F (fully associated) link
A link that directly connects to signaling endpoints, generally SSPs.

facility
(1) Any or all of the physical elements of a plan used to provide communications services. (2) A component of an operating system. (3) A transmission path between two or more points, provided by a common carrier.

fading
A phenomenon, generally of microwave or radio transmission, where atmospheric, electromagnetic, or gravitational influences cause a signal to be deflected or diverted away from the target receiver. The reduction in intensity of the power of a received signal.

Fast Ethernet
A standard for high-speed Ethernet that has a rate of 100Mbps.

fast packet switching
A packet processing technology that has streamlined protocol handling, including Frame Relay and ATM.

fault
A condition that causes any physical component of a system to fail to perform in acceptable fashion.

fault tolerance
The capability of a program or system to operate properly even if a failure occurs.

FCC (Federal Communications Commission)
A regulatory agency established by the Communications Act of 1934, charged with regulating all electrical and radio communications in the United States.

FDD (Frequency Division Duplexing)
A full-duplex technique that it is used when there is a significant contiguous spectrum allocated and when synchronization between the base stations is not possible. Each direction (incoming and outgoing) occupies a different portion of the frequency band, and a rather large portion of the spectrum is consumed.

FDDI (Fiber Distributed Data Interface)
A 100Mbps, fiber-based token-passing ANSI standard. It consists of dual fiber-optic counter-rotating rings, each capable of supporting 100Mbps data rates. FDDI is defined for fiber-optic cable, but it has a twisted-pair alternative called CDDI. FDDI II is an enhanced version of FDDI that supports isochronous transmission (for voice and video) as well as the packet-oriented (both asynchronous and synchronous) traffic handling of FDDI.

FDM (Frequency-Division Multiplexing)
A technique of dividing the bandwidth of a communications line into multiple smaller units of bandwidth, each of which supports an independent information stream.

FDMA (Frequency Division Multiple Access)
A multiple access technique used in analog cellular systems, in which each user is assigned to a different frequency.

FECN (forward explicit congestion notification)
A bit in the Frame Relay header by which the network can inform the receiver of network congestion.

femtosecond
0.000000000000001 (that is, 10?15) second.

FEP (front-end processor)
A dedicated communications system that intercepts and handles activity for the host. It may perform line control, message handling, code conversion, and error control, as well as such application functions as control and operation of special-purpose terminals.

FHHS (Frequency Hopping Spread Spectrum)
A spread spectrum technique in which the frequency hopping varies in a known pattern, and separate error correction must be included.

fiber-optic waveguides
Thin filaments of glass through which a light beam can be transmitted for long distances by means of multiple internal reflections. Occasionally, other transparent materials, such as plastic, are used.

fiber optics
A technology that uses light as digital information carrier. Fiber-optic cables (light guides) are a direct replacement for coaxial cables and twisted-wire pairs. The glass-based transmission facilities occupy far less physical volume, yet provide a tremendous amount of transmission capacity, which is a major advantage in crowded underground ducts. The fibers are immune to electrical interference, which is another advantage. Also called lightwave communications, photonics, or, simply, fiber.

Fibre Channel
A high-speed interface, standardized by ANSI, that supports up to 800Mbps over 6.2 miles (10 kilometers) of fiber.

FIFO (first in, first out)
A queuing technique in which the next item to be retrieved is the item that has been in the queue for the longest time. This ensures that cells remain in the correct sequence.

file server
In local networks, a station dedicated to providing file and mass data storage services to the other stations on the network.

filter
To selectively forward data, based on criteria specified by the network manager.

firewall
A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both.

fixed wireless local loop
A stationary installation that dramatically cuts down on the cost of installing and maintaining the local loop. It uses a fixed antenna location, so it’s relatively easy to engineer.

flat network
A network that is constructed by using bridges or Layer 2 LAN switches. This type of network is easy to configure, and it promises better performance than hierarchical networks; it offers higher throughput and therefore also lower latencies. However, the scalability of a flat network is limited, and a flat network is subject to broadcast storms.

flat rate
A fixed cost for service. Additional charges may be applied for additional services or usage if so specified.

flow control
A system that uses buffering and other mechanisms, such as controls that turn a device on and off, to prevent data loss during transmission.

