A AAL (ATM Adaptation Layer) - A collection of standardized protocols that adapt user traffic to the cell format. The AAL is subdivided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) sublayer. There are several types of AALs - AAL1, AAL2, AAL3/4 and AAL5 - to support the various AAL service classes. ABR (Available Bit Rate) - One of five ATM Forum-defined service categories. In this service type, the network makes the best effort to pass the maximum number of cells but does not guarantee cell delivery. Supports variable bit rate data traffic with flow control, a minimum guaranteed data transmission rate and specified performance parameters. In exchange for regulating user traffic flow, the network offers minimal cell loss of accepted traffic. Traffic parameters are PCR and MCR. QoS parameters are CLR and CER. Address - A coded representation of the origin or destination of data. ADPCM (Adaptive Differential Pulse Code Modulation) - An ITU standard technique for encoding analog voice signals into a digital form at 32 kbps (half the standard PCM rate). Agent - In SNMP, the word agent refers to the managed system. AMI (Alternate Mark Inversion) - A bipolar coding scheme in which successive ones (marks) must alternate in polarity. Analog - A continuous wave or signal (such as human voice). Analog Loopback - A testing technique that isolates faults in transmission equipment by performing a loopback on the data at the analog (line) side of the modem. Analog Transmission - The transmission of a continuously variable signal, as opposed to a discrete (digital) one. ANSI - American National Standards Institute. AR (Access Rate) - The data rate of the user access channel in a Frame Relay network. The speed of the access channel determines how fast (maximum rate) the end user can inject data into a Frame Relay network. ARQ (Automatic Request for Repeat or Retransmission) - A communications feature where the receiver asks the transmitter to resend a block or frame because errors were detected by the receiver. ASCII (American Standard Code for Information Interchange) - A seven-level code (128 possible characters) used for data transfer. Asynchronous Transmission - A transmission method that sends units of data one character at a time. Characters are preceded by start bits and followed by stop bits, which provide synchronization at the receive terminal. Also called start-stop transmission. ATM - Asynchronous Transfer Mode A standard (ITU) implementation of cell relay, which is a packet switching technique using packets (cells) of a fixed length. It is asynchronous in the sense that the recurrence of cells containing information from an individual user is not periodic. ATM-ARP (ATM Address Resolution Protocol) - An address resolution protocol for mapping ATM and IP addresses (each host is assigned a unique IP address). It can be used for discovering LAN hosts attached to an ATM network or in classical IP over ATM. ATM Forum - A worldwide organization that promotes and sets standards for ATM networks and equipment. Attenuation - Signal power loss through equipment, lines or other transmission devices. Measured in decibels. AWG - The American Wire Gauge System, which specifies wire width.

B Balanced - A transmission line in which voltages on the two conductors are equal in magnitude, but opposite in polarity, with respect to ground. Bandwidth - The range of frequencies passing through a given circuit. The greater the bandwidth, the more information can be sent through the circuit in a given amount of time. Baseband - Refers to transmission of a digital or analog signal at its original frequency, as an unmodulated signal. Baud - Unit of signaling speed equivalent to the number of discrete conditions or events per second. If each signal event represents only one bit condition, baud rate equals bps (bits per second). Bc (Committed Burst Size) - the maximum amount of data (in bits) that the network agrees to transfer during a time interval Tc. Be (Excess Burst Size) - The maximum amount of uncommitted data (in bits) in excess of Bc that a Frame Relay network can try to deliver during time interval Tc. The network treats Be data as discard-eligible. BECN (Backward Explicit Congestion Notification) - A bit set by a Frame Relay network to notify an interface device (DTE) that congestion avoidance procedures should be initiated by the sending device. BERT (Bit Error Rate Test/Tester) - A device used to test the bit error rate of a communications circuit (i.e. the ratio of received erroneous bits to received bits, usually a number referenced to a power of 10). Bipolar - A signaling method (in T1/E1 ) represents a binary "1" by alternating positive and negative pulses, and a binary "0" by absence of pulses. BISDN - Broadband Integrated Services Digital Network. The next generation of ISDN, which is intended to carry digital data, voice, and video. ATM provides the switching fabric and SONET or SDHthe physical transport. Bit - The smallest unit of information in a binary system. Represents either a one or zero ("1" or "0"). Bit Interleaving/Multiplexing - A process used in time division multiplexing where individual bits from different lower speed channel sources are combined (one bit from one channel at a time) into one continuous higher speed bit stream. Bps (Bits Per Second) - A measure of data transmission rate in serial transmission. Bridge - A device interconnecting local area networks at the OSI Data Link Layer, filtering and forwarding frames according to media access control (MAC) addresses. Broadband - Wideband technology capable of supporting voice, video and data, possibly using multiple channels. Buffer - A storage device. Commonly used to compensate for differences in data rates or event timing when transmitting from one device to another. Also used to remove jitter. Bus - A transmission path or channel. A bus is typically an electrical connection with one or more conductors, where all attached devices receive all transmissions at the same time. Byte - A group of bits (normally 8 bits in length).

