Index - A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A-Z
FACILITY/FACILITIES (TRANSMISSION FACILITY/TRANSMISSION FACILITIES) - Typically refers to transmission lines or circuits. A user's facilities are the physical circuits or circuit paths available to make calls.
FACSIMILE (FAX) - Graphic transmission of pictures, maps or documents via communications circuits using terminal devices that scan documents, transforming scanned images into coded data-like signals and reproduce likenesses of original documents at a distant point. Has a long history dating to the 1840's as a method of telegraphy; archaic press implementations of this century used the names Wirephoto and Telephoto for early, slow photographic transmissions.
FCC (FEDERAL COMMUNICATIONS COMMISSION) - A board of Presidentially-appointed commissioners empowered to regulate interstate and international communications and all uses of radio in the United States. Operates under the Communications Act of 1934 and several more recent laws; promulgates its own regulations interpreting those laws, as Title 47 of the Code of Federal Regulations ( CFR 47 , 47CFR ).
FEATURE GROUP A (FGA,FG A) - Subscriber-line-side originating and terminating LEC switched access for which an originating subscriber dials a 7-digit telephone number that essentially connects to another subscriber- type 2-wire line to an IEC. The IEC must operate like a subscriber, interpreting ringing signal as a demand for a call, returning a distinctive tone to signal the caller for input additional tone-generated digits of an account code and the called number; does not provide the function of supervision signaling to tell the IEC when to start billing on inward calls.
FEATURE GROUP B (FGB,FG B) - Trunk-side originating and terminating LATA access for which an originating subscriber dials a 950-WXXX number (where W=0,1 and XXX is the Carrier Access Code), which is translated to its corresponding IEC's trunk group. Optional rotary dial service and ANI may be available; permits IECs to offer coin-free local access and credit card calling. For example, dialing 950-1022 at any phone nationwide should place the caller on MCI's network, returning a tone ready for the caller's desired number and account code; this is the number and sequence dialed by card-reading payphones.
FEATURE GROUP C (FGC,FG C) - Trunk-side local switching office access for AT&T; generally on a direct basis between each LEC switching machine and AT&T's nearest switching machine.
FEATURE GROUP D (FGD,FG D) - Also referred to as "Equal Access," Feature Group D is trunk-side local exchange switching machine access affording call supervision to IECs, alternative access code use (10XXX), optional calling-party identification, recording of access-charge billing details, and presubscription to a customer- specified IEC, for convenient use of 1+ dialing to the user's choice of IEC's.
FIBER OPTICS - a technology using light as a digital information bearer. Fiber optic cables ( light guides ) are a direct replacement for conventional wire, coaxial cable and many forms of radio, including microwave. Fiber optic lines actually cost less, occupy less space and provided far more transmission capacity than earlier methods, while providing superior quality due to virtual immunity to electrical interference.
FILE SERVER (SERVER) - In LANs, a station, often microcomputer-based, that provides the mass storage and file access to users on the LAN. Server capabilities vary widely; some even include ability to " gateway " to other communications means or " bridge " to a similar nearby LAN, or even determine for users if those functions are needed and decide the route to the user's destination, in which case they are called " routers ".
FIRMWARE - Permanent or semipermanent control coding built into a software-operated computer device that operates an application program, instruction set, operating routine or other user-oriented instructions to a computer; often resident in a ROM ( Read Only Memory ) chip to simplify installation.
FLAG - In data transmission, an indicator of an expected event like the beginning or end of a block of data. In CCITT standards for X.25 networks, the 8-bit character 01111110 has been uniquely established with the name "Flag" to be used at the beginning and end of a block; many other proprietary protocols have adopting it, even if the rest of their data block is not CCITT-compliant.
FLOW CONTROL - In data communications, the use of buffering and other mechanisms that operate to avoid data loss in case the receiver cannot keep up with the transmitter. The ASCII control characters X-ON and X-OFF are frequently-used examples; they are returned in the reverse direction as an instruction for the sender to hold or restart held transmission.
FOOTPRINT - 1.) In physical hardware planning, the floor space or desktop space a unit requires; 2.) In satellite communications, the area on the surface of the earth that a given satellite covers; this technology has reached a high state of development, with satellite coverages capable of being quite shaped and tailored to the purpose of the satellite.
