ATM Glossary

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A

AAL: ATM Adapation Layer: The standards layer that allows multiple applications 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.
AAL Connection: Association established by the AAL between two or more next higher layer entities.
AAL-1: ATM Adaptation Layer Type 1: AAL functions in support of constant bit rate, time-dependent traffic such as voice and video.
AAL-2: ATM Adaptation Layer Type 2: This AAL is still undefined by the International Standards bodies. It is a placeholder for variable bit rate video transmission.
AAL-3/4: ATM Adaptation Layer Type 3/4: AAL functions in support of variable bit rate, delay-tolerant data traffic requiring some sequencing and/or error detection support. Originally two AAL types, i.e. connection-oriented and connectionless, which have been combined.
AAL-5: ATM Adaptation Layer Type 5: AAL functions in support of variable bit rate, delay-tolerant connection-oriented data traffic requiring minimal sequencing or error detection support.
ABR: Available Bit Rate: ABR is an ATM layer service category for which the limiting ATM layer transfer characteristics provided by the network may change subsequent to connection establishment. A flow control mechanism is specified which supports several types of feedback to control the source rate in response to changing ATM layer transfer characteristics. It is expected that an end-system that adapts its traffic in accordance with the feedback will experience a low cell loss ratio and obtain a fair share of the available bandwidth according to a network specific allocation policy. Cell delay variation is not controlled in this service, although admitted cells are not delayed unnecessarily.
ACM: Address Complete Message: A BISUP call control message from the receiving exchange to sending exchange indicating the completion of address information.
ACR: Attenuation to Crosstalk Ratio: One of the factors that limits the distance a signal may be sent through a given media. ACR is the ratio of the power of the received signal, attenuated by the media, over the power of the NEXT crosstalk from the local transmitter, usually expressed in decibels (db). To achieve a desired bit error rate, the received signal power must usually be several times larger than the NEXT power or plus several db. Increasing a marginal ACR may decrease the bit error rate.
ACR: Allowed Cell Rate: An ABR service parameter, ACR is the current rate in cells/sec at which a source is allowed to send.
Address Prefix: A string of 0 or more bits up to a maximum of 152 bits that is the lead portion of one or more ATM addresses.
Address Resolution: Address Resolution is the procedure by which a client associates a LAN destination with the ATM address of another client or the BUS.
Adjacency: The relationship between two communicating neighboring peer nodes.
Administrative Domain: A collection of managed entities grouped for administrative reasons.
ADPCM: Adaptive Differential Pulse Code Modulation: A reduced bit rate variant of PCM audio encoding (see also PCM). This algorithm encodes the difference between an actual audio sample amplitude and a predicted amplitude and adapts the resolution based on recent differential values.
ADTF: ACR Decrease Time Factor: This is the time permitted between sending RM-cells before the rate is decreased to ICR (Initial Cell Rate). The ADTF range is .01 to 10.23 sec. with granularity of 10 ms.
AFI: Authority and Format Identifier: This identifier is part of the network level address header.
Aggregation: Token A number assigned to an outside link by the border nodes at the ends of the outside link. The same number is associated with all uplinks and induced uplinks associated with the outside link. In the parent and all higher-level peer group, all uplinks with the same aggregation token are aggregated.
AHFG: ATM-attached Host Functional Group: The group of functions performed by an ATM-attached host that is participating in the MPOA service.
Ai: Signaling ID assigned by Exchange A.
AIM: ATM Inverse Multiplexer: A term discontinued because of conflict with an established product. Refer to AIMUX.
AIMUX: ATM Inverse Multiplexing: A device that allows multiple T1 or E1 communications facilities to be combined into a single broadband facility for the transmission of ATM cells.
AIR: Additive Increase Rate: An ABR service parameter, AIR controls the rate at which the cell transmission rate increases. It is signaled as AIRF, where AIRF = AIR*Nrm/PCR.
AIRF: Additive Increase Rate Factor: Refer to AIR.
AIS: Alarm Indication Signal: An all ones signal sent down or up stream by a device when it detects an error condition or receives an error condition or receives an error notification from another unit in the transmission path.
Alternate Routing: A mechanism that supports the use of a new path after an attempt to set up a connection along a previously selected path fails.
AMI: Alternate Mark Inversion: A line coding format used on T1 facilities that transmits ones by alternate positive and negative pulses.
Ancestor Node: A logical group node that has a direct parent relationship to a given node (i.e., it is the parent of that node, or the parent's parent, ...).
ANI: Automatic Number Identification: A charge number parameter that is normally included in the Initial Address Message to the succeeding carrier for billing purposes.
ANM: Answer Message: A BISUP call control message from the receiving exchange to the sending exchange indicating answer and that a through connection should be completed in both directions.
ANSI: American National Standards Institute: A U.S. standards body.
API: Application Program Interface: API is a programmatic interface used for interprogram communications or for interfacing between protocol layers.
API_connection: Native ATM Application Program Interface Connection: API_connection is a relationship between an API_endpoint and other ATM devices that has the following characteristics:
APPN: Advanced Peer to Peer Network: IBM network architecture for building dynamic routing across arbitrary network topologies. Intended as an eventual replacement for SNA, IBM's static routed, hierarchical network architecture.
ARE: All Routes Explorer: A specific frame initiated by a source which is sent on all possible routes in Source Route Bridging.
ARP: Address Resolution Protocol: The procedures and messages in a communications protocol which determines which physical network address (MAC) corresponds to the IP address in the packet.
ASP: Abstract Service Primitive: An implementation-independent description of an interaction between a service-user and a service-provider at a particular service boundary, as defined by Open Systems Interconnection (OSI).
Assigned Cell: Cell that provides a service to an upper layer entity or ATM Layer Management entity (ATMM-entity).
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.
ATM: Asynchronous Transfer Mode: A transfer mode in which the information is organized into cells. It is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic.
ATM Address: Defined in the UNI Specification as 3 formats, each having 20 bytes in length including country, area and end-system identifiers.
ATM Layer Link: A section of an ATM Layer connection between two adjacent active ATM Layer entities (ATM-entities).
ATM Link: A virtual path link (VPL) or a virtual channel link (VCL).
ATM Peer-to-Peer Connection: A virtual channel connection (VCC) or a virtual path connection (VPC).
ATM Traffic Descriptor: A generic list of traffic parameters that can be used to capture the intrinsic traffic characteristics of a requested ATM connection.
ATM User-User Connection: An association established by the ATM Layer to support communication between two or more ATM service users (i.e., between two or more next higher entities or between two or more ATM-entities). The communications over an ATM Layer connection may be either bidirectional or unidirectional. The same Virtual Channel Identifier (VCI) issued for both directions of a connection at an interface.
ATS: Abstract Test Suite: A set of abstract test cases for testing a particular protocol. An "executable" test suite may be derived from an abstract test suite.
Attenuation: The process of the reduction of the power of a signal as it passes through most media. Usually proportional to distance, attenuation is sometimes the factor that limits the distance a signal may be transmitted through a media before it can no longer be received.

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B

B-ICI: B-ISDN Inter-Carrier Interface: An ATM Forum defined specification for the interface between public ATM networks to support user services across multiple public carriers.
B-ICI SAAL: B-ICI Signaling ATM Adaptation Layer: 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: Broadband ISDN: A high-speed network standard (above 1.544 Mbps) that evolved Narrowband ISDN with existing and new services with voice, data and video in the same network.
B-LLI: Broadband Low Layer Information: This is a Q.2931 information element that identifies a layer 2 and a layer 3 protocol used by the application.
B-TE: Broadband Terminal Equipment: An equipment category for B-ISDN which includes terminal adapters and terminals.
BBC: Broadband Bearer Capability: A bearer class field that is part of the initial address message.
BCD: Binary Coded Decimal: A form of coding of each octet within a cell where each bit has one of two allowable states, 1 or 0.
BCOB: Broadband Connection Oriented Bearer: Information in the SETUP message that indicates the type of service requested by the calling user.
BCOB-A: Bearer Class 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).
BCOB-C: Bearer Class C: Indicated by ATM end user in SETUP message for connection-oriented, variable bit rate service. The network may perform internetworking based on AAL information element (IE).
BCOB-X: Bearer Class 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.
BECN: Backward Explicit Congestion Notification: A Resource Management (RM) cell type generated by the network or the destination, indicating congestion or approaching congestion for traffic flowing in the direction opposite that of the BECN cell.
BER: Bit Error Rate: A measure of transmission quality. It is generally shown as a negative exponent, (e.g., 10-7 which means 1 out of 107 bits are in error or 1 out of 10,000,000 bits are in error).
BHLI: Broadband High Layer Information: This is a Q.2931 information element that identifies an application (or session layer protocol of an application).
Bi: Signaling ID assigned by Exchange B.
BIP: Bit Interleaved Parity: A method used at the PHY layer to monitor the error performance of the link. A check bit or word is sent in the link overhead covering the previous block or frame. Bit errors in the payload will be detected and may be reported as maintenance information.
BIS: Border Intermediate System.
BISUP: Broadband ISDN User's Part: A SS7 protocol which defines the signaling messages to control connections and services.
BN: Bridge Number: A locally administered bridge ID used in Source Route Bridging to uniquely identify a route between two LANs.
BN: BECN Cell: A Resource Management (RM) cell type indicator. A Backwards Explicit Congestion Notification (BECN) RM-cell may be generated by the network or the destination. To do so, BN=1 is set, to indicate the cell is not source-generated, and DIR=1 to indicate the backward flow. Source generated RM-cells are initialized with BN=0.
BOM: Beginning of Message: An indicator contained in the first cell of an ATM segmented packet.
Border Node: A logical node that is in a specified peer group, and has at least one link that crosses the peer group boundary.
BPDU: Bridge Protocol Data Unit: A message type used by bridges to exchange management and control information.
BPP: Bridge Port Pair (Source Routing Descriptor): Frame header information identifying a bridge/LAN pair of a Source route segment.
Broadband: A service or system requiring transmission channels capable of supporting rates greater than the Integrated Services Digital Network (ISDN) primary rate.
Broadband Access: An ISDN access capable of supporting one or more broadband services.
Broadcast: Data transmission to all addresses or functions.
BT: Burst Tolerance: BT applies to ATM connections supporting VBR services and is the limit parameter of the GCRA.
Btag: Beginning Tag: A one octet field of the CPCS_PDU used in conjunction with the Etag octet to form an association between the beginning of message and end of message.
BUS: Broadcast and Unknown Server: This server handles data sent by an LE Client to the broadcast MAC address ('FFFFFFFFFFFF'), all multicast traffic, and initial unicast frames which are sent by a LAN Emulation Client.
BW: Bandwidth: A numerical measurement of throughput of a system or network.

