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The H.248 protocol, also known as MEGACO (MEdia GAteway COntrol) or the Gateway Control Protocol, defines a VoIP network in which basic functions (such as voice coding and decoding) are performed by one device, and higher functions.



H.264, also called MPEG-4 Part 10 or AVC (Advanced Video Coding), is a video compression technology that provides reduced bitrates for high-quality video transmission in various applications such as 3G cell phones, iChat, HDTV (High-Definition TeleVision



H.323 is a standard teleconferencing protocol suite that provides audio, data and video conferencing. It allows for real-time point-to-point and multipoint communication between client computers over a packet-based network that does not provide a guaranteed quality of service.



In WiMAX, handover is the process of passing a mobile station's connection from one of a service provider's base stations to another. This process usually happens when a mobile station detects a stronger signal from another base station.


Handover Ranging

In WiMAX, a mobile station sends this request in order to perform roaming from one base station to another.


Half Duplex

Data transmission that can occur in two directions over a single line, but only one direction at a time.


Hard Disk Drive Power Saving

To reduce power usage, some devices automatically put an installed hard disk to sleep if it is idle for a pre-defined length of time. The NSA wakes it up when you use it again.



Hybrid Automatic Repeat-Request (H-ARQ) is similar to Automatic Repeat reQuest (ARQ) but works in the WiMAX physical (PHY) layer. In H-ARQ, forward error correction (FEC) bits are also added to the ED bits. HARQ performs better than ARQ in poor signal conditions.



The physical aspect of computers, telecommunications, and other information technology devices.


Hardware Monitor

A device uses the hardware monitor to observe temperature, voltage and fan speed readings.



A hash is a mathematical function (or algorithm) that generates a message digest from plain text input. Se also message digest.



A bit-oriented (the data is monitored bit by bit), link layer protocol for the transmission of data over synchronous networks.


HDMI (High Definition Multimedia Interface)

HDMI is an interface that allows high quality, all-digital video and audio streams. Both the video and audio streams are carried through the same interface so you only need to use one (HDMI) cable.


Hello Time

In RSTP (or STP), this is the time interval in seconds between tree configuration messages generated by all devices in RSTP or the root device in STP.


Heuristic Analysis

Heuristic-based signatures use algorithms based often on statistics to judge whether a warning is warranted. An example of this type of signature is one that would be used to detect a port sweep. This signature might look for the presence of a threshold number of unique ports being probed on a particular device. See also Protocol Decode, Protocol Anomaly Detection and Traffic Flow Anomaly.


Hexadecimal Notation

Hexadecimal notation is a base-16 number as opposed to decimal (base-10) or binary (base 2). This number representation uses 0-9 along with the letters a-f to represent the (decimal) numbers 10 to 15. The right-most digit represents ones, the next represents multiples of 16, then 16 squared (256), 16 cubed (4096) and so on. MAC addresses are usually written in hexadecimal notation, for example 00:a0:c5:01:23:43.


High Availability DNS

A DNS server maps a domain name to a port IP address. If that port loses its connection, high availability allows the router to substitute another portIP address for the domain name mapping.


High definition video

High definition video usually uses 1080 or 720 vertical lines of resolution. This is higher resolution than standard definition video. High definition video uses a 16:9 ratio.


A standard for high-speed wireless LANs that supports data rates up to 54 Mbps. This system is similar to 802.11a and uses the same 5 GHz frequency band.


Home Gateway

This is an intelligent network device located in the home. Users can access the home gateway device from a remote location. Examples of home gateways include computers, routers or modems, LAN access points, WLAN access points, and digital set-top boxes.


HomePlug 1.0

HomePlug 1.0 was the first HomePlug specification. It provides a peak PHY-rate of 14Mbps. It was first introduced in June, 2001 and has since been replaced by HomePlug AV. On May 28, 2008 Telecommunications Industry Association (TIA) incorporated HomePlug 1.0 powerline technology into the newly published TIA-1113 international standard. TIA-1113 defines modem operations on user-premises electrical wiring. The new standard is the world's first multi-megabit powerline communications standard approved by an American National Standards Institute (ANSI)-accredited organization.


There are also HomePlug 1.0 with Turbo adapters that still may be found on the market. These comply with the HomePlug 1.0 specification but feature a faster, proprietary mode that increases the peak PHY-rate to 85Mbps.


HomePlug AV

The HomePlug AV specification, which was introduced in August 2005, provides sufficient bandwidth for applications such as HDTV and VoIP. HomePlug AV offers a peak data rate of 200Mbps at the physical layer, and about 80Mbps at the MAC layer. HomePlug AV devices are required to coexist, and optionally to interoperate, with HomePlug 1.0 devices. Typical data rates realized at the application layer by over 95% of the units in a typical home will be more close to 20-30Mbps, not the numbers claimed by HomePlug. This is well known and documented in the industry. Further, the reliance of HP AV on CSMA technology, versus TDMA where specific data connections or flows are allocated specific timeslots on the wire, is problematic for anything but "best effort" internet traffic. The use of HD IPTV over HP AV is quite rare due to this reliance on CSMA and HP AV's less than robust noise mitigation techniques.