FORWARD CHANNEL - The communications path from the originator of information toward the recipient; a path in the return direction (perhaps established for error checking and/or flow control ) is called the " reverse channel ."
FORWARD ERROR CORRECTION (FEC) - An error-correcting technique that avoids the need for any reverse channel by enabling self-correction of errors at the receiver; operates by adding information (at the expense of throughput) to enable the receiver to determine what the error was and the correct information to substitute for the error. Operates by using "convolutional coding" which means adding a repeating sequence to the information that is known to the receiver. By subtracting the convolutional code and performing other tests, the receiver can insert what should have been received with a high degree of accuracy; now extends to making error- correcting data modems called Trellis-coded (trellis coded) modems.
FOUR WIRE CIRCUITS (FOUR WIRE CHANNELS) - Circuits which using two separate transmission paths of two wires each or their virtual equivalent, for simultaneous two-way ( Full Duplex ) transmission, as opposed to regular local lines which usually only have two wires to carry conversations in both directions. One set of wires carries information in one direction; the other in the opposite direction. Contrast with Two wire circuits ; two wire channels .
FRAME - 1.) In digital communications, a group of bits or characters sent serially over a channel; generally a logical unit of information between data link layer entities that contains its own control information for addressing and error checking. Synonomous with " block ." 2.) In communications wiring, an openwork structure upon which terminal connections for wires can be run to interconnect different devices; typically called a " distributing frame " or " distribution frame ."
FRAMING - A control procedure used with digital transmission to permit the receiver to synchronize itself with the transmitter's sequence of bits or characters; framing bits in more recent systems also carry alarm and control messages in entities like T-1 carrier systems. This makes framing bits in communications carrier systems closely akin to control characters in data communications.
FREQUENCY - The number of complete cycles of an event (in communications typically an alternating current signal) per unit of time; usually expressed by means of the unit ' Hertz ," named after Heinrich Hertz an early German investigator of the properties of high-frequency alternating current waves. FREQUENCY DIVISION MULTIPLEXING (FDM) The analog method of deriving multiple transmission channels from a single physical facility; divides of an available frequency range (bandwidth) into various subchannels. Widely used in many formats by virtually every kind of telecommunications technology; the " carrier systems " of telephony. Also called Frequency Division Multiplex ; Frequency Division Mux .
FREQUENCY RESPONSE - The measurement of how well a transmission channel or device transports all the frequencies sent into it; a measure of how faithfully signals are transported or reproduced. In Bell terms, the arithmetic is reversed and the name " amplitude response " or " amplitude variation " is used for the same expression.
FREQUENCY SHIFT KEYING (FSK) - One of the more basic and durable forms of transmitting binary information; in FSK, one of the binary states is represented by one known frequency and the other by another known frequency. The receiver produces outputs only when one of the two known frequencies is received in the absence of the other. Applications of FSK abound in every form of telecommunications; computer data modems used FSK exclusively for speeds up to 120 bps for years, and the classic FDM of telegraphers was 24 FSK channels on a voice channel to produce what was called " carrier telegraph " and variations on that name.
FRESNEL ZONE - The area in open space that must be practically free of obstructions for a microwave radio path to function properly; some degree of Fresnel consideration is required in the immediate vicinity of the form of microwave radio used on satellite links.
FULL DUPLEX (FDX) - A circuit which allows independent transmission information in both directions
simultaneously. - Synonym: In wire telephony, 4 wire circuit.
FULL PERIOD - In private line telephone and telegraph service, a circuit rented for the exclusive use of a single customer on a month-by-month basis. Archaic in the sense that part-time circuits have not been rented for years.
FUNCTIONAL TEST - Operation carried out under normal working conditions (but not necessarily full operating load) to verify that a circuit or device operates correctly. Compare to Acceptance test .
FX (FOREIGN EXCHANGE) SERVICE - A telephone service allowing a customer the appearance of a local presence in a distant city, by connecting a local phone line into a distant local telephone exchange switching machine. Accomplished over a leased private line circuit purchased from either an LEC or IEC as appropriate to the distance being covered. In certain LEC territories, local distances are sufficient or tariff limits make appropriate connection to a Central Office within the same " exchange area " of central offices, and the distinctive name " Foreign Central Office " ( FCO ) is applied to these short-haul "FX" types of circuit; the term " FEX " is used for FX in some non-Bell LECs.