1000Base-SX -- A gigabit Ethernet standard for HDX and FDX fiber @ 850nm wavelength.

1000Base-LX -- A gigabit Ethernet standard for HDX and FDX fiber @ 1300nm wavelength.

100Base VG AnyLan -- The IEEE specification for 100 Mbps Token Ring and Ethernet implementations over four pair UTP. The Media Access Control (MAC) layer is not compatible with the 802.3 MAC layer.

100Base-T Fast Ethernet -- A 100 Mbps technology based on the Ethernet/CD network access method.

10BaseT -- The IEEE 802.3 specification for Ethernet over unshielded twisted pair (UTP).

AAL -- ATM Adaptation Layer. The standards layer that allows multiple appliations to have data converted to and from the ATM cell. A protocol used that translates higher layer services into the size and format of an ATM cell.

Advanced Data Communications Control Procedure (ADCCP) -- ANSI counterpart to HDLC. One of more than 20 protocols transported by FastComm FRADs.

Advanced Intelligent Network (AIN) -- Carrier offering more than 'pipes' to users.

Advanced Peer-to-Peer Networking (APPN) -- IBM SNA facility that provides distributed processing based on Type 2.1 network nodes and Logical Unit (LU) 6.2.

Advanced Program-to-Program Communications (APPC) -- Implementation of SNA LU 6.2 sessions that permits personal computers in an SNA network to communicate in real time with the mainframe host and other networks.

Allowed Cell Rate -- An ABR service parameter, ACR is the current rate in cells/sec at which a source is allowed to send.

American National Standards Institute (ANSI) -- The coordinating body for voluntary standards groups within the United States. ANSI is a member of the International Organization for Standardization (ISO).

American Standard Code for Information Interchange (ASCII) -- This is the code that most computers use to represent displayable characters. An ASCII file is a straightforward text file without special control characters.

ANSI T1.403.T1E1 -- The performance-monitoring, data-link, and network-interface requirements for ESF CSUs as defined by the Exchange Carriers Standards Association. T1.403 specifies automatic performance reports transmitted to the network once per second via the data link. (In an E1 environment, Performance Monitor is the equivalent of T1.403).

Application Program Interface (API) -- Means of communication between programs to give one program transparent access to another.

Asymmetrical Digital Subscriber Line (ASDL) -- A new standard for transmitting at speeds up to 7 Mbps over a single copper pair.

Asynchronous Balance Mode (ABM) -- A communication mode used in HDLC that allows either of two workstations in a peer-oriented point-to-point configuration to initiate a data transfer.

Asynchronous Time Division (ATD) -- ETSI proposal for pure cell relay, without SONET or
other framing.

Asynchronous Time Division Multiplexing -- A multiplexing technique in which a transmission capability is organized in a priori unassigned time slots. The time slots are assigned to cells upon request of each application's instantaneous real need.

Asynchronous Transfer Mode (ATM) -- (1) The CCITT standard for cell relay wherein information for multiple types of services (voice, video, data) is conveyed in small, fixed-size cells. ATM is a connection-oriented technology used in both LAN and WAN environments. (2) A fast-packet switching technology allowing free allocation of capacity to each channel. The SONET synchronous payload envelope is a variation of ATM. (3) ATM is an international ISDN high speed, high-volume, packet switching transmission protocol standard. ATM currently accommodates transmission speeds from 64 Kbps to 622 Mbps.

Automatic Number Identification (ANI) -- A charge number parameter that is normally included in the Initial Address Message to the succeeding carrier for billing purposes.

Automatic Repeat Request (ARQ) -- A feature that automatically initiates a request for retransmission when an error in transmission is detected.

Available Bit Rate (ABR) -- A class of service in which the ATM network makes its "best effort" to meet traffic bit rate requirements.

B Channel -- In ISDN, a full-duplex, 64 Kbps channel for sending data.

B-ICI Signaling ATM Adaptation Layer (B-ICI SAAL) -- A signaling layer that permits the transfer of connection control signaling and ensures reliable delivery of the protocol message. The SAAL is divided into a Service Specific part and a Common part (AAL5).

B-ISDN Inter-Carrier Interface (B-ICI) -- An ATM Forum defined specification for the interface between public ATM networks to support user services across multiple public carriers.

Backward Explicit Congestion Notification (BECN) -- A bit in the frame relay header. The bit is set by a congested network node in any frame which is traveling in the reverse direction of the congestion. (In frame relay, a node can be congested in one direction of frame flow but not in the other.)

Bandwidth -- (1) Measure of the information capacity of a transmission channel. (2) The difference between the highest and lowest frequencies of a band that can be passed by a transmission medium without undue distortion, such as the AM band 535 to 1705 kilohertz.

Baseband -- Transmission scheme in which the entire bandwidth, or data-carrying capacity, of a medium (such as a coaxial cable) is used to carry a single digital pulse, or signal, between multiple users. Because digital signals are not modulated, only one kind of data can be transmitted at a time. Contrast with broadband.

Basic Rate Interface (BRI) -- ISDN standards and specifications for provision of low-speed ISDN services. Supports two "B" channels of 64 Kbps each and one "D" channel of 16 Kbps on a single wire pair.

Baud (Bite at Unit Density) -- A measure of the speed of transmission of data; number of elements transmitted per second.

Binary Synchronous Communication/Bisinc-Character -- Oriented data link protocol for half-duplex applications. Supported on FastComm FRADs.

Bridge/Router -- A device that can provide the functions of a bridge, router, or both concurrently. Bridge/router can route one or more protocols, such as TCP/IP and/or XNS, and bridge all other traffic.

Broadband -- A data-transmission scheme in which multiple signals share the bandwidth of a medium. This allows the transmission of voice, data, and video signals over a single medium. Cable television uses broadband techniques to deliver dozens of channels over one cable.

Broadband Bearer Capability (BBC) -- A bearer class field that is part of the initial address message.

Broadband Connection Oriented Bearer/Class A (BCOB-A) -- Indicated by ATM end user in SETUP message for connection-oriented, constant bit rate service. The network may perform internetworking based on AAL information element (IE).

Broadband Connection Oriented Bearer/Class X (BCOB-X) -- Indicated by ATM end user in SETUP message for ATM transport service where AAL, traffic type, and timing requirements are transparent to
the network.

Broadband Integrated Services Digital Network (B-ISDN) -- A technology suite designed for multimedia. The two transmission types are: ATM (Asynchronous Transfer Mode) and STM (Synchronous Transfer Mode).

Broadband Inter Carrier Interface (BICI) -- A carrier-to-carrier interface like PNNI (private network-to-network interface) but lacking some information offered by PNNI. Carriers are not likely to let their switches share routing information or detailed network maps with their competition's equipment. BICI now supports only permanent virtual circuits between carriers; the ATM Forum is currently addressing switched virtual circuits.

Broadband Inter -- Switching System Interface (B-ISSI). Between ATM nodes.

Broadcast Address -- A special address that is reserved for simultaneous broadcast to all stations.

Broadcast Domain -- Defines the set of all devices which will receive broadcast frames originating from any device within the set. Broadcast domains are normally bounded by routers.

Broadcast Storm Firewalls -- A mechanism that limits the rate at which broadcast/multicast packets are forwarded through the system.

Brouter -- Concatenation of "bridge" and "router." Used to refer to devices which perform both bridging and routing functions.

Buffer -- A storage area used for handling data in transit. Buffers are often used to compensate for differences in processing speed between network devices.

Byte -- The fundamental unit that a computer uses in its operation. It is a group of adjacent binary digits, usually 8, often used to represent a single character.

C-Notched Noise -- The C-message frequency weighted noise on a voice channel with a holding tone, which is removed at the measuring and through a notch (very narrow band) filter.

Caching -- (1) Speeds information processing by storing information from a transaction to use for later transactions. (2) Storing or buffering data in a temporary location, so that the information can be retrieved quickly by an application program.

Carrier Sense Multiple Access/Collision Detection (CSMA/CD) -- A channel access mechanism wherein devices wishing to transmit first check the channel for a carrier. If no carrier is sensed for some period of time, devices can transmit. If two devices transmit simultaneously, a collision occurs and is detected by all colliding devices, which subsequently delays their retransmissions for some random length of time. CSMA/CD access is used by Ethernet and IEEE 802.3.

