F

f - 1. Symbol for frequency. 2. femto. From Danish or Norwegian femten, translating as fifteen and referring to one quadrillionth (10 ^-15 ). See also femtocell.

Fabry-Perot laser - A general purpose laser light source with a relatively narrow spectral width in the range of 3–6 nm. As a Fabry-Perot laser oscillates at several wavelengths, it emits a narrow range of less intense wavelengths around the center wavelength in which the power is concentrated. For example, a Fabry-Perot laser operating at a nominal wavelength of 1310 nm might also emit weaker signals at wavelengths ranging from 1307–1313 nm. This spectral width causes some amount of chromatic dispersion, which limits bandwidth in single-mode fiber (SMF) systems due to pulse spreading, which limits the bit rate. Fabry-Perot lasers Perot lasers are more precise than LEDs, are moderately fast at 1 Gbps or less, and are moderately priced.The Fabry-Perot laser is named for Charles Fabry and Alfred Pérot, who together invented the Fabry-Perot interferometer, or etalon, which formed the basis for this type of laser. See also DFB laser, etalon, laser, LED, and wavelength.

face time - Time spent with people, face-to-face. Face time is highly productive.Telephone conversations are less so, but still very productive. E-mail and instant messaging are not good substitutes for looking people in the eye or at least hearing their voices. People will rediscover that some day.

facilities bypass - Referring to the technologies that a service provider or user organization uses to bypass the local loop facilities of the incumbent local exchange carrier (ILEC) in order to gain access to an interexchange carrier (IXC) or Internet service provider (ISP). Facilities bypass generally involves wireless technologies. See also ILEC, ISP, and IXC.

facsimile (fax) - From the Latin facere simile, which translates to make similar.Technology that enables the transmission of images between paired transmitters and receivers.The transmitting fax scans the image document from top to bottom and from left to right, looking for dots of color — most machines support only black and white, some systems will also support 256 levels of grayscale, and some will support a large color palette — at various levels of resolution, as measured in lines per inch (lpi) and dots per inch (dpi). The fax machine translates the dots into data bits, which it compresses in order to reduce transmission time, and transmits through a network to the receiving machine. If the local loop is analog in nature, as is usually the case, an embedded modem transmits the digital data by modulating the analog waveforms. Just in case you were wondering about the origin and evolution of facsimile technology, Edward Davy invented the first practical facsimile machine in 1837, but abandoned the invention soon thereafter. Alexander Bain (1811–1877), a Scottish clockmaker and inventor, revived the concept and patented the recording telegraph in 1843.The first commercial facsimile service was established in 1865 by Giovanni Casselli over a circuit between Paris and Lyon, France. Circuits were added to other cities, and Casselli sent 5,000 faxes in the first year using his patented Pantelegraph machine, which was based on the Bain recording telegraph.A number of other inventors developed various wireline facsimile devices over the next 50 years or so, but all soon failed in the face of competition from the much more functional and practical electric telegraph (1844), invented by Samuel F.B. Morse (1791–1872) and Alfred Vail (1807–1859). Fax development began anew in the 1920s, but it was not until the 1970s that the ITU-T set international interoperability standards and the technology found some level of market acceptance.The ITU-T developed standards for fax machines in stages, designated as Group I, Group II, Group III, and Group IV.The Group III specification (1980) succeeded in making facsimile truly affordable for mass business markets. The Group IV specification (1984) addressed digital technology, which led to the development of highcapacity, networked fax servers and made fax broadcasting and fax-on-demand (FOD) possible. It is possible for a personal computer (PC) to emulate a fax machine through the use of a fax board and fax software, although this approach never achieved general market acceptance. See also compression, fax emulation,grayscale, Group I, Group II, Group III, Group IV, ITU-T, modem, modulation, and resolution.

Facsimile over Internet Protocol (FoIP) - See FoIP.

fair use policy - A policy of some Internet service providers (ISPs) that imposes bandwidth restrictions on users who exhibit patterns of system usage that exceed certain thresholds for extended periods of time. Direct broadcast satellite (DBS) providers offering two-way Internet access have fair use policies in place as bandwidth is so highly limited and a small number of highly active users can consume large amounts of bandwidth, leaving little for consumption by others. More traditional terrestrial ISPs in various countries (e.g.,Australia and South Africa) have similar policies with respect to international usage such as web surfing and file transfers. See also bandwidth, DBS, Internet, and ISP.

fallback modem - 1. A modem that has the ability to adjust its transmission rate downward in the event that the connection quality degrades. For example,V.90 modems also are V.34bis modems. Assuming that the terminating modem is V.34bis, the V.90 modem adjusts its maximum downstream rate of 56 kbps downward to the V.34bis maximum transmission rate of 33.6 kbps. See also downstream, dynamic rate adaption, modem, transmission rate, V.34bis, and V.90. 2. A modem that dials a connection only when a primary, usually leased, line has failed.

Family Radio Service (FRS) - See FRS.

fantail circuit - Also known as a multi-drop circuit and, more formally, as a point-to-multipoint circuit.A dedicated circuit that connects a single device (i.e., point) to multiple devices, with the circuit fanning out like a tail from the headend.The drops, or tail circuits, connect to the main circuit through a simple bridge. Fantail circuits generally are phrased in the context of a wide area network (WAN), and generally are provided as a carrier service. See also bridge, drop, headend, point-to-multipoint circuit, and WAN.

far-end cross talk (FEXT) - See FEXT.

farther - 1. More distant in space or time, particularly where there is a notion of physical distance. 2. Erroneously used to mean to a greater extent. See also further.

FAST (Framed ATM over SONET/SDH Transport) - A specification from the ATM Forum (July 2000) that defines the mechanisms and procedures required to support the transport of variable-length datagrams, known as ATM frames, over an ATM infrastructure using SONET/SDH facilities. FAST is similar to Data Exchange Interface (DXI) and Frame User Network Interface (FUNI), which are designed for access to an ATM network over relatively low-speed plesiochronous transmission facilities. FAST, however, is designed for access and/or inter-switch trunking over very high speed SONET/SDH transmission facilities. See also ATM, ATM Forum, datagram, DXI, frame, FUNI, plesiochronous, SDH, and SONET.

fast busy signal - A signal indicating to the calling party that network resources are not available to process the call. Synonymous with reorder tone. See also busy signal.

fast packet services - Referring to a group of packet services operating at broadband speeds and including asynchronous transfer mode (ATM), frame relay, and Switched Multimegabit Data Service (SMDS). See also ATM, broadband, frame relay, and SMDS.

fast retrain - A feature of asymmetric digital subscriber line (ADSL) modems that allows the transmission rate to resume normal levels after having been reduced by the power back-off feature in order to avoid interference between voice and data channels. See ADSL, modem, power back-off, and transmission rate.

fat access point - See fat AP.

fat AP (fat Access Point) - In wireless local area networks (WLANs), an AP with sufficient program logic and processing power to allow it to enforce policies relating to access and usage, rather than working under the supervision of a centralized controller. (A fat AP may use information from a RADIUS server, for example.) A network based on fat APs is more costly and complex, but offers the advantage of faster access as they can act independently rather than having to consult a centralized controller for authentication and other security purposes. In a mobile application, users moving between AP zones of coverage realize faster handoffs with fat APs. See also authentication, RADIUS, server, thin AP, and WLAN.

fat client - In contrast to a thin client, a fat client possesses considerable resources (e.g., memory, hard drive storage, and processing power) and functionality independent of a server. See also client, client/server, server, and thin client.

FATE (Frame-based ATM Transport over Ethernet) - A specification from the ATM Forum (February 2000 and July 2002) that allows ATM Adaptation Layer Type 5 (AAL5) services to be provided over Ethernet by transporting ATM data within an Ethernet frame. FATE has particular application in the context of an ATM-based ADSL environment interfacing to an Ethernet local area network (LAN) through a switch or hub on the customer premises. See also AAL5, ADSL, ATM, ATM Forum, Ethernet, hub, LAN, and switch.

fatware - Software that is so rich in feature content or so bloated with inefficient design or poorly written code that it consumes excessive resources, such as RAM, hard disk storage, and processing power. See also RAM and software.

fault management - An element of network management, fault management includes the detection of alarms and alerts, test and acceptance, and network recovery. Network elements (NEs) generate alarms and alerts are to indicate catastrophic failures or severe performance degradations.A network management system receives and correlates alarms and alerts from multiple NEs, and perhaps disables a failed port and enables another, or perhaps reroutes traffic around a failed switch or router after testing the alternate route. See also NE and network management.

fax (facsimile) - See facsimile.

fax emulation - Application software that enables a personal computer or fax server to behave like (i.e., function as) a fax machine.

