Glossary-H
A Data Communication Historical Series
By Bob Pollard

HOME                 INDEX

H CHANNEL:  

High-speed Channel: Full-duplex ISDN primary rate channel operating at 384 kbps.

H.225.0:

An ITU standard that governs H.225.0 session establishment and packet functions. H.225.0 describes several different protocols.

H.245:

An ITU standard that governs H.245 endpoint control

H.320:

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

H.323:

H.323 allows dissimilar communication devices to communicate with each other by using a standardized communication protocol. H.323 defines a common set of CODEC(s), call setup and negotiating procedures, and basic data transport methods.

H.323 RAS:

Registration, Admission, and Status: The RAS signaling protocol performs registration, admissions, bandwidth changes, and status and disengage procedures between the VoIP gateway and the gatekeeper.

H.450.2:

Call transfer supplementary service for H.323

H.450.3:

Call diversion supplementary service for H.323

HAIRPIN:

Refers to a telephony function where a received call is returned back in the direction of the originating call. For example, if a call cannot be routed over IP to a gateway that is closer to the target (receiving) telephone, the call typically is sent back out the local zone, back the way from which it came.

HAIRPINNING:

An incoming PSTN call is looped back out onto the PSTN. This is done if the call cannot be delivered using IP. It also can be used by a trunk gateway to deliver a modem call to a NAS.

HALF-DUPLEX TRANSMISSION (HDX):

The common use definition is a circuit designed for transmission in either direction but not both directions simultaneously. Contrast with FDX, full-duplex transmission.

HALF DUPLEX (HDX):

See Half-Duplex Transmission

HAMPS:

Host AUTODIN Message Processing System

HAMMING CODE:

In data transmission, a code with added redundant bits for error detection purposes

HANDOFF:

The transfer of a wireless call in progress from one transmission site to another site without a disconnect sequence.

HANDSET:

The part of a telephone that contains both receiver and mouthpiece

HANDSHAKE / HANDSHAKING:

1) A preliminary process performed by MODEMs and/or terminals and computers to verify that a communications link has been established and it is OK to proceed with data transmission. A part of the protocol designated procedures. Handshaking activity also occurs during message (data) transmission when functional and procedural control is required.

2) Signals between a wireless phone and a wireless system to accomplish a connection and call setup controls.

HANDWRITING INPUT DEVICE:

A tool, such as a digital pen and tablet, used to enter text by writing instead of typing. Along with writing tablets, users can use 3-D drawing or Computer Aided Drafting (CAD) tablets, or a tablet-PC. Writing is also possible by moving the mouse on the mouse pad.

HARDCOPY:

The printed output of a computer, in readable form

HARD DISK:

A mass storage device for computer data that consists of a hermetically sealed enclosure that holds stacked, rotating, magnetic coated discs accessed by multiple read/write heads.

HARDWARE:

Refers to the physical equipment that represents a computer system, including mechanical or electromechanical devices; some form of hardware is required in order to run the software (programs).

HARDWARE COMPRESSION:

A feature available on some tape devices that automatically compresses the data being stored on the device: This is usually an option that is turned on or off in a backup program.

HARDWARE CONFIGURATION:

Resource settings that have been allocated for a specific device: Each device on the computer has a hardware configuration, which may consist of IRQ lines, DMA, an I/O port, or memory address settings.

HARDWARE INTERFACE:

Refers to the physical hardware (port) used in order to connect computers to devices, data terminals and a MODEM or data terminals to devices or a MODEM.

HARDWARE PROFILE:

Data that describes the configuration and characteristics of specific computer equipment: This information can be used to configure computers for accessing and using peripheral devices.

HARDWARE RAID:

Dual-storage processors that improve data availability and performance and create data protection information: They are located in an external storage subsystem, freeing the CPU from performing RAID parity, striping, and also rebuild overhead calculations. This intelligent circuit board controls the disk drives.

HARDWARE TYPE:

A classification for similar devices: For example: Imaging Device is a hardware type for digital cameras and scanners.

HARDWIRED:

Refers to a permanent connection of data communications lines or cables and other related devices

HARDWIRED FRONT END PROCESSOR (FEP):

Refers to a non-programmable Front End Processor (FEP)

HARMONIC:

The combined multiple frequencies of some basic or fundamental frequencies. A frequency generated could be an unwanted frequency.

HARMONIC DISTORTION:

An impairment of a transmission line caused by an unwanted, erroneous (harmonic) frequency

HASP:

Houston Automatic Spooling Priority: A synonym for Job Entry Subsystem (JES), HASP is an IBM control program for the transmission of jobs to computers and the control of devices and data lines.