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C

CAC: Connection Admission Control: Connection Admission Control is defined as the set of actions taken by the network during the call set- up phase (or during call re-negotiation phase) in order to determine whether a connection request can be accepted or should be rejected (or whether a request for re-allocation can be accommodated).
Call: A call is an association between two or more users or between a user and a network entity that is established by the use of network capabilities. This association may have zero or more connections.
CAS: Channel Associated Signaling: A form of circuit state signaling in which the circuit state is indicated by one or more bits of signaling status sent repetitively and associated with that specific circuit.
CBDS: Connectionless Broadband Data Service: A connectionless service similar to Bellcore's SMDS defined by European Telecommunications Standards Institute (ETSI).
CBR: Constant Bit Rate: An ATM service category which supports a constant or guaranteed rate to transport services such as video or voice as well as circuit emulation which requires rigorous timing control and performance parameters.
CCR: Current Cell Rate: The Current Cell Rate is an RM-cell field set by the source to its current ACR when it generates a forward RM-cell. This field may be used to facilitate the calculation of ER, and may not be changed by network elements. CCR is formatted as a rate.
CCS: Common Channel Signaling: A form signaling in which a group of circuits share a signaling channel. Refer to SS7.
CD-ROM: Compact Disk-Read Only Memory: Used by a computer to store large amounts of data. Commonly used for interactive video games.
CDF: Cutoff Decrease Factor: CDF controls the decrease in ACR (Allowed Cell Rate) associated with CRM.
CDV: Cell Delay Variation: CDV is a component of cell transfer delay, induced by buffering and cell scheduling. Peak-to-peak CDV is a QoS delay parameter associated with CBR and VBR services. The peak-to- peak CDV is the ((1-a) quantile of the CTD) minus the fixed CTD that could be experienced by any delivered cell on a connection during the entire connection holding time. The parameter "a" is the probability of a cell arriving late. See CDVT.
CDVT: Cell Delay Variation Tolerance-ATM layer functions may alter the traffic characteristics 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 ran domness 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.
CE: Connection Endpoint: A terminator at one end of a layer connection within a SAP.
CEI: Connection Endpoint Identifier: Identifier of a CE that can be used to identify the connection at a SAP.
Cell: A unit of transmission in ATM. A fixed-size frame consisting of a 5-octet header and a 48-octet payload.
Cell Header: ATM Layer protocol control information.
Cells in Frames: Cells In Frames is a protocol established by the CIF Alliance which specifies how to transport ATM protocol over Ethernet, Token Ring and other frame protocols. CIF uses software at the workstation instead of a new hardware Network Interface Card to do QOS scheduling and ABR flow control.
CER: Cell Error Ratio: The ratio of errored cells in a transmission in relation to the total cells sent in a transmission. The measurement is taken over a time interval and is desirable to be measured on an in-service circuit.
CES: Circuit Emulation Service: The ATM Forum circuit emulation service interoperability specification specifies interoperability agreements for supporting Constant Bit Rate (CBR) traffic over ATM networks that comply with the other ATM Forum interoperability agreements. Specifically, this specification supports emulation of existing TDM circuits over ATM networks.
Child Node: A node at the next lower level of the hierarchy which is contained in the peer group represented by the logical group node currently referenced. This could be a logical group node, or a physical node.
Child Peer Group: A child peer group of a peer group is any one containing a child node of a logical group node in that peer group. A child peer group of a logical group node is the one containing the child node of that logical group node.
CI: Congestion Indicator: This is a field in a RM-cell, and is used to cause the source to decrease its ACR. The source sets CI=0 when it sends an RM-cell. Setting CI=1 is typically how destinations indicate that EFCI has been received on a previous data cell.
CIP: Carrier Identification Parameter: A 3 or 4 digit code in the initial address message identifying the carrier to be used for the connection.
CIR: Committed Information Rate: CIR is the information transfer rate which a network offering Frame Relay Services (FRS) is committed to transfer under normal conditions. The rate is averaged over a minimum increment of time.
CL: Connectionless Service: A service which allows the transfer of information among service subscribers without the need for end-to- end establishment procedures.
CLP: Cell Loss Priority: This bit in the ATM cell header indicates two levels of priority for ATM cells. CLP=0 cells are higher priority than CLP=1 cells. CLP=1 cells may be discarded during periods of congestion to preserve the CLR of CLP=0 cells.
CLR: Cell Loss Ratio: CLR is a negotiated QoS parameter and acceptable values are network specific. The objective is to minimize CLR provided the end-system adapts the traffic to the changing ATM layer transfer characteristics. The Cell Loss Ratio is defined for a connection as: Lost Cells/Total Transmitted Cells. The CLR parameter is the value of CLR that the network agrees to offer as an objective over the lifetime of the connection. It is expressed as an order of magnitude, having a range of 10-1 to 10-15 and unspecified.
CMIP: Common Management Interface Protocol: 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.
CMR: Cell Misinsertion Rate: 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.
CNR: Complex Node Representation: A collection of nodal state parameters that provide detailed state information associated with a logical node.
COD: Connection Oriented Data: Data requiring sequential delivery of its component PDUs to assure correct functioning of its supported application, (e.g., voice or video).
COM: Continuation of Message: An indicator used by the ATM Adaptation Layer to indicate that a particular ATM cell is a continuation of a higher layer information packet which has been segmented.
Common Peer Group: The lowest level peer group in which a set of nodes is represented. A node is represented in a peer group either directly or through one of its ancestors.
Communication endpoint: An object associated with a set of attributes which are specified at the communication creation time.
Configuration: The phase in which the LE Client discovers the LE Service.
Connection: An ATM connection consists of concatenation of ATM Layer links in order to provide an end-to-end information transfer capability to access points.
Connection: In switched virtual connection (SVC) environments the LAN Emulation Management entities set up connections between each other using UNI signaling.
Connectionless: Refers to ability of existing LANs to send data without previously establishing connections.
Control Connections: A Control VCC links the LEC to the LECS. Control VCCs also link the LEC to the LES and carry LE_ARP traffic and control frames. The control VCCs never carry data frames.
Corresponding Entities: Peer entities with a lower layer connection among them.
CPCS: Common Part Convergence Sublayer: The portion of the convergence sublayer of an AAL that remains the same regardless of the traffic type.
CPCS-SDU: Common Part Convergence Sublayer-Service Data Unit: Protocol data unit to be delivered to the receiving AAL layer by the destination CP convergence sublayer.
CPE: Customer Premises Equipment: End user equipment that resides on the customer's premise which may not be owned by the local exchange carrier.
CPN: Calling Party Number: A parameter of the initial address message that identifies the calling number and is sent to the destination carrier.
Crankback: A mechanism for partially releasing a connection setup in progress which has encountered a failure. This mechanism allows PNNI to perform alternate routing.
CRC: Cyclic Redundancy Check: A mathematical algorithm that computes a numerical value based on the bits in a block of data. This number is transmitted with the data and the receiver uses this information and the same algorithm to insure the accurate delivery of data by comparing the results of algorithm and the number received. If a mismatch occurs, an error in transmission is presumed.
CRF: Cell Relay Function: This is the basic function that an ATM network performs in order to provide a cell relay service to ATM end-stations.
CRF: Connection Related Function: A term used by Traffic Management
CRM: Missing RM-Cell Count: CRM limits the number of forward RM-cells which may be sent in the absence of received backward RM-cells.
CRM: Cell Rate Margin: This is a measure of the difference between the effective bandwidth allocation and the allocation for sustainable rate in cells per second.
CRS: Cell Relay Service: A carrier service which supports the receipt and transmission of ATM cells between end users in compliance with ATM standards and implementation specifications.
CS: Convergence Sublayer; The general procedures and functions that convert between ATM and non-ATM formats. This describes the functions of the upper half of the AAL layer. This is also used to describe the conversion functions between non-ATM protocols such as frame relay or SMDS and ATM protocols above the AAL layer.
CSU: Channel Service Unit: An interface for digital leased lines which
CT: Conformance Test: Testing to determine whether an implementation complies with the specifications of a standard and exhibits the behaviors mandated by that standard.
CTD: Cell Transfer Delay: This is defined as the elapsed time between a cell exit event at the measurement point 1 (e.g., at the source UNI) and the corresponding cell entry event at measurement point 2 (e.g., the destination UNI) for a particular connection. The cell transfer delay between two measurement points is the sum of the total inter-ATM node transmission delay and the total ATM node processing delay.