Fax over Internet Protocol (FoIP) - See FoIP.

fax relay - Also known as demod/remod, fax relay is one of the implementation methods described by the ITU-T Recommendation T.38 specification for Fax over Internet Protocol (FoIP). Fax relay addresses the demodulation of standard analog fax transmissions from originating machines equipped with modems, and their remodulation for presentation to matching destination devices. Fax relay depends on a low latency IP network in order to avoid session time-out. See also facsimile, fax spoofing,FoIP, latency,modulation, and T.38.

fax spoofing - An implementation methods described by the ITU-T Recommendation T.38 specification for Fax over Internet Protocol (FoIP). Fax spoofing is used for facsimile transmissions over IP networks characterized by relatively long and unpredictable levels of packet latency that could cause a session between conventional fax machines to time out. Fax spoofing compensates for both increased latency and jitter by padding the line with occasional keep-alive packets to keep the session active, rather than allowing it to time out.Thereby,T.38 spoofs, or fools, the receiving device into thinking that the incoming transmission is over a real-time, synchronous voice network. See also facsimile, FoIP, jitter, latency, session, spoofing, and T.37.

fax-on-demand (FOD) - See FOD.

FC (Fibre Channel) - See Fibre Channel.

FC/IP (Fibre Channel over Internet Protocol) - A specification that extends Fibre Channel (FC) to operate through secure tunnels over long haul public Internet Protocol (IP) networks. See also Fibre Channel, IP, and tunnel.

FCC (Federal Communications Commission) - An independent United States government agency, directly responsible to Congress and charged with regulating interstate and international communications by radio, television, wire, satellite, and cable. The FCC’s jurisdiction covers the 50 states, the District of Columbia, and U.S. possessions.The FCC was established by the Communications Act of 1934. See also Communications Act of 1934.

FCS (Frame Check Sequence) - A 16- or 32-bit field containing the cyclic redundancy check (CRC) character sequence used to check the integrity of both the payload and control fields of a frame, such as a Synchronous Data Link Control (SDLC) frame. See also CRC, frame, and SDLC.

FDD (Frequency Division Duplex) - A means of providing duplex (bidirectional) communications in wireless networks, FDD makes use of separate frequencies for forward and backward channels. FDD is used with both analog and digital wireless technologies, including cordless telephony and cellular. See also analog, cellular, channel, cordless telephony, digital, duplex, frequency, and wireless.

FDDI (Fiber Distributed Data Interface) - The ANSI standard (X3T9-5) for a dual, counter-rotating, fiber optic, token-passing ring LAN.The specification pegs the signaling rate at 125 Mbps and the transmission rate (i.e., data rate) at 100 Mbps due to the 4B/5B line coding technique. FDDI is intended for backbone applications, interconnecting major computing resources such as high speed switches, routers, and servers.As the FDDI maximum frame size is 9000 symbols (1 symbol = 4 bytes), Ethernet and Token Ring frames can easily be encapsulated within FDDI frames for backbone transport. FDDI specifies devices separations of as much as 1.2 miles (2 kilometers) over multimode fiber (MMF) and 37.2 miles (62 kilometers) over single-mode fiber (SMF), with excellent error performance.The dual counter-rotating ring provides considerable redundancy, but requires that all directly connected devices be dual-attached, which adds to the cost and complexity. In consideration of the high cost and fragility of optical fiber, standards were developed to extend connectivity to workstations via unshielded twisted pairs.Those standards are known variously as CDDI (Copper Distributed Data Interface) and TPDDI (Twisted Pair Distributed Data Interface). FDDI is considered obsolete, having been overwhelmed by simpler, higher speed switched Ethernet technologies such as 1000Base-LX, 1000Base-SX, and 10GBase-LR, LW. See also 10GBase-LR, 1000Base-LX, 1000Base-SX, 4B/5B, ANSI, backbone, CDDI, Ethernet, fiber optics, frame, LAN, LW, MMF, signaling rate, SMF, symbol, token passing, Token Ring, and TPDDI.

FDM (Frequency Division Multiplexing) - A multiplexing method by which multiple low speed incoming transmissions can share a single high speed outgoing analog circuit. An analog voice conversation requires bandwidth of 4 kHz. A voice grade analog local loop, therefore, provides an analog channel of 0–4,000 Hz. Such a loop is a two-wire circuit, comprising two physical conductors in a pair configuration. It is possible to equip a four-wire circuit to support multiple 4 kHz channels. In order to do so, a FDM multiplexer, or mux, must be placed on each end of the circuit.The muxes subdivide the bandwidth of the circuit into 4 kHz channels, each of which can support a voice grade transmission. So, an FDM mux might multiplex 24 voice grade channels of 4 kHz onto a four-wire circuit with total bandwidth of 96 kHz. All 24 channels coexist on the same physical circuit, separated only by frequency. Within each channel, the voice conversation occupies the 300–3,300 Hz band and signaling and control functions take place over the 3,300–3,700 Hz band.The 0–300 Hz and 3,700–4,000 Hz bands are guard bands that provide channel separation in order to minimize the likelihood of mutual interference should the frequency channels overlap due to equipment malfunction or electromagnetic interference (EMI) from and external source.

FDMA (Frequency Division Multiple Access) - A multiplexing technique used in radio networks, FDMA derives multiple narrowband frequency channels from a wider band of assigned radio spectrum, much as frequency division multiplexing (FDM) operates in the electrical wireline domain. Using a technique known as frequency division duplex (FDD), a given call takes place on one pair of frequencies, with one for transmission in the forward direction and another for transmission in the reverse direction.At the same time, another call takes place on another pair of frequencies. The forward and reverse channels in each frequency pair are separated in frequency in order to avoid crosstalk and other forms of co-channel interference. Analog cellular systems, such as Advanced Mobile Phone System (AMPS), employ FDMA. Alternative multiplexing techniques employed in various cellular radio networks are code division multiple access (CDMA) and time division multiple access (TDMA). See also AMPS, channel, crosstalk, FDD, FDM, FDMA, frequency, multiplexer, narrowband, radio, spectrum, and TDMA.

FDX (Full DupleX) - A duplex transmission path, circuit, or channel designed to support information transfer in both directions, simultaneously.An FDX circuit can be a single physical circuit, such as a voice grade local loop. Alternatively, an FDX circuit can comprise two simplex circuits, one operating in each direction.Traditionally, T1 and E-1 circuits were provisioned over two simplex twisted pair circuits. A half-duplex (HDX) circuit differs in that it supports information transfer in both directions, but only one direction at a time. Most circuits are FDX in nature. Joseph B. Stearns of Boston, Massachusetts (United States) invented the first working FDX communications circuit, which was installed in 1872 on a onewire telegraph system using a ground return.This system effectively doubled the traffic capacity of the circuit, and at much lower cost than stringing another wire. See also HDX and simplex.

Feature Group (FG) - See FG.

FEC - 1. Forward Error Correction. An error control mode in which a detected error triggers an automatic error correction process in the receiver. FEC involves adherence to a set of specific rules of data construction and the addition of sufficient redundant data in order that the receiving device can identify, isolate, and correct a certain number (depending on the method) of errors without requiring retransmission. FEC often is used in networks where link quality is poor and bandwidth is limited, or where latency is high. FEC is used, for example, in cellular and other wireless networks in support of e-mail, short message service (SMS), and Internet access. Two commonly employed techniques are Hamming code and BCH (Bose, Chaudhuri, and Hocquengham). FEC also is used extensively in satellite communications. See also 1/3 FEC,BCH,error control,Hamming code, recognition and flagging, and recognition and retransmission. 2. Forwarding Equivalence Class In Multiprotocol Label Switching (MPLS), a class of packets, all of which are treated the same in terms of destination, priority level, and so on. See also MPLS.

FECN (Forward Explicit Congestion Notification) - Pronounced feckon. In the frame relay LAPF frame, a 1-bit field available to the network to advise devices in the forward direction, that is, in the direction of the data flow, that the frame has experienced congestion in transit. FECN thereby alerts the receiving frame relay access device (FRAD) that subsequent frames might be delayed in transit or even discarded if the congestion condition worsens. In the event the receiving FRAD detects a frame loss, it recovers by requesting a retransmission. Backward explicit congestion notification (BECN) performs a congestion control function in the reverse direction, that is, in the direction opposite the congested data flow. See also BECN, congestion, ECN, FRAD, frame, frame relay, and LAPF.

Federal Communications Commission (FCC) - See FCC.

Federal Trade Commission (FTC) - See FTC.

femto- (f) - See f and femtocell.

femtocell - An imprecise term referring to a radio cell smaller than a picocell and used to describe a very small radio cell associated with a cellular radio base station located in a home or small office. Femtocells are proposed for use as extensions of public cellular radio service into the customer premises to counteract perceived competitive threats from Generic Access Network (GAN),Wi-Fi,WiMAX, and other wireless network technologies. See also cellular radio, femto-, GAN, picocell, Wi-Fi, and WiMAX.

FEP (Front End Processor) - Synonymous with communications processor.An auxiliary processor, usually in the form of a dedicated computer, that assumes responsibilities for managing the interface between a host computer and networks, terminals, and peripherals.The FEP, thereby, relieves the host main processing unit of those responsibilities, allowing it to concentrate on running applications software.The FEP can assume responsibility for such tasks as authentication, access privileges, code translation, compression, encryption, and priority management.The FEP traditionally takes the form of a mid-range computer that manages access to a mainframe computer, although those distinctions are less significant in a contemporary data processing context. In more contemporary terms, FEP and host responsibilities both tend to be distributed across multiple servers in one or more clusters. See also authentication, compression, and encryption.

