Hardwire-Heavy Iron

Hardwire -Heavy Iron

Hardwire

To permanently connect by wire two or more devices rather than to connect them temporarily through connectors or switches. Hardwire is a term also used to represent a leased line.

Hardwire Services

An MCI definition. Services providing intercity communications facilities dedicated to the use of a specific customer, and provided through a dedicated access line from the customer to the MCI switch.

Hardwire Terminating City

City of circuit termination for hardwire services.

Harm

See Network Harm.

Harmful Interference

An FCC term for "any radiation or induction which endangers the functioning of a radio navigation service or of a safety service or obstructs or repeatedly interrupts a radio service."

Harmonic

A frequency which is an exact multiple of a fundamental frequency.

Harmonic Distortion

A problem caused when the nonlinearities in communication channels cause the harmonics of the input frequencies to appear in the output channel.

Harmonic Ringing

A way of stopping users on a party line from hearing other than their own ring. We do this by tuning the ringer in their phone to a given ringing frequency, so it only rings when their frequency comes down the line.

Harmonic Signals

Signals which are coherently related to the output frequency. In general, these signals are integer multiples of the output frequency.

Harmonica

1. A device attached to the end of a connectorized feeder cable that converts the 25 pair into individual 4, 6 or 8 wire modular channels. I have no idea why it's called a harmonica since it bears no relation to that other harmonica, also called a mouth organ, which is small rectangular wind instrument with free reeds recessed in air slots. Tones are created by inhaling and exhaling on the air slots. See also Harmonica Adapter.

Harmonica Adapter

An adapter that connects a 25-pair cable plug into 12 four- conductor RJ11 plugs or two 24 connector RJ11 plugs. These are often used as an alternative connectivity to hardwired blocks or temporary installations of key of PBX phone systems.

Harmonica Bug

See Infinity Transmitter.

Harmonics

This definition courtesy American Power Conversion Corp. In an AC power system, distortion of voltage or current waveforms may be expressed as a series of harmonics. Harmonics are voltage or current signals that are not at the desired 50 or 60 Hz fundamental frequency, but rather at some multiple frequency. For example, the fifth harmonic of 60 Hz is 300Hz. It is a characteristic of AC signals that any distortion will have components only at integer multiples of the fundamental frequency. In AC power distribution, these distortion components only occur at odd multiples of the fundamental frequency. The third harmonic voltage distortion at a typical wall outlet in the U.S. is about 3%. Harmonic voltages have virtually no effect on modern computers, but can cause overheating in some equipment.

Harry-Proof

To make something crash-proof, bug-proof, tamper-proof and idiot- proof. This term has been generously contributed by Steve Schone, of Danbury, CT. Micro Solutions, Inc., which has sold me Toshiba laptops and accessories since time immemorial. Steve coined the term, Harry-Proof, after he sent me a special "backup" package of hard disk and complex software. He claimed that he had constructed the whole system to be "Harry-Proof," by which he meant that someone of my awesome intellect couldn't destroy it or the hard disk I was trying to copy (as against erasing it) ” though he retained the right to be wrong. He was.

Hash

Total Adding up one or more information fields in order to provide a check number for error control. The addition is not intended to have any meaning other than for checking.

Hash Value

A small amount of binary data, typically around 160 bits, derived from a message by using a hashing algorithm. The hashing procedure is one-way. There is no feasible way of deriving the original message, or even any of its properties, from the hash value, even given the hashing algorithm. The same message will always produce the same hash value when passed through the same hashing algorithm. Messages differing by even one character can produce very different hash values. See Hash and Hashing.

Hashing

A cryptographic term for a small mathematical summary or digest of an original clear-text data file or message. The hash function takes a variable-length message input, and produces a fixed-length hash output which commonly is in the form of a 128- bit " fingerprint " or "message digest." For example, "Newton's Telecom Dictionary" might be hashed as "123456789123456." The resulting hashes are stored in "hash buckets" in a "hash table," with the table indexed in such a way as to speed the process of sorting through them to find a specific hash. A hash algorithm ensures data integrity through the detection of changes to the data caused either by communications errors occurring in transit, or by tampering. In combination, hashing and the use of a digital signature (digital certificate) prevent the forging of an altered message. See also Digital Certificate, Encryption, and MD5.

