IBM 990 - Manual (Page 269)

Glossary 257
local area network (LAN). A computer network
located in a user’s premises within a limited
geographic area.
Logical Channel Subsystem (LCSS). A defined subset
of the CPC hardware (subchannels, channels, and I/O
interfaces) that is used to support the operation of a
Logical Channel Subsystem. The LCSS relieves the
processor of direct I/O communication tasks, and
performs path management functions. Uses a
collection of subchannels (defined to the LCSS) to
direct a channel to control the flow of information
between its defined I/O devices and main storage.
logical control unit (LCU) . A separately addressable
control unit function within a physical control unit.
Usually a physical control unit that supports several
LCUs. For ESCON, the maximum number of LCUs that
can be in a control unit (and addressed from the same
ESCON fiber link) is 16; they are addressed from x’0’
to x’F’.
logical partition (LPAR). A set of functions that create
a programming environment that is defined by the
ESA/390 architecture. ESA/390 architecture uses this
term when more than one LPAR is established on a
processor. An LPAR is conceptually similar to a virtual
machine environment, except that the LPAR is a
function of the processor. Also, LPAR does not depend
on an operating system to create the virtual machine
environment.
logical switch number (LSN). A two-digit number
used by the I/O Configuration Program (IOCP) to
identify a specific ESCON Director.
logically partitioned (LPAR) mode. A central
processor mode, available on the Configuration frame
when using the PR/SM facility, that allows an operator
to allocate processor hardware resources among
logical partitions. Contrast with basic mode.
LPAR. See logical partition.
megabyte (MB). (1) For processor storage, real and
virtual storage, and channel volume, 2 20 or 1 048 576
bytes. (2) For disk storage capacity and
communications volumes, 1 000 000 bytes.
MT-RJ. An optical fibre cable duplex connector that
terminates both jumper cable fibres into one housing
and provides physical keying for attachment to an
MT-RJ duplex receptacle. For technical details, see the
NCITS - American National Standard for Information
Technology - Fibre Channel Standards document FC-PI.
multi-mode optical fiber. A graded-index or
step-index optical fiber that allows more than one
bound mode to propagate. Contrast with single-mode
optical fiber
.
Multiple Image Facility (EMIF). In the ESA/390
architecture and z/Architecture, a function that allows
LPARs to share a channel path by providing each
LPAR with its own set of subchannels for accessing a
common device.
National Committee for Information Technology
Standards
. NCITS develops national standards and its
technical experts participate on behalf of the United
States in the international standards activities of
ISO/IEC JTC 1, information technology.
NCITS. See National Committee for Information
Technology Standards.
ND. See node descriptor.
NED. See node-element descriptor.
node descriptor. In an ESCON and FICON
environment, a node descriptor (ND) is a 32-byte field
that describes a node, channel, ESCON Director port,
FICON Director port, or a control unit.
node-element descriptor. In an ESCON and FICON
environment, a node-element descriptor (NED) is a
32-byte field that describes a node element, such as a
DASD (Disk) device.
OEMI. See original equipment manufacturers
information.
open system. A system whose characteristics comply
with standards made available throughout the industry
and that therefore can be connected to other systems
complying with the same standards.
optical cable assembly. An optical cable that is
connector-terminated. Generally, an optical cable that
has been terminated by a manufacturer and is ready
for installation. See also jumper cable and optical
cable
.
optical cable. A fiber, multiple fibers, or a fiber bundle
in a structure built to meet optical, mechanical, and
environmental specifications. See also jumper cable,
optical cable assembly
, and trunk cable.
optical fiber connector. A hardware component that
transfers optical power between two optical fibers or
bundles and is designed to be repeatedly connected
and disconnected.
optical fiber. Any filament made of dialectic materials
that guides light, regardless of its ability to send
signals. See also fiber optics and optical waveguide.
optical waveguide. (1) A structure capable of guiding
optical power. (2) In optical communications, generally
a fiber designed to transmit optical signals. See optical
fiber.