FEX (Foreign Exchange) - See FX.

FEXT (Far-End CROSS Talk) - The unwanted coupling of energy between two circuits or channels occurring at the far end of a link, i.e., far away from the point of signal origin. It is at the far end that the attenuated downstream signal from the network, for can experience crosstalk from the strong upstream signal emanating from equipment at the customer premises. FEXT is not a large issue for V.90 modems or asymmetric digital subscriber line (ADSL) services at the customer premises, as the cables are successively smaller, containing fewer and fewer twisted pairs, from the central office (CO) to the premises, so there are fewer opportunities for signals to experience co-interference. Near-end crosstalk (NEXT) occurs at the near end of the link. It is in consideration of the phenomenon of crosstalk and the differences between NEXT and FEXT, that V.90 modems and most DSL services are asymmetric, with the higher frequencies on the downstream side (i.e., from the edge of the telco network to the customer premises) in support of greater bandwidth in that direction. See also ADSL, asymmetric, bandwidth, channel, circuit,CO, crosstalk, downstream, frequency, interference, link, NEXT, signal, upstream, and V.90.

FF (Firefox) – See Firefox. FG (Feature Group) - A trunking arrangement between a local exchange carrier (LEC) and an interexchange carrier (IXC) that enables end users to make long distance telephone calls using the IXC network. Feature groups are designated A, B, C, and D.

• FGA: A line side access method in which the caller must dial a local telephone number for IXC access, then dial the long distance telephone number and a PIN or password.The IXC gains access to the customer through a subscriber line, rather than an interoffice trunk.

• FGB: The caller must dial a 950-XXXX telephone number and then dial the long distance telephone number and a PIN or password. In the 950-XXXX telephone number, the last four digits correspond to the Carrier Identification Code (CIC) associated with each IXC.The 950-XXXX number, which can be dialed as a local call from any geographic location, directs the call setup request to a centralized database that is used to direct the call to the nearest carrier retail outlet.An FGB connection is a trunk side connection commonly provided through an access tandem (AT) switch, although it also can be provided through a direct trunking arrangement to a central office (CO) switch.An FGB connection is superior to an FGA connection.

• FGC: A traditional trunk side access service used prior to the implementation of equal access in 1984.Where it is available, which is almost universally, FGD has replaced FGC.

• FGD: An FGD trunk is sometimes referred to as an equal access trunk, as all carriers with FGD trunks in a local exchange network are afforded equal access (1+ dialing) to the subscriber at designated central office exchanges (COs), whether via a direct trunking arrangement or through an access tandem (AT) switch. FGD provides the IXC with trunk side access, connecting the IXC as an integral part of the exchange carrier network. FGD includes presubscription to a subscriber-specific IXC and a 10XXX access code for use by end users in originating and terminating connections. FGD also includes call supervision and the IXC receives calling party identification as part of the call setup, automatic number identification (ANI) data for billing purposes.

See also 10XXX, access tandem,ANI, call supervision, carrier, CIC, CO,equal access,IXC, LEC, line side, password, PIN, presubscription, trunk, and trunk side.

FGE (FiberGlass Epoxy) - FGE is a combination of fiberglass and epoxy resins used in products such as cable strength members. See also fiberglass and strength member.

FHSS (Frequency-Hopping Spread Spectrum) - A signal modulation technique employed in radio communications, FHSS transmits short bursts of data over a range of frequency channels within the wideband carrier. Each transmission is assigned a 10-bit pseudorandom binary code sequence, which comprises a series of ones and zeros in a seemingly random pattern known to both the transmitter and receiver.The original code sequence is mathematically self-correlated to yield a code that stands out from all others, at least on average. The paired transmitters and receivers recognize their assigned and correlated code sequences, which look to all others as pseudorandom noise (PN). FHSS phase-modulates the carrier wave with a continuous string of PN code symbols, or chips, resulting in a chip rate that can be much faster than the bit rate.Thereby, the noise signal occurs with much greater frequency than the original data signal and spreads the signal energy over a much wider band.The transmitter and receiver hop from one frequency to another in a carefully choreographed hop sequence under the control of the centralized base station antenna. Each transmission dwells on a particular frequency for a very short period of time (no more than 400 milliseconds for FCC-controlled applications), which may be less than the time interval required to transmit a single data packet, or symbol, or even a single bit. So, the chip rate can be faster than the bit rate. A large number of other transmissions also may share the same range of frequencies simultaneously, with each using a different hop sequence.The potential remains, however, for the overlapping of packets.The receiving device can distinguish each packet in a packet stream by reading the various codes prepended to the packet data transmissions, and treating competing signals as noise. Code division multiple access (CDMA) and Bluetooth employ FHSS, which is much like the original SS technology patented by Hedy Lamarr in 1942. See also Bluetooth, CDMA, carrier, channel, chip, chip rate, frequency, hop sequence, modulation, PN, signal, symbol, and wideband. See also Lamarr, Hedy.

Fiber Channel - See Fibre Channel.

Fiber Connection (FICON) - See FICON.

Fiber Distributed Data Interface (FDDI) - See FDDI.

fiber optic transmission system (FOTS) - See FOTS.

fiber optics - Referring variously to optical fiber as a transmission medium or a fiber optic transmission system (FOTS). See also FOTS and optical fiber.

fiber-to-the-curb (FTTC) - See FTTC.

fiber-to-the-neighborhood (FTTN) - See FTTN.

fiber-to-the-node (FTTN) - See FTTN.

fiber-to-the-premises (FTTP) - See FTTP.

fiber - See optical fiber.

fiberglass epoxy (FGE) - See FGE.

fiberglass - Also known as spun glass, fiberglass is a composite of extremely fine fibers of glass. Invented in 1938 by Russell Games Slayter of Owens-Corning for use as insulating material, the raw spun glass is used as a reinforcing agent and combined with polymers and epoxies to create what is known popularly as fiberglass, which can be drawn, shaped, and molded for a wide variety of uses.The low weight, great tensile strength, and dielectric properties of fiberglass contribute to its wide use in both rigid and nonrigid applications, including boat hulls, swimming pools, hot tubs, surfboards, thermal insulation, automobile bodies, and cable strength members. Owens-Corning remains the largest manufacturer of fiberglass, which it markets as Fiberglas®. See also strength member.

Fibre Channel (FC) - An American National Standards Institute (ANSI) specification (X.3230, 1994) for a high-speed link between computers and peripherals, primarily high speed external storage devices. Developed to replace High Performance Parallel Interface (HIPPI) and as a high-speed alternative to the distance-limited Small Computer System Interface (SCSI), Fibre Channel is intended to support applications such as data backup and mirroring, and is the predominant data link technology employed in storage area networks (SANs). Fibre Channel is connected at Layer 1, the Physical Layer, by fibre, a term the Fibre Channel industry coined to refer to a network comprising a close-knit fabric of access including both optical fiber and copper twisted pair for large data transfers with low overhead, low-latency switching, and minimal interruptions to the flow of data.The preferred physical medium is optical fiber, which can be multimode fiber (MMF) of either 62.5µ (300 meters) or 50µ (500 meters, maximum distance), or single-mode fiber (SMF) (50+ km). Fibre Channel over IP (FC/IP) technology extends Fibre Channel to operate through secure tunnels over long-haul public IP networks.The line coding technique is 8B/10B, which encodes each 8-bit byte into a 10-bit symbol, which adds a 25% overhead factor. Fibre Channel operates in full duplex (FDX) at 1 Gbps (200 MBps), 2 Gbps (400 MBps), 4 Gbps (800 MBps), and 10 Gbps (2400 MBps or 2.4 GBps). Gateways are responsible for protocol conversion to support interconnection to telecom networks such as asynchronous transfer mode (ATM) and SONET, as well as ESCON, FICON and SCSI SANs and Ethernet LANs. See also 8B/10B, ANSI, ATM, bit, byte, ESCON, Ethernet, FDX, FICON, gateway, HIPPI, IP, LAN, latency, line coding, MMF,optical fiber,overhead, Physical Layer, protocol, SAN, SCSI, SMF, SONET, symbol, tunnel, and twisted pair.

Fibre Channel over IP (FC/IP) - See FC/IP and Fibre Channel.

FICON (FIbre CONnection) - A proprietary specification for storage area network (SAN) developed by IBM for a high-speed serial interface between mainframe computers and peripherals such as external disk drives. FICON supports data transfer rates up to 400 MBps in full duplex (FDX) over distances up to 100 km. FICON is replacing the earlier and slower Enterprise System Connection (ESCON). See also Bps, ESCON, FDX, Fibre Channel, mainframe, peripheral, SAN, and serial.

fidelity - The extent to which an electronic device or process faithfully reproduces audio or visual information. Hi-fi, for example, is high fidelity.

field - Synonymous with data field. 1. A location or area in which certain data is located within a block or frame of transmitted data. See also block and frame. 2. A location or area in which certain data is located on a storage medium, particularly in a database record.

