All of the V.## recommendations from the ITU-T (formerly CCITT) have to do with data communications over analog telephone channels. As such, they’re largely obsolete. They include a series of VGC modem standards, designed to create an analog dial-up connection over two-wire lines:
V.21 – Full-duplex 300 baud & 300 b/s (originally 200 baud & 200 b/s) using FSK modulation, with a 2100 Hz handshaking tone to disable echo suppressors. The initiating DCE uses a 1750 Hz center frequency, and the answering DCE uses 1080 Hz. Both tones employ ±100 Hz shifts to indicate mark and space. In the US, the Bell 103 standard was preferred.
V.22 – Full-duplex 600 baud, 1200 b/s with D-QPSK or 600 b/s with BPSK modulation. The initiating DCE uses a 1200 Hz center frequency with phase shifts every two cycles, and the answering DCE uses 2400 Hz with a phase shift every four cycles. The modem has an 1800 Hz guard tone and fixed (non-adaptive) line equalization. The generating polynomial for the cyclic redundancy check is [x17 + x14 + 1]. In the US, the Bell 212A standard was preferred.
V.22bis – Similar to V.22, but the data rates are 2400 b/s with 16 QAM (8/4) or 1200 b/s with D-QPSK, and this standard provides an option for adaptive equalization.
V.23 – An oddball standard used for teletypes and video text. There are three versions, all using FSK modulation. Mode 1 is half-duplex at 600 b/s, with a 1500 Hz center frequency and ±200 Hz shifts. Mode 2 is half-duplex 1200 b/s, with a 1700 Hz center frequency and ±400 Hz shifts. Mode 3 is similar to mode 2, but adds a 75 b/s FSK backchannel using a tone at 420 Hz with ±30 Hz shifts. None of these versions provides line equalization.
V.26 – Full-duplex 1200 baud & 2400 b/s, with QPSK (V.26A) or 45° shifted QPSK (V.26B) modulation. The center frequency is at 1800 Hz, and the spectrum exhibits characteristic tones at 1200 & 2400 Hz (V.26A) or at 1350 and 2550 Hz (V.26B). The generating polynomial for the CRC is either [x23 + x18 + 1] or [x23 + x5 + 1]. This standard requires a conditioned, four-wire leased line.
V.26bis – Half-duplex 1200 baud, synchronous, 1200 b/s with BPSK, or 2400 b/s with QPSK and an optional 75 b/s FSK backchannel. Fixed line equalization is used.
V.26ter – Same as V.26bis, except that it has end-hybrid echo cancellation to support full-duplex as well as half-duplex operation.
V.27 – Full-duplex 1600 baud & 4800 b/s using 8PSK, with an 1800 Hz center frequency. The generating polynomial for the CRC is either [x7 + x6 + 1] or [x7 + x + 1]. This standard requires a conditioned, four-wire leased line.
V.27bis – Identical to V.27, but with a fallback option for 1200 baud & 2400 b/s using QPSK.
V.27ter – Half-duplex 1600 baud & 4800 b/s with 8 QAM (8/2), or 1200 baud & 2400 b/s with QPSK. It has an 1800 Hz center frequency, an optional 75 b/s FSK backchannel, and adaptive equalization. These are the fallback modes used by Group 3 fax.
V.29 – Half-duplex 2400 baud, synchronous, 9600 b/s with 16 QAM (8/4), 7200 b/s with 8 QAM (8/2), or 4800 b/s with QPSK. It has a 1700 Hz center frequency and adaptive equalization. The generating polynomial for the CRC is [x23 + x18 + 1]. This standard is widely used for Group 3 fax on two-wire lines.
V.32 – (adopted 1984) Full-duplex 2400 baud, 4800 b/s using QPSK, 9600 b/s using 32 QAM with trellis coding or 16 QAM without. It has an 1800 Hz center frequency, highly efficient adaptive equalization, and digital echo cancellation (necessary for FDX operation over a dial-up two-wire line – see EC). The generating polynomial for the CRC is [x23 + x18 + 1].
V.32bis – (adopted 1991) Full-duplex 2400 baud, maximum 14.4 kb/s using 128 QAM with trellis coding, plus fallback rates of 12, 9.6, 7.2, and 4.8 kb/s using smaller QAM constellations. This standard has error correction (MNP 2-4 or V.42), data compression (MNP 5 or V.42bis), and rapid rate re-negotiation – the initial handshaking takes about 7 seconds.
