EE Terminology - Numbers & Symbols

!: The factorial symbol. For a given integer n, n! (n-factorial) means the value [n × (n-1) × (n-2) … × 2 × 1]. By convention, 0! = 1.
10Base#: See 802.3.
100Base#: See 802.3.
1000BaseT: See 802.3.
1080i: 1080 pixels high, interlaced scan. See ATSC.
1080p: 1080 pixels high, progressive scan. See ATSC.
10GbE: 10-Gigabit Ethernet. Under 802.3, see 802.3ae.
10P#C: 10 Position x Contact, where x is 2, 4, 6, 8, or 10. See modular connector.
12VHPWR: 12-Volt High Power. Released in 2022 by the PCI-SIG standards body, this power-connector standard is meant to replace multiple PEG power headers on graphics cards with a single header, saving board space and reducing cable count. A 12VHPWR cable has from one to four separate 8-contact PEG plugs going to the system power supply on one end, all feeding a single 16-contact plug on the other end. Pins 1-6 carry 12VDC power, pins 7-12 are ground, and the smaller pins S1 to S4 are power-sense lines to tell the graphics card how much power is available. Since an 8-contact PEG supplies a theoretical maximum of 150W, a four-headed cable carries up to 600W.; There have been instances of the connector melting because it wasn’t fully seated and was radiating too much resistive heat from the 12V pins.
12VO: An Intel power standard for personal computers, also called ATX12VO. Instead of the older ATX12V standard’s 20-pin or 24-pin motherboard connector with 12VDC, 5.5VDC, and 3.3VDC power, it has a 10-pin motherboard cable and provides only 12VDC power. The motherboard is responsible for downconversion to the lower voltages it requires. The idea is to reduce manufacturing costs and the number of internal cables.
1337: Elite. See l337.
1394: See FireWire.
13W3: A D-sub connector used for workstation video cables. It resembles a DB-25, but with three small coaxial connectors replacing some of the pins. A monitor with a HD-15 (VGA) video port can connect to a 13W3-capable workstation through an adapter.
1553: See MIL-STD-1553.
16450, 16550, 16650, 16750, 16950: See UART.
1Base5: An implementation of the StarLAN IEEE standard on a baseband medium at 1 Mb/s. The maximum segment length is 500 meters.
1FF: First Form Factor. The original SIM card form factor. See UICC.
1G: First Generation. An after-the-fact term for the analog, FDMA-based mobile phone systems replaced by 2G, notably AMPS and NMT.
1GL: 1st Generation Language. This refers to a binary machine language used by processors.
1’s comp: See ones complement.
1U: (1); 1¾ inches. ‘U’ is an American standard unit of measure for the front-panel height of a circuit board, electronics module, or rack-mount unit.; (2); A cube 10 cm on a side, hence with volume of 1000 cm³. It’s a common unit of measure for the very small satellites known as cubesats.
21064, 21066, 21164, 21264, 21364, 21464: Members of a DEC microprocessor series. See Alpha.
232: See RS-232.
2.5 mm: For the plug/jack audio standard, see TRS.
2B1Q: Two Binary, One Quaternary. A type of modulation, also characterized as 4-level PAM.
2FA: Two-Factor Authentication. See MFA.
2FF: Second Form Factor. The mini-SIM card form factor. See UICC.
2G: Second Generation. The second generation of mobile phone systems, including GSM, IS-136, and others using CDMA or TDMA signaling. 2G systems are digital, but still have circuit-switched architectures: a constant-rate RF link is assigned for the duration of a call, regardless of the level of traffic on that call.
2GFSK: 2-state Gaussian Frequency Shift Keying. See GFSK.
2GL: Second Generation Language. This refers to an assembly language used to program microprocessors.
2K: (1); A standard for digital video production with 2048 × 1080 pixel resolution. Used casually to mean any format approximately 2000 pixels wide, especially ATSC TV (HDTV) and the 1920 × 1080 pixel HD (High Definition) resolution of computer graphics. These have an aspect ratio of 16:9 rather than 2K’s 19:10. The term “2K” came into use after manufacturers started pushing TVs advertised, inaccurately, as 4K, and later 8K.; (2); (2001) 5th generation of PowerPC chips, 0.18 µm process or smaller, clock speeds above 1 GHz.
