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H.323	Standard for packet-based video, audio, and data conferencing. Umbrella standard that describes the architecture of a conferencing system and refers to other standards (H.245, H.225.0, and Q.931) to describe its actual protocols. Defines a common set of codecs, call setup and negotiating procedures, and basic data-transport methods that allow dissimilar communication devices to communicate with each other.
H.323 RAS signaling	H.323 registration, Admission, and Status signaling. Performs registration, admissions, bandwidth changes, status, and disengage procedures between a VoIP gateway and a gatekeeper.
H.450	Component of the H.323 standard that defines signaling and procedures used to provide telephony-like services. H.450 includes other standards such as H.450.2 (call forwarding) and H.450.3 (call transfer).
hairpinning	Call-routing capability in which an incoming call on a specific gateway is signaled through the IP network and back out the same gateway.
handshake	Sequence of messages that are exchanged between two or more network devices to ensure transmission synchronization.
HCM	High-performance voice-compression module.
HDLC Protocol	High-Level Data-Link Control Protocol. 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.
hookflash	Short on-hook period usually generated by a phone-like device during a call to indicate that the phone is attempting to perform a dial-tone recall from a PBX or switch. Hookflash is often used to initiate call transfers.
HS	High speed. Fax-transmission rate used for capabilities negotiation. See also LS.
HSRP	Hot-Standby Router Protocol. Protocol that provides network redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first-hop failures in network edge devices or access circuits.
HSSI	High-Speed Serial Interface. Network standard for high-speed (up to 52 Mbps) serial connections over WAN links.
HTML	Hypertext Markup Language. The markup language used for today's web applications, providing formatting and basic document structure for presentation via browser applications.
HTTP	Hypertext Transfer Protocol. Protocol used by web browsers and web servers to transfer files.
hunt group	Number of phone lines that are associated together by the central office or PBX. When a call comes in to a hunt group, it cycles through the group of lines until it finds one that is not busy, then it rings that phone (or extension on a PBX system).
hunting	Automatic redirecting of an incoming call from a called number that is busy or does not answer through a group of designated numbers in a designated order until the call is answered. If all the lines are busy, the caller gets a busy signal.
huntstop	Ability to specify that calls should not hunt if the number that is being rung is busy or does not answer.