F

failure domain

Area in which a failure has occurred in a Token Ring, defined by the information contained in a beacon. When a station detects a serious problem with the network (such as a cable break), it sends a beacon frame that includes the station reporting the failure, its NAUN, and everything in between. Beaconing in turn initiates a process called autoreconfiguration. See also autoreconfiguration, beacon, and NAUN.

fallback

A mechanism used by ATM networks when rigorous path selection does not generate an acceptable path. The fallback mechanism attempts to determine a path by selectively relaxing certain attributes, such as delay, in order to find a path that meets some minimal set of desired attributes.

fan-out unit

Device that allows multiple devices on a network to communicate using a single network attachment.

fantail

Panel of I/O connectors that attaches to an equipment rack, providing easy access for data connections to a networking.

Fast Ethernet

Any of a number of 100-Mbps Ethernet specifications. Fast Ethernet offers a speed increase ten times that of the 10BaseT Ethernet specification, while preserving such qualities as frame format, MAC mechanisms, and MTU. Such similarities allow the use of existing 10BaseT applications and network management tools on Fast Ethernet networks. Based on an extension to the IEEE 802.3 specification. Compare with Ethernet. See also 100BaseFX, 100BaseT, 100BaseT4, 100BaseTX, 100BaseX, and IEEE 802.3.

Fast Ethernet Interface Processor

See FEIP in the Cisco Systems Terms and Acronyms section.

Fast Sequenced Transport

See FST in the Cisco Systems Terms and Acronyms section.

Fast Serial Interface Processor

See FSIP in the Cisco Systems Terms and Acronyms section.

fast switching

See fast switching in the Cisco Systems Terms and Acronyms section.

fault management

One of five categories of network management defined by ISO for management of OSI networks. Fault management attempts to ensure that network faults are detected and controlled. See also accounting management, configuration management, performance management, and security management.

FCC

Federal Communications Commission. U.S. government agency that supervises, licenses, and controls electronic and electromagnetic transmission standards.

FCS

frame check sequence. Refers to the extra characters added to a frame for error control purposes. Used in HDLC, Frame Relay, and other data link layer protocols.

FDDI

Fiber Distributed Data Interface. LAN standard, defined by ANSI X3T9.5, specifying a 100-Mbps token-passing network using fiber-optic cable, with transmission distances of up to 2 km. FDDI uses a dual-ring architecture to provide redundancy. Compare with CDDI and FDDI II.

FDDI II

ANSI standard that enhances FDDI. FDDI II provides isochronous transmission for connectionless data circuits and connection-oriented voice and video circuits. Compare with FDDI.

FDDI Interface Processor

See FIP in the Cisco Systems Terms and Acronyms section.

FDDITalk

Apple Computer's data-link product that allows an AppleTalk network to be connected by FDDI cable.

FDM

frequency-division multiplexing. Technique whereby information from multiple channels can be allocated bandwidth on a single wire based on frequency. Compare with ATDM, statistical multiplexing, and TDM.

FECN

forward explicit congestion notification. Bit set by a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action as appropriate. Compare with BECN.

Federal Communications Commission

See FCC.

Federal Networking Council

See FNC.

FEIP

See FEIP in the Cisco Systems Terms and Acronyms section.

FEP

front-end processor. Device or board that provides network interface capabilities for a networked device. In SNA, typically an IBM 3745 device.

Fiber Distributed Data Interface

See FDDI.

fiber-optic cable

Physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is more expensive, but is not susceptible to electromagnetic interference, and is capable of higher data rates. Sometimes called optical fiber.

fiber-optic interrepeater link

See FOIRL.

FID0

format indicator 0. One of several formats that an SNA TH can use. An FID0 TH is used for communication between an SNA node and a non-SNA node. See also TH.

FID1

format indicator 1. One of several formats that an SNA TH can use. An FID1 TH encapsulates messages between two subarea nodes that do not support virtual and explicit routes. See also TH.

FID2

format indicator 2. One of several formats that an SNA TH can use. An FID2 TH is used for transferring messages between a subarea node and a PU 2, using local addresses. See also TH.

FID3

format indicator 3. One of several formats that an SNA TH can use. An FID3 TH is used for transferring messages between a subarea node and a PU 1, using local addresses. See also TH.

FID4

format indicator 4. One of several formats that an SNA TH can use. An FID4 TH encapsulates messages between two subarea nodes that are capable of supporting virtual and explicit routes. See also TH.

field-replaceable unit

A hardware component that can be removed and replaced on-site. Typical field-replaceable units include cards, power supplies, and chassis components.

file transfer

Category of popular network applications that allow files to be moved from one network device to another.

File Transfer, Access, and Management

See FTAM.

File Transfer Protocol

See FTP.

filter

Generally, a process or device that screens network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.

finger

An software tool for determining whether a person has an account at a particular Internet site. Many sites do not allow incoming finger requests, but some do.

