On-Line Encyclopedia - H

The Maui Communications Networks
On-Line Encyclopedia

Alphabetical List Of Definitions For Technical Terms

H
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H.264
Also referred to as MPEG-4, it is a common format used for compression of high definition video. The standard is divided into "profiles", each with its own goals for compression (i.e. target bit-rate, image quality, color quality, etc.). An "encoder" takes a video feed (HDMI, VGA, Composite, etc.) and converts it into data bits. A "decoder" reverses the process and outputs HDMI, VGA or some other video type. Encoding and decoding is a software process in conjunction with analog/digital conversion. The software may run in a PC or in a dedicated hardware device. Decoders (and encoders) do not necessarily support all H.264 profiles and so attention must be paid when specifying equipment.

H.323
H.323 is a protocol initially used to set up, manage, and tear down Voice-over-IP telephone calls. It is a circuit management protocol designed in the context of SS7 and the PSTN. The SIP protocol performs the same functions but can also be used in a more general-purpose manner. H.323 predates SIP by a few years and SIP is currently the evolving protocol of choice for managing VoIP calls. A detailed technical discussion comparing H.323 and SIP can be referenced for more information about these two signaling methods

Hayes Modem AT Command Set
In 1977, Dennis Hayes developed (what was then) a revolutionary way of controlling a serial modem using a set of in-band commands. The Hayes SmartModem would issue an "attention" prefix (the ASCII letters "AT") followed by a command string. The commands allowed manipulation of the modem's configuration as well as its dialing and answering behavior. These became known as the "AT Commands" and, over the years, became the de facto standard in the modem marketplace. Today, most modems respond to at least the basic commands and some have extended the command set for vendor-specific purposes. Consult your modem's user manual for specific details.

Occasionally a contemporary engineer encounters a situation where a knowledge of the basic AT command set is crucial. For example, this applies when using an RS-232 Telnet connection to configure an Aruba, Cisco, or other vendor's wireless LAN switch controller with Windows' HyperTerminal. You've connected your serial cable but you don't get an opening screen. Are you connected? By typing "ATZ" you can find out! A complete list of Hayes Modem commands can be found in Wikipedia or by a web search.

High Frequency (HF)

HF sinals are generally considered to be those in the range from 3 MHz to 30 MHz. This includes the frequency bands used by cellular telephony, paging systems, and wireless medical telemetry systems (WMTS).
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