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DA: Destination Address: Information sent in the forward direction indicating the address of the called station or customer.
DA: Destination MAC Address: A six octet value uniquely identifying an endpoint and which is sent in IEEE LAN frame headers to indicate frame destination.
Data Connections: Data VCCs connect the LECs to each other and to the Broadcast and Unknown Server. These carry Ethernet/IEEE 802.3 or IEEE 802.5 data frames as well as flush messages.
DCC: Data Country Code: This specifies the country in which an address is registered. The codes are given in ISO 3166. The length of this field is two octets. The digits of the data country code are encoded in Binary Coded Decimal (BCD) syntax. The codes will be left justified and padded on the right with the hexadecimal value "F" to fill the two octets.
DCE: Data Communication Equipment: A generic definition of computing equipment that attaches to a network via a DTE.
Default Node Representation: A single value for each nodal state parameter giving the presumed value between any entry or exit to the logical node and the nucleus.
Demultiplexing: A function performed by a layer entity that identifies and separates SDUs from a single connection to more than one connection.
DES: Destination End Station: An ATM termination point which is the destination for ATM messages of a connection and is used as a reference point for ABR services. See SES.
Dijkstra's Algorithm: An algorithm that is sometimes used to calculate routes given a link and nodal state topology database.
DIR: This is a field in an RM-cell which indicates the direction of the RM- cell with respect to the data flow with which it is associated. The source sets DIR=0 and the destination sets DIR=1.
Direct Set: A set of host interfaces which can establish direct layer two communications for unicast (not needed in MPOA).
DLPI: UNIX International, Data Link Provider Interface (DLPI) Specification: Revision 2.0.0, OSI Work Group, August 1991.
Domain: Refer to Administrative Domain.
DS: Distributed Single Layer Test Method: An abstract test method in which the upper tester is located within the system under test and the point of control and observation (PCO) is located at the upper service boundary of the Implementation Under Test (IUT) - for testing one protocol layer. Test events are specified in terms of the abstract service primitives (ASP) at the upper tester above the IUT and ASPs and/or protocol data units (PDU) at the lower tester PCO.
DS-0: Digital Signal, Level 0: The 64 kbps rate that is the basic building block for both the North American and European digital hierarchies.
DS-1: Digital Signal, Level 1: The North American Digital Hierarchy signaling standard for transmission at 1.544 Mbps. This standard supports 24 simultaneous DS-0 signals. The term is often used interchangeably with T1 carrier although DS-1 signals may be exchanged over other transmission systems.
DS-2: Digital Signal, Level 2: The North American Digital Hierarchy signaling standard for transmission of 6.312 Mbps that is used by T2 carrier which supports 96 calls.
DS-3: Digital Signal, Level 3: The North American Digital Hierarchy signaling standard for transmission at 44.736 Mbps that is used by T3 carrier. DS-3 supports 28 DS-1s plus overhead.
DS3 PLCP: Physical Layer Convergence Protocol: An alternate method used by older T carrier equipment to locate ATM cell boundaries. This method has recently been moved to an informative appendix of the ATM DS3 specification and has been replaced by the HEC method.
DSE: Distributed Single-Layer Embedded (Test Method): An abstract test method in which the upper tester is located within the system under test and there is a point of control and observation at the upper service boundary of the Implementation Under Test (IUT) for testing a protocol layer, or sublayer, which is part of a multi-protocol IUT.
DSS1: Digital Subscriber Signalling System #1: N-ISDN UNI Signalling
DSS2 Setup: DSS2 Digital Subscriber Signalling System #2: B-ISDN UNI Signalling
DSU: Data Service Unit: Equipment used to attach users' computing equipment to a public network.
DTE: Data Terminal Equipment: A generic definition of external networking interface equipment such as a modem.
DTL: Designated Transit List: A list of nodes and optional link IDs that completely specify a path across a single PNNI peer group.
DTL Originator: The first switching system within the entire PNNI routing domain to build the initial DTL stack for a given connection.
DTL: Terminator The last switching system within the entire PNNI routing domain to process the connection and thus the connection's DTL.
DXI: Data Exchange Interface: A variable length frame-based ATM interface between a DTE and a special ATM CSU/DSU. The ATM CSU/DSU converts between the variable-length DXI frames and the fixed-length ATM cells.

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E.164: A public network addressing standard utilizing up to a maximum of 15 digits. ATM uses E.164 addressing for public network addressing.
E1: Also known as CEPT1, the 2.048 Mbps rate used by European CEPT carrier to transmit 30 64 kbps digital channels for voice or data calls, plus a 64 kbps signaling channel and a 64 kbps channel for framing and maintenance.
E3: Also known as CEPT3, the 34.368 Mbps rate used by European CEPT carrier to transmit 16 CEPT1s plus overhead.
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.
EFCI: Explicit Forward Congestion Indication: EFCI is an indication in the ATM cell header. A network element in an impending-congested state or a congested state may set EFCI so that this indication may be examined by the destination end-system. For example, the end- system may use this indication to implement a protocol that adaptively lowers the cell rate of the connection during congestion or impending congestion. A network element that is not in a congestion state or an impending congestion state will not modify the value of this indication. Impending congestion is the state when a network equipment is operating around its engineered capacity level.
EFS: Error Free Seconds: A unit used to specify the error 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). Also refer to SES.
ELAN: Emulated Local Area Network: A logical network initiated by using the mechanisms defined by LAN Emulation. This could include ATM and legacy attached end stations.
EMI: Electromagnetic Interference: Equipment used in high speed data systems, including ATM, that generate and transmit many signals in the radio frequency portion of the electromagnetic spectrum. Interference to other equipment or radio services may result if sufficient power from these signals escape the equipment enclosures or transmission media. National and international regulatory agencies (FCC, CISPR, etc.) set limits for these emissions. Class A is for industrial use and Class B is for residential use.
EML: Element Management Layer: An abstraction of the functions provided by systems that manage each network element on an individual basis.
EMS: Element Management System: A management system that provides functions at the element Management Layer.
End Station: These devices (e.g., hosts or PCs) enable the communication between ATM end stations and end stations on "legacy" LAN or among ATM end stations.
Entry Border Node: The node which receives a call over an outside link. This is the first node within a peer group to see this call.
EOM: End of Message: An indicator used in the AAL that identifies the last ATM cell containing information from a data packet that has been segmented.
ER: Explicit Rate: The Explicit Rate is an RM-cell field used to limit the source ACR to a specific value. It is initially set by the source to a requested rate (such as PCR). It may be subsequently reduced by any network element in the path to a value that the element can sustain. ER is formatted as a rate.
ES: End System: A system where an ATM connection is terminated or initiated. An originating end system initiates the ATM connection, and terminating end system terminates the ATM connection. OAM cells may be generated and received.
ESF: Extended Superframe: A DS1 framing format in which 24 DS0 times lots plus a coded framing bit are organized into a frame which is repeated 24 times to form a superframe.
ESI: End System Identifier: This identifier distinguishes multiple nodes at the same level in case the lower level peer group is partitioned.
ETSI: European Telecommunications Standards Institute: The primary telecommunications standards organization.
Exception: A connectivity advertisement in a PNNI complex node representation that represents something other than the default node representation.
Exit Border Node: The node that will progress a call over an outside link. This is the last node within a peer group to see this call.
Exterior: Denotes that an item (e.g., link, node, or reachable address) is outside of a PNNI routing domain.
Exterior Link: A link which crosses the boundary of the PNNI routing domain. The PNNI protocol does not run over an exterior link.
Exterior Reachable Address: An address that can be reached through a PNNI routing domain, but which is not located in that PNNI routing domain.
Exterior Route: A route which traverses an exterior link.

