half duplex Capability for data transmission in only one direction at a time between a sending station and a receiving station. Compare with full duplex and simplex. hammer drill Tool resembling an oversized electric drill used for drilling into masonry. As it turns the bit, it hammers rapidly. 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. HCC Horizontal cross-connect. Wiring closet where the horizontal cabling connects to a patch panel which is connected by backbone cabling to the main distribution facility. 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. 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. header checksum Field within an IP datagram that indicates the integrity check on the header. heartbeat See SQE. 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 Address configured on an interface to which broadcasts received on that interface will be sent. 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. hexadecimal Base 16. A number representation using the digits 0 through 9, with their usual meaning, plus the letters A through F to represent hexadecimal digits with values of 10 to 15. The right-most digit counts ones, the next counts multiples of 16, then 16^2=256, etc. hierarchical routing Routing based on a hierarchical addressing system. For example, IP routing algorithms use IP addresses, which contain network numbers, subnet numbers, and host numbers. hierarchical star topology Extended star topology where a central hub is connected by vertical cabling to other hubs that are dependent on it. 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. High-Speed Serial Interface See HSSI. highway See bus. 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 is typically used to connect supercomputers to peripherals and other devices. HLEN Number of 32-bit words in the header. 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. horizontal cross connect See HCC. 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. 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. hot swapping See OIR and power-on servicing. hot wire Ungrounded lead wire that connects the transformer and electrical devices or appliances via an electrical outlet and power plug. 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 High-Speed Communications Interface. Single-port interface, developed by Cisco, providing full-duplex synchronous serial communications capability at speeds up to 52 Mbps. HSRP Hot Standby Router Protocol. Provides 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 (up to 52 Mbps) serial connections over WAN links. HSSI Interface Processor See HIP. 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 WWW browser. See also hypertext and WWW browser. HTTP Hypertext transfer protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphic files. 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 WWW browser. See also HTML and WWW browser. hypertext markup language See HTML. Hz See hertz.