FM (frequency modulation)
One of three ways of modifying a sine wave signal to carry digital bits. The sine wave or “carrier” has its frequency modified in accordance with the information to be transmitted. The frequency function of the modulated wave may be continuous or discontinuous.

forward channel
The communications path that carries voice or data from the call initiator to the network.

forward error correction
A system that uses redundant information in received data to permit the receiver to correct transmission errors.

four-wire circuit
A circuit that contains two pairs of wire (or their logical equivalent) for simultaneous (i.e., full-duplex) two-way transmission. Two pairs of conductors, one for the inbound channel and one for the outbound channel, are connected to the station equipment.

fractional T-1/E-1
T-1/E-1 lines that have apportioned bandwidth for separate transmission channels (DS-0/64Kbps subchannels), generally in increments of four channels.

fragmentation
The process of splitting a packet into pieces when it is larger than the MTU it must transmit.

frame
(1) In data transmission, the sequence of contiguous bits bracketed by and including beginning and ending flag sequences. (2) In a TDM system, a repetitive group of signals resulting from a signal sampling of all channels, including any additional signals for synchronizing and other required system information.

frame bandwidth allocation
The sum of the committed information rates associated with all the PVCs for a specific customer.

Frame Relay
A packet-switch technology that is simpler and more powerful than the X.25 standard. Frame Relay provides a multiplexed channel between a router and a T-1/E-1 nodal processor. It increases bandwidth utilization while reducing overall equipment costs. The standard addresses data communications speeds up to 45Mbps.

framing
A control procedure used with multiplexed digital channels, such as T1 carriers, in which bits are inserted so that the receiver can identify the time slots that are allocated to such subchannel; framing bits may also carry alarm signals indicating specific alarm conditions.

Free Space Optics
An optical wireless networking option that uses low-powered infrared lasers. There are two options in Free Space Optics: point-to-point products, used to provide high speed connection between two buildings, and multiple high-speed connections through the air that operate over much shorter distances, either in a point-to-multipoint or meshed architecture.

frequency
An expression of how frequently a periodic (repetitious) wave form or signal regenerates itself at a given amplitude. It can be expressed in hertz (Hz), kilohertz (KHz), megahertz (MHz), and so on.

FSK (frequency shift keying)
A method of modulation that uses two different frequencies to distinguish between a mark (digital 1) and a space (digital 0) when transmitting on an analog line. Used in modems operating at 1,200bps or slower.

FTAM (File Transfer Access and Management)
An ISO standard that describes how to create, delete, read, and change file attributes as well as transfer and access (at file or record level) files stored at remote sites. It is an application-layer protocol.

FTP (File Transfer Protocol)
A protocol that enables a TCP/IP user on any computer to get files from another computer, or to send files to another computer. Usually implemented as application-level programs, FTP uses the Telnet and TCP protocols. The server side requires a client to supply a login identifier and password before it will honor requests.

FTTC (fiber-to-the-curb)
A system in which fiber cable extends from a switching office to a curb.

FTTH (fiber-to-the-home)
A system in which fiber cable extends from a switching office to the subscriber’s house.

full-duplex
A communication system or equipment that is capable of transmission simultaneously in both directions.

full-motion video
Moving images that the human eye perceives as being fully realistic. While there are no defined standards, full-motion video is frequently referred to as VHS-quality. Frame rates range from 24 frames per second in motion pictures, 25 frames per second in the PAL system, and 30 frames per second in the NTSC system.

FX (foreign exchange) line
A line that makes a toll call appear to be a local call.