Carrierless Amplitude and Phase Modulation (CAP) -- A modem technique applied to 50 Mbps LAN.

Category 3 Unshielded Twisted Pair (CAT-3) -- Industry standard for unshielded twisted wire pair capable of supporting voice and low-grade data traffic.

Category 5 Unshielded Twisted Pair (CAT-5) -- The highest grade of unshielded twisted-pair cable available, as defined by EIA/TIA 568. Category 5 UTP is required to run standard compliant CDDI to 100 meters.

Cell Delay Variation (CDV) -- ATM performance parameter which specifies the potential variation (+/-) from the expected average transit delay through the network over a given virtual circuit.

Cell Delay Variation Tolerance (CDVT) -- ATM layer functions may alter the traffic characteristic of ATM connections by introducing Cell Delay Variation. When cells from two or more ATM connections are multiplexed, cells of a given ATM connection may be delayed while cells of another ATM connection are being inserted at the output of the multiplexer. Similarly, some cells may be delayed while physical layer overhead or OAM cells are inserted. Consequently, some randomness may affect the inter-arrival time between consecutive cells of a connection as monitored at the UNI. The upper bound on the "clumping" measure is the CDVT.

Cell Error Ratio (CER) -- ATM performance parameter which specifies the ratio of errored cells to the total cells transmitted over a given virtual circuit.

Cell Header -- ATM Layer protocol control information.

Cell Loss Priority (CLP) -- A 1-bit field in an ATM cell header that provides a two level priority indicator. Used to bias the discarding of cells toward lower priority cells in the event of congestion. Similar to the DE bit in frame relay.

Cell Loss Ratio (CLR) -- ATM performance parameter that specifies the ratio of lost (non-delivered) cells to the total cells transmitted over a given virtual circuit.

Cell Misinsertion Rate (CMR) -- The ratio of cells received at an endpoint that were not originally transmitted by the source end in relation to the total number of cells properly transmitted.

Cell multiplexing/demultiplexing -- The ATM layer function that groups cells from different virtual paths or circuits and transmits them in a stream to the destination switch, where they are demultiplexed and routed to the appropriate end-points.

Cell Transfer Delay (CTD) -- ATM performance parameter which specifies the average transit delay of cells between a source and destination over a given virtual circuit.

Central Office (CO) -- (1) A local telephone company office which connects to all local loops in a given area and where circuit switching of customer lines occurs. (2) A local Telephone Company switching system, where Telephone Exchange Service customer station loops are terminated for purposes of interconnection to each other and to trunks. In the case of a Remote Switching Module (RSM), the term Central Office designates the combination of the Remote Switching Unit and its Host.

Central Office Local Area Network (CO-LAN) -- A data switching service based on a data PBX in a carrier's CO.

CGI (Common Gateway Interface) -- A standard that allows Web servers to run external applications such as search engines.

Circuit Switching -- Switching system in which a dedicated physical circuit path must exist between sender and receiver for the duration of the "call". Used heavily in the phone company network, circuit switching often is contrasted with contention and token passing as a channel-access method, and with message switching and packet switching as a switching technique.

Class of Service (COS) -- The categories of traffic type in ATM used to distinguish between real time and non-real time usage, as well as between variable and constant bit rages.

CMOT (CMIP over TCP) -- An effort to use the OSI network management protocol to manage TCP/IP networks. CMOT is historical, we are not aware of any running implementations of this protocol.

Committed Information Rate (CIR) -- The transport speed the frame relay network will maintain between service locations.

Common Channel Signaling -- A method of signaling in which signaling information relating to a multiplicity of circuits, or relating to a function for network management, is conveyed over a single channel by addressed messages.

Common Management Interface Protocol (CMIP) -- An ITU-TSS standard for the message formats and procedures used to exchange management information in order to operate, administer, maintain, and provision a network.

Common Part Convergence Sublayer-Service Data Unit (CPCS-SDU) -- Protocol data unit to be delivered to the receiving AAL layer by the destination CP convergence sublayer.

Common Protocol Convergence Sublayer (CPCS) -- Pads PDU to N x 48 bytes, maps control bits, adds FCS in preparation for SAR.

Competitive Access Provider (CAP) -- -Alternative to LEC for local loop to IXC or for dial tone. A company that builds and operates communication networks in metropolitan areas and provides its customers with an alternative to the local telephone company.

Competitive Local Exchange Carrier (CLEC) -- A company that builds and operates communication networks in metropolitan areas and provides its customers with an alternative to the local telephone company.

Computer Telephony Integration (CTI) -- The name given to the merger of traditional telecommunications (PBX) equipment with computers and computer applications. The use of Caller ID to automatically retrieve customer information from a database is an example of a CTI application.

Connection Admission Control (CAC) -- The function of an ATM network that determines the acceptability of a virtual circuit connection request and determines the route through the network for such connections.

Connectionless Broadband Data Service (CBDS) -- A connectionless service similar to Bellcore's SMDS defined by European Telecommunications Standards Institute (ETSI).

Connectionless Network Protocol (CLNP) -- The OSI protocol for providing the OSI Connectionless Network Service (datagram service). CLNP is the OSI equivalent to Internet IP, and is sometimes called ISO IP.

Connectionless Service (CL) -- A service which allows the transfer of information among service subscribers without the need for end-to-end establishment procedures.

ConnectionLess Transport Service (CLTS) -- OSI datagram protocol.

Constant Bit Rate (CBR) -- Delay intensive applications such as video and voice, that must be digitized and represented by a continuous bit stream. CBR traffic requires guaranteed levels of service and throughput.

Convergence -- The industry trend towards sharing network resources among disparate applications and traffic types.

Convergence Layer PDU (CS-PDU) -- Info plus new header and trailer to make a packet that is segmented into cells or SUs.

Convergence Sublayer (CS) -- The portion of
the AAL that formats information; a convergence sublayer protocol data unit (CS-PDU) before it is segmented into cells and reformats it after reassembly
at its destination.

Copper Distributed Data Interface (CDDI) -- FDDI packets transmitted over Category 5 unshielded twisted pair cable.

Customer Premises Equipment (CPE) -- (1) Telephone terminal devices, such as handsets and private branch exchanges (PBXs), located on the customer's premises. (2) Terminating equipment, such as terminals, phones, routers and modems, supplied by the phone company, installed at customer sites, and connected to the phone company network.

Cut-Through Switching -- Refers to a method of Frame Switching where the switching device commences forwarding a frame after it has determined the destination port without waiting for the entire frame to have been received on the incoming port. Also known as on-the-fly switching.

Data Communicating Equipment (DCE) -- In RS232 communications, a device implementing the interface and handshaking of a data communications device (such as a modem).

Data Exchange Interface (DXI) -- (1) ATM: A variable-length frame-based ATM interface between a DTE and a special ATM DSU/CSU. The ATM DSU/CSU converts between the variable-length DXI frames and the fixed-length ATM cells. (2) Defines the format for transmitting information that has gone through the ATM convergence sublayer.

Data Link Connection Identifier (DLCI) -- A value in frame relay that identifies a logical connection.

Data Link Control (DLC) -- The SNA layer responsible for transmission of data between two nodes over a physical link.

Data Link Switching (DLSw) -- A reliable means of transporting SNA and NetBIOS traffic in a multiprotocol router network using IP encapsulation. Defined in RFC1434 and RFC 1795.

Data Terminal Equipment (DTE) -- The part of
a data station that serves as a data source, destination, or both, and that provides for the data communications control function according to protocol. DTE includes computers, protocol translators, and multiplexers.

Demodulation -- Opposite of modulation; the process of retrieving data from a modulated carrier wave.

Destination End Station (DES) -- An ATM termination point which is the destination for ATM messages of a connection and is used as a reference point for ABR services.

Destination MAC Address (DA) -- A six octet value uniquely identifying an endpoint which is sent in IEEE LAN frame headers to indicate frame destination.

