Table of Contents




An ITU standard that governs H.225.0 session establishment and packetization. H.225.0 actually describes several different protocols: RAS, use of Q.931, and use of RTP.


An ITU standard that governs H.245 endpoint control.


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.

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.


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.

half duplex

Capability for data transmission in only one direction at a time between a sending station and a receiving station. BSC is an example of a half-duplex protocol. See also BSC. Compare with full duplex and simplex.


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

hardware address

See MAC address.


Line code type used on E1 circuits.

H channel

high-speed channel. Full-duplex ISDN primary rate channel operating at 384 Kbps. Compare with B channel, D channel, and E channel.


High-Level Data Link Control. Bit-oriented synchronous data link layer protocol developed by ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. See also SDLC.


high-data-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. Compare with ADSL, SDSL, and VDSL.


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. Compare with trailer. See also PCI.


See SQE.


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.


Interior routing protocol used principally by NSFnet nodes. HELLO allows particular packet switches to discover minimal delay routes. Not to be confused with the Hello protocol.

hello packet

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. Not to be confused with HELLO.


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.


Measure of frequency. Abbreviated Hz. Synonymous with cycles per second.

heterogeneous network

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


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

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. Compare with flat addressing.

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.

High-Energy Physics Network

See HEPnet.

High-Level Data Link Control


High Performance Computing and Communications


High Performance Computing Systems


High-Performance Parallel Interface


High Performance Routing

See HPR.

High-Speed Communications Interface

See HSCI in the "Cisco Systems Terms and Acronyms" section.

High-Speed Serial Interface



See bus.


See HIP (HSSI Interface Processor) in the "Cisco Systems Terms and Acronyms" section.


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


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


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.


Passage of a data packet between two network nodes (for example, between two routers). See also hop count.

hop count

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

hop off

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. See also node.

host address

See host number.

host name

Name given to a machine. See FQDN.

host node

SNA subarea node that contains an SSCP. See also SSCP.

host number

Part of an IP address that designates which node on the subnetwork is being addressed. Also called a host address.

Hot Standby Router Protocol

See HSRP in the "Cisco Systems Terms and Acronyms" section.

hot swapping

See OIR and power-on servicing.


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. See also the five components of the HPCC: ASTA, BRHR, HPCS, IITA, and NREN.


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. See also HPCC.


High-Performance Routing. Second-generation routing algorithm for APPN. HPR provides a connectionless layer with nondisruptive routing of sessions around link failures, and a connection-oriented layer with end-to-end flow control, error control, and sequencing. Compare to ISR. See also APPN.


See HSCI (High-Speed Communications Interface) in the "Cisco Systems Terms and Acronyms" section.


See HSRP (Hot Standby Router Protocol) in the "Cisco Systems Terms and Acronyms" section.


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

HSSI Interface Processor

See HIP in the "Cisco Systems Terms and Acronyms" section.


Hypertext Transfer Protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphic files.


Hypertext Markup Language. Simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a Web browser. See also hypertext and Web browser.


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 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.

hybrid network

Internetwork made up of more than one type of network technology, including LANs and WANs.


Pointer within a hypertext document that points (links) to another document, which may or may not also be a hypertext document.


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. See also HTML and browser.

Hypertext Transfer Protocol


Hypertext Markup Language



See hertz.

Posted: Tue Sep 21 15:17:10 PDT 1999
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