C CAS (Channel Associated Signaling) - Voice signaling based on bits taken from voice timeslots, used by many PABXs. CBR (Constant Bit Rate) - One of the five ATM classes of service. CBR supports the transmission of a continuous bit-stream of information, such as voice and video traffic, which require a constant amount of bandwidth allocated to a connection for the duration of the transmission. CC (Continuity Cell) - A cell used periodically to check whether a connection is idle or has failed. Continuity checking is one of the OAM function types for fault management. CCR (Current Cell Rate) - The currently acceptable transmission rate for an end-system as defined by RM cells within ABR. The field in the RM cell indicates the current complying cell rate (i.e., ACR) a user can transmit over a virtual connection (VC). Carrier - A continuous signal at a fixed frequency that is capable of being modulated with a second (information carrying) signal. CD (Carrier Detect) - A modem interface signal indicating to an attached terminal that the local modem is receiving a signal from the remote modem. CDP (Conditional Di-Phase) - a digital encoding technique which is a variant of Manchester encoding, and is not sensitive to polarity of wires (wires in a pair can be crossed). CDV (Cell Delay Variation) - A QoS parameter that measures the difference between the transfer delay of a single cell (CTD) and the expected transfer delay. This parameter is important for time-sensitive virtual circuits such as CBR and VBR-RT. CDVT (Cell Delay Variation Tolerance) - Used in CBR traffic, it specifies the acceptable tolerance of the CDV (jitter). Central Office (CO) - A telephone company office which connects to all local loops in a given area and where circuit switching of customer lines occurs. CER (Cell Error Rate) - A QoS parameter that measures the number of transmitted cells that are erroneous over a specific period of time (i.e., those that contain errors when they arrive at the destination). CES (Circuit Emulation Service) - An ATM service in which constant bit rate (CBR) virtual circuits use AAL1 to emulate an end-to-end physical circuit by providing a TDM-like virtual circuit between local access circuits. Channel - A path for electrical transmission between two or more points. Also called a link, line, circuit or facility. Channel Bank - Equipment that connects multiple voice channels to high speed links by performing voice digitization and Time Division Multiplexing. Voice is converted to a 64 kbps signal (24 channels into 1.544 Mbps in countries offering T1 services, such as the U.S.A.; 30 channels into 2.048 Mbps in countries offering E1 or CEPT services, such as in Europe). Characteristic Impedance - The termination impedance of an electrically uniform transmission line. CIR (Committed Information Rate) - A term used in Frame Relay which defines the information rate the network is committed to provide the user. Circuit Emulation - In ATM, a connection over a virtual circuit-based network providing service to the end users that is indistinguishable from a real point-to point, fixed-bandwidth circuit. CLLM (Consolidated Link Layer Management) - A Frame Relay network control mechanism that uses out-of-band signaling to pass status information about the physical and logical connection over the user-to-network interface. Clock - A term for the source(s) of timing signals used in synchronous transmission. Committed Rate Measurement Interval - See Tc. Composite Link - The line or circuit carrying multiplexed data which connects a pair of multiplexers or concentrators. Also called aggregate or main link. Compression - Any of several techniques that reduce the number of bits required to represent information in data transmission or storage, thereby conserving bandwidth and/or memory. Concentrator - Device that serves as a wiring hub in star-topology network. Sometimes refers to a device containing multiple modules of network equipment. Congestion - A state in which the network is overloaded and starts to discard user data (frames, cells or packets). Congestion control - In ATM networks, congestion control schemes may be based on fields within the ATM cell header (CLP, EFCI within the PTI) or may be based on a more sophisticated mechanism between the ATM end-system and ATM switches. The ATM Forum has developed a mechanism based on rate control for ABR-type traffic. In Frame Relay networks, congestion is handled by the FECN, BECN and DE bits. Constant Bit Rate - See CBR Contention - A condition arising when two or more data stations attempt to transmit at the same time using the same link or channel. Control Characters - In communications, any extra transmitted characters used to control or facilitate data transmission (for example, characters associated with polling, framing, synchronization, error checking, or message delimiting). Control Signals - Signals passing between one part of a communications system and another (such as RTS, DTR, or DCD), as part of a mechanism for controlling the system. CRC (Cyclic Redundancy Check) - A data transmission error-detection scheme. A polynomial algorithm is performed on the data, and the resultant checksum is appended at the end of the frame.The receiving equipment performs a similar algorithm. Crosstalk - Unwanted transfer of energy from one circuit to another. Crosstalk typically occurs between adjacent circuits. CRS (Cell Relay Service) - A bearer service offered by an ATM network to the end users that delivers ATM cells directly over the network. CSMA/CD - Carrier sense multiple access/collision detection. In this protocol, stations listen to the bus and only transmit when the bus is free. If a collision occurs, the packet is retransmitted after a random time-out. CSMA/CD is used in Ethernet. CSU (Channel Service Unit) - Equipment installed on customer premises at the interface to phone company lines to terminate a DDS or T1 circuit. CSUs provide network protection and diagnostic capabilities. CTD (Cell Transfer Delay) - A QoS parameter that measures the average time for a cell to be transferred from its source to its destination over a virtual connection (VC). It is the sum of any coding, decoding, segmentation, reassembly, propagation, processing and queuing delays. CTS (Clear To Send) - A modem interface control signal from the data communications equipment (DCE) indicating to the data terminal equipment (DTE) that it may begin data transmission. Current Loop - Method of data transmission. A mark (binary "1") is represented by current on the line, and a space (binary "0") is represented by the absence of current.