V.32terbo – A false “standard” implemented by AT&T Paradyne after the ITU decided not to issue V.32ter and instead began working on V.34. It was essentially V.32bis pushed to 19.2 kb/s, but achieving this rate required a near-perfect VGC. It also supported V.32, V.22, and Bell 212A modulation.
V.33 – Half-duplex 2400 baud, synchronous, 14.4 kb/s using 128 QAM or 12 kb/s using 64 QAM, both with trellis coding. It’s otherwise equivalent to V.32 without echo cancellation.
V.Fast – Prior to the release of V.34, Rockwell implemented this very similar “standard”.
V.34 – (adopted 1994) Full-duplex, with symbol rates of 2400, 2743, 2800, 3000, 3200, or 3249 baud. At 3200 baud and 9 bits per symbol with trellis coding, it achieves 28.8 kb/s. Fallback data rates are 24, 19.2, 16.8, 14.4, 7.2, and 2.4 kb/s. The constellation is very complex. It uses adaptive equalization, line probing, and protocol negotiation, with a 5-second handshake. A 1996 revision increased the top rate to 33.6 kb/s. This is sometimes called V.34+ or V.34bis.
V.90 – (issued 1998) Has a 28.8 or 56 kb/s data rate. Rockwell and US Robotics marketed 56 kb/s modems (K56flex and X2, respectively) before the release of this standard, and later issued software upgrades to make their products compliant.
ITU-T standard for data compression in modems. It’s based on the LZW algorithm. V.42bis typically achieves 2:1 or 3:1 compression. Compliant modems always use MNP 5 as a backup standard.
Two toroid-shaped regions of high-energy (i.e., very fast-moving) electrons and protons trapped by Earth’s magnetic field, hence aligned with Earth’s magnetic rather than rotational axis. Explorer I discovered them on 31 January 1958 with a Geiger counter placed on board by physicist James A. Van Allen.
The inner belt begins at 5000-7000 km altitude over the equator. It contains mainly protons with energies of 10-50 MeV, plus some electrons, oxygen ions, and low-energy protons in the 1-100 KeV range. The outer belt starts at perhaps 13,000 km altitude over the equator. It contains primarily electrons, but also protons, alpha particles, and oxygen ions, all at generally less than 1 MeV. The belts are thinnest where they curve down toward Earth around 60-70° N and S latitude. The South Atlantic Anomaly (SAA) is a low (~500 km), intense region of the inner belt over the southern Atlantic and South America.
More recent investigation has shown that solar weather can dramatically alter the belt dimensions. A transient third belt sometimes forms beyond the other two. See MEO.
Named for its inventor, American physicist Robert Van de Graaff (1901-1967). An electrostatic generator that uses a pulley, with conductive wheels and a non-conductive belt, to deposit static charge on a hollow conductive terminal isolated from its surroundings. It can then release the stored charge as an electrical arc to a grounded object that’s held close to it, or impart static charge to a contacted object that has a lower electrical potential. Created for research applications that required a very high-voltage, low-current source, it’s now mainly a spectacular educational tool/toy. Compare Tesla coil.
Verification and Validation. In software testing, verification is understood to mean testing to show that the software does what the requirements say, while validation means ensuring that the requirements are actually correct.
Volt-Amperes Reactive. Also called reactive power – the product of AC voltage and the component of AC current that’s not in phase with the voltage. The product of voltage and in-phase current is active power. See power.
Variable Resistor. A surge protection device connected across an AC input. When a surge occurs, the varistor’s resistance rapidly decreases, creating an instant shunt path for overvoltage. The varistor is often damaged in the process. See MOV.
Voltage, Base. The base voltage level for a transistor. More generally used to identify a non-primary, frequently negative supply voltage for a circuit, as opposed to the primary-source VCC or VDD.
Visual Basic Extension. A 16-bit OLE control with a .vbx file name extension, used by Microsoft’s VB 3.0 or 16-bit VB 4.0 under Windows 3.x and Windows 95. Debuted in 1991, later replaced by OCX.
