Universal Asynchronous Receiver/Transmitter. A device that acts as a translator between a processor’s parallel data and a serial input-output line. RS-232 is the most common serial interface standard, but RS-422 and RS-485 are also popular. UART ICs that have been used to provide serial ports for personal computers include:
8250 – Used in the original IBM PC, now obsolete. It has a 1-byte buffer and a top data rate of 9600 b/s.
16450 – Successor to the 8250, with the same architecture and, supposedly, a top speed of 115.2 kb/s. However, its lack of a receive buffer makes it unreliable at more than 19.2 kb/s when used with a modem. It also has problems with multi-tasking OSes, slowing to as little as 1200 b/s.
16550 – 16-byte FIFO buffer, top speed of at least 115.2 kb/s. Early units had a bug that caused them to perform like the 16450, i.e. they could buffer only one byte and had to be interrupt-controlled by the CPU. Bug-free versions are sometimes dubbed 16550A.
16650 – 32-byte FIFO buffer, top speed of 460.8 kb/s. Early units had the same problem as the early 16550, so the corrected version is sometimes called 16650V2. It’s used in V.90 modems.
16750 – 64-byte send and 56-byte receive buffer, top speed 921.6 kb/s.
16850 – 128-byte buffer, top speed 460.8 kb/s or 1.5 Mb/s.
16950 – 128-byte or 1-kB buffer.
As other types of data connections, especially USB, have proliferated, few PCs still have serial ports and their associated UARTs.
Unmanned Aerial Vehicle. More commonly called a drone, remote-controlled aircraft, or remotely piloted vehicle (RPV). As of 2015, anyone flying a UAV weighing more than 0.25 kg in a public space in the US is legally required to register it with the FAA. Flying a UAV for commercial purposes requires FAA drone pilot certification.
Universal Chiplet Interconnect Express. An industry standard for data links between chiplets, announced in 2022 as UCIe 1.0. Unlike the PCIe and CXL standards on which its electrical signals and default protocols are based, it doesn’t use an external controller, so it offers higher speed, lower latency, and lower power consumption. The initial release defines a standard package and a high-performance advanced package.
Unique Device Identification. The serial number assigned to an Apple iPhone. In 2012, reacting to privacy concerns, Apple began restricting access to it by iOS apps, which were using it to track user activities and location – but then rolled out the IDFA, which serves the same purpose.
User Datagram Protocol. A member of the TCP/IP protocol suite, where it operates at the Transport layer alongside the better-known TCP. UDP is connectionless (it does not establish a logical link), and it uses stateless protocols, which is to say, each message it sends must be complete and self-contained, with no assumption that previous messages have been received.
This lack of overhead makes UDP faster than TCP, with a smaller, simpler stack, but at the cost of reliability. It’s used where the objective is to send small, unsequenced blocks of data quickly, such as for a live audio feed. NFS uses it to set up end-to-end, application-specific communications for network PCs.
Unified Extensible Firmware Interface. A standard interface between a PC’s OS and firmware, intended to replace the BIOS, although it can also run on top of the BIOS. It’s essentially a very small OS. Derived from the EFI that Intel released in 2003, it’s governed by the Unified EFI Forum (www.uefi.org).
While the BIOS relies on the 16-bit real mode of x86 processors, UEFI is hardware-independent. It’s meant to standardize the design of motherboards, and to speed up boot times. The system boot process loads it to memory from a motherboard flash or TPM chip, a hard drive, or even a network, and it does whatever it’s programmed to do before verifying and loading the OS.
Critics say it increases complexity without providing any important advantages over a BIOS. A more serious objection is that it can be designed to enforce DRM and political or cultural censorship via technologies such as Trusted Computing.
Unattended Ground Sensor. A sensor placed in a remote area to collect and transmit information, often for months or years. Different types detect sound, movement, heat, vibrations, even radiation.
Universal Integrated Circuit Card. Usually called a SIM card, since that’s its most common use. An integrated circuit card, or smart card, serving as a standard SIM platform for GSM, 3G, or 4G mobile wireless networks.
