Photograph of TWTAs courtesy of MCL, a division of Miteq, http://www.mcl.com/index.html/prod/main.html
Type of Service (ToS or TOS)
A field within an IP packet that can be used to request priority treatment of the packet by the network. The ToS field and mechanism have been replaced by Differentiated Services (DiffServ) and DiffServ Codepoints (DSCPs).
An encoding of three fields, in which the first field indicates the type of element, the second the length of the element, and the third field the value.
Typical Operating Conditions
Optimum operating conditions for a stated number of channels.
Universal Asynchronous Receiver Transmitter
Unspecified Bit Rate
Upstream Channel Descriptor
Unidirectional Digital Cable Product
Unidirectional Digital Cable Receiver
User Datagram Protocol
Ultra High Frequency; that portion of the electromagnetic spectrum from 470 to 890 MHz, NTSC television channels 14-83, as designated by the Federal Communications Commission. [Arr11]
Unidirectional Hypertext Transport Protocol
Underwriters Laboratories; an independent not-for-profit organization helping manufacturers bring safer products and services to consumers since 1894. [Arr11]
Ultra High Frequency (UHF)
Channels above channel 13 (or from 470 MHz to 806 MHz).
Electromagnetic radiation with wavelengths between about 40 and 400 nanometers. Radiation between 40 and 200 nm is termed “vacuum ultraviolet” because it is absorbed by air and travels only through a vacuum. The “near” ultraviolet has wavelengths close to those of visible light; the “far” ultraviolet has shorter wavelengths. [Arr11]
An unbound application is not associated with a broadcast service.
The separation and discrete offering of the components of the local telephone service. Unbundling of network components facilitates the provision of pieces of the local network, such as local switching and transport, by telephone company competitors.
Undercutting (Over Polishing)
An undercut is the distance below the desired surface of a fiber connector or optical device and the actual surface of the fiber strand. Undercutting may result from excessive fiber polishing. Over polishing is the undercutting of the surface of a fiber connector or optical device due to excessive polishing. Over polishing can result from repeated cleaning maintenance of an optical connector. [Opt09]
Cable installed in subsurface conduits terminating at intervals in manholes, thus permitting the placing, replacing, or removal of cables at will. [Arr11]
Operating in one direction only. [Fib111]
Unidirectional Diagram courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/u
Unidirectional Hypertext Transport Protocol (UHTTP)
A broadcast transfer protocol, suitable for delivery of content using IP multicast.
Uniform Resource Locator (URL)
The address of an Internet site. The URL contains the protocol used for the site (e.g., http, ftp), the domain name or IP address of the site (e.g., and, optionally, the folder or page on the site where specific information is stored.
Uniformity is a measure of how evenly power is distributed between the output ports of the coupler. Uniformity applies to couplers with a nominally equal coupling ratio and is defined as the difference between the highest and lowest insertion loss between all of the coupler output ports, expressed in dB. Uniformity is a typical value across the entire bandpass. [AOF11]
A concept in which all the amplifiers in a cascade are in balance with their power inputs and outputs. Unity gain can be achieved by adjusting the receiver output, either by padding or attenuation in the node, to the proper level determined by the RF input. [Fib111]
Universal Asynchronous Receiver Transmitter (UART)
A computer's UART chip dictates the maximum rate a computer can send and receive data over its serial ports.
Universal Serial Bus (USB)
A plug-n-play standard for connecting multiple (up to 127) input/output devices to a single high-bandwidth port. The design of the bus allows hot-swapping of the devices (disconnection and reconnection without powering the computer off) and automatic configuration. The USB peripheral bus standard was developed by Compaq, IBM, DEC, Intel, Microsoft, NEC, and Northern Telecom. The original version of USB (USB1.1) supports a data rate of 12Mbps, while the second version (USB2.0) supports a data rate of 480 Mbps.
Unspecified Bit Rate (UBR)
The UBR service class is intended for delay-tolerant or non-real-time applications, or those which do not require tightly constrained delay and delay variation.
A device used to add a lower frequency to a microwave frequency.
The return signal from the user to the base station. Also, the satellite transmit antenna and all necessary electronics needed to transmit information to a communication satellite. [Arr11]
The transfer of files from a local computer “up” to a remote computer.
Universal Plug and Play
Uninterruptible Power Supply; an AC supply with full hot standby. [Arr11]
The term used to describe traffic and paths that go from the subscriber to the headend. Also known as Reverse Path or Return Path.