FIFO (First-In-First-Out) - A buffering or temporary storage method in which the entity that first exits is the one that first entered.Thereby, the entity served (e.g., processed or switched) is the one that waited the longest period of time. FIFO is commonly used in message switches such as automatic call distributors (ACDs), PBXs, switches, and routers in the absence of a priority mechanism employed to establish quality-of-service (QoS) differentiation between different types of calls, packets, or other message entities. See also LIFO and queue.

File Transfer Protocol (FTP) - See FTP.

file - 1. Program file. An electronic file containing commands and instructions for execution by a computer. See also program file. 2. Data file. An electronic file containing the work created with a program. See also data file and data set.

filter - A device that allows some signals to pass through but absorbs, attenuates, blocks, rejects, or removes all other signals, depending on their frequency (electrical) or wavelength (optical). Active filters require electrical power to operate, while passive filters do not.A low-pass filter passes all frequencies below a certain value, but blocks all others.A high-pass filter passes all frequencies above a certain value, but blocks all others.A band-pass filter passes all frequencies in a designated band, but blocks all others. See also absorption, active, attenuation, electrical, frequency, optical, passive, signal, and wavelength.

filtering bridge - A bridge that examines the destination address of an incoming frame, consults an address table, and forwards the frame only over the link toward the target device. If the frame is intended for a station on the same LAN segment, the bridge simply ignores it, rather than passing it on. Since the frames are not forwarded across other links, a filtering bridge does a great deal to relieve overall congestion on a segment-by-segment basis. Most bridges are self-learning, filtering bridges and are standardized in IEEE 802.1D, which describes the spanning tree protocol (STP). See also bridge, self-learning bridge, and STP.

filtering - Also known as image decimation. In video compression, a step that reduces the total frequency of the analog signal through a process of averaging the values of neighboring pixels or lines. For example, adjoining black and white pixels become gray pixels.Taps are the number of lines or pixels considered in this process. MPEG, for example, uses a seven-tap filter. See also analog, compression, frequency, MPEG, pixel, signal, and video.

find-me - A voice system (e.g., Centrex or PBX) feature that enables the user to preprogram telephone numbers (e.g., home office phone, cell phone, and home phone) that the system will attempt in sequence in order to complete an incoming call. If the system is unable to find the target user, it will so advise the caller and offer the opportunity to leave a message. Find-me service is defined in the advanced intelligent network (AIN) specifications as a service of the public switched telephone network (PSTN). See also AIN and PSTN.

fine print - The really small print at the bottom of an advertisement that details the conditions under which a product or service offering is made, conditions on its use, limitations on associated liability, and other matters imposed or provided for by law or regulation. Fine print is not only really tiny, but also in legal or otherwise obscure language in hopes that you will not read it or not understand it and, therefore, will make your purchasing decision in ignorance.

finger - In a rake receiver antenna system, an individual receiver that works with other receivers in a coordinated way to gather signal elements much like the tines of a garden rake work together to gather leaves. Each finger gathers a faded, or attenuated, signal element at a separate moment in time.The rake receiver employs spatial diversity and time diversity, combining and correlating the results of all four fingers to optimize the signal, thereby countering the effects of multipath fading and delay spread. Code-division multiple access (CDMA) systems employ rake receivers comprising four fingers to deal with issues of multipath interference (MPI). See also antenna, attenuation, CDMA, delay spread, MPI, multipath fading, spatial diversity, and time diversity.

Firefox (Fx, fx, or FF) - A graphical Web browser developed by the not-for-profit Mozilla Corporation and made available free of charge for Windows, Mac, and Linux computers. Firefox began as a fork of the Navigator component of the Mozilla application suite. See also Mozilla.

firewall - Security software that can actively block unauthorized entities from gaining access to internal resources such as systems, servers, databases, and networks.A firewall may also act to prevent internal users from accessing unauthorized external resources. A firewall is installed in a communications router, server, or some other device that physically and/or logically is a first point of access into a networked system. A packet-filtering firewall examines all data packets, forwarding or dropping individual packets based on predefined rules that specify where a packet is permitted to go, in consideration of both the authenticated identification of the user and the originating address of the request.A proxy firewall acts as an intermediary for user access requests by setting up a second connection to the resource.The proxy then decides if the message or file is safe.A stateful inspection firewall examines packets, notes the port numbers that they use for each connection, and shuts down those ports once the connection is terminated. See also authentication, authorization, proxy firewall, and security.

FireWire -Apple Computer terminology for IEEE 1394. See 1394.

firmware - Software programs that are stored in a computer’s read-only memory (ROM), where they are available for instantaneous use. Firmware is hard-coded and stored on a silicon chip and, therefore, is not affected by loss of electrical power, hence the term firm. See also grayware, hardware, and software.

First Computer Inquiry - In the United States, a Federal Communications Commission (FCC) inquiry (1971) that drew a firm line between data processing services and data communications services.The FCC determined that it would continue to regulate data communications services in order to avoid the possibility of AT&T’s subsidizing profit-making competitive activities with revenues from regulated telephone company activities. In the 1956 Consent Decree,AT&T had agreed to refrain from offering data processing services even through a separate subsidiary. See also Consent Decree, FCC, and Second Computer Inquiry.

first mile - More commonly referred to as the last mile and generally referring to the telco local loop, which is the link between the central office (CO) at the edge of the telco network and the user premises. In a broader contemporary context, the term applies to the physical connection between the edge of any service provider’s network and the end user’s premises. In practice, the first mile is often much longer than a mile. In the United States, UTP local loops are generally 12,000 feet or less, but often are as long as 18,000 feet. Passive optical network (PON) standards allow for local loops as long as 12 miles (20 km). Whether the first mile or the last mile, which is a matter of perspective, it is seldom exactly a mile. See also central office, local loop, and PON.

first-in-first-out (FIFO) - See FIFO.

Fixed Mobile Convergence - A term coined by the 3rd Generation Partnership Project (3GPP) for the seamless melding of fixed IP–based fixed wireless (e.g.,Wi-Fi and WiMAX) and cellular radio networks. Fixed Mobile Convergence is the ultimate goal of the 3GPP GAN (Generic Access Network). See also 3GPP, cellular radio, fixed wireless, GAN, IP, Wi-Fi, and WiMAX.

fixed satellite system (FSS) - See FSS.

fixed wireless - Referring to a group of wireless local loop (WLL) transmission systems that involve antennas permanently or semi-permanently located at the edge of a public network and at the customers’ premises, rather than being mobile. A number of 2.5G and 3G cellular standards also include fixed wireless options. See also antenna, local loop, and WLL.

flag - 1. A marker or indicator of a condition, such as an error condition, in a program or file. See also file and program. 2. In Synchronous Data Link Control (SDLC), High-level Data Link Control (HDLC), and other frame-based communications protocols, a specific eight-bit pattern that alerts the receiving device to the beginning or end of a frame, i.e., message unit.The most commonly used flag character is 01111110 in binary code (7E in hexadecimal). Flags also fill all idle time on the line between frames. Only one flag is needed between frames. See also frame, HDLC, hexadecimal notation, protocol, and SDLC.

flash memory - A type of non-volatile read-only memory (ROM) that can store data or programs, be erased, and be used again. Flash memory must be erased in blocks, rather than a byte at a time, which limits its use to applications such as a supplement to or replacement for a mechanical hard disk drive. Flash memory is unsuitable for use as main memory, or random access memory (RAM). See also RAM and ROM.

flex strength - The ability to withstand the stress of twisting and bending, flex strength is important in wire and cable applications that involve frequent bending and twisting. Illustrative applications include elevator telephone cables, telephone handset and headset cords, and microphone cords. Wires and cables designed for high flex strength commonly involve small diameter conductors and cables, stranded rather than solid core conductors, stranded or braided rather than solid metal shields, conductors and shields made of metal alloys that are flexible, and insulation made of materials that are not only flexible, but also abrasion resistant. See also break strength and tensile strength.

