Glossary - H

Half-Duplex (communications)

Communications system or circuit capable of communications in both directions, but in only one direction at a time.
Also an obsolete term for local echo.

HAN

Home Area Network (network)

HDB3

High Density Bipolar (physical layer)

HDLC

High Level Data Link Control (network, data-link layer)
An ITU-T link layer protocol standard for point-to-point and multi-point communications. Similar to SDLC.

HDSL

High bit/data rate Digital Subscriber Line/Loop (physical layer, network,communications, protocol)
A transmission system technology that allows telephone companies to use existing copper-cable plants(as opposed to optical fibre) to transmit high bit rate digital trafic. Bit rate are from 784 Kbps to 2 048 Kbps over twisted pair up to 26,246 feet. HDSL needs modems on either end of one or more twisted pair wires that deliver T1 or E1 speeds. At present T1 requires two lines and E1 requires three.
See also ADSL, HDSL, SDSL for one line HDSL, VDSL.

Header

First part of a message (frame, file...) that contains control data, routing data... There is no direct data usable by an ordinary user.
In HTML, this is a dedicated part of an HTML file where one defines keywords, the document title...
In ATM, the five bytes in a cell that supply addressing and control information, including generic flow control, virtual path identifier, virtual circuit identifier, payload type, and cell-loss priority.

HEC

Header Error Control (ATM)
A single byte containing the information needed for the transmission convergence (TC) sub-layer of the ATM physical layer (PHY) to perform error detection on the cell header. If errors are found, the cell is dropped before processing moves up to the ATM layer, where routing takesplace.

Hexadecimal - hex (mathematics)

Base 16. A number representation using the digits 0-9, with their usual meaning, plus the letters A-F (or a-f) to represent hexadecimal digits (or "hexits") with values of (decimal) 10 to 15. The right-most digit counts ones, the next counts multiples of 16, then 16^2= 256, etc..
For example, hexadecimal BEAD is decimal 48813:

        digit    weight        value
        B = 11   16^3 = 4096   11*4096 = 45056
        E = 14   16^2 =  256   14* 256 =  3584
        A = 10   16^1 =   16   10*  16 =   160
        D = 13   16^0 =    1   13*   1 =    13
                                         -----
                                BEAD   = 48813

There are many conventions for distinguishing hexadecimal numbers from decimal or other bases in programs. In C language for example, the prefix "0x" is used, e.g. 0x694A11. Hexadecimal is more succinct than binary for representing bit-masks, machines addresses, and other low-level constants but it is still reasonably easy to split a hex number into different bit positions, e.g the top 16 bits of a 32 bit word are the first four hex digits.
The term was coined in the early 1960s to replace earlier "sexadecimal", which was too racy and amusing for stuffy IBM, and later adopted by the rest of the industry.
Actually, neither term is etymologically pure. If we take "binary" to be paradigmatic, the most etymologically correct term for base 10, for example, is "denary", which comes from "deni" (ten at a time, ten each), a Latin "distributive" number; the corresponding term for base-16 would be something like "sendenary". "Decimal" is from an ordinal number; the corresponding prefix for 6 would imply something like "sextidecimal". The "sexa-" prefix is Latin but incorrect in this context, and "hexa-" is Greek. The word "octal" is similarly incorrect; a correct form would be "octaval" (to go with decimal), or "octonary" (to go with binary). If anyone ever implements a base-3 computer, computer scientists will be faced with the unprecedented dilemma of a choice between two *correct* forms; both "ternary" and "trinary" have a claim to this throne.

HIPPI

HIgh Performance Parallel Interface (physical layer)
See HPPI.

HLF

Higher Layer Functions (network)

HMMP

Hypermedia Management Protocol (communication, protocol,web)
A WBEM defined communication protocol. It could either work with or replace legacy protocol like SNMP.

HMMS

Hypermedia Management Schema (communication, protocol,web)
Originally defined by Microsoft and the Web-Based Enterprise Management (WBEM, HMMS forms the basis of the CIM data model.

HMON

HyperMedia Object Manager (web)
WBEM proposed design for a Web-based management request broker written in C++, part of CIM.

HOL

Head-of-Line (ATM)
The head position of a buffer (i.e. inside a switch). A blocking phenomenon is associated with the HOL which refers to the fact that cells in the queue have to wait for the HOL cell to depart first.

Home Page (World-Wide Web)

The top-level document relating to an individual or institution. This often has a URL consisting of just a hostname, e.g. http://www2.themanualpage.org/. All other pages on a server are usually accessible by following links from the home page.

Host

1. (Or "node") A computer connected to a network.
2. A computer to which one connects using a terminal emulator.

HPPI

High Performance Parallel Interface (communications, network)

HSSI

Height-Speed Serial Interface (standard, network)
Network standard for high-speed (up to 52 Mbps) serial connections over WAN links.
An interface between CSU/DSU and DXI.

HTML

Hypertext Markup Language (network, Internet, hypertext, World-Wide Web)
A document format with hypertext capabilities used on the World-Wide Web. Built on top of SGML.HTML supports some national characters through special escape sequences.

HTTP

Hyper Text Transfer Protocol (network, Internet, protocol)
The client-server TCP/IP protocol used on the World-Wide Web for the exchange of HTML documents. It conventionally uses port 80. Version 1.1 is the current standard.

Hub

(originally) A device connected to several other devices.
In ARCnet, a hub is used to connect several computers together. In a message handling service, a number of local computers might exchange messages solely with a hub computer. The hub would be responsible for exchanging messages with other hubs and non-local computers.
More commonly, a hub is the central signal distributor, used in a wiring topology consisting of several point-to-point segments originating from a central point.
It resynchronizes and retransmits data signals on a LAN like a repeater, but also has other capabilities such as network management.
A term applied to a multiport repeater or concentrator consisting of a chassis with slots to be populated by cards, allowing it to be configured with various numbers and combinations of LAN ports.
Multiport 10BASE-T, 10BASE2, and fiber optic (10BASE-FL, FOIRL) repeaters are considered hubs.
See also Smart wiring hub, concentrator, star topology.

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