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  • H channel The ISDN packet switched channel on Basic Rate Interface, designed to carry user information streams at different speeds, depending on type: H11=1536Kbit/s, H0=384Kbit/s and H12= 1920Kbit/s.
  • H.261 A ITU-TS standard for video compression know as Codes for Audiovisual Services at N x 384Kbit/s. It sets a common algorithm for converting analog video signals to digital, operating at or above 384Kbit/s.
  • Half Duplex A two-way means of transmission, but data can only travel in one direction at a time.
  • Half-bridge Apple Computer terminology for a device linking LANs over a low-speed link such as a telephone line or X.25 link. It is termed a half- bridge as one is required at each end of the link.
  • Handshake Part of the procedure to set up a datacommunications link. The handshake can be part of the protocol itself or an introductory process: the computers wishing to talk to each other set out the conditions they can operate under. Sometimes, the handshake is just a warning that a communication is imminent.

  • HDLC (High level Data Link Control) An ISO standard set of protocols for carrying data over a link with error and flow control. Similar to IBM's SDLC, versions of HDLC are under development for multipoint lines. The ITU-TS later adapted HDLC for its Link Access Protocol used for X.25 networks. It is a bit-oriented data link control procedure under which all data transfer takes place in frames. Each frame ends with a frame check sequence for error detection. There is a control field at the start of each frame that allows error detection, and data link setup and data link termination. HDLC is, in fact, a misnomer, as it is not a high level protocol.
  • Head-end A central point or hub in broadband networks that receives signals on one set frequency band and retransmits them or another. Every transmission has to go through the head-end in a broadband network. The head-end is the piece of hardware that enables a network to send and receive on the same piece of cable. In CATV technology, the head-end is the control center for a cable system where signals are processed and sent for distribution down the cable system.
  • Header The control information added to the beginning of a transmitted message. This may consist of packet or block address, destination, message number and routing instructions.
  • Hertz A measure of radio frequency. One Hz=one cycle per second. KHz = 1,000Hz; MHz =1,000,000Hz; GHz=1,000,000,000Hz.
  • Heterogeneous networking The networking of computers from different vendors, or the running of different operating systems.
  • HH header hub
  • HLMS (Heterogeneous LAN Management Specification) Network management specification developed by IBM and 3Com. It provides an underlying structure for the development of network management products which can function with a variety of network operating systems and adapter cards.
  • Hierarchical network A network with one host at its hub, which is the major processing center, and one or more satellite processing units. IBM's SNA was originally designed as an hierarchical network.
  • Host processor A minicomputer or mainframe attached to a network providing services to network users.

  • Hot fixing The ability to detect and mark bad sectors of a disk, then assign alternate disk sectors during routine LAN operation. This automatically updates the original defect map.
  • Hot swap The ability of a device to have parts removed after, for example, a slot-in card or fan failure, without affecting its operation. Many of today's hubs offer hot swapping without bringing down the network to charge a component.
  • HPFS (High-Performance File System) An OS/2 file system that has faster input/output than the file allocation table file system; it does not restrict file naming to eight characters with a three character extension; and is compatible with the FAT file system.
  • HSSI (High speed Synchronous Serial Interface) Interface for transferring data to or from a WAN leased line (E2, E3), or to and from a LAN via an HSSI-capable DSU and HSSI router.
  • HTML Hyper Text Markup Language
  • Hub The center of a star topology network or cabling system. A multi-node network topology that has a central multiplexer with many nodes feeding into and through the multiplexer or hub. The other nodes do not usually directly interconnect. LAN hubs are increasingly popular with the growth of structured cabling and the need for LAN management.
  • Brouter or Hybrid Bridge/Router Network interconnection device with a standard mode of operation as a router. It can support multiple OSI Layer Three routing protocols, but also supports bridging capabilities for network traffic that cannot be routed - non-routable traffic.





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