D DACS (Digital Access and Cross Connect System)- A time-slot switch which allows T1 or E1 lines to be remapped electronically at the DS-0 (64 kbps) level. Also called DCS or DXS. Data - Information represented, in digital form, including voice, text, facsimile and video. Data Link Layer - Layer 2 of the OSI model. The entity which establishes, maintains, and releases data-link connections between elements in a network. Layer 2 is concerned with the transmission of units of information, or frames, and associated error-checking. dB (Decibel) - Unit for measuring relative strength (ratio) of two signals. dBm - A measure of power in communications: the decibel in reference to one milliwatt (0 dBm = 1 milliwatt and -30 dBm = .001 milliwatt) DCD (Data and Carrier Detect) - see CD. DCE (Data Communications Equipment) - The equipment providing functions which establish, maintain and terminate a data transmission connection (such as a modem). DDS (Digital Data Service) - A trademark of AT&T, identifying a private line service for digital data communications in the data rate range between 2400 and 56,000 bps. Commonly used in countries other than the U.S.A. at 64 or 128 kbps or higher. DE (Discard Eligibility) - A bit indicating that a frame may be discarded in case of congestion on a Frame Relay network, in order to help maintain the committed information rate (CIR). Diagnostics - Procedures and systems which detect and isolate a malfunction or mistake in a communications device, network, or system. Digital - The binary ("I/0") output of a computer or terminal. In data communication, an alternating, non-continuous (pulsating) signal. Digital Loopback - A technique for testing the digital processing of a communications device. The loopback is toward the line side of a modem, but tests most of the circuitry in the modem under test. Distortion - The unwanted change in a signal's waveform occurring between two points in a transmission system. DLCI (Data Link Connection Identifier) - A unique number assigned to a PVC endpoint in a Frame Relay network. DOV (Data Over Voice) - A technology for transmitting data and voice simultaneously over twisted-pair copper wiring. DS-3 - Digital Signal level 3. Term used to refer to the 45 Mbps digital signal carried on a T3 facility. DSU (Digital Service Unit) - A user device interfacing to a digital circuit (such as DDS or T1 when combined with a CSU). The DSU converts the user's data stream to bipolar format for transmission. DTE (Data Terminal Equipment) - A device transmitting data to, and/or receiving data from, a DCE (for example, a terminal or printer). DTR (Data Terminal Ready) - A modem interface control signal sent from the DTE to the modem, usually telling the modem that the DTE is ready to transmit data. DXI - Data Exchange Interface. Protocols used between routers and DSUs for SMDS and ATM.