FLEX - A set of proprietary (Motorola) protocols for radio paging systems, FLEX largely has replaced POCSAG in the United States, and has become the de facto standard throughout most of the world, excepting Western Europe, where the ERMES standard is favored. FLEX solutions support duplex messaging and data transmission. FLEX supports as many as 5 billion addresses, with up to 600,000 supported per channel.The FLEX family of protocols includes the following:

• FLEX: 1600 bps; 25 kHz channels; simplex downstream

• ReFLEX: 1600, 3200, 6400, or 9600 bps; 25 or 50 kHz channels downstream and 12.5 kHz channel upstream; duplex

• InFLEXion: up to 112 kbps, 50 kHz channels in the narrowband PCS (N-PCS) range; duplex; supports compressed voice downstream

See also channel, compression, downstream, duplex, ERMES, narrowband, N-PCS, paging system, PCS, POCSAG, proprietary, protocol, simplex, standard, and upstream.

flow control - The process of controlling the rate of data transfer in order to prevent data loss due to congestion, flow control is an element of many data communications protocols. Between a PC and a local printer, for example, there is a simple flow control mechanism that throttles back the data transfer rate from the PC to ensure that the printer is not overwhelmed and that the printer buffer memory is not exceeded. ATM, frame relay IPv6, and TCP include more complex flow control mechanisms to ensure that switches and routers are not overwhelmed. A rate-based mechanism is an end-to-end flow-control scheme that considers resources edge to edge, communicating the level of available resources through a feedback loop. This approach requires that the transmitting end-user device adjust its rate of transmission downward in consideration of congestion. A credit-based approach either allows or disallows the end-user device to transmit data, based on end-to-end consideration of whether sufficient buffer space is available on each link of the network. See also ATM, buffer, frame relay, IPv6, and TCP.

flow - 1. Movement in a manner suggestive of a liquid. Movement in a smooth and gentle manner, like water in a stream. See also stream-oriented. 2. In telecommunications, a sequence of bits, bytes, datagrams, or packets between common endpoints identified by features such as network addresses and port numbers. See also bit, byte, datagram, endpoint, packet, and port.

flux - 1.The rate of flow of something such as energy, particles, or fluid volumes across or through a surface. Radiant flux is the time rate of energy flow of radiant energy as measures in watts or joules per second. Luminous flux is radiant flux evaluated with respect to its luminous (brightness) efficiency. See also joule, luminance, and watt. 2. The strength of an energy field acting on a specific area.

FM (Frequency Modulation) - Also known as frequency-shift keying (FSK). A signal modulation technique in which the frequency of the analog carrier sine wave is varied to distinguish between a 1 bit and a 0 bit. For example, FM is the sole technique used in low-speed, Hayes-compatible modems.When no bits are transmitted, the carrier is at a reference frequency of 1700 Hz. A unibit FM technique impresses one bit on each baud by shifting the carrier to 2200 Hz when transmitting a 1 bit, and to 1200 Hz when transmitting a 0 bit.At 2400 baud, therefore, the transmission rate is 2400 bps.The benefits of dibit transmission can be realized by defining four frequencies, with each sine wave or set of sine waves representing a 2-bit pattern (00, 01, 10, and 11).Thereby, at 2400 baud, the transmission rate is 4800 bps. FM often is used in conjunction with amplitude modulation (AM) and phase-shift keying (PSK). A variation on the FSK theme is Gaussian frequency-shift keying (GFSK), which is used in DECT and Bluetooth wireless networks. See also AM, analog, Bluetooth, carrier, DECT, GFSK, modulation, PSK, sine wave, and transmission rate.

FMNP (Full Mobile Number Portability) - Synonymous with wireless number portability (WNP). See WNP.

FOD (Fax-On-Demand) - A system that enables a customer to request that a document be faxed back to him or her. Traditional FOD involves a voice processing system that answers a telephone call and prompts the caller to select a document from a menu of options, enter a return fax number, and perhaps enter a credit card number for billing purposes. FOD also has been integrated with Web sites for access over the Internet, allowing customer to select documents from a menu, and enter a return fax number and billing information. FOD largely has been obsoleted by the Internet and World Wide Web (WWW), as documents easily can be attached to e-mail or printed directly from a Web page.

foil twisted pair (FTP) - Synonymous with shielded twisted pair (STP). See STP.

FoIP (Fax over IP) - A technique for facsimile (fax) transmission over the Internet or other IP-based packet network, rather than over the traditional public switched telephone network (PSTN). FoIP typically involves a fax gateway, which not only serves as a physical gate between the circuit-switched and the packet-switched networks, but also runs gateway protocols that convert from the PSTN to the IP-based packet network. Relevant IP fax standards include T.37, the specification for store-and-forward fax, and T.38, the specification for fax relay and fax spoofing. See also circuit switch, facsimile, fax relay, fax spoofing, Internet, IP, packet switch, PSTN, and store-and-forward.

follow-me - A voice service of the public switched telephone network (PSTN) that provides for call forwarding on a predetermined schedule.A telecommuter, for example, might have the network forward calls to the home office three days a week during normal business hours. Calls would be directed to the traditional office two days a week. Calls clearly outside of normal business hours automatically would be directed to a voice mail system. Follow-me service is defined in the advanced intelligent network (AIN) specifications. See also AIN, call forwarding, PSTN, telecommuting, and voice mail.

FOMA (Freedom of Mobile Multimedia Access) - The Japanese term for Universal Mobile Telecommunications System (UMTS), a 3G cellular radio standard. See UMTS.

footprint - 1. The physical space something occupies, as in the footprint of a computer or a PBX. 2.The coverage area of a satellite or other radio transmitter system.

forced account code - An account code that a KTS or PBX system requires an end user to enter prior to placing an outgoing call. See account code.

foreign exchange (FEX or FX) - See FX.

forked ringing - An IPBX feature that allows a call processor to ring multiple phones at once, rather than in turn, and serve the call to the first phone that answers. Forked ringing is enabled by the Session Initiation Protocol (SIP) signaling and control protocol. See also IPBX and SIP.

forklift upgrade - Referring to a complete system replacement. The term originated in days of yore, when mainframe computers and PBXs were so big and heavy that it literally took a forklift to remove an obsolete system from the computer room or switch room and replace it with another, usually of at least equal size and weight. See also heavy metal, mainframe computer, and PBX.

format - 1. The structure, organization, presentation, or appearance of a set of data in a document.The format of a set of data can include the coding scheme (e.g.,ASCII, EBCDIC, or Unicode) and any compression technique (e.g., GIF, JPEG, MPEG-2, or MPEG-4) that might have been employed. In an e-mail application, textual data can be formatted as plain text or rich text, with the latter supporting bold, italics, and underline.The format of the data must be compatible with the application software that attempts to read it. See also ASCII, coding scheme, compression, EBCDIC, GIF, JPEG, MPEG-2, MPEG-4, plain text, rich text, and Unicode. 2. The arrangement of fields of data in a block, frame, or cell. See also data format.

forward error correction (FEC) - See FEC.

forward explicit congestion notification (FECN) - See FECN.

forwarding equivalence class (FEC) - See FEC.

FOTS (Fiber Optic Transmission System) - An optical transmission system comprising, at the most basic level, a light source, an optical fiber, and a light detector.The light source can be in the form of a light-emitting diode (LED), vertical cavity surface-emitting laser (VCSEL), or laser diode.The optical fiber generally is one of many types of glass optical fiber (GOF), although plastic optical fiber (POF) is sometimes used.The light detector can be a positive-intrinsic-negative (PIN) diode or avalanche photodiode (APD). As is true of any transmission system, attenuation can be an issue over a long haul, so some form of amplification can be introduced. As FOTS systems generally are digital in nature, the typical approach is to apply a regenerative repeater, which detects the weak incoming signal, which it amplifies electrically, reshapes, retimes, and retransmits as an improved outgoing signal. Raman amplifiers increasingly are used, often in conjunction with repeaters. Long haul systems also typically conform to a set of ITU-T standards, with Synchronous Optical Network (SONET) being the North American version and Synchronous Digital Hierarchy (SDH) being preferred elsewhere.The optical signal is in the infrared (IR) range, within one of a number of windows, or wavelength bands, specified by the ITU-T. A single wavelength may be involved, or multiple wavelengths may coexist in a single fiber through a process known as wavelength division multiplexing (WDM), of which there are several levels, coarse (CWDM) and dense (DWDM).As a conducted system, FOTS systems are unparalleled in terms of bandwidth, error performance, signal attenuation, and security. See also APD, bandwidth, diode, diode laser, DWDM, GOF, IR, laser, LED, PIN, POF, repeater, SDH, SONET, VCSEL, wavelength, WDM, and window.

four-wave mixing (FWM) - See FWM.

four-wire circuit - A circuit that supports transmission in both directions over separate physical links or paths in support of full duplex (FDX), i.e., simultaneous two-way, transmission.The distinguishing characteristic of a four-wire circuit, as opposed to a two-wire circuit, is its ability to support multichannel communications and out-of-band signaling and control. A physical four-wire circuit, the traditional means of provisioning, is a circuit comprising four physical twisted-pair copper wires in a two-pair configuration, with one pair supporting transmission in the forward direction and the other pair supporting transmission in the reverse direction.A logical four-wire circuit comprises two-wires in a single-pair configuration.A four-wire circuit also can be provisioned as a radio circuit, which is wireless, of course. Four-wire circuits are used in bandwidth-intensive local loops, particularly multichannel loops, and backbone circuits. Specific examples include DDS, ISDN,T/E-carrier, and SDH/SONET. See Figure F-4. See also channel, circuit, DDS, E-carrier, FDX, ISDN,out-of-band signaling and control, SDH, SONET, T-carrier, twisted pair, and two-wire circuit.

fourth estate - A term first used by historian Thomas Carlyle (1795–1881) in his book, On Heroes, Hero Worship,and the Heroic in History (1841), to describe the press. Novelist Jeffrey Archer, in his book The Fourth Estate (1996), observed: “In May 1789, Louis XVI summoned to Versailles a full meeting of the ‘Estate General’. The First Estate consisted of three hundred nobles. The Second Estate, three hundred clergy. The Third Estate, six hundred commoners. Some years later, after the French Revolution, Edmund Burke, looking up at the Press Gallery of the House of Commons, said,‘Yonder sits the Fourth Estate, and they are more important than them all.’” Note: The Estate General refers to the British Parliament. The First Estate refers to the Lords Temporal and the Second Estate to the Lords Spiritual, the two of which later combined to form the House of Lords.The Third Estate refers to the House of Commons.The Fourth Estate refers to the public press, which in those days was the newspapers.The contemporary reference is to the mass media, including both print and electronic media.