Utilizing adaptive modulation on up to 1155 OFDM sub-carriers, turbo convolution codes for error correction, two-level MAC framing with ARQ and other techniques, HomePlug AV can achieve near the theoretical maximum bandwidth across a given transmission path. For security reasons, the specification includes key distribution techniques and the use of 128 bit AES encryption. Furthermore, the specification's adaptive techniques present inherent obstacles to eavesdropping and cyber attacks.


HomePlug AV2

The HomePlugAV2 project is currently under development and is the prospective next generation for the HomePlug line. Current estimates state that it will operate upon a 600Mbps transfer capability. HomePlug AV2 is fully interoperable with HomePlug AV and will be brought into the IEEE P1901 standard (this is a claim that should not be made as it presupposes the end result, which is contrary to the way IEEE is supposed to work) once the specification is completed. HomePlug AV2 offers Gigabit speed at the physical layer and 600Mbps+ at the MAC layer. The AV2 spec is expected to be completed in mid 2011, with products expected to ship in late 2011 (notice use of the term "expected" versus a more definite term). Completion of the HomePlug AV2 Marketing Requirements Document was announced in November 2009.


HomePlug GreenPhy

HomePlug Green PHY is a new specification that is a subset of HomePlug AV and is specifically designed for the requirements of the smart grid market. It has peak rates of 10Mbps and is designed to go into smart meters and smaller appliances such as HVAC/thermostats, home appliances and plug-in electric hybrid vehicles, so that data can be shared over a Home Area Network (HAN) and back to the utility. For these applications, there’s not a great need for high capacity broadband; the most important requirements are for lower power, robust, reliable coverage throughout the home, smaller size and less costly Bill of Materials. GreenPHY uses up to 75% less energy than AV. The HomePlug Powerline Alliance worked closely with utilities and meter manufacturers to develop this 700-page specification (downloadable from the HomePlug website). HomePlug Green PHY-based products will be fully interoperable with products based on HomePlug AV, IEEE 1901 or the upcoming HomePlug AV2 specification.


HomePlug Access BPL

Access Broadband Power Line (BPL) refers to a to-the-home broadband access technology. The HomePlug Alliance formed the HomePlug Access BPL Working Group, whose first charter was to develop the Market Requirements Document (MRD) for a HomePlug Access BPL specification. The Alliance made an open invitation to the BPL industry to participate in the development of or provide input for consideration in the MRD. After several months of collaboration between utilities, ISPs and other BPL industry groups, the MRD was completed in June 2005.

Hop Count

Hop count is a measure of distance between two points on the Internet. It is equivalent to the number of gateways that separate the source and destination.



Any computer on a network that is a repository for services available for other computers on the network. It is quite common to have one host machine provide several services, such as WWW and USENET.



A RAID array with a hot-spare keeps one disk on standby and automatically uses it to resynchronize the array if a disk fails. This lets the array return to its normal speed as quickly as possible.


Hotspots are public areas, such as airports, hotels, coffee shops, where end users can access the Internet via a mobile device.



High-Speed Downlink Packet Access (HSDPA) is a mobile telephony protocol, used for UMTS-based 3G networks to allow higher data transfer speeds.


HTTP (HyperText Transport Protocol)

The most common protocol used on the Internet. HTTP is the primary protocol used for web sites and web browsers. It is also prone to certain kinds of attacks.


HTTP Redirect

HTTP Redirect transparently forwards HTTP (web) traffic to a proxy server. This can speed up web browsing because the proxy server keeps copies of accessed web pages so they are readily available the next time a user needs to access that page.



HyperText Transfer Protocol over Secure Socket Layer, or HTTP over SSL is a web protocol that encrypts and decrypts web pages. Secure Socket Layer (SSL) is an application-level protocol that enables secure transactions of data by ensuring confidentiality (an unauthorized party cannot read the transferred data), authentication (one party can identify the other party) and data integrity (you know if data has been changed).


HTTP Tunnel

A HTTP tunnel allows a SIP phone to communicate with the IP-PBX when the IP-PBX is behind a firewall that blocks standard SIP communication with the IP-PBX.


Hub VPN Router

In a hub-and-spoke VPN, multiple spoke VPN routers connect to a single hub VPN router. The hub VPN router routes VPN traffic between the spoke VPN routers and itself.


Hub-and-Spoke VPN

A hub-and-spoke VPN connects VPN tunnels to form one secure network. There is a VPN connection between each spoke router and the hub router. The hub router routes VPN traffic between the spoke routers and itself.



When adjusting video setup, hue adjusts the color mix of Red, Green, and Blue (RGB).


Hybrid Dual Band

Hybrid dual band work with Wireless-G in one radio band and Wireless-N in the other radio band.