gloss_H

H

H.233

The ITU-T's data-encryption standard for real-time multimedia, H.233 is supported across a wide range of standard services, including H.320, H.323, and H.324. A related standard is H.234, which specifies how encryption keys are handled. See H.320, H.323, H.324.

H.261

An ITU-T video-compression codec ( coder -decoder) standard, H.261, which is supported by H.320, H.323, and H.324, supports CIF and QCIF images. H.261 was designed for use with ISDN and assumes data rates in multiples of 64Kbits/sec. A newer standard, H.263 (supported by H.324), improves H.261's efficiency and adds support for SQCIF-, 4CIF-, and 16CIF- sized images. See CIF, H.320, H.323, H.324.

H.320

One of the major ITU-T standards for real-time multimedia, H.320 is the standard for video conferencing over narrow- band circuit-switched WAN services such as ISDN. This standard includes specifications for T.120-based data, G.711- and G.728-based voice, H.261-based video, and H.233- and H.234-based encryption. The basic H.320 standard has been enhanced in H.323 to include basic packet-switched networks. Related standards for real-time multimedia that are not broken out separately in this glossary are H.321, for broadband ISDN and ATM; H.322, for guaranteed -bandwidth packet-switched networks; and H.310, for higher-resolution multimedia over ATM. See G.711, H.233, H.261, H.323, T.120.

H.323

An extension of the older H.320 standard, the ITU-T's H.323 covers video conferencing not only over narrow-band WAN services, but also on packet-switched networks such as corporate LANs and the Internet. H.323 is based on the Internet Engineering Task Force's (IETF's) Real-Time Protocol (RTP), H.261-based and H.263-based video, and T.120. See H.261, H.320, RTP, T.120.

H.324

The ITU-T's standard for real-time multimedia over standard POTS (Plain Old Telephone System) lines using 28.8Kbit/sec V.34 modems or better, H.324, like H.320, incorporates the T.120 standard for data sharing, H.261-based video compression, and the H.233 and H.234 encryption standards. Unlike H.320, H.324 uses the G.723 audio standard. See G.711, H.233, H.261, H.320, T.120.

Heterogeneous LAN Management (HLM).

HLM is an IEEE 802.1 specification for jointly managing mixed Ethernet and Token Ring networks with the same objects.

heterogeneous network.

A heterogeneous network is made up of a multitude of workstations, operating systems, and applications of different types from different vendors . For example, a heterogeneous network might contain 3Com Ethernet adapter cards, Dell 486 PCs, Compaq SystemPros, Novell NetWare, FTP TCP/IP, and an HP 9000 Unix host.

High Level Language API (HLLAPI).

HLLAPI is a set of tools developed by IBM to help developers write applications that conform to its Systems Application Architecture.

High-level Data Link Control (HDLC).

HDLC is an ISO standard for a bit-oriented, link-layer protocol that specifies how data is encapsulated on synchronous networks.

High-Speed Serial Interface (HSSI).

HSSI is a standard for a serial link up to 52Mbits/sec in speed over WAN links.

homogeneous network.

A homogeneous network is made up of identical or similar components -hardware, operating systems, protocols, databases, and applications.

horizontal wiring subsystem.

This part of a structured wiring system connects the users' computers in the departments. It is attached to the vertical wiring system. The horizontal wiring system is often copper cable, such as twisted pair or coax.

hub.

A hub or concentrator is a multiport repeater that brings together the connections from multiple network nodes. Modular concentrators have moved beyond their origins as wire concentration centers and often house bridges, routers, and network-management devices. Some hubs are stackable , which usually implies that they have an external backplane that permits multiple units to be connected together without taking up a "repeater hop" or using one of the ports.