E E & M Signaling - Voice transmission system that uses separate paths for signaling and voice. The "M" lead (mouth) transmits signals to the remote end of the circuit while the "E" lead (ear) receives incoming signals. E1 - The 2.048 Mbps digital carrier system common in Europe. E3 - The European standard for high-speed digital transmission, operating at 34 Mbps Echo Cancellation - A technique used in high speed modems and voice ciruits to isolate and filter out unwanted signal energy caused by echoes from the main transmitted signal. Echo-Signal - Distortion occurring when a transmitted signal is echoed back (reflected) to the originating station. EIA (Electronic Industries Association) - A standards organization in the U.S. specializing in the electrical and functional characteristics of interface equipment. EMI (Electromagnetic Interference) - Radiation leakage outside a transmission medium resulting mainly from the use of high frequency wave energy and signal modulation. EMI can be reduced by appropriate shielding. Equalizer - A device that compensates for distortion due to signal attenuation and propagation time with respect to frequency. It reduces the effects of amplitude, frequency and/or phase distortion. ESF (Extended Superframe Format) - A T1 framing format that uses the framing bit to provide maintenance and diagnostic functions. Ethernet - A local area network standardized as IEEE 802.3. Ethernet uses 10 Mbps transmission and the CSMA/CD access method. Excess Burst Size - See Be

F FCC (Federal Communications Commission) - The regulatory agency established in the United States for all interstate radio and electronic communications. FDDI (Fiber Distributed Data Interface) - An ANSI standard for fiber optic links with data rates up to 100 Mbps. FECN (Forward Explicit Congestion Notification) - A bit set by a Frame Relay network to notify an interface (DTE) that congestion avoidance procedures should be initiated by the receiving device. FEP (Front End Processor) - A communications device in the IBM/SNA environment responsible for communication between the mainframe and cluster controllers. Fiber Optics - Thin filaments of glass or plastic carrying a transmitted light beam (generated by an LED or laser). Four-Wire Circuit - A communications path consisting of two pairs of conductors (wires), one pair for transmitting and one pair for receiving. Fractional T1 - A service provided by carriers, where a full T1 link is leased to the customer, but the service charge is calculated based only on the number of timeslots used. FRAD (Frame Relay Access Device) A device responsible for framing data with header and trailer information (control information) before presenting the frame to the Frame Relay switch. Frame - A logical grouping of information sent as a link-layer unit over a transmission medium. The terms packet, datagram, segment, and message are also used to describe logical information groupings. Frame Relay - A network interface providing high speed frame or packet transmission with minimum delay and efficient line of bandwidth. It has less protocol overhead than X.25. Frame Relay Forum - A worldwide organization that promotes and sets standards for Frame Relay networks and equipment. Frame Relay Frame - A variable-length unit of data in Frame Relay format that is transmitted as pure data through a Frame Relay network. Full Duplex - A circuit or device permitting transmission in two directions at the same time. FUNI (Frame User Network Interface) - Frame-based interface to ATM supporting signaling and QoS. To interoperate with a Frame Relay end system, the ATM switch should support FRF.8, which is the Frame Relay/ATM Service Internetworking specification. Replaces the ATM-DXI. FXO (Foreign Exchange Office) - A voice interface, emulating a PABX extension, as it appears to the C.O. (Central Office) for connecting a PABX extension to a multiplexer. FXS (Foreign Exchange Subscriber) - A voice interface, emulating the extension interface of a PABX (or subscriber interface of a C.O.) for connecting a regular telephone set to a multiplexer.