Hashing Function

An algorithm that takes as input an original message and produces a fixed-length summary of that message. In order for a hashing function to be effective, the result (a.k.a. message digest) must be unique to the original message (within an acceptable range of certainty ) Hashing functions are sometimes thought of as one-way encryption schemes because they cannot be easily reversed (i.e. decrypted).

HASP

Houston Automatic Spooling Priority system. HASP is a spooling method used to permit many processes to perform operations such as printing at the same time, without actually "owning" the device. Time was, a job that wanted to output a line of print had to be physically attached to the printer. SPOOLing creates virtual devices and places the output on disk while it manages I/O to the real devices.

Haul Back Trunk

Haul back trunk is a term used by AT&T when referring to trunks that route calls off the AT&T network.

HAVi/Jini

The Home Audio-Video interoperability architecture defines protocols and interfaces to allow digital electronics to be connected and shared on a home entertainment network, in conjunction with IEEE 1394 (also called Firewire). Philips, Sony and Sun Microsystems announced plans to bridge the HAVi architecture to Sun's Jini technology, which allows devices to be recognized and operated over a network. See IEEE 1394.

Hawthorne Effect

The Hawthorne effect is the most famous psychological study in the history of the telecommunications industry. It refers to a study from 1927 to 1933 of factory workers at Western Electric's Hawthorne Plant in Illinois. It showed that regardless of the changes made in working conditions ” more breaks, longer breaks or fewer and shorter ones ” productivity increased. These changes apparently had nothing to do with the workers' responses. The workers, or so the story goes, produced more because they saw themselves as special, participants in an experiment, and their inter-relationships improved.

Sounds very compelling, wrote the New York Times on December 6, 1998. "The results of this experiment, or rather the human relations interpretation offered by the researchers who summarized the results, soon became gospel for introductory textbooks in both psychology and management science," said Dr. Lee Ross, a psychology professor at Stanford University. But only five workers took part in the study, Ross said, and two were replaced partway through for gross insubordination and low output.

A psychology professor at the University of Michigan, Dr. Richard Nisbett called the Hawthorne effect "a glorified anecdote."

"Once you've got the anecdote," he said, "you can throw away the data," reported the New York Times.

Hayes AT Command Set

Before 1981, the modem was a dumb device. It had no memory or ability to recognize commands. It simply modulated and demodulated signals between the telephone line and the computer or terminal. In 1981, Hayes Microcomputer Products, Inc. in Norcross, GA produced the first "smart" modem, appropriately named the Smartmodem 300. It was "smart" because it understood commands, such as "ATD" which means "ATtention, Dial the phone." The Hayes Standard AT Command Set (its full name ) ” a language for modems ” has been accepted as a standard by the modem industry. And now many modems claim to be 100% Hayes compatible, which may mean they are and may mean they aren't. As in all cases of claimed compatibility, one should check. You'll find the complete Hayes AT Command Set spelled out in virtually every manual of every modem which purports to be "100% Hayes Compatible." See also Class 1.

HBA

Host Bus Adapter. A printed circuit board that acts as an interface between the host microprocessor and the disk controller. The HBA relieves the host microprocessor of data storage and retrieval tasks , usually increasing the computer's performance time. A host bus adapter (or host adapter) and its disk subsystems make up a disk channel.

HBFG

Host Behavior Functional Group: The group of functions performed by an ATM- attached host that is participating in the MPOA service.

HBS

Home Base Station. A wireless PCS term. Supports the PCS 1900 air interface in combination with the PCS 1900 handset.

HCI

Host Command Interface. Mitel SX-2000 PBX to computer link. HCI is designed to work with Digital Equipment Corporation computers. See Open Application Interface.

HCL

Hardware Compatibility List. See HCL.