Destination Service Access Point (DSAP) -- Address field in header of LLC frame to identify a user within a station address (Layer 2).

Digital Access and Cross-Connect System (DACS) -- A digital switching device for routing T-1 lines and DS-0 portions of lines, among multiple
T-1 ports.

Digital Access Cross-Connect Switch (DCS) -- A digital switching device for routing time slots among multiple E1/T1 ports.

Digital Certificates -- A virtual security document which ensures the association between the user's public key and the user's identity and security privileges.

Digital Cross Connect System (DCS) -- (1) An electronic switching node that enables circuits to be cross-connected. (2) An electronic cross-connect which has access to the lower-rate channels in higher-rate multiplexed signals and can electronically rearrange (cross-connect) those channels.

Digital Data Service Unit (DSU) -- Converts RS-232 or other terminal interface to line coding for local loop transmission.

Digital Data System (DDS) -- U.S. private data transmission network, established in 1974 by AT&T and based on AT&T's Dataphone data service. DDS is a digital overlay network built on the existing loop and trunking network.

Digital Modem -- A system component which allows modem users to communicate over digital access facilities. They work by converting the PCM-encoded digital data streams sent by analogue modem users into their original analogue waveform.

Digital Signal 1 (DS-1) -- North American Digital Hierarchy signaling standard for transmissions at
2.544 Mbps. Supports 24 simultaneous DS-O signals. Term often used interchangeably with T-1, although DS-1 signals may be exchanged over other
transmission systems.

Digital Signal 3 (DS-3) -- North American Digital Hierarchy signaling standard for transmission at 44.736 Mbps. Supports 28 simultaneous DS-1 signals.

Distance Vector Multicast Routing Protocol (DVMRP) -- A metrics based algorithm for routing multicast packets.

Distributed Queue Dual Bus (DQDB) -- Communication protocol proposed by IEEE 802.6 committee for use in MANs.

DNS Spoofing -- Assuming the DNS name of another system by either corrupting the name service cache of a victim system, or by compromising a domain name server for a valid domain.

Document Conferencing -- A conferencing technology that enables customers to review a document and collaborate with others, right from their computer using either analog and modem dial-up or existing Internet accesses (LAN, dial-up, etc.)

Domain Name System (DNS) -- The distributed name/address mechanism used in the Internet.

Domestic Satellite -- A satellite that provides transmission of information between points within the United States by an authorized common carrier.

Dynamic Bandwidth Allocation (DBA) -- A process that optimizes overall network efficiency by automatically increasing or decreasing the bandwidth of a channel to accommodate changes in data flow from end-user equipment.

Dynamic Data Exchange (DDE) -- This is a method of transferring data between two Windows applications while they are running.

Dynamic Password -- An automatically generated single-use password.

Edge Device -- A physical device which is capable of forwarding packets between legacy interworking interfaces (e.g., Ethernet, Token Ring, etc.) and ATM interfaces based on data-link and network layer information but which does not participate in the running of any network layer routing protocol. An Edge Device obtains forwarding descriptions using the route
distribution protocol.

Electronic Data Interchange (EDI) -- (1) Method for passing orders, invoices, and other transactions electronically between locations or organizations. (2) The exchange of structured transactional information between autonomous computers.

Electronic Funds Transfer (EFT) -- An electronic system that transfers money and records financial transactions, replacing the use of paper.

Electronic Industries Association (EIA) -- A group that specifies electrical transmission standards.

Emulated Local Area (ELAN) -- A logical network initiated by using the mechanisms defined by LAN Emulation. This could include ATM and legacy attached end stations.

Encapsulation -- The wrapping of data in a
particular protocol header. For example, Ethernet
data is wrapped in a specific Ethernet header
before network transit.

Encryption -- Applying a specific algorithm to data
in order to alter the data's appearance and prevent other devices from reading information. Decryption applies the algorithm in reverse to restore the data to its original form.

End System to Intermediate System Protocol (ES-IS) -- The OSI protocol by which end systems such as network personal computers announce themselves to intermediate systems such as hubs.

Error Free Seconds (EFS) -- A unit used to specify the error of performance of T carrier systems, usually expressed as EFS per hour, day, or week. This method gives a better indication of the distribution of bit errors than a simple bit error rate (BER).

ES-IS -- End system to Intermediate system protocol. The OSI protocol used for router detection and address resolution.

Ethernet -- A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation. Ethernet networks operate at 10 Mbps using CSMA/CD to run over coaxial cable. Ethernet is similar to a series of standards produced by IEEE referred to as IEEE 802.3.

Excess Burst (Be) -- Transient capacity above CIR in FR net.

Explicit Forward Congestion Indicator (EFCI) -- A bit in the PTI field of the ATM cell header. The bit is set by a congested network node in any cell passing through the node.

Explorer Super Frame (ESF) -- Frame sent out by a networked device in a source route bridging environment to determine the optimal route to another networked device.

Extended Binary Coded Decimal Interexchange Code (EBCDIC) -- Usually pronounced Eb-suh-dick. The character code used by most mainframe computers. Each character is composed of eight bits, as opposed to ASCII, which is composed of seven bits.

Extended Industry Standard Architecture (EISA) -- A standard bus interface, commonly used by PCs and some UNIX workstations and servers.

Extended Superframe (EF) -- An Extended Superframe consists of 24 frames of 193 bits each (4632 bits total). In each frame, one "F bit" is followed by 24 8-bit bytes. The 8 Kbps of F-bit overhead is divided into 2 Kbps for framing, 2 Kbps of CRC-6 code for logic error checking, and a 4 Kbps Data Link for maintenance communications. As in the Superframe (D4) format, 1.536 Mbps is the available bandwidth for user information.

Exterior Gateway Protocol (EGP) -- The service by which gateways exchange information about what systems they can reach; generally, an exterior gateway protocol is any internetworking protocol for passing routing information between autonomous systems.

Extranet -- A collaborative network that uses Internet technology to link businesses with their suppliers, customers, or other businesses that share common goals.

FDDI II -- The proposed ANSI standard to enhance FDDI. FDDI II will provide isochronous transmission for connectionless data circuits and connection-oriented voice and video circuits.

Fiber Channel -- FC Fiber Channel is a high performance serial link supporting its own, as well as higher level protocols such as the FDDI, SCSI, HIPPI, and IPI. The fast (up to 1 Gbps) technology can be converted for Local Area Network technology by adding a switch specified in the Fiber Channel standard, that handles multipoint addressing.

Fiber Distributed Data Interface (FDDI) -- An ANSI-defined standard specifying a 100 Mbps token-passing network using fiber-optic cable. Uses a dual-ring architecture to provide redundancy.

Fiber In the Loop (FITL) -- Optical technology from CO to customer premises.

Fiber Optic Cable -- A transmission medium that uses glass or plastic fibers, rather than copper wire, to transport data or voice signals. The signal is imposed on the fiber via pulses (modulation) of light from a laser or a light-emitting diode (LED). Because of its high bandwidth and lack of susceptibility to interference, fiber-optic cable is used in long-haul or noisy applications.

Fiber Optical Terminating System -- Network element which terminates the optical circuit. It serves such functions as mapping the service into the SONET carrier and making opto/electrical conversions.

Fiber Optics -- A method for the transmission of information (sound, pictures, data). Light is modulated and transmitted over high purity, hair-thin fibers of glass. The bandwidth capacity of fiber optic cable is much greater than that of conventional cable or copper wire.

File Transfer Protocol (FTP) -- (1) An IP application protocol for transferring files between network nodes. (2) An Internet protocol that allows a user on one host to transfer files to and from another host over a network.

File Transfer, Access, and Management (FTAM) -- The OSI remote file service ad protocol.

Firewall -- (1) Isolation of LAN segments from each other to protect data resources and help manage traffic. (2) Hardware or software that restricts traffic to a private network from an unsecured network.

Flash EPROM -- PROM (Programmable Read-Only Memory) technology providing nonvolatile storage that can be electrically erased in the circuit and reprogrammed; developed by Intel and licensed to other semi-conductor companies.