G G.703 - An ITU standard for the physical and electrical characteristics of various digital interfaces, including those at 64 kbps and 2.048 Mbps. G.703.1 - An ITU standard for voice compression

H Half Duplex - A circuit or device capable of transmitting in two directions, but not at the same time. HDLC (High-level Data Link Control) - An international standard communication protocol defined by ISO. HDSL (High Bit-Rate Digital Subscriber Line) - A high-performance twisted pair transmission technology, best known as an enhanced transport mechanism for T1 or E1 service. It is designed for the local loop between a customer's premises and an area exchange central office.

I IEEE (Institute of Electrical and Electronic Engineers) - An international professional society issuing its own standards. The IEEE is a member of ANSI and ISO. IEEE 802.3 - The IEEE's specification for CSMA/CD LANs. IEEE 802.5 - The IEEE's specification for Token Ring LANs IMA (Inverse Multiplexing over ATM) - A method to pass ATM traffic over multiple E1/T1 links while maintaining the ATM quality of service and optimizing bandwidth usage. Impedance - The combined effect of resistance, inductance and capacitance on a transmitted signal. Impedance varies at different frequencies. In-Band Signaling - Signaling utilizing frequencies within the information band of a channel. Interface - A shared boundary, defined by common physical interconnection characteristics, signal characteristics, and meanings of exchanged signals. Internet Address - Also known as an IP address. This is a 32-bit hardware-independent address assigned to hosts using the TCP/IP protocol suite. IP - Internet Protocol. The network layer protocol of the TCP/IP (Internet) Protocol Suite. I-PNNI (Integrated Private Network-Network Interface) - Protocol used to exchange reachability information between routers that augment or replace protocols such as OSPF and IPX and is compatible with PNNI. This enables the integration of existing router-based connectionless networks with ATM networks. ISDN (Integrated Services Digital Network) - A carrier provided service that allows a variety of switched digital data and voice transmission to be accommodated simultaneously. ISO (International Standards Organization) - An international organization involved in writing communications standards. ITU (International Telecommunication Union) - A European-based, international advisory committee recommending worldwide standards for transmission.

J Jitter - The deviation of a transmission signal in time or phase. It can introduce errors and loss of synchronization in high speed synchronous communications.

K

L LAN (Local Area Network) - A high volume data transmission facility connecting a number of communicating devices (computers, terminals and printers) within a single room, building, campus or other limited geographical area. Leased Line - A line reserved for the exclusive use of a customer, without inter-exchange switching arrangements. LMI (Local Management Interface) - Frame Relay network management mechanism that uses DLCIs 0 and 1023 to pass management messages over the user-to-network interface (UNI). Line Driver - A signal converter which conditions a digital signal to ensure reliable transmission over an extended distance. Loading - The addition of inductance to a line in order to minimize amplitude distortion. Used commonly on public telephone lines to improve voice quality, it can make the lines impassable to high speed data, and baseband modems. Loopback - A type of diagnostic test in which the transmitted signal is returned to the sending device after passing through all or part of a communications link or network.

