High-Speed Channel

Full-duplex ISDN primary rate channel operating at 384 Kbps.

Suite of ITU-T standard specifications for videoconferencing over circuit-switched media such as ISDN, fractional T-1, and switched-56 lines.

Extension of ITU-T standard H.320 that enables videoconferencing over LANs and other packet-switched networks, as well as video over the Internet.

Registration, Admission, and Status

The RAS signalling protocol performs registration, admissions, bandwidth changes, and status and disengage procedures between the VoIP gateway and the gatekeeper.

In switched-384 data service, a circuit consisting of 6 B channels, or 384 Kbps.

High-Availability Cluster Multiprocessing

Telephony term that means to send a call back in the direction that it came from. For example, if a call cannot be routed over IP to a gateway that is closer to the target telephone, the call is typically sent back out the local zone, back the way it came from.

A transmission mode which permits transmission in both directions, but only one direction at a time.

A call in progress is automatically passed from one cell or zone to another without dropping the call as the user moves.

Sequence of messages exchanged between two or more network devices to ensure transmission synchronization.

The switch over from digital CDMA to analogue during a call

Line code type used on E1 circuits.

High Bandwidth Networking

The ability to connect nodes and devices over a network that can handle digital services at the rate of 150 Mbps or more.

High-Capacity Digital Services

Service that uses DS1 digital facilities to provide communications channels to customers at 1.544 Mbps.

High Density Bipolar –3-Code

For binary signals

Hardware Description Language

High-Level Data Link Control

A synchronous, bit-oriented Link Layer for data transmission. Frame relay is an example of an HDLC-based packet protocol.

Hand-held Dynamic Mark-up Language

High-bit-rate Digital Subscriber Line

One of four DSL technologies. HDSL delivers 1.544 Mbps of bandwidth each way over two copper twisted pairs. Because HDSL provides T1 speed, telephone companies have been using HDSL to provision local access to T1 services whenever possible. The operating range of HDSL is limited to 12,000 feet (3658.5 meters), so signal repeaters are installed to extend the service. HDSL requires two twisted pairs, so it is deployed primarily for PBX network connections, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks.

High-Definition Television

Expected to be a driving force in broadband technology; of great concern to telecoms carriers, which are counting on being the principal providers of support services for HDTV satellite transmission and for the distribution of signals via fibre-optic cables.

End point of a broadband network. All stations transmit toward the headend; the headend then transmits toward the destination stations.

Control information placed before data when encapsulating that data for network transmission.

Header Error Control

Algorithm for checking and correcting an error in an ATM cell. Using the fifth octet in the ATM cell header, ATM equipment will check for an error and correct the contents of the header. The check character is calculated using a CRC algorithm allowing a single bit error in the header to be corrected or multiple errors to be detected.

High-Energy Physics Network

Research network that originated in the United States, but that has spread to most places involved in high-energy physics. Well-known sites include Argonne National Laboratory, Brookhaven National Laboratory, Lawrence Berkeley Laboratory, and the SLAC.

Network consisting of dissimilar devices that run dissimilar protocols and in many cases support dissimilar functions or applications.

Hybrid Fibre-Coaxial

Technology being developed by the cable TV industry to provide two-way, high-speed data access to the home using a combination of fibre optics and traditional coaxial cable.

See HCDS

Scheme of addressing that uses a logical hierarchy to determine location. For example, IP addresses consist of network numbers, subnet numbers, and host numbers, which IP routing algorithms use to route the packet to the appropriate location.

The complex problem of routing on large networks can be simplified by reducing the size of the networks. This is accomplished by breaking a network into a hierarchy of networks, where each level is responsible for its own routing.

High-Performance Parallel Interface

High-performance interface standard defined by ANSI. HIPPI is typically used to connect supercomputers to peripherals and other devices.

Home Location Register

A permanent database used in mobile systems to identify subscribers and to contain subscriber data related to features and services.

Conformity of a product or specification to international standards, such as ITU-T, CSA, TUV, UL, or VCCI. Enables portability across company and international boundaries.

Short on-hook period usually generated by a telephone-like device during a call to indicate that the telephone is attempting to perform a dial-tone recall from a PBX. Hookflash is often used to perform call transfer.

The transmission of a data packet between two network nodes - for example between two routers.

In data networking, measurement of the distance between a source and destination. It is used to compare route length. If a packet passes through six routers between source and destination nodes, the hop count for the packet will be six when it arrives at its destination.

Point at which a call transitions from H.323 to non-H.323, typically at a gateway.

Computer system on a network. Similar to node, except that host usually implies a computer system, whereas node generally applies to any networked system, including access servers and routers.

Part of an IP address that designates which node on the sub-network is being addressed. Also called a "host address."

Hybrid Private/Public Networking

The creation of a network using both private leased lines and public switched facilities (digital dial-up bandwidth). The goals of combining both networking technologies are increased performance and flexibility at reduced cost.

High Performance Routing

Second-generation routing algorithm for APPN. HPR provides a connectionless layer with non-disruptive routing of sessions around link failures, and a connection-oriented layer with end-to-end flow control, error control, and sequencing.

High Speed Circuit Switched Data

An enhancement to the GSM mobile communications system that enables up to four 14.4 Kbps channels to be combined to provide 57.6 Kbps data transfer. Part of GSM Phase 2, HSCSD is suited for videoconferencing and multimedia transmission.

High-Speed Serial Interface

A serial interface that operates at speeds up to 52Mbps at distances up to 50 feet. It is similar to, but faster than, RS-232 and V.35 serial interfaces.

HyperText Markup Language

The authoring language used to create hypertext documents for the World Wide Web. Like the Standard Generalized Markup Language (SGML), on which it is based, HTML identifies the types of information in a document rather than the exact way it is to be presented. The presentation is left to the software that converts the contents to a suitable format for viewing.

HyperText Transport Protocol

The application protocol for moving hypertext files across the Internet. This protocol requires an HTTP client program on one end of a connection and an HTTP server on the other.

Generally, a term used to describe a device that serves as the centre of a star-topology network. (2) Hardware or software device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (where they repeat signals sent through them) or passive (where they do not repeat, but merely split, signals sent through them). (3) In Ethernet and IEEE 802.3, an Ethernet multiport repeater, sometimes called a concentrator.

A hybrid network is one in which some links are capable of sending and receiving only analogue signals while others handle digital signals only. Another definition is the division of a network into public and private sections.

Hertz

Measure of frequency. Synonymous with cycles per second. (2) The international unit for measuring frequency, equivalent to the older unit of cycles per second. One megahertz (MHz) is one million hertz. One gigahertz (GHz) is one billion hertz. The standard US electrical power frequency is 60 Hz, the AM broadcast radio frequency band is 535 - 1605 kHz, the FM broadcast radio frequency band is 88 -108 MHz, and wireless 802.11b LANs operate at 2.4 GHz.