Forward Explicit Congestion Notification (FECN) -- A bit in the frame relay header. The bit is set by a congested network node in any frame which is traveling in the same direction as the congestion. (In frame relay, a node can be congested in one direction of frame flow but not in the other).

Fractional E1 -- A carrier service that offers data rates between 64 Kbps and 2.048 Mbps (E1) in increments of 64 Kbps.

Fractional E3/T3 -- Fractional E3/T3 refers to the leasing of portions of E3/T3 bandwidth (a specific number of time slots) by carriers. FE3 or FT3 allows for more economical networking in some applications.

Fractional T-1 -- A WAN communications service that provides the user with some portion of a T1 circuit which has been divided into 24 separate 64 Kb channels. Fractional E-1 is in Europe.

Frame Relay -- High-performance interface for packet-switching networks. Considered more efficient than X.25 which it is expected to replace. Frame relay technology can handle "bursty" communications that have rapidly changing bandwidth requirements.

Frame Relay Service -- A connection oriented service that is capable of carrying up to 4096 bytes
per frame.

Frequency Modulation (FM) -- Radio transmission covering 88-108 megahertz on the broadcast band.
It is less susceptible to interference than AM broadcasting. Also used in other frequency bands
for two way communications in land mobile and marine services.

Frequency-Division Multiplexing (FDM) -- Division of a transmission facility into two or more channels by splitting the frequency band transmitted by the facility into narrower bands, each of which is used to constitute a distinct channel.

Gateway -- A set of functions intended to facilitate electronic access by users to remote services and vice versa. Gateways are intended to provide a single source through which users can locate and gain access to a wide variety of service. Gateways typically offer a directory of services available through them, and provide billing for these services.

Generic Cell Rate Algorithm (GCRA) -- An ATM Forum-developed traffic shaping algorithm that utilizes the traffic parameters defined on a given virtual circuit to smooth wide fluctuations in cell traffic volumes and to enforce the traffic limits defined in those parameters.

Generic Connection Admission Control (GCAC) -- This is a process to determine if a link has potentially enough resources to support a connection.

Gigabit -- One billion bits.

Gigabit Ethernet -- A 1 Gbps standard for Ethernet.

Gigabit Ethernet Alliance -- An association of Gigabit Ethernet manufacturers and suppliers
formed for the purpose of promoting Gigabit
Ethernet Technology.

Group Address -- A single address that refers to multiple network devices. Synonymous with
multicast address.

Groupware -- A network-based application that lets users collaborate.

H.320 -- A video conferencing protocol which defines a standard for video conferencing over ISDN and other narrow-band transmission media. H.320 is one in a series of "H" protocols, set forth by the International Telecommunication Union (ITU), that define the central technologies used in multimedia tele-conferencing.

H.323 -- A standard approved by the International Telecommunication Union (ITU) that defines how audiovisual conferencing data is transmitted across networks. In theory, H.323 should enable users to participate in the same conference even though they are using different videoconferencing applications. Although most videoconferencing vendors have announced that their products will conform to H.323, it's too early to say whether such adherence will actually result in interoperability.

High Performance Parallel Interface (HIPPI) -- An emerging ANSI standard which extends the computer bus over fairly short distances at speeds of 800 and 1600 Mbps. HIPPI is often used in a computer room to connect a supercomputer to routers, frame buffers, mass-storage peripherals, and other computers.

High Performance Routing (HPR) -- A form of dynamic call routing in the PSTN.

High Speed Data Link Control (HDLC) -- A protocol defined by the International Standards Organization and used in X.25 communications. It specifies an encapsulation method for data on synchronous serial data links. Various manufacturers have proprietary versions of HDLC, including IBM's SDLC.

High Speed Serial Interface (HSSI) -- Standard for a serial interface at high speeds (64 Kbps and higher up to 52 Mbps) between DTE and DCE equipment over very short distances. Used for the physical connection between a router and a DSU.

High-bit-rate Digital Subscriber Line (HDSL) -- A standard defined by the TlE1.4 standards committee, designed to be a cost-effective method of delivering T1/E1 line speeds over unconditioned copper cable.

Home Page -- The first page of a Web site or of a logical group of HTML documents.

HP Open View Windows -- A graphical user interface that integrates and presents network management applications and system management applications. It allows you to perform both system management and network management from a single terminal in a multi-vendor, distributed computing environment.

Hybrid-Fiber Coaxial (HFC) -- A technology being developed by the cable TV industry to provide two-way, high-speed data access to the home using a combination of fiber optics and traditional coaxial cable.

Hyper Text Transfer Protocol (HTTP) -- (1) The protocol most commonly used in the World Wide Web to transfer information from Web servers to Web browsers. (2) The protocol that negotiates document delivery to a Web browser from a Web server.

Hyper Text Markup Language (HTML) -- (1) The language used in the World Wide Web to create web pages with links to other documents, rich text enhancements (bold, italic, etc.) and so on. The "source" file for what you see on a web page is written in HTML. (2) The language with which World Wide Web documents are formatted. It defines fonts, graphics, hypertext links, and other details. HTML is an implementation of SGML.

Impulse Noise -- Any momentary occurrence of the noise on a channel over a specified level threshold. It is evaluated by counting the number of occurrences which exceed the threshold.

Initial Cell Rate (ICR) -- An ABR service parameter, in cells/sec, that is the rate at which a source should send initially and after an idle period.

Integrated IS-IS -- Routing protocol based on the OSI routing protocol IS-IS but with support for IP or other networks. Integrated IS-IS implementations send only one set of routing updates and are more efficient than two separate implementations.

Integrated Services Digital Network (ISDN) -- (1) The recommendation published by CCITT for private or public digital telephone networks where binary data, such as graphics, digitized voice, and data transmission, pass over the same digital network that carries most telephone transmissions. (2) An overall application of the technology to provide for both newer digital and more traditional telephone services in an integrated network that incorporates the new network and interfacing standards which are being adopted worldwide.

IntelliMux Service -- A service whereby customers can reconfigure a specific voice grade, DDS and/or individual channels within a High Capacity DS1 service connected at a digital cross-connect system.

Interactive Voice Response (IVR) -- Term used to describe systems that provide information in the form of recorded messages over telephone lines in response to user input in the form of spoken words or more commonly DTMF sugnalling. Examples include banks that allow you to check your balance from any telephone and automated stock quote systems.

Interexchange Carrier (IXC) or Interexchange Common Carrier -- Any individual, partnership, association, joint-stock company, trust, governmental entity, or corporation engaged for hire in interstate or foreign communication by wire or radio, between two or more exchanges.

Interior Gateway Protocol (IGP) -- The protocol used to exchange routing information between collaborating routers in the Internet. RIP and OSPF are
examples of IGPs.

Interior Gateway Routing Protocol (IGRP) -- Learns best routes through LAN Internet (TCP/IP).

Intermediate System to Intermediate System Protocol (IS-IS) -- The OSI protocol by which intermediate systems exchange routing information.

International Organization for Standardization (ISO) -- Best know for the 7 layer OSI Reference Model.

International Record Carrier (IRC) -- A common carrier engaged in providing overseas telecommunications service. Services furnished traditionally include telex, private line service, and alternate voice data service.

Internet Assigned Numbers Authority (IANA) -- The entity responsible for assigning numbers in the Internet Suite of Protocols.

Internet Control Message Protocol (ICMP) -- A network-layer Internet protocol that provides message packets to report errors and other information relevant to IP packet processing. Documented in RFC 792.

Internet Gateway Protocol (IGRP) -- A proprietary IGP used by Cisco System's routers.

Internet Protocol (IP) -- A Layer 3 (network layer) protocol that contains addressing information and some control information that allows packets to be routed. Documented in RFC 791.

Internet Protocol Version 6 (IPng, IPv6) -- IPv6 is a new version of the Internet Protocol which is designed to be an evolutionary step from its predecessor, version 4. There are many RFCs defining various portions of the protocol, its auxiliary protocols, and the transition plan from IPv4. The core RFCs are 1883 through 1886.

Internet Service Provider (ISP) -- (1) Any of a number of companies that sell Internet access to individuals or organizations at speeds ranging from 300 Bps to OC-3. (2) A business that enables individuals and companies to connect to the Internet by providing the interface to the Internet backbone.