M MAC - Media Access Control. A protocol that defines the way workstations gain access to transmission media, most widely used in reference to LANs. For IEEE LANs, the MAC layer is the lower sublayer of the data link layer protocol. Mark - In telecommunications, this means the presence of a signal. A mark is equivalent to a binary 1. A mark is the opposite of a space ( 0 ). Master Clock - The source of timing signals (or the signals themselves) that all network stations use for synchronization. MBS (Maximum Burst Size) - A traffic parameter that specifies the maximum number of ATM cells in a burst that can be transmitted at the peak rate (PCR). MCR (Maximum Cell Rate) - An ABR traffic parameter (in cells per second) that gives the slowest rate at which the network controls the flow of the source on an ABR virtual connection (VC). MIB - Management Information Base. A collection of objects that can be accessed via a network management protocol such as SNMP. The objects represent values that can be read or changed. Modem (Modulator-Demodulator) - A device used to convert serial digital data from a transmitting DTE to a signal suitable for transmission over extended distances. It also reconverts the transmitted signal to serial digital data for acceptance by a receiving DTE. Modem Eliminator - A device used to connect a local terminal and a computer port. The modem eliminator replaces the pair of modems ordinarily required. Modulation - The alteration of a carrier wave in relation to the value or samples of the data being transferred. MPMLQ (Multipulse Maximum Likelihood Quantization) - A speech compression technology which is the ITU compression standard G.723.1. It is very effective at low bit rates, maintaining a minimal level of distortion. MPOA (Multi-Protocol Over ATM) - IETF-defined specifications and procedures that enable Network Layer protocols to operate directly on top of ATM and provide end-to-end internetworking between hosts in an ATM and non-ATM environment. Multidrop - A communications configuration in which multiple devices share a common transmission facility (or multipoint line), although generally only one may transmit at a time. Usually used with some kind of polling mechanism to address each connected terminal with a unique address code. Multiplexer (Mux) - A device allowing two or more signals to pass over and share a common transmission path simultaneously. Multipoint Line - see Multidrop.

N NDIS - A standard interface specification for PC network adapter cards developed by Microsoft to separate the communications protocol from the PC networking hardware. The driver is able to run multiple protocol stacks concurrently. Network - (1) An interconnected group of nodes. (2) A series of points, nodes, or stations connected by communications channels; the collection of equipment through which connections are made between data stations. Network Management System - Systems equipment used in monitoring, controlling, and managing a data communications network. NNI (Network Node Interface or Network-to-Network Interface) - ITU-T standard interface between nodes within the same network. The ATM forum distinguishes between two standards, one for private networks called PNNI and one for public networks known as public NNI. Node - A point of interconnection to a network. NRZ (Non-Return to Zero) - A binary encoding scheme representing ones and zeroes by opposite and alternating high and low voltages, in which there is no return to a zero (reference) voltage between encoded bits. NRZI (Non-Return to Zero Inverted) - A binary encoding scheme which inverts the signal 1 and leaves the signal unchanged or a 0. Also called transition coding.

O OAM - Operation Administration and Maintenance Management framework defined by the ITU. OAM cells are special-purpose ATM cells exchanged between two ATM entities for network fault and performance management, analysis and fault isolation. ODI - Open Data Link Interface, a standard interface specification developed by Novell to enable PC adapter cards to run multiple protocol stacks. OSI (Open Systems Interconnection) Model - A seven-layer model of network communications developed by the International Standard Organization (ISO).

P Packet - An ordered group of data and control signals transmitted through a network, as a subset of a larger message. Packet Switching - A data transmission technique, which divides user information into discrete data envelopes called packets, and sends the information packet by packet. Parity Bit - An additional non-information bit added to a group of bits to ensure that the total number of l bits in the character is even or odd. Payload - The 48-byte segment of the ATM cell containing user data. Any adaptation of user data via the AAL will take place within the payload. PBX (Private Branch Exchange) - A private telephone exchange. PCM (Pulse Code Modulation) - A procedure for adapting an analog signal (such as voice) into a 64 kbps digital bit stream for transmission. PCR (Peak Cell Rate) - An ATM traffic parameter (in cells per second) that characterizes the source and gives the maximum rate at which cells can be transmitted. It is calculated as the reciprocal of the minimum intercell interval (the time between two cells) over a given virtual connection (VC). PHY (Physical layer) - The bottom layer of the ATM protocol reference model, it is subdivided into two sublayers: Transmission Convergence (TC) and Physical Medium (PM). It provides ATM cell transmission over the physical interfaces that interconnect the ATM devices. Physical Layer - Layer 1 of the OSI model. The layer concerned with electrical, mechanical, and handshaking procedures over the interface connecting a device to the transmission medium. PNNI (Private Network-Network Interface) - The interswitch interface within a private ATM domain. The PNNI trunking protocol providing for hierarchical ATM-layer routing and QoS support. Policing - A method for verifying that the incoming VC complies with the user's service contract. Polling - see Multidrop. Port - The physical interface to a computer, multiplexer, for connection of terminals and modems. Protocol - A formal set of conventions governing the formatting and relative timing of message exchange between two communicating systems. PSTN (Public Switched Telephone Network) - The telecommunications network commonly accessed by ordinary telephones, key systems, PBX trunks and data equipment. PVC (Permanent Virtual Connection) - A permanent, virtual connection established by the network management between an origin and a destination. PVP (Permanent Virtual Path) - A set of permanent virtual channels (PVCs) that exist between two cross points.