FPLMTS (Future Public Land Mobile Telecommunications System) - An initiative of the ITUR that defined a vision for a single global standard for digital wireless networks. FPLMTS was subsequently replaced by International Mobile Telecommunications-2000 (IMT-2000). See also IMT-2000.

fps (frames per second) - The number of frames transmitted per second. See frame, frame rate, and second.

fractal transform - A technique for video compression. Fractal compression reduces an image into extremely small independent blocks which it translates into mathematical equations that are used to form a codebook that creates a mathematical model of the image. So, the image, itself, becomes the basis for the ad hoc codebook. Vector quantization is similar, but uses a standard codebook. Fractal compression is extremely efficient, but highly processor-intensive. Fractal compression can be either lossless or lossy in nature. See also compression, fractal, lossless compression, lossy compression, transforms, vector quantization, and video.

fractal - From the Latin fractus, translating as broken or fractured. An irregular or fragmented geometric shape that can be repeatedly subdivided into parts, each of which is a smaller copy of the whole. In words, a complex irregular object that is self-similar. Examples of fractal objects include mountain ranges, clouds, and lightening bolts. See also fractal transform.

fractional T1 (FT1) - See FT1.

FRAD (Frame Relay Access Device or Frame Relay Assembler/Disassembler) - A device that assembles and disassembles frame relay frames. A FRAD is data communications equipment (DCE) that can be in the form of a standalone device, although it is generally embedded under the skin of a router or other device.The FRAD connects from the customer premises across a digital local loop, such as an E-1 or T1, to the frame relay network device (FRND) at the edge of the carrier network.The FRAD can be located at the network edge in support of dial-up users. See also carrier,DCE,E-1, frame, frame relay, FRND, local loop, router, and T1.

fragment-free switch - A type of LAN matrix switch that quickly stores the first 64 octets of the frame before it reads the address and quickly flows it through the switching matrix, bit by bit. As most errors occur at the beginning of a frame, this approach eliminates the possibility that runt frames, i.e., truncated frames, will be transmitted. See also cut-through switch, LAN, LAN switch, matrix switch, and switch.

fragmentation - Referring to the process by which a switch or router breaks up or divides a large datagram. If the receiving network cannot accommodate a datagram of a given total length, it must be fragmented.There must be some form of fragmentation control to ensure that the fragments can be re-associated when they exit the network and that the datagram can be reconstituted. In Internet Protocol version 4 (IPv4), for example, fragmentation control requires that each fragment contain a copy of an identification field and certain other fields in the header.The IPv4 header contains a fragment offset field that identifies where a fragment fits in the complete set of fragments that comprise the original datagram.This field is used to sequence the fragments correctly, as they may arrive at the destination device out of sequence. See also datagram, IPv4, IPv6, network, router, and switch.

frame check sequence (FCS) - See FCS.

frame rate - In video communications, the rate at which frames of still images are transmitted.Video is a series of still images transmitted in succession to create the perception of fluidity of motion. Motion picture quality is considered to be a frame rate of 24 frames per second (fps) and broadcast television quality is considered to be 30 fps. If the frames are transmitted at a slow rate, the result is a poor quality, herkyjerky video that creates a strobe-light effect. Particularly below 15 fps, quality suffers noticeably, as the fluidity of motion is lost even though the image quality may be high. See also scanning.

frame relay access device (FRAD) - See FRAD.

frame relay assembler/disassembler (FRAD) - See FRAD.

Frame Relay Forum (FRF) - A not-for-profit special interest group of manufacturers, vendors, carriers, and others with interests in the development and promotion of frame relay technology. The Frame Relay Forum developed a number of Implementation Agreements (IAs) that address interoperability issues. The Frame Relay Forum merged with the ATM Forum and MPLS Forum to form the MFA Forum. See also frame relay, Implementation Agreement, and MFA Forum.

frame relay network device (FRND) - See FRND.

frame relay - The ITU-T I.122 Recommendation (1988), Framework for Providing Additional Packet Mode Bearer Services established the basic framework for a packet communications mode over an integrated services digital network (ISDN). The access protocol specified in ITU-T Q.922 (1992) is Link Access Procedure for Frame Mode Services (LAPF), which is an adaptation of the Link Access Procedure Data channel (LAPD) signaling protocol developed for ISDN. Frame relay was originally intended as an ISDN framing convention for a bearer service, i.e., information-bearing service, anticipated for the ISDN D channel.The D channel is intended primarily for signaling and control purposes, in support of Signaling System 7 (SS7).The D channel runs at 16 kbps for ISDN basic rate interface (BRI) and at 64 kbps for primary rate interface (PRI). Certainly during the early stages of frame relay development, 16 kbps was not a particularly limiting signaling rate, particularly in the context of X.25 packet switching, which often was limited to 9.6 kbps. Over time, however, it became clear that ISDN was far too slow for data communications, even at B channel rates of 64 kbps, and certainly at D channel rates of 16 kbps. So, frame relay became a distinct service, independent of ISDN. In the context of the OSI Reference Model, frame relay standards address the Physical Layer and Data Link Layer, and do not specify internal network operations. Frame relay is analogous to a streamlined and supercharged version of X.25.Although both are designed to support bursty data traffic, frame relay is intended specifically for LAN-to-LAN traffic, but also is used in support of SDLC and many other legacy protocols.Access to a frame relay network is generally over a dedicated digital circuit in the form of a DDS, a Fractional T1, an E-1 (2.048 Mbps), or a T1 (1.544 Mbps). Access via E-3 (34 Mbps) or T3 (45 Mbps) circuits is also generally available. Frame relay statistically multiplexes frames of data over virtual circuits (VCs), with specifications providing for both permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). (Note: SVCs are virtually non-existent because of their additional complexity and cost and the fear of carriers that such a service would cannibalize the PSTN.) The user interface is in a frame relay access device (FRAD) that can be implemented on the customer premises and is analogous to an X.25 packet assembler/disassembler (PAD). Like X.25, frame relay is intended for bursty data traffic, although it works well with fixed bit rate applications, for which it offers assured bandwidth.Although both X.25 and frame relay can support voice, video, and audio, the inherently unpredictable levels of latency and loss over such a highly shared network translate into quality of service (QoS) issues. As frame relay specifies a completely digital network, error performance is excellent.Therefore, frame relay does not attempt to correct any errors created in transit, but simply discards errored frames. It is the responsibility of the receiving user equipment to discover and recover from such an action.As frame relay guarantees frame delivery in the order sent, there is no frame sequence numbering, and there are no acknowledgements of any sort provided.As a result, the load on the computational and bandwidth resources of the network is reduced, frame processing and forwarding are speeded up considerably, and latency is reduced significantly.There are, however, a number of congestion control mechanisms that variously work to provide some assurances of acceptable performance.Whereas frame relay specifies a variable size payload up to 4,096 octets, the Frame Relay Forum (now MFA Forum) developed an Implementation Agreement (IA) that sets the maximum size at 1,600 octets for purposes of interconnectivity and interoperability.This frame size easily supports the largest standard 802.3 Ethernet frame of 1,518 octets. See also B channel, BRI, Data Link Layer, D channel, DDS, E-1, E-3, Fractional T1, FRAD, frame, IA, ISDN, ITU-T, latency, LAPD, LAPF, MFA Forum, OSI Reference Model, packet switch, PAD, Physical Layer, PRI, PVC, QoS, SDLC, signaling and control, signaling rate, SS7, statistical time division mulltiplex, SVC, T1, T3, and X.25.

Frame UNI (FUNI) - See FUNI.

