H.225
H.225, a key protocol in the H.323 VOIP architecture defined by ITU-T, is a standard to cover narrow-band visual telephone services defined in H.200/AV.120-Series Recommendations. It specifically deals with those situations where the transmission path includes one or more packet-based networks, each of which is configured and managed to provide a non-guaranteed QoS, which is not equivalent to that of N-ISDN, such that additional protection or recovery mechanisms beyond those mandated by Rec.

H.225.0
H.225.0 is the specific document number for RAS, use of Q.931, and use of RTP under the H.323 VOIP architecture. It is also called as H.225.

H.235
H.235 is the security recommendation for the H.3xx series systems. In particular, H.235 provides security procedures for H.323-, H.225.0-, H.245- and H.460-based systems. H.235 is applicable to both simple point-to-point and multipoint conferences for any terminals which utilize H.245 as a control protocol.

H.245
H.245, a control signaling protocol in the H.323 multimedia communication architecture, is for the exchange of end-to-end H.245 messages between communicating H.323 endpoints/terminals. The H.245 control messages are carried over H.245 control channels. The H.245 control channel is the logical channel 0 and is permanently open, unlike the media channels. The messages carried include messages to exchange capabilities of terminals and to open and close logical channels.

H.248
H.248, also known as Media Gateway Control protocol (Megaco), is for the control of elements in a physically decomposed multimedia gateway, enabling the separation of call control from media conversion. Megaco is a result of joint efforts of the IETF and the ITU-T Study Group 16. Therefore, the IETF defined Megaco is the same as ITU-T Recommendation H.248.

H.261
H.261 is the video coding standard of the ITU. It was designed for data rates which are multiples of 64Kbit/s and is sometimes called p x 64Kbit/s (p is in the range 1-30). These data rates suit ISDN lines, for which this video codec was originally designed. H.261 transports a video stream using the real-time transport protocol, RTP, with any of the underlying protocols that carry RTP.

H.263
The H.263, by the International Telecommunications Union (ITU), supports video compression (coding) for video-conferencing and video-telephony applications. H.263 was developed to stream video at bandwidths as low as 20K to 24K bit/sec and was based on the H.261 codec. As a general rule, H.263 requires half the bandwidth to achieve the same video quality as in the H.261. As a result, H.263 has largely replaced H.261. H.263 uses RTP to transport video streams.

H.264
The H.264, also named Advanced Video Coding (AVC), is the MPEG-4 Part 10. H.264 is jointly developed by ITU and ISO. H.264 supports video compression (coding) for video-conferencing and video-telephony applications. The H.264 video codec has a very broad rang of applications that covers all forms of digital compressed video from, low bit-rate Internet streaming applications to HDTV broadcast and Digital Cinema applications with nearly lossless coding. H.264 is designed as a simple and straightforward video coding with enhanced compression performance, to provide a “network-friendly” video representation.

H.323
H.323, a protocol suite defined by ITU-T, is for voice transmission over internet (Voice over IP or VOIP). In addition to voice applications, H.323 provides mechanisms for video communication and data collaboration, in combination with the ITU-T T.120 series standards. H.323 is one of the major VOIP standards, on a par with Megaco and SIP.

H.450.2
H.450.2 is the call transfer supplementary service in the H.323 VOIP architecture.

H.450.3
H.450.3 is the call diversion supplementary service in the H.323 VOIP architecture.

H Channel: High-Speed Channel
High-speed channel (H-Channel) is the full-duplex ISDN primary rate channel operating at 384 kbps.

Hagelbarger Code
Hagelbarger code is a convolutional code that enables error bursts to be corrected as long as there are relatively long error-free intervals between the error bursts.

Halftone Characteristic
Halftone characteristic refers to the following definitions in telecommunication: (1) in facsimile system, the relationship between the density of the recorded copy and the density of the original. (2) in facsimile system, the relationship between the amplitude of the facsimile signal to either the density of the original or the density of the recorded copy when only a portion of the system is under consideration.

Hamming Code
Hamming code, sometimes referred to as an Error Correction Code (ECC), is an algorithm that can be used to detect errors in individual bits of transmitted data, and sometimes (dependent on the exact code used) correct that error. Although not particularly powerful, they are one of the "perfect" codes in that its standard array has all of the error patterns that can exist for single errors.