Upstream Channel Descriptor (UCD)
The MAC Management Message used to communicate the characteristics of the upstream physical layer to the cable modems.
Uniform Resource Locator
Universal Serial Bus
A native or OCAP™-based application running on a set-top receiver that decodes and executes a binary-encoded enhanced television application.
User Datagram Protocol (UDP)
A protocol residing on top of IP that is used for end-to-end transmission of user messages. Unlike TCP, UDP is an unreliable protocol, which means that it does not contain any retransmission mechanisms. Thus, UDP packets are not guaranteed to make it through the network.
User Interface (UI)
A user interface is the sensory and behavioral aspects of a program that are presented to a user. The term is generally used to denote the menuing and navigational constructs of a program.
Coordinated Universal Time
Abbreviation for ultraviolet. That portion of the electromagnetic spectrum in which the longest wavelength is just below the visible spectrum, extending from approximately 4 nm to 400 nm. [Fib111]
DC Half Wave Voltage; the voltage, applied to the DC input port of the modulator needed to shift the phase by radians. For intensity modulators VDC corresponds to the voltage required to switch from an “OFF” state (minimum transmission) to an “ON” state (maximum transmission). VDC applies for intensity modulators only since phase modulators usually have no DC electrodes. VDC is measured with no electrical signal applied to the RF input port of the modulator, and with an optical power meter connected to the output fiber of the modulator. [Jer04]
RF Half Wave Voltage; the voltage, applied to the RF input port of an intensity modulator needed to shift the phase by radians and to move from an “OFF” state (minimum transmission) to an “ON” state (maximum transmission). For digital applications, the RF electrical signal is generated by a PRBS generator and the output optical signal is displayed by a high speed digital sampling oscilloscope. An eye diagram is displayed and the amount of voltage delivered by the RF driver/amplifier required to optimize the RF extinction ratio is recorded as VRF. [Jer04]
Voice Activation Detection
Value-Added Reseller (VAR)
Refers to distributors that also provide other services such as systems integration or network management.
Variable Bit Rate (VBR)
A type of telecommunications service characterized by a service bit rate specified by statistically expressed parameters that allow the bit rate to vary within defined limits.
Vertical Blanking Interval
Variable Bit Rate
Video Carrier Level [Arr11]
See vertical cavity surface-emitting laser.
Virtual Channel Table
Very high-speed digital subscriber line that utilizes existing copper infrastructure of the telephone companies. VDSL offers video and data transmission rates up to 52 Mbps up to 2,700 feet. See also DSL.
Velocity of Light
The velocity of light (speed) in a vacuum is 2,997,925 meters per second or 186,280 miles per second. For rough calculations the figure of 3,000,000 meters per second is generally used. [Arr11]
Velocity of Propagation
Velocity of signal transmission. In free space, electromagnetic waves travel with the speed of light. In coaxial cables, this speed is reduced. Commonly expressed as percentage of the speed in free space.
Vertical Blanking Interval (VBI)
A portion of the television signal that does not contain visual data. In NTSC, the VBI are lines 1 through 21 in each field.
Vertical Blanking Pulse
A pulse used during the vertical retrace period at the end of each scanning field to extinguish illumination from the electron beam. [Arr11]
The term Vertical Retrace is used to describe the movement of the electron beam as it makes its way from the bottom right part of the television screen back up to the top of the screen on the left side. The painting beam always finishes in the bottom right corner and always starts again in the top left corner, whether the television is using an interlacing scan or a progressive scan to show the image. [Glo07]
Vertical Retrace Signal
The Vertical Retrace Signal is the part of a television broadcast signal that tells the electron beam to return to the top left corner of the screen to begin painting again. This is an important part of the television broadcast as it tells the television basically when the new image needs to be started, about 30 times a second. [Glo07]
Vector Quantization (VQ)
A digital video compression technique based on Shannon’s distortion-rate theory which states that the performance of data compression systems improves if blocks of data are coded, with larger coded data blocks giving better performance. [Fib111]
Very High Frequency (VHF)
Channels 2-13 (54-88 MHz and 174-216 MHz).
Very Small Aperture Terminal (VSAT)
Small earth stations with a satellite dish usually 4-6 feet in diameter used to receive high speed data transmissions; can also transmit slow-speed data.