Voltage, Collector. The voltage at the collector terminal of a bipolar junction transistor (BJT), or the DC power input for a circuit based on BJTs. It’s positive if the transistors are npn, negative for pnp. It pairs with a GND and/or a VEE input. Contrast VDD.
Voltage-Controlled Oscillator. A resonant circuit with an output frequency set by the input voltage. In 2013, researchers built the first graphene VCO.
Vertical-Cavity Surface-Emitting Laser. A semiconductor laser with its resonant cavity extending from the bottom to the top of the chip. This short cavity (typically > 500 µm) has little lasing power, but it produces a nearly circular, highly collimated beam. It can be directly modulated, supporting relatively high-frequency modulation (about 10 Gb/s).
Voltage-Controlled Crystal Oscillator. A type of VCO. The externally generated tuning voltage is applied to (typically) a piezoelectric crystal or a varactor diode in the crystal circuit. See XO.
Voltage, Drain. The voltage at the drain terminal of a field-effect transistor (FET), or the DC power input for a circuit based on FETs. It can be positive or negative. It pairs with a GND and/or a VSS input. Contrast VCC.
Voltage, Emitter. The voltage at the emitter terminal of a bipolar junction transistor (BJT), or the input ground or negative voltage for a circuit based on BJTs. Contrast VSS.
Verilog Hardware Description Language. A programming language for describing and simulating circuits, and for programming them into PLDs. Verilog began as a private effort for circuit simulation only, and was later adopted as the Verilog HDL standard by IEEE. Its syntax closely resembles C. A Verilog program is called a module. It covers the interface ports, body, and add-ons for the device being simulated. Verilog-AMS is a specialized tool for analog & mixed signal (AMS) design.
The practice of keeping a copy of all versions of a digital file or set of files. It's used for software source code as well as documents. Some popular version-control software programs:
CVS – Concurrent Versions System. Open-source version-control software dating to 1986. A more capable implementation of CVS called CVSNT was released under GPL in 1998. WinCVS is a MS Windows freeware GUI application that installs and manages CVSNT.
PVCS Version Manager – Originally stood for Polytron Version Control System. Dates to 1985.
Subversion – Commonly abbreviated SVN. Popular open-source version-control software, released in 2004, maintained and updated by the Apache Software Foundation. Users first upload a repository of controlled files to the SVN server. They can then download copies of the controlled files, and modify them without affecting the originals. A user with write permission can upload changed or added files, creating a new revision of the repository. However, Subversion retains every version of every file, along with data showing which users made which changes. Compare CVS.
The recommended (though not mandatory) way of creating a software project in Subversion that will be used by multiple developers is to have a trunk folder with the latest revision, and two other folders called branch and tag for alternate versions of the trunk. A branch is a way to preserve and share work-in-progress changes to trunk files until they’re ready to be merged back into the trunk and become the newest revision. A tag is just a snapshot of the trunk at a particular date and time. Although created in the same way as a branch, a tag isn’t meant to be modifed. It just points to a revision of trunk code rather than having actual copies, so modifying it would change the trunk in potentially harmful ways.
There’s a companion program called TortoiseSVN that uses MS Windows Explorer to invoke Subversion tools.
Video Electronics Standards Association. A standards-setting body, responsible for VBE and VLBus in the 1990s. More recently, they’ve created the DisplayPort video connection standard. They also do mounting standards for computer FPD monitors, specifying form factors for a mounting arm connection to the back of the monitor.
Video Frequency. The output of a receiver’s IF demodulator. It still contains all of the information in the original RF carrier, at some frequency below IF but above baseband.
Voice Frequency. 300-3000 Hz. This is the frequency range occupied by most components of human speech, so the corresponding band of the RF spectrum is often given this name as well.
Virtual File Allocation Table. A driver added to Windows 95 to support file names longer than the old 8.3 standard, which FAT can’t read. It’s therefore not a true file system, just a translator for FAT16 or FAT32. The VFAT runs in protected mode and uses 32-bit code.
Variable Frequency Control. A device that enables an electric motor to run at less than full speed & power consumption by sensing the load and adjusting the level and frequency of AC power input. It’s also called an adjustable speed drive (ASD) or variable speed drive (VSD).
Voice Frequency Telegraph. An obsolete analog technology for sending one or more telegraph signals over a voice grade channel. It typically uses FSK modulation and ASCII source coding.