Handset
Handset refers to any of the various forms of manual telephone instruments held in hands that can receive and send voice.

Hardwired
Hardwired refers to: (1) equipment which is wired together so that its function if fixed as compared to equipment in which plug-in components may be varied to change the function. (2) equipment units which are permanently cabled together as compared to those which may be flexibly interconnected at a distributing frame.

HCO: Hearing Carry Over
Hearing Carry Over (HCO) refers to a method for using TTYs (Teletypewriter). HCO allows a person with a speech disability to hear a response from their party directly. HCO is often used in conjucntion with a Relay Service.

HCS: Header Check Sequence
Header Check Sequence (HCS) is a 16-bit cyclic redundancy check (CRC) check sequence (CS) that is derived from bits from the first 8 octets (excluding flags) of a UIH format packet.

HD: High Definition
High Definition (HD) is a video format consisting of withering 720p (active lines of progressive ) or 1080 which could be either active lines of progressive, 1080p, or interlaced, 1080i.

HDSL: High Data Bit Rate DSL
High Data Bit Rate Digital Subscriber Line (HDSL), also known as High Speed DSL or High data-rate DSL, provides equal bandwidth for both downloads and uploads. HDSL is one of oldest DSL technologies. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection.

HDSL2: 2nd generation HDSL
HDSL2, the 2nd generation HDSL (High Data Bit Rate Digital Subscriber Line), is a variant of HDSL which delivers 1.5 Mbps service each way, supporting voice, data, and video using either ATM (asynchronous transfer mode), private-line service or frame relay over a single copper pair. This ATIS standard (T1.418) for this symmetric service gives a fixed 1.5 Mbps rate both up and downstream. HDSL2 does not provide standard voice telephone service on the same wire pair. HSDL2 differs from HDSL in that HDSL2 uses one pair of wires to convey 1.5 Mbps whereas ANSI HDSL uses two wire pairs.

HDSL4: 4th generation HDSL
HDSL4, the 4th generation HDSL (High Data Bit Rate Digital Subscriber Line), is an enhanced version of HDSL2 which achieves about 30% greater distance than HDSL or HDSL2 by using two pairs of wire (thus, 4 conductors), whereas HDSL2 uses one pair of wires.

HDB3: High Density Bipolar 3
High Density Bipolar 3 (HDB3) refers to a bipolar signaling technique. It relies on the transmission of both positive and negative pulses. High Density Bipolar 3 is based on Alternate Mark Inversion (AMI), but extends this by inserting violation codes whenever there is a run of 4 or more 0's.

HDT: Host Digital Terminal
Host Digital Terminal (HDT) is the interface between a cable distribution system and the telephony switch, where the HDT acts as a digital multiplexer. It provides T1/E1 links to the telephony switch at 1.544 Mb/s and accepts 64 kb/s digital signals from lines on the subscriber side, usually in a T1/E1 format. Most vendors have designed the HDTs with an open interface to the telephony switch, allowing the service provider to choose different vendors for the switch and the HDT. On the subscriber side, however, the connection to the Network Interface Unit (NIU) is proprietary, requiring the cable operator to purchase both the HDTs and NIUs from the same vendor. Having an open interface to the switch also enables a cable operator to obtain telephony switching from another company through alliances or leasing agreements, so the operator doesn't need its own digital switch in the early stages of telephony offerings.

Headset
Headset refers to a voice communicatins sending and receiving device worn on the head.

Hierarchical Network
Hierarchical Network is a network that includes two or more different classes of switching systems in a defined arrangement.

High Tone
High Tone is a generic tone used with various interruption patterns for the specific tones. High Tone is 4800 Hz at - 17dBm0. On some systems manufactured before 1974, High Tone was 400 Hz or 500 Hz at 61 - 71 dBmC. The following are a list of some specific High Tones: Partial Dial Tone, Permanent Signal, Coin Return (test) Tone, Coin Return Tone, Number Checking Tone, Interception Loopback Tone, Warning Tone, Order Tone, Station Ringer Test and Class of Service.