Q Quality of Service (QoS) - Backup In ATM, for the set of parameters and their value which determine the performance of a given virtual circuit.

R Redundancy/Redundant Card or Power - Backup components used to ensure uninterrupted operation of a system in case of a failure. Repeater - A device which automatically amplifies, restores, or reshapes signals to compensate for distortion and/or attenuation prior to retransmission. RFC 1483 - Specifies the encapsulation of multiprotocol data for transmission over an ATM network. RFC 1483 make use of AAL5 in the support of PVCs and SVCs. The two methods defined in this RFC are VC muxing and LLC/SNAP encapsulation. RFC 1490 - Specifies the encapsulation of multiprotocol data for transmission over Frame Relay. Frame Relay over ATM uses this encapsulation in combination with AAL5. RMON - The Remote Monitoring MIB, which allows a network monitoring device to be configured and read from remote locations. Routing - The process of selecting the most efficient circuit path for a message. RSVP (Resource reSerVation Protocol) - A protocol developed for supporting different QoS classes for IP applications. RTS (Request To Send) - A modem control signal sent from the DTE to the modem, which tells the modem that the DTE has data to send. RTT (Round Trip Time) - The round trip time it takes for a packet to travel between a source and a network device. In ATM, RTT is usually measured in numbers of cells.

S SAR (Segmentation And Reassembly) - Segments the information frames into cells at the source and reassembles these cells back into information frames at the destination. These activities occur at the lower part of the AAL. Each AAL type has its own SAR format. SCR (Sustainable Cell Rate) - A traffic parameter that characterizes a bursty source and specifies the maximum average rate at which cells can be sent over a given virtual connection (VC). It can be defined as the ratio of the MBS to the minimum burst interarrival time. SDH - Synchronous Data Hierarchy. The European standard for using optical media as the physical transport for high speed long-haul networks. SDLC - Synchronous Data Link Control. An IBM protocol for use in SNA environment. SDLC is a bit oriented protocol, similar to HDLC. Serial Transmission - A common mode of transmission, where the character bits are sent sequentially one at a time instead of in parallel. Sharing Device - A device that enables sharing of a single resource (modem, mux or computer port) among several devices (terminals, controllers or modems). Shielding - The protective enclosure surrounding a transmission medium, designed to minimize electromagnetic interference (EMI/RFI). Short Haul Modem - A modem designed for use in transmitting over relatively short distances across unloaded metallic circuits. Also called a line driver or limited distance modem (LDM). SMDS - Switched Multimegabit Data Service. A specification for a connectionless packet - switched data service. SNA - Systems Network Architecture. IBM's layered communications protocol architecture. SNMP - Simple Network Management Protocol now widely used. The network management protocol of the TCP/IP protocol suite. SONET - Synchronous Optical Network. A standard for using optical media as the physical transport for high-speed long-haul networks. SONET basic speeds start at 51.84 Mbps and go up to 2.5 Gbps. Space - In telecommunications, the absence of a signal. Equivalent to a binary 0. Statistical Multiplexer (STM or STDM) - A device connecting multiple channels to a single link by dynamically allocating timeslots to the channels based on their transmission activity. STP (Shielded Twisted Pairs) - General term for cabling systems that are designed specifically for data transmission, and where the cables are shielded. Sub-rate Multiplexing - In the U.S. this refers generally to time division multiplexing at data rates less than 64 kbps. SVC (Switched Virtual Circuit) - A logical connection between two points that is established dynamically and exists during transmission only. In ATM networks, the SVC connection is established via signaling. End systems transmit their UNI 3.1 or 4.0 signaling request via the Q.2931 signaling protocol. Synchronous Transmission - Transmission in which data bits are sent at a fixed rate, with the transmitter and receiver synchronized.

