual (Page 29)

HP Ultrium drives technical reference manual, volume 5: UNIX configuration guide 29
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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.