Telecom Dictionary

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Half Duplex
Half duplex is a transmission path capable of transmitting signals in both directions, but only in 
one direction at a time.

HDLC 
high-level data link control

Header
The header is control information appended to a segment of user data for control, synchronization, routing and sequencing of a transmitted data packet or frame.

Hertz (Hz)
Hertz is the measurement that distinguishes electromagnetic waveform energy; number of 
cycles, or complete waves, that pass a reference point per second; measurement of frequency, 
by which one hertz equals one cycle per second.

Horizontal Wiring
In a premises distribution system, horizontal wiring is the connection between the 
telecommunications outlet in work areas and the telecommunications closet.

Hub
In local area networks (LANs), a hub is a wiring concentrator equipment used in hierarchical star 
physical wiring topologies. Those directly connected to terminals or other user devices are of ten 
referred to as local hubs or concentrators. Central hubs are those at the highest hierarchical 
level. Hubs often provide the means for interconnecting 10BaseT, coaxial or fiber optic cable LAN 
segments. Intelligent hubs may implement multiport bridging and network management 
functions.

HVAC 
heating, ventilation and air conditioning