T Tc (Committed Rate Measurement Interval) - The time interval during which the user can send only Bc-committed amount of data and Be excess amount of data. Tc is used to measure only incoming traffic. The duration of the Tc is usually proportional to the burstiness of the traffic and is usually computed as Tc = Bc/CIR. T1 - An AT&T term for digital carrier facility used to transmit a DS1-formatted digital signal at 1.544 Mbps. A T1 frame has 24 timeslots or channels. TCP/IP - Transmission Control Protocol/Internet Protocol. Also known as the Internet Protocol Suite. This suite of protocols is used on the Internet and is widely used for heterogeneous internetworking. TDM (Time Division Multiplexer) - A device which divides the time available on its composite link among its channels, usually interleaving bits (Bit TDM) or characters (Character TDM) of data from each terminal. Throughput - The amount of information transferred through the network between two users in a given period, usually measured in the number of packets per second (pps). Timeslot - A portion of a serial multiplex of timeslot information dedicated to a single channel. In T1 and E1, one timeslot typically represents one 64 kbps channel. TM (Traffic Management) - Set of actions and operations performed by the network to guarantee the operability of the network. TM is exercised in the form of traffic control and flow control. The ATM traffic management includes the following: CAC, FRM, NRM, Priority Control (PC), Traffic Shaping (TS) and UPC , the goal of which is to maintain the required QoS. Token Ring - A local area network standardized as IEEE 802.5. A supervisory frame, or token, is passed from station to adjacent station sequentially. Stations wishing to gain access to the network must wait for the token to arrive before transmitting data. Traffic policing - Mechanism whereby any traffic which violates the traffic contract agreed to at connection setup is detected and discarded. Traffic Shaping - A method of smoothing bursty traffic in order to present a more uniform traffic rate to the network. Trunk - A single circuit between two points, both of which are switching centers or individual distribution points. A trunk usually handles many channels simultaneously.

U Unbalanced Line - A transmission line in which a single conductor is used to transmit a signal, in reference to ground (for example, in a coaxial cable). Undefined Bit Rate (UBR) - An economical class of on a moment-to-moment basis, without guaranteeing service levels UNI (User Network Interface) - The interface, defined as a set of protocols and traffic characteristics, between the CPE and the ATM network. UNI 4.0 - This UNI specification refers to signaling issues in ABR, VP and QoS negotiations. UTP (Unshielded Twisted Pair) - General term for all cabling systems used for transmission of data which are not shielded.

V VBR-NRT (Variable Bit Rate Non Real Time) - One of the two VBR service types for transmitting traffic where timing information is not critical. Since this service type is delay-tolerant, it is well suited for bursty traffic such as data communications. VBR-RT (Variable Bit Rate Real Time) - One of the two VBR service types for transmitting traffic that depends on timing and control information. It is suitable for carrying delay-sensitive traffic such as packetized video and audio. VC (Virtual Channel) - A connection established between end users, where packets are forwarded along the same path and bandwidth is not permanently allocated until it is used. VCC (Virtual Channel Connection) - An end to end connection consisting of a concatenation of two or more Virtual Channels between two end points. VCCs may be bundled into a VPC. Voice Compression - The conversion of an analog voice signal into a digital signal using minimum bandwidth (16 kbps or less). Voice Digitization (Voice Encoding) - The conversion of an analog voice signal into digital symbols for storage or for transmission (examples: ADPCM, CVSD, or PCM). VP (Virtual Path) - A term to describe a set of Virtual Channels (VCs) between cross points, grouped together. VPC (Virtual Path Connection) - An end-to-end connection consisting of two or more Virtual Path (VP) links.

W WWW (World Wide Web) - The internet-based global information system based on a combination of information retrieval and hypertext techniques.

X X-ON/X-OFF (Transmitter On/Transmitter Off) - Control characters used for flow control, instructing a terminal to start transmission (X-ON) and end transmission (X-OFF).

Y

Z