2LMES: Two-Line Mean Element Set. A NORAD ephemeris system for describing the motion of a satellite using six orbital elements: semi-major axis a (size of the orbit), eccentricity e (shape of the orbit), inclination i (orientation with respect to the equator), argument of perigee ω (point of perigee with respect to the surface), right ascension of the ascending node Ω (ascending and descending orbit locations with respect to the equatorial plane), and true/mean anomaly ν (where the satellite is in the orbit with respect to the perigee). The 2LMES consists of two 69-character lines of data, like this:; Breakdown for line 1: Bloc 1 is the satellite’s NORAD Catalog number and classification; bloc 2 is the international launch designator; bloc 3 is the epoch year and day, including a fractional portion thereof; bloc 4 is the first-time derivative of the mean motion; bloc 5 is the second-time derivative of the mean motion (exponential, with a leading decimal point assumed); bloc 6 is the B* drag coefficient term (exponential, with a leading decimal point assumed); bloc 7 (a single character) is the orbital model used to generate the data (0 means SGP4/SDP4); bloc 8 is the element set number and a single-character modulo-10 checksum.; Breakdown for line 2: Bloc 1 is the satellite’s NORAD Catalog number; bloc 2 is degrees of inclination; bloc 3 is degrees right ascension of the ascending node; bloc 4 is eccentricity (decimal point assumed); bloc 5 is argument of perigee; bloc 6 is degrees of mean anomaly; bloc 7 is mean motion in revolutions/day; bloc 8 is the revolution number at epoch, and a single-character modulo-10 checksum.
2’s comp: See twos complement.
3.5 mm: For the plug/jack audio standard, see TRS.
3D printing: Three-Dimensional Printing. A new RP technology, also called additive manufacturing, that started out using extruded plastics (e.g., ABS or PLA) and 3D computer models to produce physical objects, building them up as a series of quick-hardening layers. The precise size of the printed object is sometimes slightly larger or smaller than intended. Compare the much older CNC.; Manufacturers want 3D printing that can handle ceramics and hard metals without the molds required by older RP techniques such as selective laser sintering (SLS) and its descendant, selective laser melting (SLM). As of 2019, there are industrial 3D printers with this capability, though they’re expensive and relatively slow. They print a part in layers from wire or powdered metal, using a laser, electron beam, or oven to melt each layer before laying down the next. This technique is called direct energy deposit (DED) or, for a variant that handles much smaller parts, powder bed fusion (PBF). There are also very large 3D printers that can create buildings or other large objects using various materials.
3D XPoint: Pronounced “crosspoint”. (2016) A non-volatile memory technology developed by Intel and Micron. Rather than the conventional 2-dimensional planar layout, it uses a 3-dimensional grid of ultra-fine wires. Each contact point between wires is a one-bit data cell with its value determined by a selector, a device that sets either low or high resistance. The data cells themselves are probably some type of phase-change memory. 3D XPoint reportedly has about 1000 times the speed and lifespan of NAND-gate flash memory. It’s still not fast enough to compete with DRAM as PC main memory, but is used in Intel’s Optane series of ultra-fast SSDs for data centers.
3FF: Third Form Factor. The micro-SIM card form factor. See UICC.
3G: Third Generation. The generation of mobile phone standards based on the ITU-R’s International Mobile Telecommunications 2000 (IMT-2000) project, to support worldwide roaming. Whereas 2G mobile phone systems were designed primarily for voice, 3G systems, first appearing in 2001-2002, can offer multiple services at a variety of data rates up to 2 Mb/s (less for users in motion). 3G networks use W-CDMA signaling, and are packet- rather than circuit-switched so they can dynamically adjust user bandwidth.; UMTS was the most common 3G implementation worldwide. In the US, the bands the ITU chose for 3G services were largely allocated, slowing adoption. US carriers ended 3G service in 2022, freeing the spectrum they have rights to for 4G and 5G signals.
3GL: Third Generation Language. This refers to a high-level programming language that can be compiled into machine language. Most traditional programming languages are of this type.
3GPP: Third Generation Partnership Project. International standards-setting body formed originally to develop specifications for global 3G mobile phone systems, although they continued with 4G and beyond. See also LTE.
3GPP2: Third Generation Partnership Project 2. International standards group for 3G mobile phone systems based on CDMA2000, such as IS-95.
