H.225.0
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.
H.245
An ITU standard that governs H.245 endpoint control.
H.320
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
H.323 allows dissimilar communication devices to communicate with each other by using a standardized communication protocol. H.323 defines a common set of CODECs, 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.
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.
hardware address
See MAC address.
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.
header
Control information placed before data when encapsulating that data for network transmission. Compare with trailer.
Hello protocol
Protocol used by OSPF systems for establishing and maintaining neighbor relationships.
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.
hijack attack
Form of active wire tapping in which the attacker seizes control of a previously established communication association.
hop
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.
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.
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.
HTTP
Hypertext Transfer Protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphic files.
hub
In Ethernet and IEEE 802.3, an Ethernet multiport repeater, sometimes called a concentrator.
hyperlink
Pointer within a hypertext document that points (links) to another document, which might or might not also be a hypertext document.
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.
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