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- H.261
- An International Telegraph and Telephone Consultative Committee standard for video compression (also known as px64). H.261 will be the worldwide standard and, therefore, must accommodate both the North American National Television System Committee (NTSC) and European PAL protocols.
- H/S LAN (High-Speed Local-Area Network)
- A LAN operating at 4 Mbits per second or more to facilitate rapid exchange of files, mail, etc., within a broader departmental or divisional environment.
- HACMP (High Availability Cluster Multiprocessing)
- AN IBM RS/6000 product originally designed to provide a high-availability configuration (by enabling automated failover from one RS/6000 to another). HACMP comprises:
• Cluster management software to restart an application or subsystem on an alternate RS/6000 processor after a failure of the primary processor.
• Distributed lock manager software to coordinate and synchronize process execution on multiple processors in a cluster.
• Configuration and topology software management tools to enable developers to build cluster-aware applications.
- Half Duplex
- Pertaining to the capability to transmit in two directions but not simultaneously.
- HDLC (High-Level Data Link Control)
- A bit-oriented data link protocol developed by the International Standards Organization and used in Open Systems Interconnection.
- Helical Scan Recording
- In tape recording, when the tape is wrapped around a transport drum at an angle, track length is created that is nearly 10 times longer than the tape width. These diagonal tracks can increase track density and data transfer rates as well.
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- Help System
- A consistent, system-supplied mechanism for system and application programs to offer on-line, context-sensitive help to end users.
- Heterogeneous
- A system composed of hardware, operating systems, middleware or applications from several different vendors.
- Hierarchical Database
- A database that is organized in a tree structure, in which each record has one owner. Navigation to individual records takes place through predetermined access paths.
- High Availability
- A high probability that a system at any given time will be operational for useful work, and has the ability to recover quickly and with minimum disruption to the user in case of failure. Those characteristics of a data processing system that reduce or eliminate its vulnerability to unscheduled outages, such as power failures, code defects or hardware failures. One element of 24 x 7.
- Hiperspace (High-Performance Space)
- Introduced with ESA/370, a hiperspace is, in essence, an address space occupied only by data and to which access is controlled by hardware via special instructions.
- HLL (High-Level Language)
- A procedural programming language that offers a higher level of abstraction than assembly language. Examples of HLLs: COBOL, FORTRAN, PL/I, C, Pascal and Ada. The term is interchangeable with "3GL."
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- HP-UX
- Hewlett-Packard’s System V-based Unix implementation, with real-time extensions added.
- HPR (High-Performance Routing)
- IBM’s technology to enable users to send Systems Network Architecture traffic over frame and cell-based networks, making for more efficient communications. For example, the HPR version of Advanced Peer-to-Peer Network (APPN) is generally superior to Transmission Control Protocol/Internet Protocol (TCP/IP), which uses reactive congestion control, while APPN is predictive, resulting in higher network utilization and throughput for APPN. Because it is newer, APPN can better take advantage of increasingly error-free, high-speed digital transmission facilities. TCP/IP’s Domain Name Service and limited (now insufficient) addressing capacity are inferior in performance to APPN’s topology database and use of logical unit names. APPN supports a prioritization/class of service mechanism for different types of network applications, while individual router vendors have been forced to add their own nonstandard prioritization capabilities to TCP/IP.
- HSDL (High-Speed Subscriber Data Line)
- A local phone line with full-duplex T1 capabilities. As corporate internets grow, bigger "pipes" are needed to maintain network performance. Using inverse multiplexers that combine multiple E1 circuits will be the best way to deliver that bandwidth between the user site and the carrier’s central office. But the use of HSDL technology, as an alternative to inverse multiplexing of E1 circuits, will be a "dark horse" alternative to inverse multiplexing. Unlike inverse multiplexing, HSDL would require new support systems and maintenance procedures.
- HSM (Hierarchical Storage Manager)
- An earlier version of IBM’s Data Facility Hierarchical Storage Manager, introduced in 1975 with the IBM 3850 Mass Storage Subsystem.
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- HSSI (High-Speed Serial Interface)
- Standard for a serial interface used to connect to T3 lines. HSSI operates over a shielded cable at speeds of up to 52 Mbps and distances of up to 50 feet. Functionally, it serves the same purpose as lower-speed serial interfaces such as V.35 and RS-232, in that it provides the interface to DCE for wide-area-network (WAN) communications.
- Huffman Code
- A code used for one-dimensional data compression in the International Telecommunications Union Telecommunications Standards Sector Group III digital facsimile standard.
- Hybrid Modeling
- A term, coined by Gartner Group, that is used to signify second-generation, dimension-driven, constraint-based solids modeling technology beyond first-generation parametric modeling. Hybrid modelers offer multiple design input mechanisms, flexible constraint management, and robust interoperability with legacy computer-aided design data.
- Hypertext
- Software technology used to create and store simple and complex navigational paths across computerized data.
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- Hz (Hertz)
- A measure of electromagnetic frequency equivalent to cycles per second.
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