HBA:

Host Bus Adapter: An SCSI-2 (Small Computer System Interface) adapter that plugs into a host and lets the host communicate with a device. The HBA usually performs the lower level of the SCSI protocol and normally operates in the initiator role.

HCM(s):

High-performance voice Compression Modules: Modules that provide voice compression according to the voice compression coding algorithm (CODEC) specified when the Cisco MC3810 multi-service concentrator is configured.

HDB3:

High Density Binary 3:

1) Zero suppression line coding used on E1 links.

2) Line code type used on E1 circuits.

HDLC:

High-Level Data Link Control: Standard bit-oriented communication line protocol developed by ISO.

HDMI (High-Definition Multimedia Interface):

A compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency (RF) coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA. HDMI connects digital audio/video sources (such as set-top boxes, upconvert DVD players, HD DVD players, Blu-ray Disc players, AVCHD camcorders, personal computers (PCs), video game consoles such as the PlayStation 3, Xbox 360, and AV receivers) to compatible digital audio devices, computer monitors, video projectors, and digital televisions.

HDS:

Historical Data Server: An Administrative Workstation with a special database that holds ICM historical data. In a normal configuration, historical data is stored only in the central database. When the HDS option is used, the historical data is also stored on the HDS machine, which must be a real-time distributor. Other Admin Workstations at the site can read historical data from the HDS rather than accessing the central database.

HDSL:

High-bit-rate Digital Subscriber Line: A type of DSL communications. Independent Local Exchange Carriers (ILEC) have been using HDSL as an alternative to T-1 lines for years, so the service is often sold to subscribers at a high cost. HDSL provides symmetric data speeds up to 1.5 Mbps. It also requires the use of two pairs (4 wires) of local loop lines. The U.S. DSL standards body, ANSI T-1, has approved a new generation of HDSL called HDSL-2 that offers several enhancements over traditional HDSL. Most important is that HDSL-2 only requires a single twisted pair of local loop wires. HDSL-2 can deliver data communication service speeds to a distance of 18,000 feet without repeaters.

HDX:

See Half Duplex

HEAD:

An individual assembly that magnetically reads records or erases information on a storage device: Example: disk, magnetic tape, etc. See Magnetic Head.

HEADEND:

1) End point of a broadband network: All stations transmit toward the head-end; the head-end then transmits toward the destination stations.

2) Head-End: The upstream, transmit end of a tunnel

HEAD END UNIT:

Refers to a Local Area Network (LAN) hardware device used in a broadband network; uses separate frequencies for multiple services.

HEAD END HOP OFF (HEHO):

A traffic engineering process where calls are completed by using long distance facilities directly off the switch that serves that location.

HEAD-ON COLLISION:

A condition in message telephony when two switching exchanges seize a both-ways trunk at the same instant and attempt to send outbound call instructions to each other.

HEADER:

The address part of a message: Including other control information, appearing before the text and following the Transmission Identifier Line (TI). Headers are formatted differently for teletypewriter and other data media, but basically the header is divided into three parts: Header data up to and including the Start-of-Routing sequence, the Routing Indicator(s) field and the Security line. The Precedence Field is included in the first part of the Header.

HEARTBEAT:

Same as Signal Quality Error: Transmission sent by a transceiver back to the controller to let the controller know the collision circuitry is functional.

HEC:

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 checks for an error and corrects 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.

HELLO:

Interior routing protocol used principally by NSF-net nodes. HELLO allows particular packet switches to discover minimal delay routes.

HELLO PACKET:

A multicast packet that is used by routers for neighbor discovery and recovery: Hello packets also indicate that a client is still operating and network-ready.

HELLO PROTOCOL:

Protocol used by OSPF systems for establishing and maintaining neighbor relationships.

HEPNET:

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 SLAC.

HERTZ (HZ):

Named after the discoverer of alternating current (AC) and is used to express cycles per second

HETEROGENEOUS NETWORK:

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

HEURISTIC ANALYSIS:

Behavior-based analysis: Computer program / anti-virus software that identifies a potential virus. Often heuristic scanning produces false alarms when a clean program behaves like a virus.

HEXADECIMAL:

A numbering system used in computer systems, with 16 combinations represented by the symbols 0 through 9 and A through F (A base 16 numbering system).

HF:

See High Frequency

HFC:

Hybrid Fiber Coax

HFE:

Hardware Forwarding Engine

HIBERNATION:

An operational state in which the computer shuts down after saving everything in memory to the hard drive: When the computer is brought out of hibernation, all programs and documents that were originally open are restored to the desktop.