Internet Telephony -- Generic term used to describe various approaches to running voice telephony over IP.

Internetwork -- A collection of networks interconnected by routers that function (generally) as a single network. Sometimes called an internet, which is not to be confused with the Internet.

Internetwork Packet Exchange, Network Protocol (IPX) -- LAN protocol developed by Novell for NetWare.

Internetworking -- General term used to refer to the industry that has arisen around the problem of connecting networks together. The term can refer to products, procedures, and technologies.

Interoperability (IOP) -- The ability of equipment from different manufacturers (or different implementations) to operate together.

Interstate IntraLATA -- Interstate services between two points within the same LATA which are not used in connection with InterLATA channels.

Inverse Multiplexing -- A method of combining individually dialed low-speed circuits into a single high-speed data stream.

IP Address -- The 32-bit address defined by the Internet Protocol in RFC 791. It is usually represented in dotted decimal notation

IP and ARP over ATM -- An adaptation of TCP/IP and its address resolution protocol (ARP) for ATM defined by the Internet Engineering Task Force in Requests for Comment 1483 and 1577. It places IP packets and ARP requests directly into protocol data units (PDUs) and converts them into ATM cells.

IP Datagram -- The fundamental unit of information passed across the Internet. Contains source and destination addresses along with data and a number of fields which define such things as the length of the datagram, the header checksum, and flags to say whether the datagram can be (or has been) fragmented.

IP Next Generation (IPng) -- Collective term used to describe the efforts of the Internet Engineering Task Force to define a new version of the Internet Protocol (IP) which can handle larger IP addresses to cope with the explosive growth of the Internet. Also known as Ipv6.

IP Spoofing -- An attack whereby a system attempts to illicitly impersonate another system by using its IP network address.

IS-IS -- Intermediate system to Intermediate system protocol. The OSI protocol by which intermediate systems exchange routing information.

ISDN BRI -- A digital access line that is divided into three channels. Two of the channels, called B channels, operate at 64 Kbps and are always used for data or voice. The third D channel is used for signaling at 16 Kbps.

ISDN Centrex -- A service provided by local telephone companies to customer premises, in which a central office digital switch performs in lieu of a customer PBX in an ISDN system. ISDN Centrex uses one B channel and one D channel to provide an array of digital voice and data capabilities.

ISDN PRI -- Based physically and electrically on an E1 circuit, but channelized so that two channels are used for signaling and 30 channels are allocated for user traffic. ISDN PRI is available in E1 and T1 frame formats, depending on country.

Java -- A object-oriented programming language developed by Sun Microsystems. Applets written in Java include their own software players, so you can download and run them on any computer.

Kerberos -- A component of MIT's Project Athena. Kerberos is the security system, based on symmetric key cryptography.

LAN Emulation (LANE) -- A technique for legacy LAN MAC-layer protocols like Ethernet and token ring, to work transparently across an ATM network.

LAN Emulation Client (LEC) -- ATM Forum-defined specifications in support of LAN-to-LAN connectivity, called LAN Emulation. LEC defines that set of functions implemented in an ATM network that provide LAN DTEs with information regarding the location of the other LAN Emulation services.

LAN Emulation Network-to-Network Interface (LNNI) -- Enables each vendors' implementation of LAN emulation to interoperate. This is essential for building multivendor ATM networks.

LAN Emulation Server (LES) -- ATM Forum-defined specifications in support of LAN-to-LAN connectivity called LAN emulation. LES defines that set of functions implemented in an ATM network in support of LAN-to-LAN connection establishment.

LAN Emulation User-Network Interface (L-UNI) -- The definition of how legacy LAN protocols and applications will coexist within an ATM network.

LAN Segmentation -- Dividing LAN bandwidth into multiple independent LANs to improve performance.

Layer 3 Switching -- The emerging Layer 3 switching technology integrates routing with switching to yield very high routing throughput rates in the millions-of-packets-per-second range. The movement to Layer 3 switching is designed to address the downsides of the current generation of layer 2 switches, which are functionally equivalent to bridges. These downsides for a large, flat network include being subject to broadcast storms, spanning tree loops, and address limitations that drove the injection of routers into bridged networks in the late 1980s. Currently, Layer 3 switching is represented by a number of approaches in the industry.

Leaky Bucket Algorithm -- A form of flow control that checks an arriving data stream against the traffic-shaping parameters specified by a sending node. Cells arriving at an ATM network switch are placed in a memory buffer ("bucket"), which is allowed to reach its capacity but not overflow. The bucket is "leaky" in that it allows cells to flow out to their destinations, which allows more cells to be added to the memory buffer.

Leased Line -- A permanent physical connection between two locations that forms a private Wide Area Network (WAN). The connection is made via lines leased from a telephone company.

Lightweight Directory Access Protocol (LDAP) -- This protocol provides access for management and browser applications that provide read/write interactive access to the X.500 Directory.

Line Interface (LINF) -- Card used in the HyperSwitch.

Line Overhead (LOH) -- 18 bytes of overhead accessed, generated, and processed by line terminating equipment. This overhead supports functions such as locating the SPE in the frame, multiplexing, or concatenating signals, performance monitoring, automatic protection switching, and line maintenance.

Line Terminating Equipment (LTE) -- Network elements such as add/drop multiplexers or digital cross-connect systems which can access, generate, and process Line Overhead.

Line-Side Connection -- A connection of a transmission path to the line side of a local exchange switching system.

Link -- Physical connection between two nodes in a network. It can consist of a data communication circuit or a direct channel (cable) connection.

Link Access Procedure D (LAPD) -- A layer 2 protocol defined by CCITT (original name of ITU-T). This protocol reliably transfers blocks of information across a single Layer 1 link and supports multiplexing of different connections at Layer 2.

Link Service Access Point (LSAP) -- Logical address of boundary between layer 3 and LLC sublayer 2

Local Access and Transport Area (LATA) -- A geographic area established for the provision and administration of communications service. It encompasses one or more designated exchanges, which are grouped to serve common social, economic, and other purposes.

Local Exchange Company (LEC) -- A telephone company that provides customer access to the worldwide public switched network through one of its central offices.

Local Loop -- The line from a telephone customer's premises to the telephone company Central Office.

Logical IP Subnetwork (LIS) -- An IP subnetwork is a single network on which all devices have a direct communications path to all other devices.

Logical Link Control (LLC) -- IEEE-defined sub layer of the OSI link layer. LLC handles error control, flow control, and framing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants.

Logical Link Control, type 2 (LLC2) -- A connection-oriented OSI logical link control sub layer protocol.

Loss of Cell Delineation (LOC) -- A condition at the receiver or a maintenance signal transmitted in the PHY overhead indicating that the receiving equipment has lost cell delineation. Used to monitor the performance of the PHY layer.

Loss of Frame (LOF) -- A condition at the receiver or a maintenance signal transmitted in the PHY overhead indicating that the receiving equipment has lost frame delineation. This is used to monitor the performance of the PHY layer.

Loss of Pointer (LOP) -- A condition at the receiver or a maintenance signal transmitted in the PHY overhead indicating that the receiving equipment has lost the pointer to the start of cell in the payload. This is used to monitor the performance of the PHY layer.

Loss of Signal (LOS) -- A condition at the receiver or a maintenance signal transmitted in the PHY overhead indicating that the receiving equipment has lost the received signal. This is used to monitor the performance of the PHY layer.

LU6.2 -- Set of services that support program to program communications.

Mail Gateway -- A machine that connects two or more electronic mail systems (especially dissimilar mail systems on two different networks) and transfers messages between them. Sometimes the mapping and translation can be quite complex, and generally it requires a store-and-forward scheme whereby the message is received from one system completely before it is transmitted to the next system after suitable translations.

Maximum Burst Size (MBS) -- ATM performance parameter defining the duration of transmission at peak rate that would be accepted on a given ATM virtual circuit.

Maximum Cell Delay Variance (MCDV) -- The maximum two-point CDV objective across a link or node for the specified service category.

Maximum Cell Loss Ratio (MCLR) -- The maximum ratio of the number of cells that do not make it across the link or node to the total number of cells arriving at the link or node.

