Glossary I

H

half duplex

The capacity to transfer data in only one direction at a time between a sending unit and a receiving unit. See also: full duplex.

handshake

Any series of transmissions exchanged between two or more devices on a network to ensure synchronized operations.

H channel

high-speed channel: A full-duplex, ISDN primary rate channel operating at a speed of 384Kbps. Compare to: B channel, D channel, and E channel.

HDLC

High-Level Data Link Control: Using frame characters, including checksums, HDLC designates a method for data encapsulation on synchronous serial links and is the default encapsulation for Cisco routers. HDLC is a bit-oriented synchronous Data Link layer protocol created by ISO and derived from SDLC. However, most HDLC vendor implementations (including Cisco's) are proprietary. See also: SDLC.

helper address

The unicast address specified, which instructs the Cisco router to change the client's local broadcast request for a service into a directed unicast to the server.

hierarchical addressing

Any addressing plan employing a logical chain of commands to determine location. IP addresses are made up of a hierarchy of network numbers, subnet numbers, and host numbers to direct packets to the appropriate destination.

hierarchical network

A multi-segment network configuration providing only one path through intermediate segments, between source segments and destination segments.

hierarchy

See: hierarchical network.

HIP

HSSI Interface Processor: An interface processor used on Cisco 7000 series routers, providing one HSSI port that supports connections to ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.

holddown

The state a route is placed in so that routers can neither advertise the route nor accept advertisements about it for a defined time period. Holddown is used to allow time for bad information about a route to be propagated all routers in the network. A router generally places a route in holddown if a directly connected link fails.

hop

The movement of a packet between any two network nodes. See also: hop count.

hop count

A routing metric that calculates the distance between a source and a destination. RIP employs hop count as its sole metric. See also: hop and RIP.

host address

Logical address configured by an administrator or server on a device. Logically identifies this device on an internetwork.

HSCI

High-Speed Communication Interface: Developed by Cisco, a single-port interface that provides full-duplex synchronous serial communications capability at speeds up to 52Mbps.

HSRP

Hot Standby Routing Protocol: A protocol that provides high network availability and provides nearly instantaneous hardware failover without administrator intervention. It generates a Hot Standby router group, including a lead router that lends its services to any packet being transferred to the Hot Standby address. If the lead router fails, it will be replaced by any of the other routers-the standby routers-that monitor it.

HSSI

High-Speed Serial Interface: A network standard physical connector for high-speed serial linking over a WAN at speeds of up to 52Mbps.

hub

Physical layer devices that are really just multiple port repeaters. When an electronic digital signal is received on a port, the signal is reamplified or regenerated and forwarded out all segments except the segment from which the signal was received.