HIERARCHICAL ADDRESSING:

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.

HIERARCHICAL NETWORK STRUCTURE:

A network specification where all functions are categorized into specific areas or layers each having a specific role.

HIERARCHICAL ROUTING:

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.

HIERARCHICAL SWITCHING:

Refers to a switching methodology used in a Local Area Network (LAN) where the switching is done in stages. In a star topology, this would be called star switching.

HIGH CONTRAST:

A display feature that instructs programs to change the color scheme to a high-contrast scheme and to increase legibility whenever possible

HIGH FREQUENCY (HF):

Refers to frequencies between 3 and 30 mega-Hertz

HIGH PASS:

High Pass Filter: An electrical component that allows only a predetermined frequency level to pass or all frequencies above a certain level.

HIGH PERFORMANCE OPTION (HPO):

An alternate means of conditioning communication circuits, similar to D1 conditioning

HIGH SPEED CIRCUITRY:

Circuits with a clock rate in the range of 8 to 500 MHz.

HIGH-SPEED COMMUNICATIONS INTERFACE (HSCI):

A single-port interface developed by Cisco that provides full-duplex synchronous serial communications at speeds up to 52 Mbps

HIGH SPEED PRINTER:

A printer that operates at a speed compatible with computer processing speed and usually operates in an on line mode; see HSP

HIGH SPEED READER:

A high-speed electronic or optical computer input reading device. See HSPTR for information on a past AUTODIN paper tape reader.

HIGH WATER MARK:

A counter that reports the highest number of DS0s in use at one time

HIGHWAY:

Another term for ‘bus’: See Bus

HIJACK ATTACK:

Form of active wire tapping in which the attacker seizes control of a previously established communication association.

HIJACKING:

A computer system attack where an active and established program session is intercepted and used by the attacker: Hijacking can occur locally if, for example, a legitimate user leaves a computer unprotected. Remote hijacking can occur via the Internet.

HIP:

HSSI Interface Processor: Interface processor on the Cisco 7000 series routers. The HIP provides one HSSI port that supports connections to ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.

HIPPI:

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

HIT:

When used in reference to the World Wide Web hit means a single request from a web browser for a single item from a web server. In order for a web browser to display a page that contains 3 graphics, 4 hits would occur at the server; one for the HTML page, and one for each of the 3 graphics.

HIT(s), LINE HITS:

A maintenance term used to describe intermittent line noise or other forms of interference causing data communications errors.

HIVE:

   A section of the registry that appears as a file on the hard drive: The registry sub-tree is divided into hives, named for their resemblance to the cellular structure of a beehive. A hive is a discrete body of keys, sub-keys, and values that is rooted at the top of the registry hierarchy. A hive is backed by a single file and a .log file, which are in the systemroot\System32\Config or the systemroot\Profiles\username folders.

   By default, most hive files (Default, SAM, Security, and System) are stored in the systemroot\System32\Config folder. The systemroot\Profiles folder contains the user profile for each user of the computer. Because a hive is a file, it can be moved from one system to another. However, a user must use the Registry Editor to edit the file.

HLD:

High-Level Designator: Designator that logically identifies the peer session endpoints; used if the multiplex in the circuit is set to group.

HLR:

Home Location Register: A database that contains information about mobile network subscribers. The HLR registers subscribers for a particular service provider. The HLR stores ‘permanent’ subscriber information, including the service profile, location information, and the activity status of the mobile user.

HMAC:

Hash-based Message Authentication Code: HMAC is a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, for example, MD5, SHA-1, in combination with a secret shared key.

HMAC-MD5:

Hashed Message Authentication Codes with MD5 (RFC 2104): A keyed version of MD5 that enables two parties to validate transmitted information using a shared secret.

HMM:

Hex MICA Module: Contains six discrete MODEM(s)

HOAX:

E-mail, usually a chain letter(s), that describes some devastating type of virus. Normally there won’t be a file attachment and there won’t be a reference to anyone who can validate the claim.

HOLDDOWN:

State into which a route is placed so that routers neither advertise the route nor accept advertisements about the route for a specific length of time (hold-down period). Hold-down is used to flush bad information about a route from all routers in the network. A route typically is placed in hold-down when a link in that route fails.

HOLDING TIME:

Refers to the length of time a communication channel is in use for each transmission, which includes message and operating time.

HOLE:

Refers to the vulnerability of software and/or hardware design that allows circumvention of system security protocols

HOLLERITH CODE:

A 12-bit IBM code set used by data processing card punch machines. The same cards were used by the Compound Terminals connected to the AUTODIN system, but a modified 8 bit code set was used by the U.S Air Force as a compromise between the conflicting commercial codes.