High Water Mark
High Water Mark in telecom refers to a counter that reports the highest number of DS0s that were in use at one time.

High-Priority Emergency Service Call
High-priority emergency service call is a call identified as receiving emergency service treatment, which has been marked as requiring special treatment by the emergency service administration. The call has an associated CESID and transfer number.

High-Usage Group
High-Usage Group refers to a trunk group that is the primary direct route between two switching systems. The group is provided with an alternate route for overflow traffic in order to provide an acceptable level of blocking.

Hit
Hit, in telecommunication, refers to a random and momentary disturbance in a transmission path which could result in a transmission error.

Holding Time
Holding Time is the length of time an attempt, call, or message occupies a component of the network that is normally expressed in seconds or minutes.

Home Tandem
Home Tandem is a tandem of a higher office class to which another tandem of an end office has a final trunk group. It may exist for all or defined subsets of tandem switched traffic.

Homing Arrangement
Homing Arrangement refers to the last-choice trunk group(s) between switching systems in a specific routing ladder.

Hookflash
Hookflash, a type of signaling, is a brief interruption in the loop current on loopstart trunks that the attached system does not interpret as a call disconnect. Once the PBX or PSTN senses the hookflash, it generally puts the current call on hold and provides a secondary dial tone or access to other features such as transfer or call waiting access. A hookflash is done by momentarily pressing down the cradle on a telephone. Some telephone handsets have a button called 'flash' or 'recall' that sends a 'timed loop break', or 'calibrated flash' which is a hookflash that has a precise timing.

Hop Off
In traditional telecom, Hop Off refers to a point at which a signal or call leaves a network and moves to another network. For example, a call made on a phone set can hop off the local telephone network to long distance network. Conversely, a call coming through a major network can hop off that network to a smaller local network or private branch exchange (PBX). In voice over IP (VoIP), Hop Off is a point or gateway at which a call moves from an H.323 network to a network that uses some other protocol, typically at a gateway.

Horizontal Wiring
Horizontal Wiring is the wiring scheme that encompasses all cables from a work-area wallplate or network connection to the telecommunications closet. The outlets, cable, and cross-connects in the closet are all part of the horizontal wiring, which gets its name because the cable typically runs horizontally above ceilings or along the floors.

Howler Tone
Howler Tone is one of the telephone system audible tones that is used in older offices to inform a customer that their receiver is off-hook. It has been superseded by the Receiver Off-Hook Tone. The Howler Tone was a 480 Hz tone incremented in volume every second for ten seconds until it reaches +40 VU.

HPF: High Pass Filter
High Pass Filter (HPF), sometimes called a low-cut filter, is a filter that high frequencies can be transmitted well and frequencies lower than the cutoff frequency are attenuated or reduced. The actual amount of attenuation for a particular frequency varies from filter to filter.

HPPI: High Performance Parallel Interface
High Performance Parallel Interface (HPPI) is a standard point-to-point protocol for transmitting large amounts of data at up to billions of bits per second over relatively short distances, mainly on local area networks (LANs). The proponents of HIPPI believe that its use can make computers, interconnected storage devices, and other resources on a local area network function as though they were all within a single supercomputer.

Hundred Call Seconds
Hundred Call Seconds is a measure of traffic intensity. It is one traffic path occupied for 100 seconds per hour. 36 hundred call seconds equal to one Erlang.

Hunt
Hunt is a term that describes the automatic process by which a switch progresses from contact to contact, usually seeking an idle path.

Hybrid Balance
Hybrid balance is an expression of the degree of electrical symmetry between two impedances connected to two conjugate sides of a hybrid set or resistance hybrid. It is usually denoted in dB.

Hybrid Coil
Bybrid Coil is a transformer-like device which is designed to provide the interface between a two-wire and four-wire circuit. It has four ports designed such that a signal input to one port will be split evenly to the two adjacent ports with no signal coupled to the opposite port. One port is connected to the two-wire line. The two adjacent ports are connected to the four-wire line. The opposite port is connected to a balanced network to cancel any stray signals.

Hybrid Fiber Coaxial Network
Hybrid Fiber Coaxial Network is a telecommunication technology using optical fiber cable and coaxial cable in different portions of a network to carry broadband content.