Half-Wave Rectifier -
A rectifier that delivers direct current output during alternate half-waves
only of the incoming AC signal input.
Handoff - The
process by which the Mobile Telephone Switching Office passes a cellular phone
conversation from one radio frequency in one cell to another radio frequency in
another. It is performed so quickly that callers donít notice.
Hands-free - A
feature thatís included with most of todayís car phones. It permits the driver
to use his cellular car phone without lifting or holding the handset to his
ear. An important safety feature.
Handshaking - The
electrical exchange of predetermined signals when a connection is made between
two modems or other devices carrying data.
Hardware - The
actual physical computing machinery, as opposed to software, which is the list
of instructions to operate the hardware, or the firmware, which is combination
hardware/software that is burned into a Programmable Read Only Memory chip or
Hardwire - (1) To
connect equipment or components permanently, in contrast to using switches,
plugs, or connectors; and (2) the wiring-in of fixed logic or read-only storage
that cannot be altered by program changes.
- A problem of the nonlinearities in a communication channel causing the
harmonics of the input frequencies to appear in the output channel.
Additional frequencies, multiples of the fundamental, appearing in the output
waveform of an alternator. (Reducing harmonics to tolerable working limits is
an element of good design practice.)
HDB3 (High Density Bipolar)
- A bipolar coding method that does not allow more than 3 consecutive zeroes.
HDLC (High Level Data
Link Control) - An ITU-TSS link layer protocol standard for
point-to-point and multi-point communications.
Header - Protocol
control information located at the beginning of a protocol data unit.
Heat Shrinkable Sleeve
- Sleeve of polyethylene placed around a plastic cable splice. Application of
heat by a hot air blower shrinks the sleeve and makes a tight fit around the
cable on each side of the splice.
Heat Shrinkable Tubing
- Tubing which shrinks to about half its original diameter when heat (such as
from a hot air blower) is applied. Sometimes used to provide oversheaths for
Heat Sink - Device
which conducts heat away from a heat-producing component so that it stays
within its safe working temperature range.
HEC (Header Error Control)
- A CRC code located in the last byte of an ATM cell header that is used for
checking cell header integrity only.
Hello Packet - A
type of PNNI Routing packet that is exchanged between neighboring logical
HF (High Frequency)
- Any of the radio frequencies in the band between 3 and 30 MHz.
Source Routing - A stack of DTLs representing a route across a PNNI
routing domain such that a DTL is included for each hierarchical level between
and including the current level and the lowest visible level in which the
source and destination are reachable.
High Capacity Service
- Tariffed, digital-data transmission service equal to, or in excess of, T1
data rates (1,544 Mbit/s).
High Tone - Tone of
480 Hz at -17dBm.
High Voltage - A
voltage higher than that used for power distribution. The lower limit is
usually taken as either 5000V (Bell) or 8700V (National Electrical Safety
High Voltage Alarm -
An alarm indicating that the DC voltage being fed to the switching equipment
(normally 50V) has risen above a preset warning level.
HIPPI (High Performance
Parallel Interface) - ANSI standard that extends the computer bus over
fairly short distances at speeds of 800 and 1600 Mbps.
Hit - (1) A
transient disturbance to a communication medium; and (2) a comparison of two
items of data that occurs when specified conditions are satisfied.
Hop by Hop Route -
A route that is created by having each switch along the path use its own
routing knowledge to determine the next hop of the route, with the expectation
that all switches will choose consistent hops such that the call will reach the
desired destination. PNNI does not use hop-by-hop routing.
Horizontal Link - A
link between two logical nodes that belong to the same peer group.
Horn Alert - A
feature that automatically blows the carís horn if a call comes in, alerting
the recipient standing nearby.
Host - In a
network, the primary or controlling computer in a multiple computer
Hot - Live wire. A
conductor carrying a signal is said to be a hot conductor, i.e. the wire
carrying the signal as opposed to a ground wire.
Hot Cut - The
conversion from an old to a new phone system that occurs instantly as one is
removed from the circuit and the other is brought in. Also known as a Flash
Hot Standby -
Backup equipment kept turned on and running in case some equipment fails
Hot Swappable -
Components that can be removed and replaced while the switch is in operation.
Housing - A
removable protective cover, particularly for external plant equipment.
HPUX - The
Hewlett-Packard version of UNIX.
HSSI (High-Speed Serial
Interface) - A serial communications connection that operates at
speeds of up to 1.544 Mbps.
Hub - A device that
connects several other devices, usually in a star topology.
HV1 (High Voltage) -
A discrete lead designation which is connected to report a high voltage
condition at the plant batteries. The voltage level reported is at the minimum
limit and is usually considered a minor alarm. The alarm output can be grounded
or looped and can be from a make or break contact, depending upon the model of
the power plant.
HV2 (High Voltage) -
A discrete lead designation which will be connected under a maximum high
voltage plant battery situation and is considered a major alarm. Grounded or
looped connections from make or break contacts with respect to the plant
Hz (Hertz) - Unit
of frequency (formerly cycles per second).