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

H.320
An ITU-T standard addressing video telephony specifications over ISDN and other circuit switched networks and services.

H.323
A model for the exchange of data for the Telecommunication. 
An ITU-T standard, an extension of H.320, addressing audiovisual communication on LANs, including corporate intranets and packet-switched networks. 
H.323 is an umbrella standard that describes the architecture of the conferencing system, and refers to a set of other standards H.245, H.225 and Q.931
to describe its actual protocol. 

H.323 RAS
Registration, Admission and Status. 
The RAS signaling function performs registration, admissions, bandwidth changes, status and disengage procedures between the VOIP gateway and the gatekeeper.

H.324
An ITU-T standard addressing High Quality Video and Audio Compression over POTS modems.

H channel
Full duplex ISDN primary rate channel operating at 384 Kbps. High-speed bearer channels. 

HDB3
High-Density Bipolar coding. 

HDLC
High Level Data Link Control 
HDLC manages synchronous, code-transparent, serial information transfer over a link connection. ISO standard bit-oriented, link-layer protocol derived from SDLC. HDLC specifies an encapsulation method of data on synchronous serial data links.
Manage the error check on the data link level. SDLC, LAP and LAPB are part of the HDLC. A bit-oriented (physical) data link protocol developed by ISO for full-duplex communications. 

HDSL
High bit rate Data Subscriber Line / High data rate Digital Subscriber Line
Using the regular copper telephone line to transmit up to 784 K/bits. Same speed for Upload and Download.  Modems on either end of one or more twisted pair wires that deliver T1 or E1 speeds. 

HEC
Header Error Control. The last byte of an ATM cell header. Guards against the misdelivery of cells due to header or single bit errors. 

HELLO
This protocol is used by OSPF systems for establishing and maintaining neighbor relationships.

HFC
Hybrid Fiber-Coaxial Cable - Distribution cabling concept using both fiber-optic and coaxial cable.

HOC
Highest Outgoing Channel

HOP COUNT
A routing metric used to measure the distance between a source and a destination. 
ex: RIP uses hop count as a routing metric. The maximum allowable hop count for RIP is 16.
The hop is unit of measurement indicating the passage of a packet through one router or bridge. 
The number of routers and bridges a packet traverses between a source and destination. A header field in the packet stores the hop count. Each router or bridge in the path drops the packet if the hop count exceeds a predetermined value and adds one to the hop count if it does not. 

HPR
High Performance Routing. 
An APPN enhancement that includes Automatic Network Routing and the Rapid Transport Protocol.

HRC
Hybrid Ring Control - FDDI

HSRP
Hot Standby Routing Protocol - Cisco ¨. 
Proprietary protocol that provides a redundancy mechanism when more than one router is connected to the same segment/subnet of an Ethernet/FDDI/Token-Ring network.

HSSI
High Speed Serial Interface. 
High-level serial interface supporting up to 52 Mb/s and offering LAN-like performance over a wide area. Cisco ¨ Full duplex sync. serial comm. up to 52 Mbps. 

HSTR
High Speed Token Ring - 100 Mbits

HUB
A device that serves as the center of a star topology network. For Ethernet, a hub is a multi-port repeater. The term is also used to refer to a hardware/software device that contains multiple independent but connected modules of network equipment. Hub can be active, whether they repeat signals sent through them or passive, where they split signals through them.
A physical layer device, connected to other devices, that restores a signal's amplitude and timing for transfer across a network. Known as a repeater in most IEEE 802.3 standards and also called a concentrator. The hub usually resides at the core of a star topology network and is used for the transfer of messages. Hubs provide Ethernet, token ring, and/or FDDI functions; accept host, internetworking, and network management modules; and provide retiming/repeater, bridging, and/or network management functionality. A typical hub has multiple user ports to which computers and peripheral devices are attached.