Frame-based ATM Transport over Ethernet (FATE) - See FATE.

frame - 1. A structure in the form of a structure of metal uprights and cross pieces with termination points on each side into which components can be mounted and conductors can be mechanically connected.The term is applied to distribution frames for cables and is the origin of the term mainframe computer. 2. In asynchronous serial data communications, a transmission unit comprising a character of data and one or two parity bits, preceded by a start bit, succeeded by a stop bit.Asynchronous communications are said to be character-framed. See also asynchronous transmission. 3. In synchronous data communications protocols such as HDLC and SDLC, a message unit. A frame comprises control data, address data, user data, and an error control mechanism.The frame is preceded by a beginning flag and succeeded by an ending flag.The data field of an SDLC frame, for example, can comprise as many as 4,096 octets and the various control fields add another four or six octets. See also SDLC and synchronous transmission. 4. In digital carrier systems E-carrier, J-carrier, and T-carrier, a collection of time slots that repeats every 125 microseconds. In a channelized application, each time slot constitutes a channel. In an unchannelized application, the entire collection of time slots constitutes a channel. Note: A T1 and J1 frame is always preceded by a framing bit, which is used for synchronization and other control purposes. An E-1 frame does not require a framing bit, as time slots 0 and 16 serve those functions. See also carrier, channel, E-carrier, framing bit, J-carrier, synchronous, T-carrier, and time slot. 5. In video communications, a single photographic image that is one of many in a motion picture. See also frame rate.

Framed ATM over SONET/SDH Transport (FAST) - See FAST.

frames per second (fps) - See fps.

framing bit - A bit that precedes a T1 or J1 frame and is used for synchronization and other control purposes. See also bit, frame, J1, and T1.

free space optics (FSO) - See FSO.

free space transmission - The transmission of radio or optical signals in free space, i.e., space devoid of physical obstructions that might hinder signal propagation. In this context, the term physical obstruction suggests trees, buildings, hills, mountains, and other significant material objects.The term does not suggest atomic, molecular, or particulate matter that commonly is present in the atmosphere. Neither does it suggest water vapor, rain, snow, sleet, or hail. Free space transmission does not include radio or optical transmission through waveguides. See also free space, FSO, transmission medium, and waveguide.

free space - 1. A theoretical region of space utterly devoid of physical matter, gravitational fields, and electromagnetic fields. 2. A region devoid of physical obstructions that might hinder the propagation of electromagnetic signals. See also free space transmission.

Freedom of Mobile Multimedia Access (FOMA) - The Japanese term for Universal Mobile Telecommunications System (UMTS), a 3G cellular radio standard. See UMTS.

Freephone - The term used in some countries to refer to toll free service. See also toll free service.

freeware - Software given away free of charge, typically over the Internet or through a user group. Individual software developers often make beta versions of copyrighted software available as freeware for user trials, but charge for subsequent versions refined as a result of end user input. Some developers never charge for its use and even forbid others to do so. Some freeware is available in commercial versions, with charges for support, but not for licensing. Freeware is generally not public domain software, as there usually are restrictions on its use and modification. See also beta, copyright, open source, public domain, and software.

frequency (f) - The Institute of Electrical and Electronics Engineers (IEEE) defines frequency as the number of complete cycles of sinusoidal variation per unit time, with the unit of time generally being that of one second. Plotting y = sin x, where x is expressed in radians, yields a sine wave. (From the Latin radius, a radian is a unit of plane angular measurement equivalent to the angle between two radii that enclose a section of a circle’s circumference [arc] equal in length to the length of a radius.There are 2 π radians in a circle.) A complete sine wave entails a cycle as measured from a point of zero (0) amplitude to a point of maximum positive amplitude (+A), through zero to a point of maximum negative amplitude (–A), and back to a point of zero (0) amplitude.The frequency of an alternating current, for example, is the half number of times the polarity of a continuous waveform reverses direction, or polarity, per second.The frequency of the waveform is the number of times per second it makes a complete cycle from zero voltage (0V) to its maximum positive voltage (+V) or amplitude, back to zero (0 V), to its maximum negative voltage (–V) or amplitude, and back to zero (0V). Frequency traditionally is measured in cycles per second (cps) or, in more contemporary terms, Hertz (Hz). See also amplitude, electromagnetic spectrum, Hz, and sine wave.

frequency band - A continuous group, or range, of frequencies with an upper limit and a lower limit. In analog terms, bandwidth and channel width are defined as a range of frequencies. See also analog, bandwidth, channel, and frequency.

frequency diversity - The use of multiple paired transmit and receive antennas operating at different frequencies.As the likelihood is that the signals will not suffer the same level of attenuation at different frequencies, the receiver with the strongest signal assumes control of the transmission. Frequency diversity is sometimes employed in microwave systems. See also attenuation, frequency, and microwave.

frequency division duplex (FDD) - See FDD.

frequency division multiple access (FDMA) - See FDMA.

frequency division multiplexer (FDM) - A device that performs Frequency Division Multiplexing (FDM). See FDM.

frequency division multiplexing (FDM) - See FDM.

frequency modulation (FM) - See FM.

frequency spectrum - See electromagnetic spectrum.

frequency-hopping spread spectrum (FHSS) - See FHSS.

frequency-shift keying (FSK) - Also known as Frequency Modulation (FM). See FM.

Fresnel reflection - The reflection of a portion of the light incident on a planar surface (i.e., the interface) between two media having different refractive indexes.The reflected portion of the light remains in the same plane as the incident light and reflects at the same angle to the normal as the incident ray.The polarization is known as parallel. The extent to which the incident is reflected, rather than refracted, depends on the difference in the index of refraction (IOR) of the individual media (e.g., air and glass), the angle of incidence, the light polarization, and the direction in which the light is passing (e.g., from air to glass or from glass to air). See also angle of incidence; Fresnel,Augustin-Jean; Fresnel refraction; IOR; polarization; reflection; and refraction.

Fresnel refraction - The bending of light as it crosses the planar surface between two media having different refractive indexes.The refracted portion of the light crosses the interface and enters the plane of the incident medium, changes velocity, and bends while the reflected portion of the light remains in the same plane as the incident light, remains at the same velocity, and reflects at the same angle to the normal as the incident ray.The polarization is known as perpendicular.The extent to which the incident is refracted, rather than reflected, the difference in the index of refraction (IOR) of the individual media (e.g., air and glass), the angle of incidence, the light polarization, and the direction in which the light is passing (e.g., from air to glass or from glass to air). See also angle of incidence; Fresnel,Augustin-Jean; Fresnel reflection; IOR; polarization; reflection; and refraction.

Fresnel, Augustin-Jean (1788–1827) - A French physicist who made significant contributions to the field of wave optics, particularly with respect to explaining the phenomenon of diffraction. See also diffraction, Fresnel reflection, Fresnel refraction, optics, and wave.

Friis, Harald T. (1893–1976) - An electrical engineer with a focus on radio systems, Friis was director of a research team at Bell Telephone Laboratories that developed the first microwave transmission systems. The first primitive systems, used in military applications in the European and Pacific theaters of World War II, could handle up to 2,400 voice conversations over five channels.The first public demonstration was conducted between the West Street lab and Neshanic, New Jersey in October 1945, and construction began on the first experimental microwave telephone network in 1947. Friis also was instrumental in the development of several radar systems and various millimeter-wave systems, and made fundamental contributions to the understanding of the role of noise in radio receivers. See also Bell Labs, microwave, noise, and radar.

FRND (Frame Relay Network Device) - Pronounced friend. A device at the edge of the frame relay carrier network that provides the user network interface (UNI).The FRND connects across a digital local loop, such as an E-1 or T1, to the frame relay access device (FRAD) on the customer premises. See also carrier, E-1, FRAD, frame relay, local loop, T1, and UNI.

front end processor (FEP) - See FEP.

FRS (Family Radio Service) - In the United States, the Federal Communications Commission (FCC) regulates FRS as one of the family of CB (Citizens Band) Radio Services. FRS uses mobile handsets that operate in the 462.5625–467.7125 MHz band at a maximum power level of 0.5 watts (W) and have a range of less than 1 mile. FRS is intended for personal and business use by family, friends, and associates. Dual mode sets are available that also operate as General Mobile Radio Service (GMRS) radios. See also CB Radio Service, FCC, GMRS, personal radio services, and watt.

FSAN (Full Service Access Network) - Pronounced ef san. An initiative of a consortium of vendors for a ATM-based passive optical network (APON).The specification subsequently was ratified by the ITU-T and incorporated within the G.983.1 standard. See also APON, ATM, ITU-T, and PON.

FSK (Frequency-Shift Keying) - Also known as frequency modulation (FM). See FM.

FSO (Free Space Optics) - Point-to-point airwave transmission systems that use focused infrared (IR) light beams between transmitters and receivers, much as microwave systems use focused radio beams. FSO requires optical line of sight (LOS) and will work through clear windows, depending on their chemical composition, but not through walls or other opaque objects. FSO transmitters use laser light sources in the form of either vertical cavity surface-emitting lasers (VCSELs) or laser diodes. FSO systems suffer from environmental interference, particularly fog, which absorbs, diffuses, and reflects the light beam, much as fog affects the beam from the headlights of an automobile. Under optimum conditions, transceiver separation is limited to about two to five kilometers, which limits the technology to short haul applications. Unlike radio systems, FSO systems are immune to electromagnetic interference (EMI) and radio frequency interference (RFI), require no licensing, and are inherently secure. FSO systems commonly operate at rates of 1.544 Mbps (T1), 2.048 Mbps (E-1), 34 Mbps (E-3) and 45 Mbps (T3), and are capable of rates up to 10 Gbps (OC-192). Systems running at up to 160 Gbps have been demonstrated in the labs. See also absorption, airwave transmissions, diffusion, EMI, infrared, laser diode, LOS, reflection, RFI, short haul circuit, and VCSEL.