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Fairness: As related to Generic Flow Control (GFC), fairness is defined as meeting all the agreed quality of service (QOS) requirements, by controlling the order of service for all active connections.
FC: Feedback Control: Feedback controls are defined as the set of actions taken by the network and by the end-systems to regulate the traffic submitted on ATM connections according to the state of network elements.
FCS: Frame Check Sequence: Any mathematical formula which derives a numeric value based on the bit pattern of a transmitted block of information and uses that value at the receiving end to determine the existence of any transmission errors.
FDDI: Fiber Distributed Data Interface: A 100 Mbps Local Area Network standard that was developed by ANSI that is designed to work on fiber-optic cables, using techniques similar to token-ring.
FEBE: Far End Block Error: A maintenance signal transmitted in the PHY overhead that a bit error(s) has been detected at the PHY layer at the far end of the link. This is used to monitor bit error performance of the link.
FEC: Forward Error Correction: A technique for detection and correction of errors in a digital data stream.
FG: Functional Group: A collection of functions related in such a way that they will be provided by a single logical component. Examples include the Route Server Functional Group (RSFG), the IASG (Internetwork Address Sub-Group), Coordination Functional Group (ICFG), the Edge Device Functional Group (EDFG) and the ATM attached host Behavior Functional Group (AHFG).
Flush Protocol: The flush protocol is provided to ensure the correct order of delivery of unicast data frames.
Foreign Address: An address that does not match any of a given node's summary addresses.
Forwarding Description: The resolved mapping of an MPOA Target to a set of parameters used to set up an ATM connection on which to forward packets.
FRS: Frame-Relay Service: A connection oriented service that is capable of carrying up to 4096 bytes per frame.
FRTT: Fixed Round-Trip Time: This is the sum of the fixed and propagation delays from the source to the furthest destination and back.

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G.703: ITU-T Recommendation G.703, "Physical/Electrical Characteristics of Hierarchical Digital Interfaces".
G.704: ITU-T Recommendation G.704, "Synchronous Frame Structures Used at Primary and Secondary Hierarchy Levels".
G.804: ITU-T Recommendation G.804, "ATM Cell Mapping into Plesiochronous Digital Hierarchy (PDH)".
GCAC: Generic Connection Admission Control: This is a process to determine if a link has potentially enough resources to support a connection.
GCRA: Generic Cell Rate Algorithm: The GCRA is used to define conformance with respect to the traffic contract of the connection. For each cell arrival the GCRA determines whether the cell conforms to the traffic contract. The UPC function may implement the GCRA, or one or more equivalent algorithms to enforce conformance. The GCRA is defined with two parameters: the Increment (I) and the Limit (L).
GFC: Generic Flow Control: GFC is a field in the ATM header which can be used to provide local functions (e.g., flow control). It has local significance only and the value encoded in the field is not carried end-to-end.

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H-Channel: H-Channels are ISDN bearer services that have pre-defined speeds, starting and stopping locations on a PRI and are contiguously transported from one PRI site through networks to another PRI site.
H0 Channel: A 384 kbps channel that consists of six contiguous DS0s (64 kbps) of a T1 line.
H10 Channel: The North American 1472 kbps channel from a T1 or primary rate carrier. This is equivalent to twenty-three (23) 64 kbps channels.
H11 Channel: The North American primary rate used as a single 1536 kbps channel. This channel uses 24 contiguous DS0s or the entire T1 line except for the 8 kbps framing pattern.
H12: The European primary rate used as a single 1920 kbps channel (30 64 kbps channels or the entire E1 line except for the 64 kbps framing and maintenance channel.
HBFG: Host Behavior Functional Group: The group of functions performed by an ATM-attached host that is participating in the MPOA service.
HDLC: High Level Data Link Control: An ITU-TSS link layer protocol standard for point-to-point and multi-point communications.
Header: Protocol control information located at the beginning of a protocol data unit.
HEC: Header Error Control: Using the fifth octet in the ATM cell header, ATM equipment may check for an error and corrects the contents of the header. The check character is calculated using a CRC algorithm allowing a single bit error in the header to be corrected or multiple errors to be detected.
Hello Packet: A type of PNNI Routing packet that is exchanged between neighboring logical nodes.
Hierarchically Complete Source Route: A stack of DTLs representing a route across a PNNI routing domain such that a DTL is included for each hierarchical level between and including the current level and the lowest visible level in which the source and destination are reachable.
Hop-by-Hop Route: A route that is created by having each switch along the path use its own routing knowledge to determine the next hop of the route, with the expectation that all switches will choose consistent hops such that the call will reach the desired destination. PNNI does not use hop-by-hop routing.
Horizontal Link: A link between two logical nodes that belong to the same peer group.
Host Apparent Address: A set of internetwork layer addresses which a host will directly resolve to lower layer addresses.

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I.356: ITU-T Specifications for Traffic Measurement.
I.361: B-ISDN ATM Layer Specification.
I.362: B-ISDN ATM Layer (AAL) Functional Description.
I.363: B-ISDN ATM Layer (AAL) Specification.
I.432: ITU-T Recommendation for B-ISDN User-network Interface.
IASG: Internetwork Address Sub-Group: A range of internetwork layer addresses summarized in an internetwork layer routing protocol.
ICD: International Code Designator: This identifies an international organization. The registration authority for the International Code Designator is maintained by the British Standards Institute. The length of this field is two octets.
ICR: Initial Cell Rate: An ABR service parameter, in cells/sec, that is the rate at which a source should send initially and after an idle period.
IDU: Interface Data Unit: The unit of information transferred to/from the upper layer in a single interaction across the SAP. Each IDU contains interface control information and may also contain the whole or part of the SDU.
IEC: Inter-exchange Carrier: A long distance telephone company.
IEEE: Institute of Electrical and Electronics Engineers: A worldwide engineering publishing and standards-making body for the electronics industry.
IEEE 802.3: A Local Area Network protocol suite commonly known as Ethernet. Ethernet has either a 10 Mbps or 100 Mbps throughput and uses Carrier Sense Multiple Access bus with Collision Detection CSMA/CD. This method allows users to share the network cable. However, only one station can use the cable at a time. A variety of physical medium dependent protocols are supported.
IEEE 802.5: A Local Area Network protocol suite commonly known as Token Ring. A standard originated by IBM for a token passing ring network that can be configured in a star topology. Versions supported are 4 Mbps and 16 Mbps.
IETF: Internet Engineering Task Force: The organization that provides the coordination of standards and specification development for TCP/IP networking.
ILMI: Integrated Local Managment Interface: An ATM Forum defined interim specification for network management functions between an end user and a public or private network and between a public network and a private network. This is based on a limited subset of SNMP capabilities.
Induced Uplink: An uplink "A" that is created due to the existence of an uplink "B" in the child peer group represented by the node that created uplink "A". Both "A" and "B" share the same upnode, which is higher in the PNNI hierarchy than the peer group in which uplink "A" is seen.
Inside Link: Synonymous with horizontal link.
Instance ID: A subset of an object's attributes which serve to uniquely identify a MIB instance.
Interior: Denotes that an item (e.g., link, node, or reachable address) is inside of a PNNI routing domain.
Internal Reachable Address: An address of a destination that is directly attached to the logical node advertising the address.
IOP: Interoperability: The ability of equipment from different manufacturers (or different implementations) to operate together.
IP: Internet Protocol: Originally developed by the Department of Defense to support interworking of dissimilar computers across a network. This protocol works in conjunction with TCP and is usually identified as TCP/IP. A connectionless protocol that operates at the network layer (layer 3) of the OSI model.
IPX: Novell Internetwork Packet Exchange: 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.
IS: Intermediate System: A system that provides forwarding functions or relaying functions or both for a specific ATM connection. OAM cells may be generated and received.
ISO: International Organization for Standardization: An international organization for standardization, based in Geneva, Switzerland, that establishes voluntary standards and promotes global trade of 90 member countries.
ITU-T: International Telecommunications Union Telecommunications: ITU-T is an international body of member countries whose task is to define recommendations and standards relating to the international telecommunications industry. The fundamental standards for ATM have been defined and published by the ITU-T (Previously CCITT).
ITU H.222: An ITU-T Study Group 15 standard that addresses the multiplexing of multimedia data on an ATM network.
ITU Q.2100: B-ISDN Signaling ATM Adapation Layer Overview.
ITU Q.2110: B-ISDN Adapation Layer -- Service Specific Connection Oriented Protocol.
ITU Q.2130: B-ISDN Adapation Layer -- Service Specific Connection Oriented Function for Support of Signaling at the UNI.
ITU Q.2931: The signaling standard for ATM to support Switched Virtual Connections. This is based on the signaling standard for ISDN.
ITU Q.931: The signaling standard for ISDN to support SVCs. The basis for the signaling standard developed for Frame Relay and ATM.
ITU Q.933: The signaling standard for Frame Relay to support SVCs. This is based on the signaling standard for ISDN.
IUT: Implementation Under Test: The particular portion of equipment which is to be studied for testing. The implementation may include one or more protocols.
IWF: Interworking Function

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Joining: The phase in which the LE Client establishes its control connections to the LE Server.
JPEG: Joint Photographic Experts Group: An ISO Standards group that defines how to compress still pictures.