Vertical Cavity Surface-emitting Laser (VCSEL)
A type of laser that emits light perpendicular to the plane of the wafer it is grown on. They have very small dimensions compared to conventional lasers and are very efficient. [Fib111]
VCSEL Diagram courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/v
Vestigial Side Band (VSB)
In amplitude-modulated transmissions, a portion of only one sideband of a modulated carrier. The modulated carrier is passed through a filter having a graduated cut-off characteristic near the carrier frequency. A substantial portion of the modulated carrier is suppressed in this fashion. [Arr11]
Vestigial Sideband Transmission
A modified double-sideband transmission in which one sideband, the carrier, and only a portion of the other sideband are transmitted. See also sideband. [Fib111]
Voice Frequency [Arr11]
Video Graphics Array; a high-resolution color standard for computer monitors. [Fib111]
Voltage, gate-to-source; a measurement and phenomenon applicable to III-V compound semiconductor devices employing any type of field effect transistor (FET) construction, including radio frequency (RF) gallium arsenide (GaAs) Metal-Semiconductor Field Effect Transistors (MESFETs), Junction Field Effect Transistors (JFETs), Metal-Oxide Semiconductor Field-Effect Transistor (MOSFETs), High Electron Mobility Transistors (HEMTs), and Pseudomorphic High Electron Mobility Transistor (pHEMT) devices. See also FET (a transistor in which most current flows in a channel whose effective resistance can be controlled by a transverse electric field) within this document. [Fre11]
Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Diagrams courtesy of http://www.electronics-tutorials.ws/transistor/tran_6.html
Very High Frequency; that portion of the electromagnetic spectrum from 30 to 300 MHz, NTSC television channels 2 to 13 and most FM radio, as designated by the Federal Communications Commission. [Arr11]
A term pertaining to the bandwidth and spectrum of the signal which results from television scanning and which is used to produce a picture.
Pertaining to the signal which carriers a television picture. Also, describing the four megahertz wide band of frequencies which constitutes a television signal. [Arr11]
The frequency band utilized to transmit a composite video signal.
Conducting conferences via a video telecommunications system. [Fib111]
A television that accepts unmodulated baseband signals to reproduce a broadcast. [Arr11]
A telephone-like service with a picture as well as sound. [Fib111]
The original video signal before it is modulated and converted to radio-frequency and broadcast or cablecast. A home television set reconverts radio-frequencies to a video signal.
Allows the end-user subscriber to select at any time movies they wish to view from a large selection of titles and categories stored on a remote server. Service may also provide VCR functionality, (stop, pause, etc.) which allows the end-user subscriber to control the “play back” of the server from the remote control. Or a television service where viewers can select and watch video content for viewing at any time.
Used to electronically record sight and sound for instant playback. Videotape comes in half-inch, three-quarters, one and two-inch widths and can be erased and re-recorded.
Videotape Recorder (VTR)
A device which allows the recording and playback of magnetic tape sound and picture recordings.
The generic term used to refer to a two-way interactive system for the delivery of computer-generated data into the home, usually using the television set as a display device. Some of the more often used specific terms are “viewdata” for telephone-based systems (narrowband interactive systems); “wideband broadcast” or “cabletext” for systems utilizing a full video channel for information transmission; and “wideband two-way teletext” for systems which could be implemented over two-way cable television systems. In addition, hybrids and other transmission technologies, such as satellite, could be used for delivery of videotex services on a national scale.
Viewers Per Viewing Household (VPVH)
A demographic percentage which indicates how many persons per 100 or per 1,000 households are viewing. For example, a VPVH of 80 K2-11 means that for every 100 households viewing, there are an estimated 80 children ages 2 to 11.
Violence Chip (V-Chip)
A term used to describe a microchip which will permit parental control over rated television programs.
Virtual Channel (VC)
The communication channel that provides for the sequential unidirectional transport of ATM cells.
Virtual Channel Table (VCT)
Data declared as part of the Service Information standard defined by SCTE.
This term refers to a predefined path through a network that provides a connection-oriented session between two applications.
Virtual Private Network (VPN)
A term that is applied to either voice or data networking. In essence, a VPN is a portion of a public network that has been logically partitioned for private use.