The first type of UICC, called the 1FF (First Form Factor) or just SIM, measures 85.6 × 53.98 × 0.76 mm, the size of a credit card. The 2FF mini-SIM is 25 × 15 × 0.76 mm, with the same contact arrangement as the 1FF. The 3FF micro-SIM is 15 × 12 × 0.76 mm. The 4FF nano-SIM announced in 2012 has 8 contacts and measures 12.3 × 8.8 × 0.67 mm. There are adapters for connecting the cards to interfaces meant for other sizes. The obstacle to even smaller cards is that they would be too small for human handling.
2FF (mini-SIM), 3FF (micro-SIM), and 4FF (nano-SIM) cards
User Identification. In Windows and Unix, a number specifying a system user, for purposes of assigning common security permission settings. In Windows, each UID must be unique. Unix also places each user under a GID.
Unique Identifier Header. Also called a supercookie. A random-looking character string that Verizon Wireless networks inject into Web requests made by their mobile subscribers. Although the UIDH does change, each is unique to a particular customer, enabling user tracking. AT&T Wireless was doing the same thing, but stopped in late 2014 when users began to notice. Other ISPs might be using this tactic as well.
Although Verizon has since been legally compelled to offer users an opt-out, the only sure defense against this sort of user tracking is to go through a VPN.
Underwriters Laboratories, Inc. A non-profit product safety testing and certification company founded in 1894, at https://www.ul.com. Some overseas manufacturers of electronics use counterfeit UL labels.
A power-storage capacitor with a complex internal structure that gives it vastly more electrical surface area than standard types. It has very short charge/discharge time and very high current potential, making it useful for, e.g., regenerative braking. Modern ultracapacitors extend into the thousands of farads, and have much longer life relative to batteries, but so far have much lower charge density.
The terms ultracap and supercap are interchangeable, although the latter typically refers to an older application: short-term backup power to chips and microcircuits to protect against interruption of main power, and to handle spikes in current draw that could overtax or harm the main, especially if the main is batteries.
The electric double-layer capacitor (EDLC) is an ultracap that uses two porous electrodes made of conductive polymers immersed in an electrolyte.
Upper Memory Block. In MS DOS, an area of upper memory that isn’t being used by the system and so is free for other uses by a memory manager. The command DOS=UMB means, load DOS in a UMB if possible. See RAM.
Unified Modeling Language. An industry standard visual language for designing systems, software or otherwise, distinct from and prior to the use of an actual programming language. It’s especially useful in creating a blueprint for complex real-time systems to maximize timeliness and efficiency. It provides constructs for building a model. Users model a system in any of several ways, depending on what makes the most sense from their point of view: user case diagram, class diagram, state diagram, physical diagram, etc.
UML takes an object-oriented approach. It has an offshoot called Systems Modeling Language (SysML) that doesn’t.
Universal Mobile Telecommunications System. An obsolete packet-switched 3G digital mobile phone standard based on the 2G GSM, developed by ETSI and its spinoff 3GPP in accordance with the ITU’s IMT 2000. Its radio interface was W-CDMA, with a 5 MHz uplink channel in the 1885-2025 MHz range and a 5 MHz downlink at 2110-2200 MHz. It offered data rates up to 1920 kb/s, although performance was more often limited to 384 kb/s. Deployment began in Japan (2001) and then Europe (2003). Some providers later upgraded UMTS networks to LTE.
Translation of “bukimi no tani”, a term from a 1970 essay by Japanese engineer Masahiro Mori. The level of animatronics or computer-graphics rendering of human features at which increasing realism is creepy rather than appealing. Getting out of the Uncanny Valley requires either perfect realism (possible as of 2008-2010 only with very complex programming and banks of dedicated servers) or a retreat to something comfortably, cartoonishly unrealistic.
Unlicensed National Information Infrastructure. A new (2001) portion of the US spectrum, consisting of two bands covering 5.15-5.35 GHz and 5.725-5.825 GHz. Used by the IEEE 802.11a wireless LAN standard.