Maximum Cell Rate (MaxCR) -- The maximum capacity usable by connections belonging to the specified service category.

Maximum Cell Transfer Delay (MCTD) -- This is the sum of the fixed delay component across the link or node and MCDV.

Maximum Information Rate (MIR) -- Refer to Program Clock Reference (PCR).

Maximum Transmission Unit (MTU) -- The maximum packet size, in bytes, that a particular interface will handle.

Media Access Control Layer Address (MAC Layer Address) -- Also called hardware address or physical address. A data-link layer address associated with a particular network device. Contrasts with network or protocol address which is a network layer address.

Media Access Control Sub Layer (MAC Sub layer) -- As defined by the IEEE, the lower portion of the OSI reference model data link layer. The MAC sub layer is concerned with media access issues, such as whether token passing or contention will be used.

Media Attachment Unit (MAU) -- In IEEE 802.3, a device that performs IEEE 802.3 Layer 1 functions, including collision detection and injection of bits onto the network.

Metropolitan Area Network (MAN) -- A data communication network covering the geographic area of a city (generally, larger than a LAN but smaller than a WAN). FDDI can provide a private MAN, while IEEE 802.6 can provide a public MAN.

MIB Attribute -- A single piece of configuration, management, or statistical information which pertains to a specific part of the PNNI protocol operation.

MIB Object -- A collection of attributes that can be used to configure, manage, or analyze an aspect of the PNNI protocol's operation.

Minimum Cell Rate (MCR) -- ATM performance parameter which specifies the minimum rate for cell transmission that a network must guarantee to a user on a given virtual circuit. Also, a field in an RM cell specifying the smallest value to which the ER field can be set.

Motion Picture Experts Group (MPEG) -- (1) Industry organization developing standards and specifications for the encoding and transmission of video information over various media and network technologies.

Multi-User Dungeon (MUD) -- Adventure, role playing games, or simulations played on the Internet. Devotees call them "text-based virtual reality adventures." The games can feature fantasy combat, booby traps, and magic. Players interact in real time and can change the "world" in the game as they play it. Most MUDs are based on the Telnet protocol.

Multimode -- Used to describe optical fiber that allows more than one mode of light signal transmission.

Multimode Fiber -- Optical fiber with a core diameter of 62.5 or 50 microns. Dispersion of light is greater than single mode fiber so distances are less.

Multiplexer (MUX) -- A technique that enables several data streams to be sent over a single physical line. It is also (ISO), a function by which one connection from a layer is used to support more than one connection to the next higher layer.

Multipoint -- The connection of more than two locations to affect a teleconference, as opposed to only point-to-point connections.

Narrowband -- Mobile or portable radio services which can be used to provide services to both individuals and businesses such as paging and data services.

Narrowband Integrated Services Digital Network (N-ISDN) -- Services include basic rate interface (2B+D or BRI) and primary rate interface (30B+D-Europe and 23B+D-North America or PRI). Supports narrowband speeds at/or below 1.5 Mbps.

Network Access Point (NAP) -- Another name for an Internet Exchange Point.

Network Interface Card (NIC) -- The circuit board or other hardware that provides the interface between a communicating DTE and the network.

Network Node Interface (NNI) -- Set of ATM Forum-developed specifications for the interface between two ATM nodes in the same network. Two variations are being developed; an interface between nodes in a public network (called a public Nni or just NNI) and an interface between nodes in a private network (called a private NNI or P-NNI).

Network Operations Center (NOC) -- Any center tasked with the operational aspects of a production network. These tasks include monitoring and control, trouble-shooting, user assistance, and so on.

Network Services Access Point (NSAP) -- OSI generic standard for a network address consisting of 20 octets. ATM has specified E.164 for public network addressing and the NSAP address structure for private network addresses.

Network to Network Interface (NNI) -- Interface between ATM network nodes defined in the ATM Forum's user network interface (UNI).

Network-Level Firewall -- A firewall in which traffic is examined at the network protocol packet level.

Neural Networks -- Non-linear predictive models that learn through training and resemble biological neural networks in structure.

Non-return To Zero (NRZ) -- A signal that does
not return to the zero level between successive transmitted ones.

Novell Internetwork Packet Exchange (IPX) -- A built-in networking protocol for Novell Netware. It was derived from the Xerox Network System protocol and operates at the network layer of the OSI protocol model.

On-Line Transaction Processing -- Processing that supports the daily business operations. Also known as operational processing and OLTP.

Open Network Architecture (ONA) -- The structure of a communications network that permits all types of terminals and users to have access to any network capability without compatibility restrictions.

Open Shortest Path First (OSPF) -- Routing protocol for TCP/IP networks.

Operations, Administration and Maintenance (OAM) -- ATM management cells defined by the ATM Forum in the UNI 3.0 specification for the management of ATM devices.

Optical Carrier (OC-x) -- Fundamental unit used in SONET (Synchronous Optical NETwork)
hierarchy -- Indicates an optical signal and x represents increments of 51.84 Mbps. OC-1, -3, and -12 equal optical rates of 51, 155, and 622 Mbps.

Optical Carrier 12 (OC-12) -- The optical carrier level equivalent to SONET STS-12 at 622.08 Mbps.

OSI Reference Model -- Seven-layer network architecture model of data communication protocols developed by ISO and CCITT. Each layer specifies particular network functions such as addressing, flow
control, error control, encapsulation, and reliable message transfer.

OSI Transport Protocol Class 4 (TP4) -- Error Detection and Recovery Class. This is the most powerful OSI Transport Protocol, useful on top of any type of network. TP4 is the OSI equivalent to TCP.

OSI Transport Protocol Class O (TPO) -- Simple Class. This is the simplest OSI Transport Protocol, useful only on top of an X.25 network (or any other network that does not lose or damage data).

Out-of-Band Signaling -- An exchange access signaling feature which allows customers to exchange call control and signaling information over a communications path which is separate from the message path.

Packet Assembler-Disassembler (PAD) -- The mechanism for disassembling packets at the sending end and assembling them to form the complete message at the receiving end. Traditionally used in X.25 networks.

Packet Filtering -- A second layer of filtering on top of the standard filtering provided by a traditional transparent bridge. Can improve network performance, provide additional security, or logically segment a network to support virtual workgroups.

Packet Loss Rate -- The measure of loss, over time, of data packets as a percentage of the total traffic transmitted.

Payload -- That portion of a frame or cell that carries user traffic, that is, the frame or cell exclusive of any headers or trailers.

Peak Cell Rate (PCR) -- Parameter defined by the ATM forum for ATM traffic management.

Permanent Virtual Circuit (PVC) -- A defined virtual link with fixed end-points that are set-up by the network manager. A single virtual path may support multiple PVCs.

Physical Layer (PHY) -- The bottom layer of the OSI and ATM protocol stack, which defines the interface between ATM traffic and the physical media.
The PHY consists of two sublayers: the transmission
convergence (TC) sublayer and the physical medium-dependent (PMD) sublayer.

Physical Medium Dependent (PMD) -- A sublayer of the physical layer that interfaces directly with the physical medium and performs the most basic bit transmission functions of the network.

Ping (Packet Internet Grouper) -- Refers to the ICMP echo message and its reply. Often used to test the reachability of a network device.

Point to Point Tunneling Protocol (PPTP) -- Manages the separation of traffic streams via tunneling over PPP.

Point-to-Point Protocol (PPP) -- (1) Successor to SLIP. Provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits. (2) A protocol which allows a computer to use a modem and a regular telephone line to make a TCP/IP connection directly to the Internet. PPP is gradually replacing SLIP for this purpose.

Points of Presence (POP) -- A term used by Internet service providers to indicate the number of geographical locations from which they provide access to the Internet.

Primary Rate Interface -- ISDN interface to primary access, consisting of a single 64 Kbps D channel plus 23 or 30 B channels for voice and/or data.

Private Network-to-Network Interface (P-NNI) -- A routing protocol that allows multiple vendors' ATM switches to be integrated. It automatically and dynamically distributes routing information, enabling any switch to determine a path within the network.