That part of the spectrum to which the human eye is sensitive, usually defined as wavelengths between 390 and 780 nanometers. [Arr11] Electromagnetic radiation visible to the human eye; wavelengths of 400-700 nm. [Fib111]
Virtual Local Area Network; a logical local area network (or LAN) that extends beyond a single traditional LAN to a group of LAN segments, given specific configurations. Because a VLAN is a logical entity, its creation and configuration is done completely in software. [Tec113]
Variable Optical Attenuator; a component that allows the attenuation of selected optical signals or wavelengths. VOAs are integral in dense wavelength division multiplexing (DWDM) systems, as they are used to dynamically compensate for skewed gains in network wavelength amplification. The amplifiers used in typical DWDM systems have varying degrees of "flatness" - signals of certain wavelengths are amplified more or less than signals of other wavelengths. After several chains of amplification, the wavelengths receiving the most amplification may be powerful enough to saturate the receive electronics, while the wavelengths receiving the least amplification are just strong enough to register at the receiver. VOAs can then attenuate only the most powerful wavelengths, bringing the entire signal into conformance with DWDM receiver specifications. VOA s are also crucial in protection switching functions, as a cable cut can reduce the number of aggregate wavelengths on a fiber. Optical amplifiers have a fixed amount of amplification, which is typically linked to amplifier pump laser current; the amplification is then spread across all of the relevant wavelengths. If a broken or disconnected fiber results in the amount of wavelengths being cut in half, the remaining wavelengths will be amplified twice as much. This effect is often cascaded through multiple amplifiers; the end receiver component may then receive a signal that is significantly higher than the component's operating specification for optical power, rendering the received signal unintelligible. VOAs can prevent failure during a protection switching event by attenuating the offending wavelengths until they are in the operating range for receiver input power. [IEC06]
Voice over Frame Relay.
VoFR dial peer
Dial peer connected via a Frame Relay network. VoFR peers point to specific VoFR devices.
A circuit capable of carrying one telephone conversation or its equivalent; the standard sub-unit in which telecommunication capacity is counted. The U.S. analog equivalent is 4 kHz. The digital equivalent is 64 kbit/s in North America and in Europe. [Fib111]
Voice over Frame Relay
Voice over Frame Relay enables a router to carry voice traffic (for example, telephone calls and faxes) over a Frame Relay network. When sending voice traffic over Frame Relay, the voice traffic is segmented and encapsulated for transit across the Frame Relay network using FRF.12 encapsulation. [Cis00]
A unit of electrical pressure. One volt is the amount of pressure that will cause one ampere of current in one Ohm of resistance. [Arr11]
A measure of the electrical force that causes current flow in a circuit, expressed in volts. [Arr11]
Volt-Ohm Meter; a piece of test equipment used to measure various electrical parameters such as voltage, resistance, current and capacitance. [Arr11]
Velocity of propagation; the speed of a transmitted signal in a cable, usually somewhat less than the velocity of light. Different types of cables exhibit different VOPs. VOP is expressed as a decimal number and is a necessary value for computing the location of a cable fault when using a TDR. [Arr11]
Voice Activation Detection (VAD)
Allows a data network carrying voice traffic to detect the absence of audio and conserve bandwidth by preventing the transmission of “silent packets” over the network. Most conversations include about 50% silence. VAD is also called “silence suppression”.
Voice-over-Internet Protocol (VoIP)
VoIP services are a provision of voice telephony via the use of packet-switched networks running Internet Protocol (IP) networks rather than traditional circuit switching. CableLabs has developed the PacketCable specifications for the delivery of IP-based multimedia services, including voice services, over the DOCSIS 1.1 access network.
A term that refers to various sets of technologies that are used to enable voice applications across the Internet.
Voltage Tuned Oscillator (VTO)
An electronic circuit whose output oscillator frequency is adjusted by voltage. Used in downconverters and satellite receivers to select from among transponders. [Arr11]
Abbreviation for Volts, AC. Voltage using alternating current. [Fib111]
Abbreviation for Volts, DC. Voltage using direct current. [Fib111]
Virtual Private Network; a protected information-system link utilizing tunneling, security controls, and end-point address translation giving the end user the impression that a dedicated line exists between nodes. [Fib111]
Viewers Per Viewing Household
Very Small Aperture Terminal; generally, an inexpensive Ku-band receive antenna that receives one transponder of a satellite that is transmitting high speed data. [Arr11]
Vestigial Sideband Amplitude Modulation [Gar90]
A device that seizes the outer conductor of a coaxial hard line cable. The cable center conductor extends through this type of connector and is retained within the equipment housing. Another name for feed-thru connector. [Arr11]
Voltage Standing Wave Ratio; a ratio of the difference between the minimum and maximum voltage along a transmission line caused by addition and subtraction of reflected signal wave. [Arr11]
Wide Area Network
Wide Area Telecommunications Service
The unit of electric power, equal to the rate of work when a current of one ampere flows under a pressure of one volt. For direct currents, it is equal to the product of the voltage and current, or the product of circuit resistance by the square of the current. For alternating currents it is equal to the product of effective volts and effective current times the circuit power factor. [Arr11]
1. A periodic variation of an electric voltage or current. 2. A wave motion in any medium: mechanical as in water, acoustical as sound in air, electrical as current waves on wires, or electromagnetic as radio light waves through space. [Arr11]
A graphical representation of the rise and fall of the electrical potential (voltage) on a pair of wires or some other signal over time. A classic example of a waveform is the spiky line displayed on a cardiograph machine used to analyze the electrical activity created as the heart beats. In telephony, analog waveforms are translated into a series of binary values, called samples. These samples are taken 8,000 times a second, sent on to their destination, where they are translated back into a series of 8,000 changes in voltage that almost resembles the original analog signal.