HCO

Hearing Carry Over. A reduced form of Telecommunication Relay Service (TRS) where the person with a speech disability is able to listen to the other end user and, in reply, the Communications Assistant speaks the text as typed by the person with the speech disability. The Communications Assistant does not type any conversation.

HCS

1. Hundred Call Seconds. One hundred seconds of telephone conversation. See CCS.

2. Hard Clad Silica.

HCT

Hardware Compatibility Test. Microsoft came up with this definition and concept when it found several computers didn't run its software as well as they should. Basically the Microsoft Hardware Compatibility Test (HCT) is a series of tests for verifying the compatibility of hardware systems with Windows NT. The TCT Test Manager is an application that provides a way to launch the tests, keep track of test results and return the results to Microsoft. Microsoft maintains a Windows NT Hardware Compatibility List (HCL). If you send back the test results and your stuff passes , your hardware will be included on Microsoft's list of hardware that works with NT.

HD

Half Duplex circuit.

HD Radio

See High Definition Radio.

HDB3

High Density Bipolar 3. Specified in the ITU-T G.703 recommendation, HDB3 is a line coding technique used in E-1 circuits. Similar to the B8ZS line coding technique used in North American T-1 circuits, HDB3 substitutes strings of four zeros with one of four bipolar violation codes. The first violation code is a single pulse in bit position 4, and matches the polarity of the last preceding pulse. Subsequent violation codes also always are in bit position 4, and are of the opposite polarity of the previous violation code. HDB3 supports clear channel communications of 64 Kbps per channel, as does B8ZS in the T-1 environment. See also B8ZS.

HDD

Hard Disk Drive.

HDLC

High level Data Link Control. A link layer protocol (Layer 2 of the OSI Reference Model) standard for point-to-point and point-to-multipoint communications. HDLC was based on IBM's SDLC (Synchronous Data Link Control) and ANSI's ADCCP (Advanced Data Communication Control Procedure). HDLC encapsulates packet data in a frame (i.e., yet another packet), with the frame header and trailer including various control information such as an error control mechanism. Variants on HDLC include Frame Relay, LAP-B (Link Access Procedure-Balanced), Link Access Procedure-Data channel), PPP (Point-To-Point Protocol), and SDLC (Synchronous Data Link Control).

HDMAC

Another potential high definition TV standard. HDMAC was spawned by Britain's Independent Broadcasting Authority. Unlike Japan's Hi-Vision, HDMAC has the attraction of being compatible with existing TV sets, i.e. those in Europe.

HDML

Handheld Device Markup Language, which is Unwired Planet of Redwood Shores, CA's modification of standard HTML for use on mobile phones. HDML is a text-based markup language which uses HyperText Transfer Protocol (HTTP) and is compatible with all Web servers. HDML is designed to display on a smaller screen such as one might find on a cellular phone, PDA, pager, or PCS device. The basic structural unit for HDML is a "card," while that of HTML is a "page." HDML allows the mobile user to access the Internet, and send, receive and redirect e-mail. As PCS devices are graphics-challenged, a Web site must be HDML-enabled in order to allow access by such devices. www.wapforum.org. See also WAP.

HDSL

High-bit-rate Digital Subscriber Line, also known as HDSL1. The most mature of the xDSL technologies, HDSL allows the provisioning of T-1/E-1 local loop circuits much more quickly and at much lower cost than through conventional means. In the U.S., HDSL delivers T-1 at 1.536 Mbps over a four-wire loop of two pairs. E-1 capacity of 2.048 Mbps originally required three pairs, but now also requires only two pairs. Unlike ADSL, HDSL bandwidth is symmetric, as equal bandwidth is provided in each direction.

The traditional approach of provisioning T-1/E-1 access loops on copper wires requires specially-conditioned UTP (Unshielded Twisted Pair), with repeaters spaced every 6,000 feet in order to compensate for signal attenuation at the high carrier frequencies required. Each pair supports simplex (one-way) transmission at 1.544 Mbps, of which 1.536 Mbps is usable for data transmission, with the remaining .008 Mbps being used for signaling and control purposes. In combination, the two simplex circuits yield a full-duplex circuit.