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LAN: Local Area Network: A network designed to move data between stations within a campus.
LANE: LAN Emulation: The set of services, functional groups and protocols which provide for the emulation of LANS utilizing ATM as a backbone to allow connectivity among LAN and ATM attached end stations.
LAPD: Link Access Procedure D: 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.
Layer Entity: An active element within a layer.
Layer Function: A part of the activity of the layer entities.
Layer Service: A capability of a layer and the layers beneath it that is provided to the upper layer entities at the boundary between that layer and the next higher layer.
Layer User Data: Data transferred between corresponding entities on behalf of the upper layer or layer management entities for which they are providing services.
LB: Leaky Bucket: Leaky Bucket is the term used as an analogous description of the algorithm used for conformance checking of cell flows from a user or network. See GCRA, UPC and NPC. The "leaking hole in the bucket" applies to the sustained rate at which cells can be accommodated, while the "bucket depth" applies to the tolerance to cell bursting over a given time period.
LE: LAN Emulation. Refer to LANE.
LE_ARP: LAN Emulation Address Resolution Protocol: A message issued by a LE client to solicit the ATM address of another function.
Leadership Priority: The priority with which a logical node wishes to be elected peer group leader of its peer group. Generally, of all nodes in a peer group, the one with the highest leadership priority will be elected as peer group leader.
Leaky Bucket: An informal term for the Generic Cell Rate Algorithm.
LEC: Local Exchange Carrier: A telephone company affiliate of a Regional Bell Operating Company or an Independent Telephone Company.
LEC: LAN Emulation Client: The entity in end systems which performs data forwarding, address resolution, and other control functions.
LECID: LAN Emulation Client Identifier: This identifier, contained in the LAN Emulation header, indicates the ID of the ATM host or ATM-LAN bridge. It is unique for every ATM Client.
LECS: LAN Emulation Configuration Server: This implements the policy controlled assignment of individual LE clients to different emulated LANs by providing the LES ATM addresses.
LES: LAN Emulation Server: This implements the control coordination function for the Emulated LAN, examples are enabling a LEC to join an ELAN, resolving MAC to ATM addresses.
LGN: Logical Group Node: LGN is a single node that represents the lowest level peer groups in the respective higher level peer group.
LIJP: Leaf Initiated Joint Parameter: Root screening options and Information Element (IE) instructions carried in SETUP message.
Link: An entity that defines a topological relationship (including available transport capacity) between two nodes in different subnetworks. Multiple links may exist between a pair of subnetworks. Synonymous with logical link.
Link Aggregation Token: Refer to Aggregation Token.
Link Attribute: A link state parameter that is considered individually to determine whether a given link is acceptable and/or desirable for carrying a given connection.
Link Connection: A link connection (e.g., at the VP-level) is a connection capable of transferring information transparently across a link without adding any overhead, such as cells for purposes for monitoring. It is delineated by connection points at the boundary of the subnetwork.
Link Constraint: A restriction on the use of links for path selection for a specific connection.
Link Metric: A link parameter that requires the values of the parameter for all links along a given path to be combined to determine whether the path is acceptable and/or desirable for carrying a given connection.
Link State Parameter: Information that captures an aspect or property of a link.
LNNI: LANE NNI: The standardized interface between two LAN servers (LES-LES, BUS-BUS, LECS-LECS and LECS-LES).
LOC: Loss of Cell Delineation: 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.
LOF: Loss of Frame: 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.
Logical Group Node: A logical node that represents a lower level peer group as a single point for purposes of operating at one level of the PNNI routing hierarchy.
Logical Link: An abstract representation of the connectivity between two logical nodes. This includes individual physical links, individual virtual path connections, and parallel physical links and/or virtual path connections.
Logical Node: An abstract representation of a peer group or a switching system as a single point.
Logical Node ID: A string of bits that unambiguously identifies a logical node within a routing domain.
LOP: Loss of Pointer: 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.
LOS: Loss of Signal: 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.
LPF: Low Pass Filter: In an MPEG-2 clock recovery circuit, it is a technique for smoothing or averaging changes to the system clock.
LSAP: Link Service Access Point: Logical address of boundary between layer 3 and LLC sublayer 2.
LSB: Least Significant Bit: The lowest order bit in the binary representation of a numerical value.
LSR: Leaf Setup Request: A setup message type used when a leaf node requests connection to existing point-to-multipoint connection or requests creation of a new multipoint connection.
LT: Lower Tester: The representation in ISO/IEC 9646 of the means of providing, during test execution, indirect control and observation of the lower service boundary of the IUT using the underlying service provider.
LTE: SONET Lite Terminating Equipment: ATM equipment terminating a communications facility using a SONET Lite Transmission Convergence (TC) layer. This is usually reserved for end user or LAN equipment. The SONET Lite TC does not implement some of the maintenance functions used in long haul networks such as termination of path, line and section overhead.
LUNI: LANE UNI: The standardized interface between a LE client and a LE Server (LES,LECS and BUS).

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M1: Management Interface 1: The management of ATM end devices.
M2: Management Interface 2: The management of Private ATM networks or switches.
M3: Management Interface 3: The management of links between public and private networks.
M4: Management Interface 4: The management of public ATM networks.
M5: Management Interface 5: The management of links between two public networks.
MAC: Media Access Control: IEEE specifications for the lower half of the data link layer (layer 2) that defines topology dependent access control protocols for IEEE LAN specifications.
MAN: Metropolitan Area Network: A network designed to carry data over an area larger than a campus such as an entire city and its outlying area.
Managed System: An entity that is managed by one or more management systems, which can be either Element Management Systems, Subnetwork or Network Management Systems, or any other management systems.
Management Domain: An entity used here to define the scope of naming.
Management System: An entity that manages a set of managed systems, which can be either NEs, subnetworks or other management systems.
MaxCR: Maximum Cell Rate: This is the maximum capacity usable by connections belonging to the specified service category.
MBS: Maximum Burst Size: In the signaling message, the Burst Tolerance (BT) is conveyed through the MBS which is coded as a number of cells. The BT together with the SCR and the GCRA determine the MBS that may be transmitted at the peak rate and still be in conformance with the GCRA.
MCDV: Maximum Cell Delay Variance: This is the maximum two-point CDV objective across a link or node for the specified service category.
MCLR: Maximum Cell Loss Ratio: This is 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.
MCR: Minimum Cell Rate: An ABR service traffic descriptor, in cells/sec, that is the rate at which the source is always allowed to send.
MCTD: Maximum Cell Transfer Delay: This is the sum of the fixed delay component across the link or node and MCDV.
Metasignaling: ATM Layer Management (LM) process that manages different types of signaling and possibly semipermanent virtual channels (VCs), including the assignment, removal and checking of VCs.
Metasignaling VCs: The standardized VCs that convey metasignaling information across a User-Network Interface (UNI).
MIB: Management Information Base: A definition of management items for some network component that can be accessed by a network manager. A MIB includes the names of objects it contains and the type of information retained.
MIB Attribute: A single piece of configuration, management, or statistical information which pertains to a specific part of the PNNI protocol operation.
MIB Instance: An incarnation of a MIB object that applies to a specific part, piece, or aspect of the PNNI protocol's operation.
MIB Object: A collection of attributes that can be used to configure, manage, or analyze an aspect of the PNNI protocol's operation.
MID: Message Identifier: The message identifier is used to associate ATM cells that carry segments from the same higher layer packet.
MIR: Maximum Information Rate: Refer to PCR.
MMF: Multimode Fiberoptic Cable: Fiberoptic cable in which the signal or light propagates in multiple modes or paths. Since these paths may have varying lengths, a transmitted pulse of light may be received at different times and smeared to the point that pulses may interfere with surrounding pulses. This may cause the signal to be difficult or impossible to receive. This pulse dispersion sometimes limits the distance over which a MMF link can operate.
MPEG: Motion Picture Experts Group: An ISO Standards group dealing with video and audio compression techniques and mechanisms for multiplexing and synchronizing various media streams.
MPOA: Multiprotocol over ATM: An effort taking place in the ATM Forum to standardize protocols for the purpose of running multiple network layer protocols over ATM.
MPOA Client: A device which implements the client side of one or more of the MPOA protocols, (i.e., is a SCP client and/or an RDP client. An MPOA Client is either an Edge Device Functional Group (EDFG) or a Host Behavior Functional Group (HBFG).
MPOA Server: An MPOA Server is any one of an ICFG or RSFG.
MPOA Service Area: The collection of server functions and their clients. A collection of physical devices consisting of an MPOA server plus the set of clients served by that server.
MPOA Target: A set of protocol address, path attributes, (e.g., internetwork layer QoS, other information derivable from received packet) describing the intended destination and its path attributes that MPOA devices may use as lookup keys.
Mrm: An ABR service parameter that controls allocation of bandwidth between forward RM-cells, backward RM-cells, and data cells.
MSB: Most Significant Bit: The highest order bit in the binary representation of a numerical value.
MT: Message Type: Message type is the field containing the bit flags of a RM-cell. These flags are as follows: DIR = 0 for forward RM-cells = 1 for backward; RM-cells BN = 1 for Non-Source Generated (BECN), RM-cells = 0 for Source Generated RM-cells CI = 1 to indicate congestion = 0 otherwise NI = 1 to indicate no additive increase allowed = 0 otherwise RA -- Not used for ATM Forum ABR.
MTP: Message Transfer Part: Level 1 through 3 protocols of the SS7 protocol stack. MTP 3 (Level 3) is used to support BISUP.
Multicasting: The transmit operation of a single PDU by a source interface where the PDU reaches a group of one or more destinations.
Multiplexing: A function within a layer that interleaves the information from multiple connections into one connection.
Multipoint Access: User access in which more than one terminal equipment (TE) is supported by a single network termination.
Multipoint-to-Multipoint Connection: A Multipoint-to-Multipoint Connection is a collection of associated ATM VC or VP links, and their associated nodes, with the following properties:
Multipoint-to-Point Connection: A Point-to-Multipoint Connection may have zero bandwidth from the Root node to the Leaf Nodes, and non-zero return bandwidth from the Leaf Nodes to the Root Node. Such a connection is also known as a Multipoint-to-Point Connection. Note that UNI 4.0 does not support this connection type.