Voice Grade Channel. The PCM standard voice channel is 4 kHz wide, but the usable (in-band) range is roughly 300-3400 Hz. Anything in the channel but outside this range (typically network signaling) is out-of-band. See FDM.
VHSIC Hardware Description Language. A programming language for describing and simulating circuits, and for implementing them in PLDs. More difficult and more flexible than Verilog, VHDL defines entities (components or subsystems), and describes their architecture by behavior (one or more processes acting on inputs to produce outputs) or by structure (a set of components and I/O signals). VHDL-AMS is a specialized tool for analog & mixed signal (AMS) design.
Video Home System. An analog video cassette standard with ½" wide tape, introduced by JVC in 1976. To support the high signal rate needed for video without spooling the tape intolerably fast, it writes the signal to tape in short, parallel, diagonal strips, and the read-head spool is angled so that the tape runs diagonally across it.
S-VHS (Super VHS) is a backward-compatible improved version. D-VHS (Digital VHS) uses the same cassette with digital encoding. VHS-C (Compact) is a physically smaller cassette meant for camcorders, but can be used with an adapter in a conventional VHS device. It was probably the greater recording capacity of VHS that beat Sony’s Betamax cassette in the market, but the question is still debated.
From “visual”. The Unix command-line text editor. Like Morse code, it’s been eclipsed by easier-to-learn software but persists as a shibboleth. Compare Emacs.
Latin for “road”. Also called a through-silicon via (TSV). In a multi-layer circuit, a hole with conducting lining to connect traces on two or more layers. Thermal vias are to conduct heat rather than electricity.
Vendor Identifier. A 16-bit tag embedded in a USB device. The host computer uses the combination of VID and PID to determine which device driver to use. Manufacturers can register PID/VID combinations with the USB Implementers Forum, for a fee. See also GUID, HID.
An optional mode for 80386 and some later microprocessors that permits multi-tasking by chopping extended memory into 1 MB pieces, each with its own copy of MS DOS running. DOS programs are still restricted to 640 kB in each memory segment. This is how Windows 3.x operates.
Virtual Instrument System Architecture. An API from the VXI plug-and-play consortium. It links applications with hardware interfaces such as GPIB, VXI, GPIB-VXI, TCP/IP, and serial ports.
A technique devised by American engineer Andrew Viterbi in 1967 to find the optimum path through a Markov chain, which is a sequence of possible states (i.e., a finite state machine) with defined probabilities for the state transitions. Its most common application by far is soft-decision, maximum-likelihood trellis decoding of FEC convolutional codes, to determine the most probable transmitted sequence from a received sequence – in plainer language, separating a digital signal from random noise.
This is the development that made convolutional codes useful. It works by finding the trellis path that has the minimum Hamming distance relative to the received sequence. Every digital cell phone in existence relies on it.
Variable-Length Array. A set of data values used in a computer program, with the number of values in the set determined at run time rather than at compile time as in a fixed-length array. VLAs can be a security vulnerability.
VESA Local Bus. An early 1990s local bus standard that VESA created for the 80486 processor. It lost out to PCI largely because it required more modification to work with the later Pentium motherboards.
VideoLAN Client, originally. Popular open-source multimedia player software created by contributors to a France-based GPL project called VideoLAN, resident online at www.videolan.org. It handles video DVDs as well as a variety of audio formats.
Very Low Frequency. 3-30 kHz. Because VLF radio waves can penetrate deep underwater, the US Navy uses this band to send low-data-rate transmissions to submarines. Antennas must be proportional to carrier wavelength, so VLF transmitting antennas are enormous. The receiving antenna is supposedly a very long wire unspooled behind the sub. See RF.
Very Large Scale Integrated Circuit. A dated way of describing the density of an integrated circuit (IC). It means a chip with more than 1000 elements, as opposed to MSI and LSI.
Virtual Machine. A software simulation of a computer environment, for the benefit of guest software that runs entirely within the simulation. This can be for security reasons, or to run software that requires a different OS.
A process virtual machine runs like a normal application, supports a single process, and terminates when that process ends. The JVM (Java Virtual Machine), for hosting the interpreter that converts Java byte code into platform-specific software, is a process VM.