HDSL, which involves special electronics at both the CO and the customer premise , delivers the same transmission capacity over standard UTP at distances up to 12,000 feet on 24 AWG (American Wire Gauge) wire and up to 9,000 feet on 22 AWG wire, without the requirement for repeaters. The UTP loop may be bridged, although loading coils are not tolerated. This is accomplished through full-duplex transmission at 784 Kbps over each pair of the four-wire circuit; 768 Kbps is usable for data transmission, with the remaining 16 Kbps being required for signaling and control. In the aggregate, the yield is 1.536 Mbps (T-1). The lower transmission rate on each pair implies a much lower carrier frequency. As lower frequency signals can travel much longer distances without experiencing unacceptable levels of attenuation (loss of signal strength), the requirement for repeaters is obviated for distances up to 12,000 feet. Note: T-1 requires 1.5 MHz over each pair, one supporting upstream transmission and the other supporting downstream transmission. While several options exist for electrical encoding, all of them are unibit schemes, impressing a single bit on each baud. HDSL operates at frequencies ranging from 80 KHz to 240 KHz, depending on the specific techniques employed. In order to improve the data transmission speed at a given carrier frequency, HDSL uses the 2B1Q electrical encoding scheme, which also is known as 4 PAM (Pulse Amplitude Modulation); this scheme is a dibit coding scheme, which allows to bits to be impressed on each baud. Echo cancellation allows each pair to support full-duplex (simultaneous two-way) transmission over each pair.

HDSL has been deployed aggressively by LECs for some years . Well over 500,000 systems reportedly are in service. Reportedly, as many as 70% of all U.S. T-1 circuits employ HDSL. Although both the COT (Central Office Termination) and the RT (Remote Termination) require the placement of HDSL electronics, the overall carrier costs of provisioning are much reduced. No special circuit engineering, no physical inspection of cable plant, and no repeater acquisition and placement is required. Additionally, the circuit can be provisioned much more quickly, which fact results in much happier customers and much faster revenue generation. In fact, several LECs have lowered their T-1 rates in consideration of the lower costs.

At the time of this writing, a proposal for a new variation on the HDSL theme recently was proposed as a standard. HDSL2, based on technology from Adtran Inc., provides the same capability over a single pair, although the local loop length is limited to about 10,000 feet. This technology also is known as SDSL (Single line DSL). S-HDSL (Single-line HDSL) is a variation on this non-standard variation (It gets confusing, doesn't it? Remember that this is an emerging technology.) run at speeds of 768 and 384 Kbps for loop lengths of 12,000 feet and 18,000 feet, respectively. See also DSL, ADSL, HDSL2, IDSL, RADSL, SDSL, T-1, T1E1.4 and VDSL. www.adtran.com and www.adsl.com.

HDSL2

High-bit-rate Digital Subscriber Line version 2. Also known as G.shdsl (G. is the ITU-T Recommendation series under which HDSL2 is being considered ) and SDSL (Single line DSL). On January 6, 1998, Level One Communications, ADC Telecommunications, Adtran, PairGain Technologies and the Siemens Semiconductor Group today announced agreement within the American National Standards Institute (ANSI) TIE1.4 committee on the basis on an HDSL2 standard. A provisional agreement, T-1/E-1 contribution number 41/97-471, has been approved marking a milestone within the ANSI HDSL2 standards effort. The elements agreed upon were line code in the form of advanced Trellis code, a precoding mechanism, spectral shaping, equalization circuits, and forward error correction. These elements make up the core of the HDSL2 standard. The agreement reached is expected to accelerate the development HDSL2 technology and promote industry interoperability. The HDSL2 standard proposal will enable service providers to deliver full T-1 (1.544 Mbps), and potentially E-1 (2.048 Mbps), performance over a single twisted pair cable, and over a longer reach than HDSL, now also known as HDSL1. Over 24 AWG (American Wire Gauge)wire, HDSL2 is intended to support a data rate of 1.536 Mbps (T- 1) over a distance of up to 13,200 feet, 768 Kbps up to 17,700 feet, and 384 Kbps up to 22,500 feet. Over 25 AWG wire, the distances are 9,200 feet, 12,400 feet, and 15,500 feet, respectively. The modulation technique originally approved by the ANSI T1E1.4 committee is 16 PAM (Pulse Amplitude Modulation), an improvement on the 2B1Q (also known as 4 PAM) technique used in HDSL1. Subsequently, OPTIS (Overlapped Phase Trellis-code Interlocked Spectrum) also was approved. See also HDSL, SDSL, and T1E1.4.