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N-ISDN: Narrowband Integrated Services Digital Network: 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.
Native Address: An address that matches one of a given node's summary addresses.
NDIS: Network Driver Interface Specification: Refer to 3COM/Microsoft, LAN Manager: Network Driver Interface Specification, October 8, 1990.
NE: Network Element: A system that supports at least NEFs and may also support Operation System Functions/Mediation Functions. An ATM NE may be realized as either a standalone device or a geographically distributed system. It cannot be further decomposed into managed elements in the context of a given management function.
NEF: Network Element Function: A function within an ATM entity that supports the ATM based network transport services, (e.g., multiplexing, cross-connection).
Neighbor Node: A node that is directly connected to a particular node via a logical link.
NEL: Network Element Layer: An abstraction of functions related specifically to the technology, vendor, and the network resources or network elements that provide basic communications services.
NEXT: Near End Crosstalk: Equipment that must concurrently receive on one wire pair and transmit on another wire pair in the same cable bundle must accommodate NEXT interference. NEXT is the portion of the transmitted signal that leaks into the receive pair. Since at this point on the link the transmitted signal is at maximum and the receive signal has been attenuated, it may be difficult to maintain an acceptable ACR with the received signal if the cable media allows large amounts of crosstalk leakage to occur. Foiled or shielded cables generally have less crosstalk than unshielded varieties.
NM: Network Management Entity: The body of software in a switching system that provides the ability to manage the PNNI protocol. NM interacts with the PNNI protocol through the MIB.
NML: Network Management Layer: An abstraction of the functions provided by systems which manage network elements on a collective basis, so as to monitor and control the network end-to-end.
NMS: Network Management System: An entity that implements functions at the Network Management Layer. It may also include Element Management Layer functions. A Network Management System may manage one or more other Network Management Systems.
NMS Environment: A set of NMS which cooperate to manage one or more subnetworks.
NNI: Network Node Interface: An interface between ATM switches defined as the interface between two network nodes.
Nodal Attribute: A nodal state parameter that is considered individually to determine whether a given node is acceptable and/or desirable for carrying a given connection.
Nodal Constraint: A restriction on the use of nodes for path selection for a specific connection.
Nodal Metric: A nodal parameter that requires the values of the parameter for all nodes along a given path to be combined to determine whether the path is acceptable and/or desirable for carrying a given connection.
Nodal State Parameter: Information that captures an aspect or property of a node.
Node: Synonymous with logical node.
NPC: Network Parameter Control: Network Parameter Control is defined as the set of actions taken by the network to monitor and control traffic from the NNI. Its main purpose is to protect network resources from malicious as well as unintentional misbehavior which can affect the QoS of other already established connections by detecting violations of negotiated parameters and taking appropriate actions. Refer to UPC.
Nrm: An ABR service parameter, Nrm is the maximum number of cells a source may send for each forward RM-cell.
NSAP: Network Service Access Point: 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.
NSR: Non-Source Routed: Frame forwarding through a mechanism other than Source Route Bridging.
NT: Network Termination: Network Termination represents the termination point of a Virtual Channel, Virtual Path, or Virtual Path/Virtual Channel at the UNI.
NTSC: National Television System Committee: An industry group that defines how television signals are encoded and transmitted in the US.
Nucleus: The interior reference point of a logical node in the PNNI complex node representation.
nx64K: This refers to a circuit bandwidth or speed provided by the aggregation of nx64 kbps channels (where n= integer > 1). The 64K or DS0 channel is the basic rate provided by the T Carrier systems.

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OAM: Operations Administration and Maintenance: A group of network management functions that provide network fault indication, performance information, and data and diagnosis functions.
Octet: A term for eight (8) bits that is sometimes used interchangeably with "byte" to mean the same thing.
ODI: Open Data-Link Interface: This refers to Novell Incorporated, Open Data-Link Interface Developer's Guide, March 20, 1992.
One Hop Set: A set of hosts which are one hop apart in terms of internetwork protocols TTLs (TTL=0 -on the wire+).
OOF: Out of Frame. Refer to LOF.
OSI: Open Systems Interconnection: A seven (7) layer architecture model for communications systems developed by the ISO for the interconnection of data communications systems. Each layer uses and builds on the services provided by those below it.
OSPF: Open Shortest Path First: A link-state routing algorithm that is used to calculate routes based on the number of routers, transmission speed, delays and route cost.
OUI: Organizationally Unique Identifier: The OUI is a three-octet field in the IEEE 802.1a defined SubNetwork Attachment Point (SNAP) header, identifying an organization which administers the meaning of the following two octet Protocol Identifier (PID) field in the SNAP header. Together they identify a distinct routed or bridged protocol.
Outlier: A node whose exclusion from its containing peer group would significantly improve the accuracy and simplicity of the aggregation of the remainder of the peer group topology.
Outside Link: A link to an outside node.
Outside Node: A node which is participating in PNNI routing, but which is not a member of a particular peer group.

