Specifications

ATAPI For Streaming Tape QIC-157 Rev B
31
5. Streaming Tape Device Model
Streaming Tape Devices optimize their use in storing or retrieving user data sequentially. Since access
is sequential over a long medium, position changes typically take a long time, when compared to
direct-access Devices with a short medium.
The recording medium is tape cartridges or cassettes of various lengths of a flexible substrate coated
with a semi-permanent magnetic material. The recording medium is encapsulated into cartridges
containing both a supply peel and a take-up reel. Several American National Standards and QIC
(Quarter-Inch Cartridge) standards exist covering the construction of cassettes and cartridges for
interchange as well as recording techniques for many of the formats or density combinations.
A complete unit composed of the recording medium and its physical carrier is called a volume.
Volumes have an attribute of being mounted or demounted on a suitable transport mechanism.
Mounted is the state of a volume when the Device is physically capable of executing commands that
cause the medium to be moved. A volume is demounted when it is being loaded, threaded, unloaded,
unthreaded, or when not attached to the Device.
Ready is the state of the Device when medium access and non-medium access commands can be
executed. The Device is not ready when no volume is mounted or, from the Host's perspective,
whenever all medium access commands report “Check Condition” status and a NOT READY Sense
Key. Some Devices may have a separate switch function that places the Device in a not ready state
even when a volume is mounted.
The write enabled or write protected state determines when the Host may write information on a
volume. This attribute is usually controlled by the user of the volume through manual intervention
(e.g., thumbwheel switch).
5.1. Medium Attributes
The recording medium has two physical attributes called beginning-of-medium (BOM) and end-of-
medium (EOM). Beginning-of-medium is at the end of the medium that is attached to the take-up
reel. End-of-medium is at the end of the medium that is attached to the supply reel. In some cases, the
medium is permanently affixed to one or both of the reel hubs.
The entire physical length of medium is not usable for recording data. For most volumes, a length of
the medium is reserved before the beginning-of-medium and after the end-of-medium position. This
is done to provide sufficient tape wraps onto the reel hub(s) and to ensure that recording starts in an
undamaged section of the medium.
The position on the medium where a pattern of recorded signal may be written by one write
component is called a track. A Device may write or read from one or more tracks at a time, depending
on the format.
On a new volume, recording of one or more tracks begins after mounting the volume and moving from
beginning-of-medium toward end-of-medium. The number of tracks written at one time is called a
track group. For recorded volumes, reading in the forward direction follows the same course of tracks
as when writing.
If not all tracks are recorded at the same time, and the Device reverses direction when approaching
end-of-medium and begins writing on remaining tracks, the recording method is called serpentine.
For serpentine Devices that record only one track at a time, each physical track represents one track
group.