HDT

Host Digital Terminal. A cable telephony term. A HDT is located at the head end (i.e., the point of signal origin) of the traditional CATV network. In telephony terms, the HDT is at the edge of the carrier network. The HDT interfaces T-carrier (e.g., T-1 and T-3) circuits from the Central Office (CO) switch in support of voice services over the coaxial cable network. The HDT modulates those voice signals over the broadband coax cable, interfacing with a Remote Service Unit (RSU) at the subscriber premises, where the process is reversed. See RSU for more detail.

HDTP

1. Handheld Device Transport Protocol ” a wireless-optimized protocol sitting between the UP.Browser's HDML interpreter and a datagram transport (typically UDP). HDTP provides security and reliability for the transport in a way that is significantly more efficient (optimized for wireless and communication with a minimal number of IP addresses) than is TCP. HDTP is the protocol in use on UP's UP.Link Platform V2.x. Version 3 sees UP migrating towards WAP - the new Wireless Application Protocol, from the WAP Forum. www.wapforum.org.

2. Hoofddirectie Telecommunicatie en Post. Directorate for Telecommunications and Posts, The Netherlands.

HDTV

High Definition TeleVision. HDTV is a new TV standard producing a better quality picture and better quality sound. The hallmark of HDTV is the high resolution of display and the wide rectangular screen. HDTV produces a picture quality approaching 35mm film and sound approaching compact disc quality. HDTV screens use a width to height ratio of 16:9 (also referred to as 1.78:1). An analog TV (i.e. today's version) has a 4:3 ratio (or 1.33:1). HDTV should not be confused with Digital Television (DTV). DTV is an umbrella term for the transmission of pure digital television signals, along with the reception and display of those signals on a digital TV set. This is designed to replace the old analog signals. The digital signals might be broadcast over the air or transmitted by a cable or satellite system to your home. In your home, a decoder receives the signal and uses it, in digital form, to directly drive your digital TV set. HDTV is a type of digital television. It is high-resolution digital television combined with Dolby Digital surround sound (AC-3). Standard Definition Digital Television (SDTV) is also a type of DTV, without the superior resolution that HDTV offers. Standards for HDTV are still somewhat up in the air. There are two main standards for HDTV:

1. 1080 vertical lines by 1920 horizontal pixels, which is favored by many mainstream broadcasters and television networks. It produces high-quality pictures, but leaves less bandwidth for other channels and data services. This is called 1080i and is the most common in North America. The i stands for interlacing which means that every 60th of a second, all the odd-numbered lines are drawn, then in the next 60th of a second, the even lines. The p stands for progressive scan.

2. 720 vertical lines by 1280 horizontal pixels, a format favored by cable TV companies and the PC industry, as it leaves more bandwidth available.

If you decide to buy an HDTV, here are a few things to look out for:

• Integrated HDTV. This is a high resolution set with the HD-Tuner is built-in. It comes with a higher price tag, and could become obsolete if standards change. But I'm dubious they will if your TV has both 720p and 1080i.

• 'HDTV-Capable' or 'HDTV-Ready' television sets. Generally more affordable, these TV sets (also called HDTV Monitors) can display both the existing NTSC analog signals and the new ATSC digital signals. However, they require an external, High Definition tuner (set top box) to receive and display HDTV programs.