Protocol Data Unit (PDU) -- A discrete piece of information like a frame or a packet in the appropriate format for encapsulation and segmentation in the payload of a cell.

Proxy -- The mechanism whereby one system "fronts for" another system in responding to protocol requests. Proxy systems are used in network management to avoid having to implement full protocol stacks in simple devices, such as modems.

Public Key Cryptography -- A security scheme in which a different key is used for encryption and decryption. Key-1 is the public key; that is, everyone knows it. Key-2 is private so that only the recipient knows it. In this scheme, it is computationally impossible to derive the identity of key-2 from key-1.

Public Switched Network -- The combined transmission facilities of the world's telephone companies and administrations, including all those circuits available to subscribers on an unrestricted basis.

Pulse Code Modulation (PCM) -- A technique for converting an analog voice signal to digital format. Requires sampling the analog signal 8 thousand times per second and converting the sample to an 8-bit value. Hence, PCM requires a 64 Kbps digital channel to carry one voice connection.

Quality of Service (QoS) -- Term for the set of parameters and their values which determine the performance of a given virtual circuit.

Regional Bell Operating Company (RBOC) -- The acronym for the local telephone companies created in 1984 as part of the break-up of AT&T. The seven RBOCs are Ameritech, Bell Atlantic, Bell South, NYNEX, Pacific Telesis Group, Southwestern Bell, and U.S. West.

Remote Access Server -- Access equipment at a central site that connects remote users with corporate LAN resources.

Remote Authentication Dial-In User Service (RADIUS) -- A security administration standard that functions as an information clearinghouse, storing
authentication information about users and administering multiple security systems across complex networks.

Reverse Address Resolution Protocol (RARP) -- A part of the TCP/IP protocol suit used to determine a destination host's IP address using its hardware MAC address. Commonly used by diskless workstations.

RJ-11-Standard 4 -- Wire connectors for phone lines.

RJ-45-Standard 8 -- Wire connectors for IEEE 802.3 1BaseT networks.

RMON -- A standard MIB defined in RFC 1271 to allow remote monitoring of networked devices.

Route -- A path through an internetwork.

Route caching -- Storage of forwarding information (based on network topology and routing policy) associated with a destination. It starts when the first packet to the destination is processed, to speed the forwarding of all subsequent packets to the same destination.

Router -- An OSI Layer 3 device that can decide which of several paths network traffic will follow based on some optimality metric. Also called a gateway (although this definition of gateway is becoming increasingly outdated), routers forward packets from one network to another based on network-layer information.

Router Cluster -- Private, high-speed switched links to each building in a campus. They are used to expand interbuilding bandwidth.

Routing Domain -- A set of routers exchanging routing information within an administrative domain.

Routing Information Protocol (RIP) -- An IGP supplied with Berkeley UNIX systems. It is the most common IGP in the Internet. RIP uses hop count as a routing metric. The largest allowable hop count for RIP is 16.

Routing Metric -- The method by which a routing algorithm determines that one route is better than another. This information is stored in routing tables. Metrics include reliability, delay, bandwidth, load, MTUs, communication costs, and hop count.

Routing Protocol -- A protocol that accomplishes routing through the implementation of a specific routing algorithm. Examples of routing protocols include IGRP, RIP, and OSPF.

Routing Table -- A table stored in a router or some other internetworking device that keeps track of routes (and, in some cases, metrics associated with those routes) to particular network destinations.

RS-232-C -- Popular physical layer interface. Virtually identical to the V.24 specification.

RS-422 -- A balanced electrical implementation of RS-449 for RS-232-C compatibility.

RS-423 -- An unbalanced electrical implementation of RS-449 for RS-232-C compatibility.

RS-449 -- Popular physical layer interface. Essentially a faster (up to 2 Mbps) version of RS-232-C capable of longer cable runs.

RSA -- A public-key cryptographic system which may be used for encryption and authentication. It was invented in 1977 and named for its inventors: Ron Rivest, Adi Shamir, and Leonard Adleman.

Secure HTTP (S-HTTP) -- An extension of HTTP for authentication and data encryption between a Web server and a Web browser.

Secure Sockets Layer (SSL) -- A transport-level technology for authentication and data encryption between a Web server and a Web browser.

Segmentation And Reassembly (SAR) sublayer -- Converts PDUs into appropriate lengths and then formats them to fit an ATM cell format. At the destination end-station, the SAR extracts payloads from the cells and converts them back into PDUs to be ultimately used by applications.

Serial Line Internet Protocol (SLIP) -- A standard for use of a regular (serial) telephone line and a modem to connect a computer as a true Internet site. This protocol is gradually being replaced by PPP.

Service Access Point (SAP) -- The point at which the services of an OSI layer are made available to the next higher layer. The SAP is named according to the layer providing the services: e.g., transport services are provided at a Transport SAP (TSAP) at the top of the Transport Layer.

Service Advertising Protocol (SAP) -- Periodic broadcast by LAN device (Netware); filtered by FRADs to reduce overhead on access links.

Session Layer -- OSI layer that provides a means for dialogue between end systems.

Shared Ethernet -- Conventional CSMA/CD Ethernet configuration to which all stations are attached by a hub and share 10 or 100 Mbps of bandwidth. Only one session can transmit at a time. This is the most popular network type today.

Shared Queue -- A Work Queue containing Forms whose access is shared with other members of a Group.

Shielded Twisted Pair (STP) -- Two-pair wire medium used in the transmission of several different protocols. It is capable of supporting CDDI for link distances of up to 100 meters. These wires have a layer of shielded insulation to reduce EMI.

Shortest Path first (SPF) -- LAN router protocol that minimizes some measure (delay) and not just "hops" between nodes.

Signal-to-C-Notched Noise Ratio -- The ratio in dB of a test signal to the corresponding C-Notched Noise.

Signaling Systems 7 -- The out of band signaling protocol developed by the Consultative Committee for International Telephone and Telegraph (CCITT) and the American National Standards Institute (ANSI).

Simple Efficient Adaptation Layer (SEAL) -- Older name for AAL5.

Simple Gateway Management Protocol (SGMP) -- The predecessor to SNMP.

Simple Internet Protocol Plus (SIPP) -- One of 3 IPng candidates.

Simple Mail Transfer Protocol (SMTP) -- Protocol governing mail transmissions. It is defined in RFC 821, with associated message format descriptions in RFC 822.

Simple Management Protocol (SMP) -- Newer and more robust than SNMP.

Simple Network Management Protocol (SNMP) -- The Internet network management protocol. SNMP provides a means to monitor and set network configuration and runtime parameters.

SMDS Interface Protocol (SIP) -- Formal name given to each layer of the SMDS Network Interface. SIP1 is the physical layer, SIP2 is the cell layer including DQDB, and SIP3 is DXI.

Socket -- A paring of an IP address and a port number.

Source Routing Transparent (SRT) Bridge -- Proposed IEEE 802.1 bridge to combine source routing (in which the source end system provides routing information) with transparent bridging.

Source Service Access Point (SSAP) -- Field in LLC frame header to identify the sending session within a physical station.

Spanning Tree -- An algorithm, the original version of which was invented by Digital Equipment Corporation, which allows redundant bridges to be used for network resilience, without the broadcast storms associated with looping. If a bridge fails, a new path to a redundant bridge is opened.

Spoofing -- A method of fooling access equipment into thinking a network connection is active even when it's not.

SubNetwork Access Protocol (SNAP) -- Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system and specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks. The SNAP entity in the end system makes use of the services of the subnetwork and performs three key functions: data transfer, connection management, and quality of service selection.

SubNetwork Protocol (SNP) -- Protocol residing in the subnetwork layer below IP that provides data transfer through the local subnet. In some systems, an adapter module must be inserted between IP and the Subnetwork Protocol to reconcile dissimilar interfaces.

Superframe (D4) -- D4 format employs a "Superframe" consisting of 12 frames of 193 bits each (2316 bits total). In each frame, one framing bit is followed by 24 8-bit bytes. The pattern of framing bits is repeated every Superframe. Framing requires 8 Kbps, leaving 1.536 Mbps for user information.