Asynchronous, serial, master-slave data bus protocol created by Microchip Technology, Inc. for embedded systems. It sends 8-bit, Manchester-encoded data words, MSb-first, at up to 100 kb/s. Compare I2C, SPI.
Originally named Un-multiplexed Information and Computing Service (UNICS) as a joke, this venerable multi-tasking OS began in 1969 with researchers Ken Thompson, Dennis Ritchie, and Rudd Canaday at AT&T’s Bell Labs. It was the first to offer a hierarchical file system, although it originally limited files to 64 kB and six-character names. It consists of the kernel (to manage access to hardware resources, including scheduling tasks on the CPU), the shell (to connect and interpret user commands), and a large set of tools and applications that add functionality. There are many versions and varieties, and also derivatives such as GNU/Linux.
Universal Power Adapter for Mobile Devices. (2015) Also called P1823. An IEEE draft standard for a DC power connector supplying between 10 and 240 W. It will have a 3.3-V CAN bus probe line to negotiate power requirements of compliant devices.
Universal Plug-and-Play. A computer-industry standard set of protocols that relies on TCP/IP and WWW protocols to achieve interoperability between devices. It allows a device to join a network, obtain an IP address, announce its name, advertise its capabilities, and obtain information on other network devices. It can use AutoIP and ARP rather than DHCP to obtain an address. It’s a gargantuan security hazard, especially for IoT devices.
Uninterruptible Power Supply. A unit that blocks power surges from reaching equipment and provides continuous power from batteries in the event of a power dip or failure. Don’t connect laser printers to it – they have sudden spikes of current draw that a UPS probably can’t handle. This is likely to cause printer malfunctions, and can damage the UPS.
Uniform (originally Universal) Resource Identifier. A character string that identifies a data resource to Web browsers. If that’s all it does, it’s a URN (Uniform Resource Name). If it also provides a path for the browser to retrieve a copy of the resource, it’s a URL (Uniform Resource Locator), the string that appears in the browser’s address bar.
The part before the colon identifies the URI type: http, file, ftp, etc. The part after the colon gives the specifics.
Uniform (originally Universal) Resource Locator. A human-readable WWW address, such as https://www.ieee.org. Internet routers map URLs to the corresponding IP addresses, e.g. 140.98.193.141, that Web servers use. See DNS.
URL shortening is a way of publishing links to Web pages that have over-long URLs. A user creates a shorter link starting with the domain name of a URL-shortening service such as http://bit.ly, http://goo.gl (now defunct), or http://tinyurl.com, and registers it with the service. The service will then redirect link requests to the actual URL using a HTTP 301 or 302 code. (301 indicates a permanent change; 302 is a temporary redirect.) This is particularly useful for text messaging, where character count must be small. Abuse by spammers and other bad actors is a persistent problem.
Universal Serial Bus. A popular PC technology originated by Intel & Microsoft and now controlled by an industry consortium called the USB Implementers Forum (USB-IF). It defines a data bus for daisy-chaining multiple peripherals (keyboard, mouse, printer, scanner, etc.) to a single computer port, and up to 127 devices to the PC as a whole, although in actual use it seldom hosts more than one device per port. It operates like a tiny LAN, dispensing with the IRQ settings, DMA channels, and I/O addresses plaguing users of the old serial ports, and it permits hot swapping of devices. It can operate over cable runs up to 5 meters. The USB thumb drive with type A connector has become the PC standard for removable storage.
Connectors are usually (usually!) color-coded to indicate the USB version. USB 1.0 connectors are white, USB 2.0 connectors are black, and USB 3.x connectors are blue. The USB specifications are designed for backward compatibility, so ports and cables/connectors built for different specifications work together as long as they’re the same form factor – unless that form factor is USB type C, for which standardization has broken down. Data speed is, of course, limited to what the oldest component in the chain can handle. The specifications:
USB 1.0 – (1996) 1.5 Mb/s or 12 Mb/s. Offers two connector standards, type A and type B. Pins 1 and 4 carry +5V power and ground, while pins 2 and 3 are the negative and positive differential signal lines, respectively. It relies on a very simple, reliable data-transfer protocol called Bulk-Only Transfer (BOT). USB was little-used until the 1998 USB 1.1 update, which is essentially a bug-fix of the original.