• 'Digital Ready' does not necessarily mean the TV will receive and display digital High Definition television programs. Verify the set you are considering will display true HDTV.

• Resolution. The ATSC Standard for High Definition Television requires a resolution of 1080 interlaced lines, or 720 progressive scan lines. Verify the set you choose is capable of working with both signals - i.e. converting (up/down) all signals to their native resolution.

• Audio. The Standard for HDTV is "Dolby A3" ” 5.1 Channel Surround Sound. 5.1 means five channels of audio ” one in the middle and two at front side ” and two channels at the back, on either side. The .1 means that there is a separate channel to drive a subwoofer . Beware, some manufacturers have their own proprietary audio systems. While many of these produce a good audio experience, to get the best sound, choose DolbyA3 Surround Audio, or better.

• Connections. Choose a model that offers the most ports with the widest selection of connector types. Also check for additional front ports ” these are convenient . You should look for composite, S-video and component video as a minimum set of analog jacks so you can use your existing analog audio equipment with the new set.

• Compatibility. If you are purchasing a separate set and tuner, be sure you verify that the tuner is compatible with the set you are purchasing, as well as the satellite/cable you expect to use, and is also capable of receiving over-the-air broadcasts. Some HD-tuners require an add-on module (8VSB) to receive OTA (Over The Air) Broadcasts. Also make sure the HDTV set will work with all your other components: VCR, DVD player, video game consoles, sound system, etc. Other questions to consider:

• Are all cables included and compatible with your audio-video components?

• Do you have a surge protector to protect your investment? If a storm approaches, we recommend not relying on the surge arrestor. Unplug the system and wait for the storm to past.

• What type of external antenna (if any) do you need for OTA (over the air) broadcasts in your area?

• Is delivery and set up included? This is especially important for the 'big-screen' projection sets, which can be heavy.

• Check for in-home maintenance/tune up contract ” this may be an important consideration for large projector systems that require periodic adjustments. HDTV does not, of course, affect a movie's plot. Samuel Goldwyn, film producer, once said, "A wide screen just makes a bad film twice as bad." See also ATV, DTV, Goldwyn, Samuel, NTSC and SDTV.

HDTV-Ready

A television set capable of displaying a full high definition picture because of its high screen resolution. See HDTV.

HDX

Half DupleX.

HDWDM

Hyper Dense Wavelength Division Multiplexing ” 128 wavelengths and higher. See DWDM.

HE

See Head End.

Head

1. A device that reads, writes, or erases data on a storage medium. The device which comes in contact with or comes very close to the magnetic storage device (disk, diskette, drum, tape) and reads and/or writes to the medium. In computer devices, it performs the same function as the head on a home cassette tape recorder.

2. A sub-component accessory which serves a translator type function for signal being input to some larger system or instrument. Many pieces of test equipment have various available plug-in "sampling heads" which allow the equipment to be used with a variety of different signal types. The most common type is probably optical-to-electrical converters which convert an optical signal for display on an oscilloscope.

3. A term for a toilet . The "head" was so named because it was in the ship's bow (the front) by the catheads. On sailing ships all smelly activities, i.e., the head, the galley, and the crew's quarters were all placed in the ship's bow so the wind could blow the smell away. That way the officers and high-quality passengers who berthed in the stern were not offended.

Head End

1. The originating point of a signal in cable TV systems. At the head end, you'll often find large satellite receiving antennas. Now increasingly spelled headend.

2. A central control device required within some LAN/MAN systems to provide such centralized functions as remodulation, re-timing, message accountability, contention control, diagnostic control, and access.

Head End Hop Off

HEHO. A method of traffic engineering whereby calls are completed by using long distance facilities directly off the switch that serves that location.

Head Fake

See Suckers Rally.

Head Landing Zone

In older hard drives, the head landing zone is an area of the hard disk set aside for take off and landing of the heads when the drive is turned on and off. In newer drives , the heads are retracted.

Head On Collision

Descriptive of a condition in message telephony when two switching exchanges seize a both ways trunk at the same instant and attempt to send outbound call instructions to each other.