Sustainable Cell Rate (SCR) -- Maximum throughput bursty traffic can achieve within a given virtual circuit without cell loss.

Switched 56 -- Switched data transmission service at 56 Kbps (as opposed to service on dedicated leased lines).

Switched Ethernet -- Configuration supporting an Ethernet hub with integrated MAC layer bridging or switching capability to provide each port with 10 or
100 Mbps of bandwidth. Separate transmissions can occur simultaneously on each port of the switching hub, and the switch filters traffic based on the destination MAC address.

Switched Multimegabit Data Service (SMDS) -- High-speed, packet switched, connectionless LAN service.

Switched Virtual Circuit (SVC) -- A virtual link, with variable end-points, established through an ATM network. With an SVC, the user defines the end-points when the call is initiated and they are subsequently terminated at the end of the call. With a PVC, the end-points are predefined by the network manager. A single virtual path may support multiple SVCs.

Synchronous Data Link Control (SDLC) -- A half-duplex IBM protocol based on HDLC; spoofed by FRADs to double data throughput on access link.

Synchronous Digital Hierarchy (SDH) -- ITU-TSS international standard for transmission over optical fiber.

Synchronous Optical Network (SONET) -- A set of standards for transmitting digital information over optical networks. "Synchronous" indicates that all pieces of the SONET signal can be tied to a single clock.

Synchronous Time-Division Multiplexing -- A method of TDM in which time slots on a shared transmission line are assigned to devices on a fixed, predetermined basis.

Synchronous Transport Signal 1 (STS-1) -- A SONET frame including overhead and payload capacity. The basic SONET frame is the STS-1. STS-1s can be multiplexed or concatenated with no additional overhead.

T1 -- Digital transmission facility operating with a nominal bandwidth of 1.544 Mbps. Also known as Digital Signal Level 1.

T3 -- Digital transmission facility operating at 45 Mbps bandwidth. Composed of 28 DS-1 channels in many cases. Also known as DS-3.

Tariff -- A statement by a communications company submitted for approval to the Federal Communications Commission that sets forth the services offered and the rates, terms, and conditions for the use of those services.

Thin Client -- A 'thin storage' client in a network application environment. The client downloads the program (java applets, for example) from the server and performs processing just like a PC, but does not store applications or data locally. All programs and data are on the server, minimizing management costs on the client side.

Time Division Multiplexing (TDM) -- Technique where information from multiple channels may be allocated bandwidth on a single wire based on time
slot assignment.

Time to Live (TTL) -- A field in the IP header which indicates how long this packet should be allowed to survive before being discarded. It is primarily used as a hop count.

Token Ring -- As defined in IEEE 802.5, a communications method that uses a token to control access to the LAN. The difference between a token bus and a token ring is that with a token ring, the LAN does not use a master controller to control the token. Instead, each computer knows the address of the computer that should receive the token next. When a computer with the token has nothing to transmit, it passes the token to the next computer in line.

Transmission Control Protocol/Internet Protocol (TCP/IP) -- The common name for the suite of protocols developed by the U.S. Department of Defense in the 1970s to support the construction of world-wide internetworks. TCP and IP are the two best-known protocols in the suite. TCP corresponds to Layer 4 (the transport layer) of the OSI reference model. It provides reliable transmission of data. IP corresponds to layer 3 (the network layer) of the OSI reference model and provides connectionless datagram service.

Trivial File Transfer Protocol (TFTP) -- A simplified version of FTP allowing the transfer of files from one computer to another over a network.

Tunneling -- Tunneling refers to encapsulation of protocol A within protocol B, such that A treats B as though it were a datalink layer. Tunneling is used to get data between administrative domains which use a protocol that is not supported by the internet connecting those domains.

Twisted Pair (TP) -- Cable consisting of two 18 to 24 AWG (American Wire Gauge) solid copper strands twisted around each other. The twisting provides a measure of protection from electromagnetic and radio-frequency interference.

Undefined Bit Rate (UBR) -- Traffic class defined by the ATM Forum.

Unicast Address -- An address specifying a single network device.

Uniform Service Order Code -- A three or five character alphabetic, numeric, or an alphanumeric code that identifies a specific item of service or equipment. Uniform Service Order Codes are used in the Telephone Company billing system to generate recurring rates and nonrecurring charges.

Unshielded Twisted Pair (UTP) -- Four-pair wire medium used in the transmission of many different protocols such as Ethernet, 10BaseT, and CDDI.

Unspecified Bit Rate (UBR) -- An ATM service type in which the ATM network makes a "best effort" to meet the transmitter's bandwidth requirements; essentially a "send and pray" service like that available from today's networks.

User Datagram Protocol (UDP) -- A connectionless transport-layer protocol belonging to the Internet protocol family.

User Network Interface (UNI) -- The protocol to define connections between ATM end-stations and the ATM switch including signaling, cell structure, addressing, traffic management and adaptation layers.

User Parameter Control (UPC) -- Traffic policing to ensure that the defined peak traffic rate is not exceeded in the ATM switch.

Value Added Network (VAN) -- A national (or international) enhanced network that is designed expressly to carry data communications. VANs also provide billing and other special services to their customers.

Variable Bit Rate (VBR) -- Applications which produce traffic of varying bit rates, like common LAN applications, which produce varying throughput rates.

Variable Bit Rate/non-real time (VBR/nrt) -- One of five ATM Forum-defined service types. Supports variable bit rate traffic with average and peak traffic parameters which can tolerate variable but predictable transit delays.

Variable Bit Rate/real time (VBR/rt) -- One of five ATM Forum-defined service types. Supports variable bit rate traffic which requires strict timing control, such as packetized voice or video, with average and peak traffic parameters.

Very Small Aperture Terminal (VSAT) -- Satellite dish under 1m.

Virtual Channel -- A defined route between two end nodes that may access multiple virtual paths.

Virtual Channel Connection (VCC) -- Virtual channels in two or more sequential physical circuits can be concatenated to create an end-to-end connection, called a VCC. A VCC is a specific instance of a SVC or PVC. A VCC may traverse one end-to-end VPC or several sequential VPCs.

Virtual Channel Identifier (VCI) -- The 16-bit number in an ATM cell header identifying the specific virtual channel on which the cell is traversing on the current physical circuit.

Virtual Circuit (VC) -- (1)A portion of a virtual path or a virtual channel used to establish a virtual connection between two end nodes. (2) Logical channels established as a result of the call initiation procedure to a network address that exists for a period of time.

Virtual IP -- A function provided on the Catalyst with the Virtual Network Services software which enables the creation of logically separated switched IP workgroups across Catalyst switch ports.

Virtual LAN -- Membership to a Virtual LAN is defined administratively independent of the physical network topology. A virtual LAN segment is a unique broadcast domain.

Virtual Path -- A group of virtual channels, which can support multiple virtual circuits.

Virtual Path Connection (VPC) -- Virtual paths in two or more sequential physical circuits can be concatenated to create a logical connection, called a VPC. VPCs must be pre-configured. All cells traversing VCs in a VPC are routed the same way.

Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI) -- Combined, these fields identify a connection in the ATM network.

Virtual Private Network (VPN) -- A network service offered by public carriers in which the customer is provided a network that in many ways appears as if it is a private network (customer-unique addressing, network management capabilities, dynamic reconfiguration, etc.) but which, in fact, is provided over the carrier's public network facilities.

Virtual Reality Modeling Language (VRML) -- VRML is a language much like HTML for getting data on the Web. The 3D nature of VRML is where the two specs depart.

VoiceLAN -- The synergy amid a group of technologies which together allow for the convergence of voice, computing, and other types of communications to coexist on a Local Area Network.

Wavelength Division Multiplexing (WDM) -- Two or more colors of light on one fibre.

Wide Area Network (WAN) -- A network that encompasses interconnectivity between devices over a wide geographic area. Such networks require public rights-of-way and operate over long distances.

Workgroup -- A group of workstations and servers that commonly exchange data. This term is also used to describe a group of people who work together.

X.400 -- International standard for a store-and-forward message handling system in a multivendor environment.

X.500 -- The CCITT and ISO standard for electronic directory services.

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