3U: Either 5¼ inches, or three 1000 cm³ cubes. See 1U.
4004: See Intel.
404: The error code returned by HTTP when a WWW server is unable to find the requested Web page.
485: See RS-485.
488: See GPIB.
480i: 480 pixels high, interlaced scan. See ATSC.
480p: 480 pixels high, progressive scan. See ATSC.
4FF: Fourth Form Factor. The nano-SIM card form factor. See UICC.
4-FSK: 4-state Frequency Shift Keying. See FSK.
4G: Fourth Generation. ITU-R standard for mobile phone networks finalized in 2012, officially called IMT-Advanced. It uses IP-based packet switching compatible with IPv6, and OFDMA rather than spread-spectrum signaling. Its main advantage over 3G is higher data rates, which depend heavily on MIMO antenna arrays.; IMT-Advanced specifies data rates of at least 1 Gb/s for low-mobility subscribers (i.e., those not in a moving vehicle) and 100 Mb/s for high mobility. The LTE and IEEE standard 802.16m (WiMAX) networks that providers deployed as “4G” fell short of that, so providers got the ITU-R to weaken the standard. This is why some sources talk about 3.5G or 3.75G. The later LTE Advanced can meet the original 4G requirements. WiMAX has been the less successful of the two implementations.
4GFSK: 4-state Gaussian Frequency Shift Keying. See GFSK.
4GL: Fourth Generation Language. This refers to a programming language that’s designed to be more similar to natural language than the 3GL types. Database access languages, such as SQL, are often 4GL.
4K: A standard for digital video production with 4096 × 2160 pixel resolution, double the width × height of 2K. With compression, it supports up to 250 Mb/s data rate. The term is widely misapplied to UHD (Ultra High Definition) displays. UHD, besides being a broadcast and display standard (see graphics), is 3840 × 2160 pixels – an aspect ratio of 16:9 rather than 4K’s 19:10. For comparison, HDTV is 2048 pixels wide. Compare 8K.
4P#C: 4 Position x Contact, where x is 2 or 4. See modular connector.
5G: 5th Generation. The mobile phone network standard meant to replace 4G, debuting in 2019. Some of the initial rollouts in the US were just 4G with minor tweaks and misleading names such as “5G E”. The 5G standard continues 4G’s reliance on OFDM signaling, and defines three different frequency ranges, although a given mobile service provider might not use all three:; low-band – The same 600-900 MHz band as 4G phones, with slightly faster data rates.; mid-band – 3.7 to 3.98 GHz. Providers sometimes call this the C band, although it falls outside the IEEE-defined C band (4 to 8 GHz – see microwave band). Data rates are reportedly as high as 900 Mb/s.; high-band – The little-used EHF part (30-300 GHz) of the RF spectrum, which is little-used because it suffers strong atmospheric attenuation and is easily blocked, including by walls, windows, and bodies. 5G signals most often occupy 24.25 to 29.5 GHz, but other frequencies in EHF are in use or are expected in the future. The high data rates cited for 5G assume signaling in this band. However, EHF’s shorter range and greater susceptibility to interference means that cell towers must be closer together to maintain coverage, making high-band 5G more expensive and confining it to urban areas.; In addition to the increased costs from high-band signaling, 5G will require smarter servers and data centers to support IoT applications.
5GL: Fifth Generation Language. This refers to a programming language that uses a visual development environment. Almost all instances of 5GL are simply visual toolkits for 3rd-generation languages. The G language of NI’s LabVIEW is the star exception.
5VSB: 5 Volts Standby. A feature of the ATX power supply specification. It provides 5 volts for low-power standby features even when the system is nominally off.
680x0: See Motorola.
68k: See Motorola.
6G: 6th Generation. The mobile-phone network standard planned to replace 5G, expected to use frequencies around 94 GHz.
6P#C: 6 Position x Contact, where x is 2, 4, or 6. See modular connector.
6U: Either 10½ inches, or six 1000 cm³ cubes. See 1U.
6x86: See Cyrix.
720p: 720 pixels high, progressive scan. See ATSC.
783: A type of very-low-loss coaxial cable with 75 Ω characteristic impedance.
8008: See Intel.
80186: See Intel.
