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flash memory. A data storage device that is
programmable, erasable, and does not require
continuous power. The chief advantage of flash memory
over other programmable and erasable data storage
devices is that it can be reprogrammed without being
removed from the circuit board.
flow control. (1) In SNA, the process of managing the
rate at which data traffic passes between components
of the network. The purpose of flow control is to
optimize the rate of flow of message units with minimum
congestion in the network; that is, to neither overflow
the buffers at the receiver or at intermediate routing
nodes, nor leave the receiver waiting for more message
units. (2) See also
pacing
.
fragment. See
fragmentation
.
fragmentation. (1) The process of dividing a datagram
into smaller parts, or fragments, to match the
capabilities of the physical medium over which it is to be
transmitted. (2) See also
segmenting
.
frame. (1) In Open Systems Interconnection
architecture, a data structure pertaining to a particular
area of knowledge and consisting of slots that can
accept the values of specific attributes and from which
inferences can be drawn by appropriate procedural
attachments. (T) (2) The unit of transmission in some
local area networks, including the IBM Token-Ring
Network. It includes delimiters, control characters,
information, and checking characters. (3) In SDLC, the
vehicle for every command, every response, and all
information that is transmitted using SDLC procedures.
frame level. Synonymous with
data link level
. See
link
level
.
frame relay. (1) An interface standard describing the
boundary between a user’s equipment and a fast-packet
network. In frame-relay systems, flawed frames are
discarded; recovery comes end-to-end rather than
hop-by-hop. (2) A technique derived from the integrated
services digital network (ISDN) D channel standard. It
assumes that connections are reliable and dispenses
with the overhead of error detection and control within
the network.
front-end processor. A processor such as the IBM
3745 or 3174, that relieves a main frame from the
communication control tasks.
G
gateway. (1) A functional unit that interconnects two
computer networks with different network architectures.
A gateway connects networks or systems of different
architectures. A bridge interconnects networks or
systems with the same or similar architectures. (T)
(2) In the IBM Token-Ring Network, a device and its
associated software that connect a local area network to
another local area network or a host that uses different
logical link protocols. (3) In TCP/IP, synonym for
router
.
general data stream (GDS). The data stream used for
conversations in LU 6.2 sessions.
general data stream (GDS) variable. A type of RU
substructure that is preceded by an identifier and a
length field and includes either application data, user
control data, or SNA-defined control data.
H
header. (1) System-defined control information that
precedes user data. (2) The portion of a message that
contains control information for the message such as
one or more destination fields, name of the originating
station, input sequence number, character string
indicating the type of message, and priority level for the
message.
heap memory. The amount of RAM used to
dynamically allocate data structures.
Hello. A protocol used by a group of cooperating,
trusting routers to allow them to discover minimal delay
routes.
hello message. (1) A message sent periodically to
establish and test reachability between routers or
between routers and hosts. (2) In the Internet suite of
protocols, a message defined by the Hello protocol as
an Interior Gateway Protocol (IGP).
heuristic. Pertaining to exploratory methods of
problem solving in which solutions are discovered by
evaluation of the progress made toward the final result.
high-level data link control (HDLC). In data
communication, the use of a specified series of bits to
control data links in accordance with the International
Standards for HDLC: ISO 3309 Frame Structure and
ISO 4335 Elements of Procedures.
high-performance routing (HPR). An addition to the
Advanced Peer-to-Peer Networking (APPN) architecture
that enhances data routing performance and reliability,
especially when using high-speed links.
hop. (1) In APPN, a portion of a route that has no
intermediate nodes. It consists of only a single
transmission group connecting adjacent nodes. (2) To
the routing layer, the logical distance between two
nodes in a network.
hop count. (1) A metric or measure of distance
between two points. (2) In Internet communications, the
number of routers that a datagram passes through on
its way to its destination. (3) In SNA, a measure of the
number of links to be traversed in a path to a
destination.
Glossary 699