HOME GATEWAY:

A router or access server that terminates VPDN tunnels and PPP sessions

HOME FOLDER:

A folder, usually on a file server, administrators can assign to individual users or groups. Administrators use home folders to consolidate user files onto specific file servers for easy backup. Home folders are used by some programs as the default folder for the Open and Save As dialog boxes. Home folders are sometimes referred to as ‘home directories’.

HOME LOOP:

Refers to a short, local transmission path (circuit)

HOME PAGE:

1) The Web page that your browser is set to use when it starts up.

2) The more common meaning refers to the main web page for a business, organization, person or simply the main page out of a collection of web pages.

HOME-PNA:

Phone-line Networking Alliance: Is based on the specifications developed by the Home Phone Networking Alliance (HPNA). The HPNA is a consortium of key networking technology companies that created a phone-line standard for the networking industry.

HOMERF:

A digital wireless communications protocol designed for the transport of voice and multimedia content between consumer electronic devices (PC) in a residential setting; operates at 2.4 GHz.

HOMOLOGATION:

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

HOOKFLASH:

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: Hook-flash often is used to perform call transfer.

HOOT AND HOLLER:

A broadcast audio network used extensively by the brokerage industry for market updates and trading: Similar networks are used in publishing, transportation, power plants, and manufacturing.

HOP:

1) Passage of a data packet between two network nodes, for example: between two routers.

2) House Operating Program: Online message switching control program

HOP COUNT:

Routing metric used to measure the distance between a source and a destination. RIP uses hop count as its sole metric.

HOP OFF:

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

HORIZONTAL REDUNDANCY CHECK ‘Checking’ (HRC):

Data error checking: Redundant information (bits) is included in or with the data to be checked.

HOST ADDRESS:

Part of an IP address that designates which node on the sub-network is being addressed.

HOST, HOST COMPUTER:

1) The central or controlling computer in a data communications network. Database access, files, programming languages, etc. would be a function of the Host.

2) Any computer on a network that is responsible for services which is available to other computers on the network. It is quite common to have one host machine provide several services, such as E-mail and Web (Internet) access.

HOST NAME:

1) Name given to a machine (computer / mainframe)

2) The DNS (Domain Name System) name of a device on a network. These names are used to locate computers on the network. To find another computer, its host name must either appear in the Hosts file or be known by a DNS server. For most Windows computers, the host name and the computer name are the same.

HOST NODE:

SNA sub-area node that contains an SSCP

HOST NUMBER:

Part of an IP address that designates which node on the sub-network is being addressed

HOST PRIORITY:

For Network Load Balancing, a hosts precedence for handling default network traffic for TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) ports. It is used if a host within the cluster goes offline, and it determines which host within the cluster will assume responsibility for the traffic previously handled by the offline host.

HOT DOCKING:

The process of attaching a laptop computer to a docking station while the computer is running and automatically activating the docking stations video display and other functions

HOT SPARE:

In RAID systems, a spare drive in the disk array that is configured as a backup for rebuilding data in the event another drive fails.

HOT SPOT:

Hot Spot is a term used when wireless connections are available to the public at coffeehouses, parks, malls, airports, etc., all of them presently using the Wi-Fi (Wireless Fidelity) technology.

HOT STANDBY ROUTER PROTOCOL (HSRP):

A protocol that provides high network availability and allows transparent network topology changes: HSRP creates a Hot Standby router group and assigns a lead router that services all packets sent to the Hot Standby address. The lead router is monitored by other routers in the group, and if it fails, one of these standby routers inherits the lead position and the Hot Standby group address.

HOT SWAPPING:

1) Equipment online insertion and removal: Feature that permits the addition, the replacement, or the removal of cards without interrupting the system power, entering console commands, or causing other software or interfaces to shutdown.

2) Power on Servicing: Feature that allows faulty components to be diagnosed, removed, and replaced while the rest of the device continues to operate normally.

HOUSEKEEPING:

Operations within a computer: Does not contribute directly to data processing but are necessary for efficient system operation.

HOUSTON AUTOMATIC SPOOLING PRIORITY:

See HASP

HPCC:

High-Performance Computing and Communications: U.S. government-funded program advocating advances in computing, communications, and related fields. The HPCC is designed to ensure U.S. leadership in these fields through education, research and development, industry collaboration, and implementation of high-performance technology.

HPCS:

High-Performance Computing Systems: Component of the HPCC program designed to ensure U.S. technological leadership in high-performance computing through research and development of computing systems and related software.

HPO:

See High Performance Option

HPR:

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.