Any device which guides electromagnetic waves along a path defined by the physical construction of the device. [Arr11]
The distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelengths of light are measured in nanometers or micrometers. [Arr11]. Denoted by the symbol λ (Greek small letter Lambda).
Wavelength Division Multiplexing (WDM)
Technology that enables the capacity of fiber-optic lines to be increased exponentially through the use of different frequencies (or colors). As more colors are utilized, more unique communication paths are created.
A measure of how well different wavelengths are separated at the output of a wavelength division demultiplexer. It is defined as the ratio of the optical power at the two output ports of the demultiplexer at a given wavelength, expressed in dB. The minimum wavelength isolation is the lower limit to the wavelength isolation measured over the entire wavelength range of the specified bandpass. Wavelength isolation has also been referred to as far-end crosstalk. [AOF11]
Wavelength Division Multiplexing
In satellite systems, the loss of a satellite signal due to extremely heavy (and generally very localized) rain, snow, or other extreme weather. [Fib111]
An HTML document accessible on the World Wide Web using a Web browser.
The maximum system-permissible excursion of the video signal in the white direction. [After SMPTE] [ATI11]
Wide Area Network (WAN)
A computer network which usually spans larger geographic area, such as cities, counties, states, nations and planets. WAN's usually employ telephone-type topologies, like T1, T2, T5, ATM, etc. The Internet is a WAN which is held together by LANs, which network computers.
Wide Area Network (WAN) Diagram courtesy of Fiber Optics Info, http://www.fiber-optics.info/fiber_optic_glossary/w
Wide Area Telecommunications Service (WATS)
WATS permits customers to make (OUTWATS) or receive (INWATS) long distance voice or data calls and to have them billed on a bulk rather than individual call basis. The service is provided within selected service areas, or bands, by means of special private-access lines connected to the public telephone network via WATS-equipped central offices. A single access line permits inward or outward service, but not both.
Passing a wide range of frequencies. [Arr11]
The Wideband SPA is a single-wide, half-height shared port adapter that provides DOCSIS® 3.0 formatting to downstream data packets. The Wideband SPA is used for downstream data traffic only. [Cis11]
Wireless Fidelity; refers to wireless networking technology that allows computers and other devices to communicate over a wireless signal. It describes all network components that are based on one of the IEEE 802.11 standards, including 802.11a, 802.11b, 802.11g, and 802.11n. These standards were developed by the IEEE and adopted by the Wi-Fi Alliance, which trademarked the name "Wi-Fi". [Tec111]
Wireless Gigabit; a short-range wireless technology from the Wireless Gigabit Alliance (WGA). It provides up to 7 Gbps of data transmission in the unlicensed 60 GHz band over a distance of approximately 10 meters. WiGig was designed to provide a wireless technology for gaming, backup, HD connections between A/V equipment and other high-speed applications. WiGig offers an order of magnitude more bandwidth than Wi-Fi, but it throttles down to Wi-Fi speeds in noisy environments. At 60 GHz, the wavelength is only five millimeters and highly directional, and beamforming is used to continue operation when objects are in the way. However, if the signal is completely unobstructed, it can reach 20 meters and beyond. The Wireless Gigabit Alliance was formed in May of 2009, and Version 1.0 of the specification was announced at year-end. For more information, visit www.wirelessgigabitalliance.org. [PCm11]
Since hybrid circuits, such as Wilkinson power dividers, are bi-directional, they can be used to split up a signal to feed multiple low power amplifiers, with their outputs recombined to feed a single antenna with high power as shown in the diagram below. [Wik11_Wilkinson]