Head Slap

Similar to head crash but occurs while the drive is turned off. It usually occurs during mishandling or shipping. Head slap can cause permanent damage to a hard disk drive.

Head Thrashing

A term for rapid back and forth movements of the disk head of a hard drive.

Headend

The originating point of a signal in cable TV systems. At the head end, you'll often find large satellite receiving antennas.

Header

1. Protocol control information located at the beginning of a protocol data unit.

2. The portion of a message that contains information that will guide the message to the correct destination. This information contains such things as the sender's and receiver's addresses, precedence level, routing instructions, and synchronization pulses .

Header Area

The area containing preliminary information for the entire document, such as the data, company name, address, purchase order, terms, etc. An EDI (Electronic Data Interchange) term.

Header Error Control

HEC. An 8-bit CRC code contained within the header of an ATM data cell. The HEC is used for checking the integrity of the cell header at the various cell switches.

Header Information

William Safire defines it as data at the top of a credit report which the Federal Trade Commission says may be disclosed by any credit bureau with no restrictions. This includes your name, address, phone number (listed or not), social security number and mother's maiden name.

Headless Server

A server computer with no monitor attached.

Headroom

A term used in the structured cabling industry to indicate additional clearance or signal margin room above the specification. This is measured in Decibels and is related to the Attenuation to Cross-Talk Ratio (ACR).

Headset

A telephone transmitter and receiver assembly worn on the head. Headsets are now very light and very comfortable and are no longer worn only by switchboard attendants and airline clerks. They are worn by telemarketers , customer service reps. stock brokers , order entry reps, financial service professionals, and some executives who spend a lot of time on the phone.

Headset Jack

A place on a phone or console into which you can plug a headset.

Heads-up Back in the mid-twentieth century, when someone called out "heads up!" listeners knew to lift up their heads and watch out for something dangerous.

Hearing Aid Compatible

A hearing aid compatible phone may be used with inductively coupled hearing aid devices. You can find hearing aid compatible coin phones by looking for the blue grommet between the handset and the cord.

Hearing Carry Over

HCO. A form of TRS (Telecommunication Relay Service) where a person with a speech disability is able to listen to the other end user and, in reply, a Communications Assistant speaks the text as typed by the person with the speech disability. See TRS.

Hearing Designation Order

HDO. A Federal Communications Commission term, a Hearing Designation Order institutes a comparative or other adjudicatory hearing proceeding, usually before an Administrative Law Judge.

Heartbeat

Ethernet-defined SQE signal quality test function, defined in IEEE 802.3. Heartbeat is created by a circuit (normally part of the transceiver) that generates a collision signal at the end of a transmission. This signal is used by the controller interface for self-testing .

Heartbeat Support

A function that generates a frame periodically, even if no data is sent, for network management purposes.

Heat

Electromagnetic waves of a frequency between that of light waves and radio waves. A form of energy.

Heat Coil

An electrical protection device used to prevent equipment from overheating as a result of foreign voltages that do not trigger voltage limiting devices. It typically consists of a coil of fine wire around a brass tube that encloses a pin soldered with a low-melting alloy. When abnormal currents occur, the coil heats the brass to soften the solder, allowing the spring-loaded pin to move against a ground plate directing currents to ground.

Heatseeker

The person who can be depended on to purchase the latest version of any existing software product as soon as it comes on the market.

Heaven

Everybody wants to go to Heaven, but nobody wants to die - ancient Chinese fortune cookie.

Heavy Iron

Hardware, really BIG hardware. Contemporary hardware hardly qualifies. For example, the ENIAC (Electronic Numerical Integrator and Computor), built in 1946 at a cost of about $400,000, was the first large-scale electronic digital computer built. The ENIAC contained about 18,000 vacuum tubes, weighed 30 tons and occupied a footprint of 30x50 feet. In 1949, Popular Mechanics magazine forecast that "Computers in the future may...perhaps only weigh 1.5 tons." Technology marched on. Your laptop provides more horsepower than the ENIAC, has more memory, and more functionality.