HQ NAVSEA:

Headquarters – Navy

HRC:

See Horizontal Redundancy Check (Checking)

HSCI:

High-Speed Communications Interface: Single-port interface, developed by Cisco, providing full-duplex synchronous serial communications at speeds up to 52 Mbps.

HSM:

High Speed Memory (solid state): Original AUTODIN system used magnetic core based memory.

HSP:

High Speed Printer (High Speed Line Printer

HSPTR:

High Speed Paper Tape Reader

HSRP:

Hot Standby Router Protocol: A protocol that provides for high network availability and transparent network topology changes. HSRP creates a Hot Standby router group with a lead router that services all packets sent to the Hot Standby address. The lead router is monitored by other routers in the group, and if it fails, one of these standby routers inherits the lead position and the Hot Standby group address.

HSSI:

High-Speed Serial Interface: Network standard for high-speed serial connections, up to 52 Mbps, over WAN links.

HSSI INTERFACE PROCESSOR (HIP):

Interface processor on the Cisco 7000 series routers: The HIP provides one HSSI port that supports connections for ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.

HT:

Horizontal Tabulation: A format control signal that moves the printing head horizontally to a pre-determined printing position. Used with printers, CRTs and as a skip function on punched cards.

HTML:

Hypertext Markup Language: HTML is a collection of formatting commands (tags) that create hypertext documents; Web pages, to be exact. When a person points the Web browser to a URL, the browser interprets the HTML commands embedded in the page and uses them to select and format the pages text and graphic elements. HTML commands cover many types of text formatting and also have the ability to include graphics and other non-text elements.

HTTP:
Hypertext Transfer Protocol: The protocol used to transmit and receive data over the World Wide Web. When a person types an URL into the browser, a HTTP request is sent to a Web server for a page of information. URL(s) normally begin with http://.

HUB:

1) Generally, a term used to describe a device that serves as the center of a star-topology network.

2) Hardware or software device that contains multiple independent but connected modules of network and inter-network equipment. Hubs can be active; repeat signals sent through them, or passive; do not repeat, but merely split signals sent through them.

3) In Ethernet and IEEE 802.3: An Ethernet multi-port repeater sometimes called a concentrator.

HUBBED MODE:

A mode in which the ARP/MARS (Address Resolution Protocol) provides ATM (Asynchronous Transfer Mode) addresses to requesting clients in the form of a Multicast Server (MCS) list value. In this mode, the ARP/MARS acts as a multicast server, providing active forwarding of all multicast and broadcast traffic destined for IP (Internet Protocol) addresses contained within the ranges specified in the list.

HUBBING POINT:

A central connection of several circuits to one shared circuit: similar to a concentrator. In the simplest form the connected terminals would operate on a contention basis, one at a time.

HUE:

The position of a color within the color spectrum: For example, green is between yellow and blue. This attribute can be set using Display in Control Panel.

HYBRID:

1) A combination of two or more technologies: For example, Digital Data Communications Message Protocol (DDCMP), a character/bit data communications protocol, would be considered a hybrid

2) A communications electronics circuit that performs the wire conversions necessary for the connection of a 2 wire local loop with a 4 wire long-haul facility; sometimes called a ‘ two wire / four wire converter’.

HYBRID ENCRYPTION:

Refers to an application of cryptography that combines two or more encryption algorithms; particularly a combination of symmetric and asymmetric encryption.

HYBRID NETWORK:

Inter-network made up of more than one type of network technology, including LANs and WANs

HYPER GROUP:

(Jumbo Group): A Frequency Division Multiplexing (FDM) carrier system that contains 3600 Voice Frequency (VF) or telephone channels in six master groups.

HYPERLINK:

1) Colored and underlined text or a graphic that a user clicks to go to a file: a location in a file; an HTML page on the World Wide Web, or an HTML (Hypertext Markup Language) page on an intranet. Hyperlinks can also go to newsgroups and to Gopher, Telnet, and FTP (File Transfer Protocol) sites.

2) In Windows folders, hyperlinks are text links that appear in the folders left pane. Users can click these links to perform tasks, such as moving or copying a file, or to go to other places on the computer, such as the My Documents folder or Control Panel.

HYPERTEXT:

1) A non-sequential way of presenting information: Hypertext links information in a complex web of associations, powered by hyper-links; essentially a way to browse through information.

2) Electronically stored text that allows direct access to other texts by way of encoded links:  Hypertext documents can be created using HTML, and often integrate images, sound, and other media that are commonly viewed using a browser.

Hz:

Hertz, (replaced Cycles-Per-Second), named after Heinrich Hertz