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 don’t notice.

Hands-free - A feature that’s included with most of today’s 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 chips.

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.

Harmonic Distortion - A problem of the nonlinearities in a communication channel causing the harmonics of the input frequencies to appear in the output channel.

Harmonics - 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 cable splices.

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 nodes.

HF (High Frequency) - Any of the radio frequencies in the band between 3 and 30 MHz.

Hierarchically Complete 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 Code).

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 car’s horn if a call comes in, alerting the recipient standing nearby.

Host - In a network, the primary or controlling computer in a multiple computer installation.

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 Cut.

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 design.

Hz (Hertz) - Unit of frequency (formerly cycles per second).