A VoIP signaling protocol, which is the ITU version of MEGACO.
Also see MEGACO.

An ITU-T recommendation which consists of a set of signaling and media format specifications for multimedia communications over packet networks. The most prominent application of H.323 is in IP Telephony (VoIP).
Also see VoIP.

Unauthorized person who tries to gain access to a computer, a server, or a network for fun or illegal purpose. Hacker also refers to a network security expert, a high profile profession in a security company.

Communication in both directions (receive and transmit), but only in one direction at a time.

High bit rate Digital Subscriber Line. Symmetric DSL service providing data rate of 2,048 bps (E1) over three pairs of copper wires or 1,544 bps (T1) over two pairs of copper wires. HDSL is defined in ITU-T G.991.1.
Also see DSL. Compare with SHDSL.

High Definition TV. A television system that has higher resolution than conventional formats (i.e. NTSC, SECAM, PAL). HDTV broadcast requires broader frequency spectrum.

A numeral system with 16 as its base. It is represented by 16 digits, namely 0 - 9 and A - F. For example 20 in decimal system is represented by 14 (read: "one four", NOT "fourteen") in hexadecimal system. Conversely, 2F in hexadecimal equals 2x16¹+15x16⁰ or 47 in decimal system. Each digit position is equivalent with 16 raised to the power of its order from the rightmost position. The count starts at zero.
MAC address and IPv6 address are written in hexadecimal format. Many programming languages also prefers hexadecimal format. This is because 1 byte (or 8 bits) can be shortened to 2 hexadecimal digits, thus makes the writing shorter.
Also see binary.

High Frequency. Frequency spectrum in the range of 3 MHz - 30 MHz, corresponding to wavelength from 100 m to 10 m.  A.k.a. shortwave. HF transmission can reach distant place on earth through reflections by the ionosphere, hence it is often used in terrestrial radio communication.

Hybrid Fiber Coax. Refers to a CATV network which consists of fiber optic infrastructure from the CATV operator to a neighborhood hub and coaxial cables from the hub down to subscribers' homes.
Also see CATV.

A powerline network standard for connecting computers and devices in the home that was developed by HomePlug Powerline Alliance. HomePlug Powerline Alliance is an industry association that was formed in March 2000 to promote home powerline network standards and interoperability among products from multiple vendors.
HomePlug standards can be classified into in-the-home and to-the-home technologies.
HomePlug 1.0 and HomePlug AV are in-the-home technologies. HomePlug 1.0 supports data rate of up to 14 Mbps. HomePlug AV (audio video) standard expands the powerline network to connect consumer electronics, such as TV, stereo, and other audio video equipment. It will provide raw data rate of up to 200 Mbps at physical layer or around 100 Mbps effective rate, comparable to Fast Ethernet.
HomePlug BPL (Broadband over Power Line) is to-the-home technology that brings access to the Internet over power (electricity) network.
Also see Powerline Network.

Home Phoneline Networking Alliance. An industry association that was founded in June 1998 to promote a unified standard in home phoneline network and interoperability among phoneline network products.
There have been several versions of HomePNA standards. HomePNA 1.0, 1.1, 2.0, and 3.0. HomePNA 1.0 supports data rate of 1 Mbps. HomePNA 2.0 that can reach data rate up to 32 Mbps is supported by most phoneline networking products now. While the newest standard HomePNA 3.0 has maximum data rate of 128 Mbps and can be extended to 240 Mbps.
Also see Phoneline Network.

Wireless LAN technology using RF as its physical medium that supports data as well as voice. HomeRF was promoted by HomeRF WG (Working Group) that was founded in March 1998. HomeRF has been abandoned due to the growing popularity of Wi-Fi.

A computer or a device that provides control for other devices and hosts applications used by other devices.

Refers to an area under wireless LAN or Wi-Fi coverage.
Also see Wi-Fi.

Handheld PC.

A network component that acts as a central connection point in a star configuration. A passive hub functions as a junction box that relays signals it received from a sending node to all other nodes. An active hub acts as a repeater that amplifies the incoming signals and transmits stronger signals to all other nodes. Hub operates at Layer 1 of the OSI model.
Also see OSI model.

High Speed Circuit Switched Data. Data transmission over GSM network using circuit-switching technology that employs multiple time slots to carry the data. Its maximum data rate is 57.6 kbps.
Compare with GPRS and EDGE.

High Speed Downlink Packet Access. An enhancement to 3G (UMTS-WCDMA) network that gives peak shared downlink data rate of up to 14 Mbps (i.e. 7 times that of 3G) in a 5 MHz FDD channel and at least two-fold increase in network capacity. HSDPA replicates wireless LAN and fixed-line broadband experience to mobile users.
The performance improvement is achieved by shifting scheduling and retransmission functions from the Radio Network Controller (RNC) down to the Base Station (BTS) thus closer to the air interface and the user, adding a high-speed downlink shared channel (HS-DSCH), and applying adaptive modulation and coding (AMC).
HSDPA is standardized in 3GPP Release 5.
Also see 3G.

High Speed Packet Access. The combination of HSDPA and HSUPA.

High Speed Uplink Packet Access. An further enhancement to 3G (UMTS-WCDMA) network after HSDPA that promises peak shared uplink data rate of up to 5.8 Mbps. HSDPA enhanced with HSUPA will allow symmetrical services to run over 3G cellular network. While HSDPA has been rolled out in a few 3G networks, HSUPA is currently in trial phase.
HSDPA is standardized in 3GPP Release 6.

Hyper Text Transfer Protocol. An Application Layer protocol that functions to request Web pages to a Web server. HTTP is the most visible protocol to users because it is typed in a browser address every time a user visits a Website.
Also see Application Layer.

Hertz. A frequency unit which is equivalent with 1 cycle per second.
Also see frequency.