H

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.

handshake

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

hardware address

See MAC address.

HBD3

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.

HDLC

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.

HDSL

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, 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.

headend

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

header

Control information placed before data when encapsulating that data for network transmission. Compare with trailer. See also PCI.

heartbeat

See SQE.

HEC

header error control. An algorithm for checking and correcting an error in an ATM cell. Using the fifth octet in the ATM cell header, ATM equipment may 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.

HELLO

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.

helper address

See helper address in the Cisco Systems Terms and Acronyms section.

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 the Stanford Linear Accelerator Center (SLAC).

hertz

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.

hierarchical addressing

A 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.

High-Energy Physics Network

See HEPnet.

High-Level Data Link Control

See HDLC.

High Performance Computing and Communications

See HPCC.

High Performance Computing Systems

See HPCS.

High-Performance Parallel Interface

See HIPPI.

High Performance Routing

See HPR.

High-Speed Communications Interface

See HSCI in the Cisco Systems Terms and Acronyms section.

High-Speed Serial Interface

See HSSI.

highway

See bus.

HIP

See HIP in the Cisco Systems Terms and Acronyms section.

HIPPI

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

holddown

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.

homologation

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.

hop

Term describing the 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 hop and RIP.

host

Computer system on a network. Similar to the term 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 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.

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. The five components of the HPCC are ASTA, BRHR, HPCS, IITA, and NREN.

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. See also HPCC.

HPR

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.

HSCI

See HSCI in the Cisco Systems Terms and Acronyms section.

HSRP

See HSRP in the Cisco Systems Terms and Acronyms section.

HSSI

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.

HTTP

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

HTML

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.

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 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 referred to as a concentrator.

hybrid network

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

hypertext

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

See HTTP.

Hypertext Markup Language

See HTML.

Hz

See hertz.