FSS (Fixed Satellite System) - A satellite that is at an altitude and in an orbit such that it maintains a fixed position relative to the Earth’s surface. An FSS is in contrast to an MSS (Mobile Satellite System). FSS is synonymous with geosynchronous earth orbiting (GEO) satellite. See also GEO, MSS, and satellite.

FT1 (Fractional T1) - A T1 circuit with some DS-0 channels disabled to provide a fraction of T1 capacity. FT1 generally is tariffed to provide 1, 2, 4, 6, 8, or 12 DS-0 channels. Subrate transmission also is available at speeds of 9.6 kbps. See also DS-0, subrate, and T1.

FTC (Federal Trade Commission) - An independent United States government agency, reporting directly to Congress, the FTC was created in 1914 to prevent unfair methods of competition in commerce. In 1938, Congress passed the Wheeler-Lea Amendment, which included a broad prohibition against unfair and deceptive acts or practices. Since then, the Commission also has been directed to administer a wide variety of other consumer protection laws, including the Telemarketing Sales Rule, the Pay-Per-Call Rule and the Equal Credit Opportunity Act. In 1975, Congress passed the Magnuson-Moss Act, which gave the FTC the authority to adopt trade regulation rules that define unfair or deceptive acts in particular industries, and that have the force of law.The FTC is organized into the Bureaus of Consumer Protection, Competition, and Economics.

FTP - 1. Foil Twisted Pair. Synonymous with shielded twisted pair (STP). See STP. 2. File Transfer Protocol (FTP). An Application Layer protocol of the TCP/IP protocol suite, FTP supports the exchange of files between two host computers across the Internet. FTP also supports interactive user interface in which humans must interact with a remote host. The specifics of the file type and data format (e.g., ASCII, EBCDIC, or binary notation; and compressed or uncompressed) can be determined from client to server. FTP also requires clients to satisfy security authorization measures in the form of login and password. FTP makes use of TELNET for control messages between the hosts, and relies on connection-oriented Transmission Control Protocol (TCP) for data transfer. FTP is defined in IETF RFC 959 (1985). See also Application Layer, ASCII, binary notation, compression, connection-oriented, EBCDIC, host, IETF, Internet, login, password, protocol, protocol suite, TCP, TCP/IP, and TELNET.

FTTC (Fiber-To-The-Curb) - Also known as FTTN (Fiber-To-The-Node) A hybrid broadband local loop configuration that involves optical fiber from the edge of the telco or CATV network to an intelligent node at the curb, with one node serving perhaps many residences or small businesses.The few hundred meters or so of the local loop from the node to the premises generally is either unshielded twisted pair (UTP) in a telco application or coaxial cable (coax) in a CATV application, although some form of wireless technology is also possible.The fiber portion of a contemporary FTTC configuration generally conforms to one of the passive optical network (PON) standards. See also broadband, CATV, local loop, node, optical fiber, PON, telco, and UTP.

FTTN (Fiber-To-The-Neighborhood or Fiber-To-The-Node) - Also known as Fiber-To-The-Curb (FTTC).A hybrid broadband local loop configuration that involves optical fiber from the edge of the telco or CATV network to an intelligent node in the neighborhood, with one node serving perhaps many residences or small businesses.The few hundred meters or so of the local loop from the node to the premises generally is either unshielded twisted pair (UTP) in a telco application or coaxial cable (coax) in a CATV application, although some form of wireless technology is also possible.The fiber portion of a contemporary FTTN configuration generally conforms to one of the passive optical network (PON) standards. See also broadband, CATV, local loop, node, optical fiber, telco, and UTP.

FTTP (Fiber-To-The-Premises) - A broadband local loop configuration involving optical fiber from the edge of the telco or CATV network to the customer premises.A contemporary FTTP loop generally conforms to one of the passive optical network (PON) standards. See also broadband, CATV, local loop, optical fiber, PON, and telco.

FUBAR (Fouled Up Beyond All Recognition) - Referring to something so mangled as to be unrecognizable.The term is generally attributed to U.S. military slang circa WWII. In contemporary usage, the spelled-out phrase is a bit more colorful and much less polite. (Note: This is a polite book, so you’ll just have to use your imagination or search elsewhere for the more colorful version.) See also SNAFU.

FUD (Fear, Uncertainty, and Doubt) factor - A marketing strategy used by a dominant company to confuse and freeze the competition by either maintaining secrecy with respect to future product or service plans or by changing them frequently and, thereby, creating concerns that competitors’ products might not be compatible with those of the dominant vendor.The term was coined by Gene Amdahl after he left IBM to found Amdahl Corporation.Amdahl was quoted as saying “FUD is the fear, uncertainty, and doubt that IBM salespeople instill in the minds of potential customers who might be considering Amdahl products.”

full duplex (FDX) - See FDX.

full mobile number portability (FMNP) - Synonymous with wireless number portability (WNP). See WNP.

full mode - A mode defined by the ITU-T T.37 standard for Fax over Internet Protocol (FoIP). Fullmode extensions include mechanisms for ensuring call completion through negotiation of capabilities between transmit and receive devices. Full-mode also provides for delivery confirmation. See also facsimile, FoIP, resolution, simple mode, T.37, and TIFF-F.

full rate ADSL (full rate Asymmetric Digital Subscriber Line) - See ADSL.

Full Service Access Network (FSAN) - See FSAN

FUNI (Frame User Network Interface) - A derivative of the DXI standard, FUNI extends asynchronous transfer mode (ATM) access to smaller sites with relatively low speed access at rates from 56 kbps to 1.544 Mbps (T1). Data enters a router, which encapsulates the data in frames similar to those employed in frame relay.The router then forwards the data to an ATM switch, which converts them to cell format. See also ATM, cell, DXI, frame, frame relay, router, switch, and UNI.

further - 1. Additional; to a greater extent. 2. More distant in place or time, especially where there is no notion of physical distance. I don’t know about you, but I was always taught to use farther when referring to physical distances, and to use further otherwise. Anyway, it drives me nuts when people, especially weathermen, point to the map and talk about further north or further east. It sounds affected, like they are trying to impress us ignorant viewers at home. See also farther and utilize.

fusion splice - In optical fiber installation and repair, the permanent joining of two fibers by melting the glass and allowing the molten ends to meld.Note:The melting point of glass is approximately 2000 degrees Celsius. See also mechanical splice, optical fiber, and splice.

Future Public Land Mobile Telecommunications System (FPLMTS) - An initiative of the ITUR that defined a vision for a single global standard for digital wireless networks. FPLMTS was subsequently replaced by International Mobile Telecommunications-2000 (IMT-2000). See also IMT-2000.

FWM (Four-Wave Mixing) A type of noise created in fiber optic transmission systems (FOTS) when wavelengths interact to create additional wavelengths. FWM occurs in optical fibers engineered with a zero dispersion point at or near the wavelengths being transmitted.The number of additional wavelengths that can result is determined by the following formula: FWM = N² (N – 1)/2 Based on this formula, 2 wavelengths will produce 2 additional wavelengths, 4 wavelengths will produce 24 additional wavelengths, and 8 wavelengths will produce 224 additional wavelengths. FWM can particularly be a problem at 1550 nm in systems using dense wavelength division multiplexing (DWDM), and especially when erbium-doped fiber amplifiers (EDFAs) are employed.The EDFAs amplify the additional wavelengths and superimpose them on the original wavelengths. Non Zero Dispersion-Shifted Fiber (NZDF) addresses this issue by shifting the optimal dispersion point slightly away from the 1550 nm range where DWDM operates. Fiber fabricated to have a zero dispersion point at 1540 nm or 1560 nm, for example, will reduce dispersion significantly and eliminate the issue of four-wave mixing. See also amplification, dispersion, DWDM, EDFA, NZDF, and wavelength.

Fx (Firefox) - See Firefox.

FX (Foreign eXchange) circuit - Also abbreviated as FEX circuit. A dedicated circuit that draws dial tone from a foreign exchange, i.e., a central office exchange (COE) other than the one in which the system resides. An FX line (KTS) or trunk (PBX) allows a user organization to avoid toll charges for long distance calls, as the leased line or trunk is billed on a flat rate basis, sensitive to bandwidth and distance, rather than a usage sensitive basis. If the user organization places a great many calls to a foreign exchange area, the costs savings can be considerable, particularly if the foreign exchange is in reasonable proximity and the FX circuit, therefore, is a relatively short haul.The interface typically found at each end of the FX circuit is of two types. FXO faces the office or switch, receives battery and ringing, and seizes the line by closing the loop. FXS faces the subscriber, gives battery and ringing voltage, and responds to a phone going off hook.