USB 1.0 type B (left) & A plugsUSB 1.0 type A portUSB 1.0 type B port
USB 2.0 – (2000) 1.5, 12, or 480 Mb/s speed, though actual maximum throughput is about 240 Mb/s. A subsequent modification to this version of the spec, made via engineering change notice (ECN), added the mini-A and mini-B connectors. Other ECNs added the micro-A and micro-B, and support for supplying power to attached devices. Mini and micro connectors have 5 pins rather than 4; the additional pin is for signaling the presence of connected devices. If a product says just USB mini or USB micro, it means mini-B or micro-B, because the mini-A and micro-A versions are much less common. Micro-B is the near-universal standard connector for charging mobile devices. It’s not clear that anyone actually makes mini-A receptacles, but there’s a mini-AB receptacle that can accept mini-A and mini-B plugs. Likewise, there’s a micro-AB receptacle for both micro plugs.
USB 2.0 type A (left) & B plugs
USB 2.0 mini-A plug (left) & portUSB 2.0 mini-AB portUSB 2.0 mini-B port (left) & plug
1
+5V
2
Data -
3
Data +
4
Device ID
5
Ground
USB 2.0 micro-A plugUSB 2.0 micro-B plug (left) and port
Wireless USB 1.0 – (2005) Transmits at about 3 GHz, has UWB capability at 6+ GHz.
USB 3.0 – (2008) Also called SuperSpeed USB or or USB 3, and retroactively renamed USB 3.1 Gen 1 and then USB 3.2 Gen 1 by the USB-IF. It supports the original BOT protocol and its 1.5, 12, and 480 Mb/s data rates. Because BOT doesn’t scale well with speed, though, it also has the ANSI standard USB Attached SCSI Protocol (UASP) to handle a new maximum 5 Gb/s rate. Maximum throughput is more like 3.2 Gb/s, due in part to the 8b/10b line coding, which adds two overhead bits for every eight data bits to help with bit-clocking. USB 3.0 ports can supply 2.5 to 4.5 W power (500-900mA at 5V). Type A and type B connectors have five new signal lines, for a total of nine. The new lines carry two SuperSpeed differential signals (one transmitting, one receiving), plus a signal ground to support them:
USB 3.0 type A plug (top) & portUSB 3.0 type B plug (top) & port
USB 3.1 – (2014) Retroactively renamed USB 3.1 Gen 2 and then USB 3.2 Gen 2 by the USB-IF. Up to 10 Gb/s, minus overhead for the 128b/132b line code (4 overhead bits for every 128 data bits), but continues to accommodate the older speeds. It supports 5V, 12V, or 20V power, with standard-size type A and type B connectors able to supply as much as 100 W (5A@20V). Power in excess of what USB 3.0 can handle requires 3.1-compliant cables and connectors.
This version of the standard also introduces the reversible type C connector, meaning the user can connect it with either orientation and it still works. It isn’t electrically symmetrical, but each port detects and adapts to plug orientation. The type C has 24 pins. For the port, these are two sets of the nine USB 3.0 pins, two additional VBUS (power) pins, two SBU (sideband use) pins that support alternate types of signaling such as DisplayPort, and two CC (configuration channel) pins that tell the host how the connector is oriented. The type C cables have only 18 wires; there’s a single wire pair for the D+ and D- differential signals because the port supports just one D+/D- (USB 2.0) interface, and the four ground pins and four VBUS pins each share a pair of wires. For this reason, although the plug also has 24 pins, two of them are unused, the CC2 pin is reassigned as VCONN to supply any power required by the cable itself rather than by the attached peripheral, and, as previously noted, the four ground and four VBUS pins between them use only four wires.
USB 3.1 type C port (top) and plug pinoutsUSB 3.1 type C plug
USB 3.2 – (2017) For no good reason that anyone can discern, the official USB-IF name is USB 3.2 Gen 2×2, with USB 3.0 and 3.1 confusingly renamed as mentioned above. Supports two data lanes at up to 10 Gb/s each over USB-C cable, doubling the maximum throughput. Its type C connectors are not necessarily electrically compatible with one another or with USB 3.1 type C.