FIP

See FIP in the Cisco Systems Terms and Acronyms section.

firewall

Router or access server, or several routers or access servers, designated as a buffer between any connected public networks and a private network. A firewall router uses access lists and other methods to ensure the security of the private network.

firmware

Software instructions set permanently or semipermanently in ROM.

flapping

Routing problem where an advertised route between two nodes alternates (flaps) back and forth between two paths due to a network problem that causes intermittent interface failures.

Flash memory

Nonvolatile storage that can be electrically erased and reprogrammed so that software images can be stored, booted, and rewritten as necessary. Flash memory was developed by Intel and is licensed to other semiconductor companies.

flash update

Routing update sent asynchronously in response to a change in the network topology. Compare with routing update.

flat addressing

A scheme of addressing that does not use a logical hierarchy to determine location. For example, MAC addresses are flat; bridging protocols must flood packets throughout the network in order to deliver the packet to the appropriate location. Compare with hierarchical addressing.

flooding

Traffic passing technique used by switches and bridges in which traffic received on an interface is sent out all of the interfaces of that device except the interface on which the information was originally received.

flow

Stream of data traveling between two endpoints across a network (for example, from one LAN station to another). Multiple flows can be transmitted on a single circuit.

flow control

Technique for ensuring that a transmitting entity, such as a modem, does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed. In IBM networks, this technique is called pacing.

flowspec

In IPv6, the traffic parameters of a stream of IP packets between two applications. See also IPv6.

FLT

Full Line Terminal. A multiplexer that terminates a SONET span. See also SONET.

FM

frequency modulation. Modulation technique in which signals of different frequencies represent different data values. Compare with AM and PAM. See also modulation.

FNC

Federal Networking Council. Group responsible for assessing and coordinating U.S. federal agency networking policies and needs.

FOIRL

fiber-optic interrepeater link. Fiber-optic signaling methodology based on the IEEE 802.3 fiber-optic specification. FOIRL is a precursor of the 10BaseFL specification, which is designed to replace it. See also 10BaseFL.

format indicator 0

See FID0.

format indicator 1

See FID1.

format indicator 2

See FID2.

format indicator 3

See FID3.

format indicator 4

See FID4.

forward channel

Communications path carrying information from the call initiator to the called party.

forward delay interval

Amount of time an interface spends listening for topology change information after that interface has been activated for bridging and before forwarding actually begins.

forward explicit congestion notification

See FECN.

forwarding

Process of sending a frame toward its ultimate destination by way of an internetworking device.

FOTS

Fiber Optics Transmission Systems. Vendor-proprietary fiber optic transmission equipment.

Fourier transform

Technique used to evaluate the importance of various frequency cycles in a time series pattern.

four-part dotted notation

See dot address.

fractional T1

See channelized T1.

FRAD

Frame Relay access device. Any network device that provides a connection between a LAN and a Frame Relay WAN. See also Cisco FRAD and FRAS in the Cisco Systems Terms and Acronyms section.

fragment

Piece of a larger packet that has been broken down to smaller units.

fragmentation

Process of breaking a packet into smaller units when transmitting over a network medium that cannot support the original size of the packet. See also reassembly.

frame

Logical grouping of information sent as a data link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control, that surround the user data contained in the unit. The terms cell, datagram, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

frame check sequence

See FCS.

frame forwarding

The mechanism by which frame-based traffic, such as HDLC and SDLC, traverses an ATM network.

Frame Relay

Industry-standard, switched data link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. See also X.25.

Frame Relay Access Device

See FRAD.

Frame Relay Access Support

See FRAS in the Cisco Systems Terms and Acronyms section.

Frame Relay bridging

Bridging technique, described in RFC 1490, that uses the same spanning-tree algorithm as other bridging functions, but allows packets to be encapsulated for transmission across a Frame Relay network.

frame switch

See LAN switch.

FRAS

See FRAS in the Cisco Systems Terms and Acronyms section.

free-trade zone

Part of an AppleTalk internetwork that is accessible by two other parts of the internetwork that are unable to directly access one another.

frequency

Number of cycles, measured in hertz, of an alternating current signal per unit time.

frequency-division multiplexing

See FDM.

frequency modulation

See FM.

front end

Node or software program that requests services of a back end. See also back end, client, and server.

front-end processor

See FEP.

FSIP

See FSIP in the Cisco Systems Terms and Acronyms section.

FST

See FST in the Cisco Systems Terms and Acronyms section.

FTAM

File Transfer, Access, and Management. In OSI, an application layer protocol developed for network file exchange and management between diverse types of computers.

FTP

File Transfer Protocol. Application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes. FTP is defined in RFC 959.

full duplex

Capability for simultaneous data transmission between a sending station and a receiving station. Compare with half duplex and simplex.

full mesh

Term describing a network in which devices are organized in a mesh topology, with each network node having either a physical circuit or a virtual circuit connecting it to every other network node. A full mesh provides a great deal of redundancy, but because it can be prohibitively expensive to implement, it is usually reserved for network backbones. See also mesh and partial mesh.

Fuzzball

Digital Equipment Corporation LSI-11 computer system running IP gateway software. The NSFnet used these systems as backbone packet switches.