A system VM hosts a complete operating system, hence also supports any other processes the hosted OS runs, assuming the host has given it the necessary resources: memory, disk space, device drivers, access permissions. VMware’s VMware Workstation and Oracle’s GPL-compliant VirtualBox, two widely used applications for system VMs, can both run on and create guest sessions of MS Windows, Mac OS, and Linux.
Because MS-DOS is a single-tasking OS, able to run just one progam at a time, MS Windows backward compatibility with DOS means creating a separate system VM for each running DOS application. Multiple Windows programs, with their support for multi-tasking, can run on newer versions of Windows under a single system VM.
Virtual Memory. A technique for increasing available system RAM by temporarily storing idle portions of a running process in a swap file on the hard drive, and loading them back into RAM when they’re needed. In MS Windows, this is handled by the Virtual Memory Manager (VMM).
From “VERSAmodule Eurocard”. A computer data bus card/backplane standard created by Motorola and two other companies in 1980, using the electrical specifications for Motorola’s 1979 VERSAbus (created for the 68000 microprocessor) and the smaller DIN-based form factor of the Eurocard. The 1994 VME64 standard has up to 64-bit data path and addressing for 6U boards, or 32-bit data path and 40-bit addressing for 3U boards. It runs at up to 80 Mb/s, and is backward-compatible with earlier versions of the standard.
Physically, it’s similar to CompactPCI, but the two are not compatible. The VME64x specification describes three connectors for VMEbus modules: the P0/J0, P1/J1, and P2/J2. The ‘P’ and ‘J’ refer to plug and jack, residing on the bus module and the backplane, respectively. The P0/J0 is a 95-pin connector.
Virtual Machine Manager. The 32-bit kernel for Windows 95. It remains in later versions of Windows that use the NT kernel, just to handle virtual machines.
Virtual Memory System. A DEC workstation OS with multiple root directories. DEC later added POSIX support and renamed it OpenVMS, but it’s the same OS. It runs on VAX and Alpha AXP CPUs. DEC was absorbed by Compaq, which merged with HP, which thereby acquired OpenVMS and the Alpha CPU.
Vector Network Analyzer. An instrument used to characterize the frequency response of circuits (see S-parameters). The name is confusing, since it doesn’t mean networks as most people understand the term. Scalar network analyzers are simpler instruments that provide no phase measurement.
Virtual Network Computing. A system for remote operation of a computer. The server, the computer to be controlled, runs a VNC process. The remote client connects to it using a remote framebuffer (RFB) protocol, and the client’s user can see and work with the server’s desktop.
Voice Encoder. A signal compressor optimized for encoding human speech, typically producing voice channel rates of 2.4 or 4.8 kb/s instead of the PCM standard 64 kb/s. Developed to reduce bandwidth use by voice lines, it has the unwanted side-effect of trashing VGC modem signals.
Vocoders are sometimes used for voice synthesis in music and entertainment, with quality deliberately degraded to sound more machinelike. Most such applications, however, rely on something simpler than a vocoder.
A NASA space probe launched on 5 Sep 1977. Now in interstellar space beyond the solar system’s heliopause, it’s the most distant known man-made object. NASA maintains a page at https://voyager.jpl.nasa.gov/ that tracks the current distance to it and to its twin, Voyager 2.
Virtual Private Network. A network-within-a-network that makes connections over public Internet channels but provides access only to selected users. It requires a network access server that checks user credentials and supports their requests for a secured connection. VPN use falls into two broad categories: security (providing secure, encrypted connections for remote users of a private or corporate intranet) and privacy (bypassing censorship, user tracking, and/or law enforcement monitoring by routing traffic through an encrypted connection). It’s difficult for external monitoring to distinguish these two cases, hence difficult for governments to allow the first case but block the second, as some have tried to do.
Clientless VPNs are those that rely on TLS or the older SSL encryption, which browsers already support.
Client-based VPNs require the user to run a VPN client based on a tunneling protocol (PPTP, or L2TP with IPsec). Tunneling protocols embed the network packets of the VPN in their own protocol wrappers – and, nowadays, encrypt the packets as well. It has become standard for operating systems to include VPN clients. For example, access the Windows Vista/7 VPN client by entering “vpn” in the desktop search bar.
