H.261: Video Compression Format (ITU)
Hacker: A hacker is a very capable programmer.In The New Hacker's Dictionary, Eric Raymond lists five connotations forthis term as applied to programming, which we paraphrase here:
1. A person who enjoys learning details of a programminglanguage or system
2. A person who enjoys actually doing the programmingrather than just theorizing about it
3. A person capable of appreciating someone else's hacking
4. A person who picks up programming quickly
5. A person who is an expert at a particular programminglanguage or system, as in "UNIX hacker"
Eric Raymond deprecates the use of this term for someone who attemptsto crack someone else's system or otherwise uses programming or expertknowledge to act maliciously. He prefers the term "cracker" for this meaning.However, as Raymond notes, journalists often miss the distinction.
Haptic Devices: a device that can both sense(receive) and send information.
Hashing: Hashing is the transformationof a string of characters into a usually shorter fixed-length value orkey that represents the original string. Hashing is used to index and retrieveitems in a database because it is faster to find the item using the shorterhashed key than to find it using the original value. As a simple example,a group of people could be arranged in a database like this:
(and many more sorted into alphabetical order)
Each of these names would be the key in the database for that person'sdata. A database search mechanism would first have to start looking character-by-characteracross the name for matches until it found the match (or ruled the otherentries out). But if each of the names were hashed, it might be possible(depending on the number of names in the database) to generate a unique
four-digit key for each name. For example:
7864 Abernathy, Sara
9802 Epperdingle, Roscoe
1990 Moore, Wilfred
8822 Smith, David
(and so forth)
A search for any name would first consist of computing the hash value(using the same hash function used to store the item) and then comparingfor a match using that value. It would, in general, be much faster to finda match across four digits, each having only 9 possibilities, than acrossan unpredictable value length where each character had 26 possibilities.
The hashing algorithm is called the hash function (and probably theterm is derived from the idea that the resulting hash value can be thoughtof as a "mixed up" version of the represented value). In addition to fasterdata retrieval, hashing is also used to encrypt and decrypt digital signatures(used to authenticate message senders and receivers). The digital signatureis transformed with the hash function and then both the hashed value (knownas a message-digest) and the signature are sent in separate transmissionsto the receiver. Using the same hash function as the sender, the receiverderives a message-digest from the signature and compares it with the message-digestit also received. They should be the same.
The hash function is used to index the original value or key and thenused later each time the data associated with the value or key is to beretrieved. Thus, hashing is always a one-way operation. There's no needto "reverse engineer" the hash function by analyzing the hashed values.In fact, the ideal hash function can't be derived by such analysis. A goodhash function also should not produce the same hash value from two differentinputs. If it does, this is known as a collision. A hash function thatoffers an extremely low risk of collision may be considered acceptable.
Here are some relatively simple hash functions that have been used:
HDL: Hardware Description Language
HDLC: High-Level Data Link Control, (IBM)
HDSL: High bit rate Digital Subscriber Line/ Loop; Modems, or more accurately 'transceivers' on either end of oneor more twisted pair wires that deliver T1 or E1speeds. At present T1 requires two lines and E1requires three. HDSL uses a coding technique called "2B1Q"to achieve bit rate of 1.5 Mbps over distances up to 18,000 feet. See SDSLfor one line HDSL.
HDT: Host Digital Terminal
HDTV: High Definition Television; offers approximatelytwice the vertical and horizontal resolution of NTSC,which is a picture quality approaching 35 mm film andhas a sound quality approaching that of a compact disc. High definitiontelevision has a resolution of approximately twice that of conventionaltelevision in both the horizontal (H) and vertical (V) dimensions and apicture aspect ratio (HxV) of 16:9. ITU-R Recommendation1125 further defines "HDTV quality" as the delivery of a television picturewhich is subjectively identical with the interlaced HDTV studio standard.
Headend: The central location on HFCnetwork that is responsible for injecting broadcast video and other signalsin the downstream direction
HFC: Hybrid Fiber Coax
HFS: Hierarchical File System, (Apple - Macintosh)
HIPPI: HIgh Performance Parallel Interface
HMD: Head Mounted Display
HOL: Head of Line
HP: Hewlett Packard, (http://www.hp.com)
HP-UX: version of Unix provided by HewlettPackard, (HP)
HPGL: Hewlett Packard Graphics Language, (HP)
HPCC: High Performance Computing and Communications;
HSC: Hughes Space and Communications; (a HughesElectronics Company).
HSE: Headend Service Element
HSSI: (High speed Synchronous Serial Interface)Interface for transferring data to or from a WAN leasedline (E2, E3), or to and from a LAN via an HSSI-capableDSU and HSSI router.
HTTP: Hyper-Text Transport Protocol
Hz: Hertz; The standard unit of frequency. OneHz equals one cycle (or vibration) per second. One kilohertz (KHz) equals1,000 cycles per second, and one megahertz (MHz) equals 1,000,000 cyclesper second. Hertz is named after German physicist Heinrich Hertz (1857-1894).