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PAD: Packet Assembler and Disassembler: A PAD assembles packets of asynchronous data and emits these buffers in a burst to a packet switch network. The PAD also disassembles packets from the network and emits the data to the non-packet device.
Parent Node: The logical group node that represents the containing peer group of a specific node at the next higher level of the hierarchy.
Parent Peer Group: The parent peer group of a peer group is the one containing the logical group node representing that peer group. The parent peer group of a node is the one containing the parent node of that node.
Path Constraint: A bound on the combined value of a topology metric along a path for a specific connection.
PBX: Private Branch eXchange: PBX is the term given to a device which provides private local voice switching and voice-related services within the private network. A PBX could have an ATM API to utilize ATM services, for example Circuit Emulation Service.
PC: Protocol Control: Protocol Control is a mechanism which a given application protocol may employ to determine or control the performance and health of the application. Example, protocol liveness may require that protocol control information be sent at some minimum rate; some applications may become intolerable to users if they are unable to send at least at some minimum rate. For such applications, the concept of MCR is defined. Refer to MCR.
PCM: Pulse Code Modulation: An audio encoding algorithm which encodes the amplitude of a repetitive series of audio samples. This encoding algorithm converts analog voice samples into a digital bit stream.
PCO: Point of Control and Observation: A place (point) within a testing environment where the occurrence of test events is to be controlled and observed as defined by the particular abstract test method used.
PCR: Program Clock Reference: A timestamp that is inserted by the MPEG-2 encoder into the Transport Stream to aid the decoder in the recovering and tracking the encoder clock.
PCR: Peak Cell Rate: The Peak Cell Rate, in cells/sec, is the cell rate which the source may never exceed.
PDH: Plesiochronous Digital Hierarchy: PDH (plesiochronous means nearly synchronous), was developed to carry digitized voice over twisted pair cabling more efficiently. This evolved into the North American, European, and Japanese Digital Hierarchies where only a discrete set of fixed rates is available, namely, nxDS0 (DS0 is a 64 kbps rate) and then the next levels in the respective multiplex hierarchies.
PDU: Protocol Data Unit: A PDU is a message of a given protocol comprising payload and protocol-specific control information, typically contained in a header. PDUs pass over the protocol interfaces which exist between the layers of protocols (per OSI model).
Peer Entities: Entities within the same layer.
Peer Group: A set of logical nodes which are grouped for purposes of creating a routing hierarchy. PTSEs are exchanged among all members of the group.
Peer Group Identifier: A string of bits that is used to unambiguously identify a peer group.
Peer Group Leader: A node which has been elected to perform some of the functions associated with a logical group node.
Peer Group Level: The number of significant bits in the peer group identifier of a particular peer group.
Peer Node: A node that is a member of the same peer group as a given node.
PES: Packetized Elementary Stream: In MPEG-2, after the media stream has been digitized and compressed, it is formatted into packets before it is multiplexed into either a Program Stream or Transport Stream.
PG: Peer Group: A set of logical nodes which are grouped for purposes of creating a routing hierarchy. PTSEs are exchanged among all members of the group.
PGL: Peer Group Leader: A single real physical system which has been elected to perform some of the functions associated with a logical group node.
PHY: OSI Physical Layer: The physical layer provides for transmission of cells over a physical medium connecting two ATM devices. This physical layer is comprised of two sublayers: the PMD Physical Medium Dependent sublayer, and the TC Transmission Convergence sublayer. Refer PMD and TC.
Physical Layer (PHY) Connection: An association established by the PHY between two or more ATM entities. A PHY connection consists of the concatenation of PHY links in order to provide an end-to-end transfer capability to PHY SAPs.
Physical Link: A real link which attaches two switching systems.
PICS: Protocol Implementation Conformance Statement: A statement made by the supplier of an implementation or system stating which capabilities have been implemented for a given protocol.
PID: Protocol Identification. Refer to OUI.
PIXIT: Protocol Implementation eXtra Information for Testing: A statement made by a supplier or implementor of an IUT which contains information about the IUT and its testing environment which will enable a test laboratory to run an appropriate test suite against the IUT.
Plastic Fiber Optics: An optical fiber where the core transmission media is plastic in contrast to glass or silica cores. Proposed plastic fibers generally have larger attenuation and dispersion than glass fiber but may have applications where the distance is limited. Plastic systems may also offer lower cost connectors that may be installed with simple tools and a limited amount of training.
PLCP: Physical Layer Convergence Protocol: The PLCP is defined by the IEEE 802.6. It is used for DS3 transmission of ATM. ATM cells are encapsulated in a 125microsecond frame defined by the PLCP which is defined inside the DS3 M-frame.
PLL: Phase Lock Loop: Phase Lock Loop is a mechanism whereby timing information is transferred within a data stream and the receiver derives the signal element timing by locking its local clock source to the received timing information.
PM: Physical Medium: Physical Medium refers to the actual physical interfaces. Several interfaces are defined including STS-1, STS-3c, STS-12c, STM-1, STM-4, DS1, E1, DS2, E3, DS3, E4, FDDI-based, Fiber Channel-based, and STP. These range in speeds from 1.544Mbps through 622.08 Mbps.
PMD: Physical Media Dependent: This sublayer defines the parameters at the lowest level, such as speed of the bits on the media.
PNI: Permit Next Increase: An ABR service parameter, PNI is a flag controlling the increase of ACR upon reception of the next backward RM-cell. PNI=0 inhibits increase. The range is 0 or 1.
PNNI: Private Network-Network Interface: A routing information protocol that enables extremely scalable, full function, dynamic multi-vendor ATM switches to be integrated in the same network.
PNNI Protocol Entity: The body of software in a switching system that executes the PNNI protocol and provides the routing service.
PNNI Routing Control Channel: VCCs used for the exchange of PNNI routing protocol messages.
PNNI Routing Domain: A group of topologically contiguous systems which are running one instance of PNNI routing.
PNNI Routing Hierarchy: The hierarchy of peer groups used for PNNI routing.
PNNI Topology State Element: A collection of PNNI information that is flooded among all logical nodes within a peer group.
PNNI Topology State Packet: A type of PNNI Routing packet that is used for flooding PTSEs among logical nodes within a peer group.
POH: Path Overhead: A maintenance channel transmitted in the SONET overhead following the path from the beginning multiplexer to the ending demultiplexer. This is not implemented in SONET Lite.
Point-to-Multipoint Connection: A Point-to-Multipoint Connection is a collection of associated ATM VC or VP links, with associated endpoint nodes, with the following properties:
Point-to-Point Connection: A connection with only two endpoints.
Port Identifier: The identifier assigned by a logical node to represent the point of attachment of a link to that node.
PRI: Primary Rate Interface: An ISDN standard for provisioning of 1.544 Mbit/s (DS1 - North America, Japan, et al) or 2.048 Mbit/s (E1 - Europe) ISDN services. DS1 is 23 "B" channels of 64 kbit/s each and one signalling "D" channel of 64 kbit/s/ E1 is 30 "B" channels of 64 kbit/s each and one signalling "D" channel of 64 kbit/s.
PRS: Primary Reference Source
Primitive: An abstract, implementation independent, interaction between a layer service user and a layer service provider.
Private ATM Address: A twenty-byte address used to identify an ATM connection termination point.
Protocol: A set of rules and formats (semantic and syntactic) that determines the communication behavior of layer entities in the performance of the layer functions.
Protocol Control Information: Information exchanged between corresponding entities, using a lower layer connection, to coordinate their joint operation.
PT: Payload Type: Payload Type is a 3-bit field in the ATM cell header that discriminates between a cell carrying management information or one which is carrying user information.
PTI: Payload Type Indicator: Payload Type Indicator is the Payload Type field value distinguishing the various management cells and user cells. Example: Resource Management cell has PTI=110, end-to-end OAM F5 Flow cell has PTI=101.
PTMPT: Point-To-Multipoint: A main source to many destination connections.
PTS: Presentation Time Stamp: A timestamp that is inserted by the MPEG-2 encoder into the packetized elementary stream to allow the decoder to synchronize different elementary streams (i.e. lip sync).
PTSE: PNNI Topology State Element: A collection of PNNI information that is flooded among all logical nodes within a peer group.
PTSP: PNNI Topology State Packet: A type of PNNI Routing packet that is used for flooding PTSEs among logical nodes within a peer group.
PVC: Permanent Virtual Circuit: This is a link with static route defined in advance, usually by manual setup.
PVCC: Permanent Virtual Channel Connection: A Virtual Channel Connection (VCC) is an ATM connection where switching is performed on the VPI/VCI fields of each cell. A Permanent VCC is one which is provisioned through some network management function and left up indefinitely.
PVPC: Permanent Virtual Path Connection: A Virtual Path Connection (VPC) is an ATM connection where switching is performed on the VPI field only of each cell. A Permanent VPC is one which is provisioned through some network management function and left up indefinitely.

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QD: Queuing Delay: Queuing Delay refers to the delay imposed on a cell by its having to be buffered because of unavailability of resources to pass the cell onto the next network function or element. This buffering could be a result of oversubscription of a physical link, or due to a connection of higher priority or tighter service constraints getting the resource of the physical link.
QoS: Quality of Service: Quality of Service is defined on an end-to-end basis in terms of the following attributes of the end-to-end ATM connection:
Q.SIG: A symmetrical adaptatation of N-ISDN signalling (DSS1) for inter-PBX signalling.

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RBOC: Regional Bell Operating Company: Seven companies formed to manage the local exchanges originally owned by AT&T. These companies were created as a result of an agreement between AT&T and the United States Department of Justice.
RD: Routing Domain: A group of topologically contiguous systems which are running one instance of routing.
RDF: Rate Decrease Factor: An ABR service parameter, RDF controls the decrease in the cell transmission rate. RDF is a power of 2 from 1/32,768 to 1.
RO: Read-Only: Attributes which are read-only can not be written by Network Management. Only the PNNI Protocol entity may change the value of a read-only attribute. Network Management entities are restricted to only reading such read-only attributes. Read-only attributes are typically for statistical information, including reporting result of actions taken by auto-configuration.
RW: Read-Write : Attributes which are read-write can not be written by the PNNI protocol entity. Only the Network Management Entity may change the value of a read-write attribute. The PNNI Protocol Entity is restricted to only reading such read-write attributes. Read-write attributes are typically used to provide the ability for Network Management to configure, control, and manage a PNNI Protocol Entity's behavior.
Registration: The address registration function is the mechanism by which Clients provide address information to the LAN Emulation Server.
Relaying: A function of a layer by means of which a layer entity receives data from a corresponding entity and transmits it to another corresponding entity.
RFC: Request For Comment: The development of TCP/IP standards, procedures and specifications is done via this mechanism. RFCs are documents that progress through several development stages, under the control of IETF, until they are finalized or discarded.
RFC1695: Definitions of Managed Objects for ATM Management or AToM MIB.
RFI: Radio Frequency Interface: Refer to EMI.
RIF: Rate Increase Factor: This controls the amount by which the cell transmission rate may increase upon receipt of an RM-cell. The additive increase rate AIR=PCR*RIF. RIF is a power of 2, ranging from 1/32768 to 1.
RISC: Reduced Instruction Set Computing: A computer processing technology in which a microprocessor understands a few simple instructions thereby providing fast, predictable instruction flow.
RM: Resource Management: Resource Management is the management of critical resources in an ATM network. Two critical resources are buffer space and trunk bandwidth. Provisioning may be used to allocate network resources in order to separate traffic flows according to service characteristics. VPCs play a key role in resource management. By reserving capacity on VPCs, the processing required to establish individual VCCs is reduced. Refer to RM-cell.
RM-Cell: Resource Management Cell: Information about the state of the net work like bandwidth availability, state of congestion, and impending congestion, is conveyed to the source through special control cells called Resource Management Cells (RM-cells).
Route Server: A physical device that runs one or more network layer routing protocols, and which uses a route query protocol in order to provide network layer routing forwarding descriptions to clients.
Router: A physical device that is capable of forwarding packets based on network layer information and that also participates in running one or more network layer routing protocols.
Routing Computation: The process of applying a mathematical algorithm to a topology database to compute routes. There are many types of routing computations that may be used. The Djikstra algorithm is one particular example of a possible routing computation.
Routing Constraint: A generic term that refers to either a topology constraint or a path constraint.
Routing Protocol: A general term indicating a protocol run between routers and/or route servers in order to exchange information used to allow computation of routes. The result of the routing computation will be one or more forwarding descriptions.
RS: Remote single-layer (Test Method): An abstract test method in which the upper tester is within the system under test and there is a point of control and observation at the upper service boundary of the Implementation Under Test (IUT) for testing one protocol layer. Test events are specified in terms of the abstract service primitives (ASP) and/or protocol data units at the lower tester PCO.
RSE: Remote Single-layer Embedded (Test Method): An abstract test method in which the upper tester is within the system under test and there is a point of control and observation at the upper service boundary of the Implementation Under Test (IUT) for testing a protocol layer or sublayer which is part of a multi-protocol IUT.
RSFG: Route Server Functional Group: The group of functions performed to provide internetworking level functions in an MPOA System. This includes running conventional interworking Routing Protocols and providing inter-IASG destination resolution.