USB4 – (2019) Based on Thunderbolt 3, with speeds up to 40 Gb/s, using type C connectors exclusively. The official name is USB4 Gen 3×2.
Every USB device contains a microcontroller hosting firmware that identifies it and its capabilities to the host computer through a unique 32-bit combination of PID and VID. Because USB has no way to verify or protect this firmware, it’s a potential malware vector against which there is currently (2014) no defense. An infected USB device can attack any computer to which it connects, and an infected computer can attempt to hack the firmware of any connected USB device.
From “Users’ Network”. A BBS variant that began in 1979 using UUCP over dial-up. Unlike a BBS, a Usenet forum isn’t tied to a single server, but can be hosted and maintained on multiple servers across a network of any size. That’s why Usenet is still going, now almost entirely on the Internet using NNTP. However, ISPs routinely block access to it for financial and legal reasons. The modern user has to buy a subscription from a Usenet service provider.
As of 2009, there are nine newsgroup hierarchies: comp., humanities., misc., news., rec., sci., soc., talk., and the wild-and-wooly alt. Each hierarchy contains many, many newsgroups, each dedicated to a specific topic. No membership or log-in is required for read access, although some groups have a moderator who decides what gets posted. Usenet is where most of Internet culture began.
Universal Service Order Code. A set of registered jack (RJ##) standards for wiring customer telephone & data equipment, introduced in the 1970s by AT&T and partially adopted by the FCC. The terms specify just the wiring but are often used, inaccurately, to mean the physical plugs and jacks. Postfixes to the RJ## have the following meanings: C = surface or flush-mounted jack, W = wall-mounted jack, S = single-line, M = multi-line, and X = complex multi-line.
Almost all RJ## installations use 4-, 6-, 8-, or occasionally 10-position ISO modular connectors, which are identified by number of positions and number of actual contacts: 6P4C (6 position, 4 contact), 8P8C (8 position, 8 contact), etc. Modular jacks accept plugs of the same size or smaller.
Common RJ## standards are listed below, although some of them have variants that are not described. The images show which pins are connected in the jacks each standard uses.
RJ11 – A single phone line, almost always using the two middle pins of a 6P2C plug and jack; pin 3 is the ring (-48V, red wire) and pin 4 is the tip (ground, green wire). RJ11 can also use 4P2C, 8P2C, and other connectors.
RJ11 jack wiring
RJ12 – A single phone line with hold function. On the usual 6P4C connectors, it uses pins 2 and 5 for the hold circuit.
RJ14 – Two phone lines. Like RJ12, it typically uses 6P4C connectors, but pin 5 is ring 2 (-48V, yellow or white wire) and pin 2 is tip 2 (ground, black wire).
RJ14 jack wiring
RJ21 – Up to 25 telephone lines, using a 50-pin connector unrelated to the ISO modular connectors.
RJ22 – Connects a phone’s handset to its base unit. Typically uses 4P2C connectors.
RJ25 – Three phone lines. Normally uses 6P6C connectors, sometimes 8P6C.
RJ25 jack wiring
RJ45 – Four phone lines. The 8-position connectors used for the original RJ45 are very similar to the 8P8C, but have an extra tab that makes them incompatible. This resemblance is why Ethernet 8P8C cables are routinely but inaccurately called “RJ45”.
RJ45 jack wiring
RJ48 – A phone line and either a 1.544 Mb/s T1 connection or another phone line, using 8P4C connectors and STP cable. Pins 7 and 8 are for the cable shield, while 3 & 6 are reserved for future use.
Coordinated Universal Time, or Temps Universel Coordonné. The order of the acronym letters is a compromise – it doesn’t exactly match either the English or the French term. The term “UTC” is favored over the older “Greenwich Mean Time”. UTC is a time-keeping system based on atomic clocks, with its time zone in Greenwich, England. The start date/time for UTC is 00:00:00 on 1 Jan 1970. UTC is adjusted every year or so to keep it within one second of clocks based on Earth’s rotation.