802.11: A.k.a. Wireless Ethernet, or, for newer types, Wi-Fi. A family of IEEE wireless LAN (WLAN) protocols, commonly paired in home or business LAN gateways with 802.3 (Ethernet). Typical real-world throughput of these standards is less than half of what’s advertised.; 802.11 – The original half-duplex wireless LAN/MAN protocol, 1997, retroactively called Wi-Fi 1. Although it’s now obsolete, later versions continue to use its frequency range and 14-channel structure (see below). It’s intended for small indoor networks, but directional antennas can dramatically extend its range. The key component of its architecture is the basic service set (BSS), a LAN cell, of which there are two types: the infrastructure BSS, which is a permanent part of the network, and the ad-hoc BSS that the network creates and discards as needed. Operating in the ISM band from 2.4 to 2.497 GHz, 802.11 offers 1 and 2 Mb/s data rates with either of two RF transmission schemes:; 1) A FHSS signal using 2GFSK or 4GFSK modulation over 75 1-MHz subchannels, or; 2) A DSSS signal using an 11-bit Barker code, DBPSK or DQPSK modulation, and one of fourteen 22-MHz wide channels. The center frequencies of the channels are spaced every 5 MHz from 2412 to 2484 MHz, so they overlap heavily. (The exception is the gap between channels 13 and 14, which is 12 MHz.) Common practice for running multiple wireless LANs in the same area is to assign each to a channel that doesn’t interfere with the others, e.g. 802.11 channels 1, 6, and 11. This approach breaks down if more than three networks overlap.; 802.11a – Not part of the Wi-Fi series, this oddball of the family operates in the UNII band above 5 GHz. This means it can’t talk to the other Wi-Fi types, has more trouble with walls, and is restricted to North America. For maximum spectral efficiency, it has a 20 MHz wide OFDM signal, and data rates up to 54 Mb/s, with throughput perhaps half that. Each of the 52 OFDM subcarriers is about 300 kHz wide, and uses BPSK, QPSK, 16-QAM, or 64-QAM modulation, plus rate ½ or rate ¾ FEC coding. 48 of the subcarriers carry data, with raw data rates from 125 kb/s to 1.5 Mb/s each, and 4 are pilot signals.; 802.11b – Also called Wi-Fi 2. This standard supports the DSSS (but not the FHSS) signaling of the original 802.11, so it has the same 14 channels spaced every 5 MHz from 2412 to 2484 MHz and pushing 1 or 2 Mb/s. It can also use those 14 channels for either of two new DSSS signals that achieve much higher data rates:; 1) 5.5 Mb/s using CCK with DQPSK modulation and a 4-chip spreading sequence, or 11 Mb/s with 8-chip rather than 4-chip spreading sequence, OR; 2) 5.5 Mb/s using rate ½ PBCC with BPSK modulation and a 256-bit spreading code, or 11 Mb/s with QPSK instead of BPSK.; Real-world throughput is much less than the theoretical limit, especially if the network uses encryption, which it should. WEP, the encryption included with the initial 802.11b standard, is vulnerable to skillful attack, so the standards committee later added WPA and WPA2.; 802.11g – Also called Wi-Fi 3. Backward-compatible with 802.11b signaling. It also offers theoretical data rates up to 54 Mb/s using the same OFDM signaling as 802.11a, but running in the 2400-2500 MHz band. Actual throughput of this new signal is more like 20-25 Mb/s, still a big advance over 802.11b.; 802.11n – Also called Wireless-N or Wi-Fi 4. Long-delayed 2009 WLAN extension with maximum theoretical throughput of 600 Mb/s, retaining backward compatibility with other Wi-Fi types. In addition to the old 2400 to 2500 MHz band, 802.11n can also use the band above 5 GHz, which it divides into 24 non-overlapping, 20-MHz wide channels with center frequencies from 5180 to 5825 MHz. For higher throughput, it can switch channel width to 40 MHz. Signaling in this band uses a 64-QAM carrier and up to four MIMO streams, i.e., exploitation of multipath.; 802.11s – Provisional standard for Wi-Fi mesh networking, in which multiple wireless nodes form