H channel The ISDN
packet switched channel on Basic Rate Interface, designed to carry
user information streams at different speeds, depending on type:
H11=1536Kbit/s, H0=384Kbit/s and H12= 1920Kbit/s.
H.261 A ITU-TS standard
for video compression know as Codes for Audiovisual Services at
N x 384Kbit/s. It sets a common algorithm for converting analog
video signals to digital, operating at or above 384Kbit/s.
Half Duplex A two-way
means of transmission, but data can only travel in one direction
at a time.
Half-bridge Apple
Computer terminology for a device linking LANs over a low-speed
link such as a telephone line or X.25 link. It is termed a half-
bridge as one is required at each end of the link.
Handshake Part of
the procedure to set up a datacommunications link. The handshake
can be part of the protocol itself or an introductory process:
the computers wishing to talk to each other set out the conditions
they can operate under. Sometimes, the handshake is just a warning
that a communication is imminent.
Head-end A central
point or hub in broadband networks that receives signals on one
set frequency band and retransmits them or another. Every transmission
has to go through the head-end in a broadband network. The head-end
is the piece of hardware that enables a network to send and receive
on the same piece of cable. In CATV technology, the head-end is
the control center for a cable system where signals are processed
and sent for distribution down the cable system.
Header The control
information added to the beginning of a transmitted message. This
may consist of packet or block address, destination, message number
and routing instructions.
Hertz A measure of
radio frequency. One Hz=one cycle per second. KHz = 1,000Hz; MHz
=1,000,000Hz; GHz=1,000,000,000Hz.
HDLC (High level Data Link Control) An
ISO standard set of protocols for carrying data over a link with
error and flow control. Similar to IBM's SDLC, versions of HDLC
are under development for multipoint lines. The ITU-TS later adapted
HDLC for its Link Access Protocol used for X.25 networks. It is
a bit-oriented data link control procedure under which all data
transfer takes place in frames. Each frame ends with a frame check
sequence for error detection. There is a control field at the
start of each frame that allows error detection, and data link
setup and data link termination. HDLC is, in fact, a misnomer,
as it is not a high level protocol.
HLMS (Heterogeneous LAN Management Specification)
Network management specification developed
by IBM and 3Com. It provides an underlying structure for the development
of network management products which can function with a variety
of network operating systems and adapter cards.
Heterogeneous networking The
networking of computers from different vendors, or the running
of different operating systems.
Hierarchical network A
network with one host at its hub, which is the major processing
center, and one or more satellite processing units. IBM's SNA
was originally designed as an hierarchical network.
Host processor A minicomputer
or mainframe attached to a network providing services to network
users.
Hot fixing The ability
to detect and mark bad sectors of a disk, then assign alternate
disk sectors during routine LAN operation. This automatically
updates the original defect map.
Hot swap The ability
of a device to have parts removed after, for example, a slot-in
card or fan failure, without affecting its operation. Many of
today's hubs offer hot swapping without bringing down the network
to charge a component.
HPFS (High-Performance File System) An
OS/2 file system that has faster input/output than the file allocation
table file system; it does not restrict file naming to eight characters
with a three character extension; and is compatible with the FAT
file system.
HSSI (High speed Synchronous Serial Interface)
Interface for transferring data to or
from a WAN leased line (E2, E3), or to and from a LAN via an HSSI-capable
DSU and HSSI router.
Hub The center of
a star topology network or cabling system. A multi-node network
topology that has a central multiplexer with many nodes feeding
into and through the multiplexer or hub. The other nodes do not
usually directly interconnect. LAN hubs are increasingly popular
with the growth of structured cabling and the need for LAN management.
Brouter or Hybrid Bridge/Router Network
interconnection device with a standard mode of operation as a
router. It can support multiple OSI Layer Three routing protocols,
but also supports bridging capabilities for network traffic that
cannot be routed - non-routable traffic.