The lower 48 states of the USA have four time zones: Eastern Standard Time (EST), Central Standard Time (CST), Mountain Standard Time (MST), and Pacific Standard Time (PST). EST is 5 hours behind UTC, annotated as UTC-5. When daylight savings time is in effect, from early April to late October or early November, “Standard” is replaced with “Daylight” and the four zones become EDT, CDT, MDT, and PDT, where EDT = UTC-4. See Earth.
Universal Transverse Mercator. A two-dimensional map & coordinate system for the Earth’s surface. It uses 60 zones, each 6° wide and extending from 80° S to 84° N. The first zone starts at the International Date Line (180°) and extends eastward. The original Mercator projection is accurate only near the equator; the transverse system generates 60 strips, each with minimal distortion near its own north-south tangent.
Unified Threat Manager. A network security device that combines the functions of a firewall, VPN, intrusion detection/prevention, anti-virus, Web filtering, etc.
Unshielded Twisted Pair. A transmission line consisting of two insulated wires twisted together. Twisted-pair cable is important for differential (balanced-line) signaling – one wire carries a signal and the other carries its inverse, and the receiver tracks the voltage differential between the two rather than referring to some arbitrary ground. Since external noise is coupled almost equally into each wire of a twisted pair, it doesn’t change the differential between them, making the line highly resistant to noise. That’s why multiple UTP lines can be packed into the same cable. See telephone, compare STP.
UMTS Terrestrial Radio Access Network. The radio interface required for UMTS base stations. It uses direct-sequence, spread-spectrum W-CDMA with hybrid CDMA/TDMA access methods in a 5 MHz channel, for proposed data rates up to 2 Mb/s. There are plans for downlink rates up to 10 Mb/s. Transmission RF should be somewhere around 2 GHz.
Unix-to-Unix Copy Protocol. A simple store-and-forward protocol that handles modem-to-modem (i.e., phone line) communications. The original basis for Usenet, it has been supplanted in that role by NNTP for Internet connections.
Unix-to-Unix encoding. An obsolete Unix command that converts a binary file into an ASCII text file so that it can (probably) pass undamaged through a mailer. For example, given a binary file called input1, the command sequence uuencode input1 output1 creates a file called output1 that contains ASCII representations of the data. The uudecode output1 command then reverses the process, stripping off any headers or trailers slapped on by the mailer and restoring the original binary. MIME and other technologies have replaced uuencode.
Ultra-violet. The part of the electromagnetic (EM) spectrum with wavelengths from 4 to 400 nm. Near UV is the range from 400 to 300 nm; 300 to 200 nm is far UV, and beyond that is extreme UV. Another classification divides UV into UV-A (400-320 nm), which tans you; UV-B (320-290 nm), which burns you; and UV-C (290-190 nm), which, mercifully, doesn’t make it through Earth’s atmosphere, but is useful for sterilizing instruments and indoor environments. As with IR, these boundaries are not universally agreed upon. See light.
Ultra-Wideband. A new, initially military technology for radio and wireless LAN that spreads a short-burst, low-power signal (10-1000 picoseconds and as little as 1 mW) across a broad spectrum (at least 500 MHz in the 3.1-10.6 GHz band), with data rates of 100-500 Mb/s. The band used to extend lower, but the FCC raised it, mainly to protect GPS. Given 3 dB attenuation points fH and fL of the power spectral density, a system meets the working definition of UWB if 2(fH - fL) / (fH + fL) > ¼.
UWB typically employs frequency-hopping OFDM, and has a very short range – 10m or so. It’s hard to detect or jam. At frequencies off the legal low end, it can penetrate concrete, giving it urban warfare potential for through-the-wall tactical radar systems. It’s the basis for the IEEE 802.15 wireless PAN standard.
Ultrasound Cross-Device Tracking. Spyware technology that embeds sound frequencies above the range of human hearing in advertisements, and employs apps running on microphone-equipped devices nearby to detect the sounds and correlate the devices (and activities on them) with the same owner.