a peer-to-peer network that shares an Internet connection.; 802.11ac – (2014) Also called Wi-Fi 5. In addition to compatibility with 802.11a and 802.11n, 802.11ac’s carriers in the 5 GHz band can have 80 or 160 MHz channel width, up to 256-QAM modulation, up to 1.3 Gb/s throughput, and up to eight MIMO streams.; 802.11ah – (2016) Like 802.11a, it’s not part of the Wi-Fi series. Signals are around 900 MHz, using less power than other Wi-Fi versions while providing up to 1 km range and greater robustness, at the cost of lower data rates. It’s designed for devices that don’t need high-speed connections, such as those that become part of the Internet of Things.; 802.11ax – (2019) Also called Wi-Fi 6. A derivative of 802.11ac, with OFDMA multiplexing for each of up to four downlink MIMO streams to boost throughput, an arrangement dubbed multi-user MIMO (MU-MIMO). It continues to use both the 2.4-GHz and 5-GHz bands, incorporates WPA3 encryption, and is claimed to achieve througput as high as 600 Mb/s for a single channel and nearly 10 Gb/s for the network.; A 2021 expansion of 802.11ax called Wi-Fi 6E adds a band spanning 5.925 GHz to 7.125 GHz. This so-called 6 GHz band has less competition from radar and satcoms than the 5 GHz band. It can support up to 14 additional 80 MHz channels or 7 additional 160 MHz channels.; 802.11bb – (2023) Also called Li-Fi, with the Li meaning light, this standard uses signaling at optical frequencies.; 802.11be – (expected 2024) Also called Wi-Fi 7. Claimed rates approach 40 Gb/s for the entire network.
802.15: IEEE standards for wireless, low-data-rate personal area networks (PANs).; 802.15.3a – Ultrawideband, in development as of 2005.; 802.15.4 – Used in ZigBee, this MAC protocol for a low-power, low-data-rate PAN has 27 channels allocated for the US and Canada. Channels 11 to 26 are from 2405 to 2480 MHz, with 5 MHz spacing and 2 MHz width. Each network member is either a coordinator or an end node. In beacon mode, the principal coordinator regularly broadcasts a synchronizing beacon frame. To save power, the network has selectable-length active periods (when members exchange data) and inactive periods (when they turn off their RF circuits). End nodes and non-primary coordinators can synchronize their active periods to the beacon, or enter prolonged sleep states and simply wait for the beacon upon waking. In non-beacon mode, the principal coordinator broadcasts a beacon to start the active interval only when needed, and some higher-level protocol has to make sure the other nodes are listening.
802.16: A family of IEEE standards defining the RF interface for a wireless MAN. The original 2002 standard, implemented as WirelessMAN™, uses 10-66 GHz line-of-sight (LOS) links. Later versions have been the basis for mobile phone networks.; 802.16a – (2003) Implemented as WiMAX (sometimes called Wider-Fi), uses single-carrier modulation or OFDM on licensed and unlicensed 2-11 GHz NLOS links. A base station can support up to 280 Mb/s and ranges up to 50 km (30 miles). WiMAX will also work with Europe’s HiperMAN.; 802.16e – (2005) Support for mobility and roaming to make it a 3G implementation.; 802.16m – A 4G implementation, also called WiMAX. First to market, but losing out to LTE networks.
802.17: IEEE standard for a resilient packet ring (RPR) network. It connects up to 256 nodes on two fiber-optic rings, one in each direction.
802.1X: An IEEE authentication protocol used in connecting to a wired or wireless network. Sometimes confused with 802.11x, where the lower-case “x” indicates any of the 802.11 family of wireless LAN standards.
802.20: IEEE standard for mobile broadband wireless, in development (2008). It has TDD multicarrier mode with 625 kHz channel spacing, and TDD/FDD OFDM wideband mode.
802.22: IEEE standard for a wireless regional area network (WRAN), which is able to operate over a large area. It exploits inactive portions of the TV spectrum using cognitive radio techniques (see SDR).