Regardless of whether or not users employ a client app, they still need access to a VPN service, which generally requires a paid subscription. Many commercial VPN services have international presence, which allows users to have local access within a specific country, bypassing region-blocking.
Split-tunneling is a client feature that lets users select which applications go through the VPN and which connect to the network without using the VPN.
OpenVPN is an open-source VPN program that uses SSL or TLS.
Voltage, Programming. A high-voltage DC input used in writing data to some types of non-volatile memory. It sometimes serves as a program-enable signal too.
Virtual Private Server. A virtual machine that runs server software, providing the same services as a dedicated physical server at much lower cost, although with less bandwidth. Because its resources aren’t shared with other customers of the cloud hosting service, it provides a consistent level of support.
Virtual Reality. Refers to any technology that immerses the user in a wholly computer-manufactured environment, aka CAVE. Contrast augmented reality. As of 2015, the first VR consumer products are approaching launch.
Video Random Access Memory. A type of RAM used on graphics cards to support video operations. Once a luxury, now increasingly a necessity in PCs, although PC video will always require some system RAM in addition to any VRAM that it has.
Early VRAM was just DRAM placed on the video card, but newer types such as the GDDR# family are custom-designed to support graphics, with dual-ported memories: one port maintains and displays the image, while the other gathers update information for the next write to the screen. This gives VRAM higher speed with lower latency than DRAM. Chip enhancements that work for DRAM are also applied to VRAM.
Volatile Random Access Memory. A type of RAM that loses state when it loses power. Because this type is so common, it’s usually called just RAM, or memory.
Visual Studio. A family of Microsoft IDEs for creating Windows applications using any of a number of programming languages, most commonly Visual C++, Visual C#, and Visual Basic. (The Visual part means it provides tools for designing Windows forms graphically, and automatically generates the code to create them. The developer must still write the code for what the forms do.) It can also develop Web pages. There are many versions, including the free-with-restrictions Visual Studio Express and Visual Studio Community. Older versions are typically unable to open projects created by a newer version.
Very Small Aperture Terminal. A small (less than 2 m) satellite dish used for independent one-way or two-way links to communications satellites. It’s a popular way to set up Internet, telephone, or messaging connections to remote locations.
Vestigial Sideband. Signal modulation technology with a QAM-like spectrum (flat, with rolloff at edges), and a vestigial sideband carrier at the lower-edge rolloff, apparently to aid receiver tuning. It uses only slightly more bandwidth than SSB. US digital TV employs a type of VSB; see ATSC.
Vector Signal Image Processing Library. An open-source API and library of functions (middleware) for creating signal- and image-processing applications. VSIPL++ is the object-oriented version.
Voltage, Source. The voltage at the source terminal of a field-effect transistor (FET), or the input ground or negative voltage for a circuit based on FETs. Contrast VEE.
Voltage Standing Wave Ratio, pronounced “viz-wahr”. Characteristic impedance mismatches between a transmission and the terminating load cause reflections of the signal. Two waves traveling in opposite directions form a standing wave. VSWR is the ratio of maximum voltage to minimum voltage over one cycle: [Vsignal + Vreflected] to [Vsignal - Vreflected]. This provides a measure of transmission inefficiency. If there is no reflection (perfect matching), VSWR is 1. A typical antenna VSWR is about 1.5. Terminating a line with a short-circuit to ground causes 100% reflection, 180° out of phase, while an open circuit causes 100% reflection with no phase shift.
Virtual (Device) Driver. A device driver for early versions of MS Windows, excluding Windows NT and everything from XP forward. It’s a core OS service that has the same priority as the kernel, and provides continued support for 16-bit apps on the later 32-bit systems. Windows 3.x loads all VxDs and keeps them loaded. Windows 95, 98, and ME load and unload them dynamically, using the kernel memory space. A VxD typically has the .386 or .vxd filename extension.
VMEbus Extensions for Instrumentation. A 1987 standard for an instrumentation bus based on VMEbus, the Eurocard, and GPIB. Similar in concept to PXI. VXI plug-and-play requires that the instrument driver package include a function panel file, C source code, and a DLL that can be loaded by an app using a conventional language (commonly C).
A scalable, network-aware RTOS developed by Wind River Systems to run on embedded devices. It requires a host system (typically a Unix variant) for program development.