802.3: The IEEE physical layer standards for the CSMA/CD bus network protocols that constitute the Ethernet LAN family. The common members of this family:; 10Base2 – Thin Ethernet, a.k.a. Thinnet or Cheapnet. It uses flexible, stranded RG58 50Ω coaxial cable with BNC connectors, and supports a 10 Mb/s data rate over a maximum distance of 185 m.; 10Base5 – Thick Ethernet, a.k.a. Thicknet. It uses thick RG8 50Ω coaxial cable with type N connectors, and supports a 10 Mb/s data rate over a maximum segment length of 500 m.; 10BaseF – 10 Mb/s Ethernet over fiber. Sub-types include 10BaseFB (synchronous fiber serial chains repeaters), 10BaseFL (asynchronous fiber, interoperable with FOIRL, greater link distances), and 10BaseFP (asynchronous fiber & passive star topology).; 10BaseT (802.3i) – Ethernet. The standard that took the 1980s networking world by storm. It differs from the original Ethernet in having a 25-byte frame header, and a star topology rather than a linear bus. It typically uses Cat3 or Cat4 cable with 8P8C modular connectors, and supports a 10 Mb/s data rate over a maximum segment length of 100 m. Although Cat3/Cat4 cable has four twisted-pair connections, 10BaseT uses only two of them, transmitting on pins 1 & 2 and receiving on pins 3 & 6.; Ethernet patch or straight-through cables (pin 1 to pin 1, pin 2, to pin 2, etc.) are used to connect computers to the hub, switch, or router that provides network connectivity. Crossover or null-modem cables (pin 1 to pin 3, pin 2 to pin 6, pin 3 to pin 1, pin 6 to pin 2, and all others straight through) connect two computers directly without going through a network device.; 100BaseFX – Fast Ethernet over two strands of multimode fiber-optic cable (one transmit, one receive) with duplex SC, ST, or MIC connectors, up to 400m distance.; 100BaseT4 – Fast Ethernet using all four pairs of a Cat3 or Cat4 cable, with each pair sending or receiving at 25 Mb/s to produce an aggregate 100 Mb/s duplex data rate.; 100BaseTX (802.3u) – Fast Ethernet. Supports a 100 Mb/s data rate at up to 100m distance, but requires at least Cat5 cable, or 150Ω STP cable with IBM type 2 connectors. Like 10BaseT, it still uses only two of the cable’s four wire pairs, but it has a different frame structure.; 1000BaseT (802.3z) – Gigabit Ethernet, or GbE. Used in Fibre Channel networks as well as over traditional cable. It achieves its namesake 1 Gb/s speed over Cat5, Cat5e, or Cat6 cable by using all four pairs, rather than reserving two pairs as backups. It uses 8B/10B coding, which converts data from 8-bit to 10-bit format. The extra two bits are for clock synchronization, hardware error detection, and control characters.; 802.3ae – 10 Gb/s Ethernet, or 10GbE. Requires optical fiber (although see 802.3ak), wavelength of 850 (S), 1300 (L), or 1550 (E) nm, and one of three types of FEC. Full duplex, hence able to do without CSMA/CD protocol.; 802.3af – A specification for distributing power through the network cables, to devices that present the signature 25 kΩ load.; 802.3ak – 10 Gb/s Ethernet over copper. Requires Cat6a cabling.; 802.3av – See EPON.; 802.3ba – (2010) Covers both 40 and 100 Gb/s Ethernet, better known as 40GbE and 100GbE. Both are designed to run over 100m multi-mode optical fiber links. 100GbE can also handle 40+ km links on single-mode fiber, and is intended for trunk lines rather than for local service.; 802.3bs – Developing standard for 400 Gb/s Ethernet (400GbE).
802.4: IEEE standard for the physical layer of a token bus LAN.
802.5: IEEE standard for the physical layer of a token ring LAN.
80286: See Intel.
80287: See Intel.
80386: See Intel.
80387: See Intel.
80486: See Intel.
80487: See Intel.
8051: See Intel.
8080: See Intel.
8085: See Intel.
8086: See Intel.
8087: See Intel.
8088: See Intel.
812.16: IEEE standard for wireless broadband networks, projected for well above 100 Mb/s.
8250: See UART.
8-FSK: 8-state Frequency Shift Keying. See FSK.
8K: A standard, little used as of 2015, for digital video production with 8192 × 4320 pixel resolution, double the width × height of 4K. The term is also used for so-called 8K UHD (Ultra High Definition) displays. “8K” UHD, a broadcast and display standard (see graphics), is 7680 × 4320 pixels – an aspect ratio of 16:9 rather than 8K’s 19:10.
8P#C: 8 Position x Contact, where x is 2, 4, 6, or 8. See modular connector.
8-PSK: 8-state Phase Shift Keying. See PSK.
8VSB: 8-level Vestigial Sideband. See ATSC.
9913: A type of very-low-loss coaxial cable with 50Ω characteristic impedance.