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SA: Source Address: The address from which the message or data originated.
SA: Source MAC Address: A six octet value uniquely identifying an end point and which is sent in an IEEE LAN frame header to indicate source of frame.
SAAL: Signaling ATM Adaptation Layer: This resides between the ATM layer and the Q.2931 function. The SAAL provides reliable transport of Q.2931 messages between Q.2931 entities (e.g., ATM switch and host) over the ATM layer; two sublayers: common part and service specific part.
SAP: Service Access Point: A SAP is used for the following purposes:
SAR: Segmentation and Reassembly: Method of breaking up arbitrarily sized packets.
Scope: A scope defines the level of advertisement for an address. The level is a level of a peer group in the PNNI routing hierarchy.
SCCP: Signaling Connection and Control Part: A SS7 protocol that provides additional functions to the Message Transfer Part (MTP). It typically supports Transaction Capabilities Application Part (TCAP).
SCP: Service Control Point: A computer and database system which executes service logic programs to provide customer services through a switching system. Messages are exchanged with the SSP through the SS7 network.
SCR: Sustainable Cell Rate: The SCR is an upper bound on the conforming average rate of an ATM connection over time scales which are long relative to those for which the PCR is defined. Enforcement of this bound by the UPC could allow the network to allocate sufficient resources, but less than those based on the PCR, and still ensure that the performance objectives (e.g., for Cell Loss Ratio) can be achieved.
SDH: Synchronous Digital Hierarchy: The ITU-TSS International standard for transmitting information over optical fiber.
SDT: Structured Data Transfer: An AAL1 data transfer mode in which data is structured into blocks which are then segmented into cells for transfer.
SDU: Service Data Unit: A unit of interface information whose identity is preserved from one end of a layer connection to the other.
SE: Switching Element: Switching Element refers to the device or network node which performs ATM switching functions based on the VPI or VPI/VCI pair.
SEAL: Simple and Efficient Adapation Layer: An earlier name for AAL5.
Segment: A single ATM link or group of interconnected ATM links of an ATM connection.
SEL: Selector: A subfield carried in SETUP message part of ATM endpoint address Domain specific Part (DSP) defined by ISO 10589, not used for ATM network routing, used by ATM end systems only.
Semipermanent Connection: A connection established via a service order or via network management.
SES: Severely Errored Seconds: A unit used to specify the error performance of T carrier systems. This indicates a second containing ten or more errors, usually expressed as SES per hour, day, or week. This method gives a better indication of the distribution of bit errors than a simple Bit Error Rate (BER). Refer also to EFS.
SES: Source End Station: An ATM termination point, which is the source of ATM messages of a connection, and is used as a reference point for ABR services. Refer to DES.
SF: SuperFrame: A DS1 framing format in which 24 DS0 timeslots plus a coded framing bit are organized into a frame which is repeated 12 times to form the superframe.
Shaping Descriptor: N ordered pairs of GCRA parameters (I,L) used to define the negotiated traffic shape of a connection.
SIPP: SMDS Interface Protocol: Protocol where layer 2 is based on ATM, AAL and DQDB. Layer 1 is DS1 and DS3.
SMDS: Switched Multi-Megabit Data Services: A connectionless service used to connect LANs, MANs and WANs to exchange data.
SMF: Single Mode Fiber: Fiber optic cable in which the signal or light propagates in a single mode or path. Since all light follows the same path or travels the same distance, a transmitted pulse is not dispersed and does not interfere with adjacent pulses. SMF fibers can support longer distances and are limited mainly by the amount of attenuation. Refer to MMF.
SN: Sequence Number: SN is a 4 octet field in a Resource Management cell defined by the ITU-T in recommendation I.371 to sequence such cells. It is not used for ATM Forum ABR. An ATM switch will either preserve this field or set it in accordance with I.371.
SN cell: Sequence Number Cell: A cell sent periodically on each link of an AIMUX to indicate how many cells have been transmitted since the previous SN cell. These cells are used to verify the sequence of payload cells reassembled at the receiver.
SNA: Systems Network Architecture: IBM's seven layer, vendor specific architecture for data communications
SNC: Subnetwork Connection: In the context of ATM, an entity that passes ATM cells transparently, (i.e., without adding any overhead). A SNC may be either a stand-alone SNC, or a concatenation of SNCs and link connections.
SNMP: Simple Network Management Protocol: Originally designed for the Department of Defense network to support TCP/IP network management. It has been widely implemented to support the management of a broad range of network products and functions. SNMP is the IETF standard management protocol for TCP/IP networks.
SONET: Synchronous Optical Network: An ANSI standard for transmitting information over optical fiber. This standard is used or accepted in the United States and Canada and is a variation of the SDH International standard.
Source Route: As used in this document, a hierarchically complete source route.
Source Traffic: A set of traffic parameters belonging to the ATM Traffic Descriptor Descriptor used during the connection set-up to capture the intrinsic traffic characteristics of the connection requested by the source.
SPE: SONET Synchronous Payload Envelope.
Split System: A switching system which implements the functions of more than one logical node.
SPTS: Single Program Transport Stream: An MPEG-2 Transport Stream that consists of only one program.
SR: Source Routing: A bridged method whereby the source at a data exchange determines the route that subsequent frames will use.
SRF: Specifically Routed Frame: A Source Routing Bridging Frame which uses a specific route between the source and destination.
SRT: Source Routing Transparent: An IETF Bridging Standard combining Transparent Bridging and Source Route Bridging.
SRTS: Synchronous residual Time Stamp: A clock recovery technique in which difference signals between source timing and a network reference timing signal are transmitted to allow reconstruction of the source timing at the destination.
SSCF: Service Specific Coordination Function: SSCF is a function defined in Q.2130, B-ISDN Signaling ATM Adaptation Layer-Service Specific Coordination Function for Support of Signaling at the User-to- Network Interface.
SSCOP: Service Specific Connection Oriented Protocol: An adaptation layer protocol defined in ITU-T Specification: Q.2110.
SSCS: Service Specific Convergence Sublayer: The portion of the convergence sublayer that is dependent upon the type of traffic that is being converted.
SS7: Signal System Number 7: A family of signaling protocols originating from narrowband telephony. They are used to set-up, manage and tear down connections as well as to exchange non-connection associated information. Refer to BISUP, MTP, SCCP and TCAP.
STC: System Time Clock: The master clock in an MPEG-2 encoder or decoder system.
STE: Spanning Tree Explorer: A Source Route Bridging frame which uses the Spanning Tree algorithm in determining a route.
STE: SONET Section Terminating Equipment: SONET equipment that terminates a section of a link between a transmitter and repeater, repeater and repeater, or repeater and receiver. This is usually implemented in wide area facilities and not implemented by SONET Lite.
STM: Synchronous Transfer Module: STM is a basic building block used for a synchronous multiplexing hierarchy defined by the CCITT/ITU-T. STM-1 operates at a rate of 155.52 Mbps (same as STS-3).
STM-1: Synchronous Transport Module 1: SDH standard for transmission over OC-3 optical fiber at 155.52 Mbps.
STM-n: Synchronous Transport Module "n" : (where n is an integer) SDH standards for transmission over optical fiber (OC-'n x 3) by multiplexing "n" STM-1 frames, (e.g., STM-4 at 622.08 Mbps and STM-16 at 2.488 Gbps).
STM-nc: Synchronous Transport Module "n" concatenated: (where n is an integer) SDH standards for transmission over optical fiber (OC-'n x 3) by multiplexing "n" STM-1 frames, (e.g., STM-4 at 622.08 Mbps and STM-16 at 2.488 Gbps, but treating the information fields as a single concatenated payload).
STP: Signaling Transfer Point: A high speed, reliable, special purpose packet switch for signaling messages in the SS7 network.
STP: Shielded Twisted Pair: A cable containing one or more twisted pair wires with each pair having a shield of foil wrap.
STS-1: Synchronous Transport Signal 1: SONET standard for transmission over OC-1 optical fiber at 51.84 Mbps.
STS-n: Synchronous Transport Signal "n" : (where n is an integer) SONET standards for transmission over OC-n optical fiber by multiplexing "n" STS-1 frames, (e.g., STS-3 at 155.52 Mbps STS-12 at 622.08 Mbps and STS-48 at 2.488 Gbps).
STS-nc: Synchronous Transport Signal "n" concatenated: (where n is an integer) SONET standards for transmission over OC-n optical fiber by multiplexing "n" STS-1 frames, (e.g., STS-3 at 155.52 Mbps STS-12 at 622.08 Mbps and STS-48 at 2.488 Gbps but treating the information fields as a single concatenated payload).
Sublayer: A logical sub-division of a layer.
Subnet: The use of the term subnet to mean a LAN technology is a historical use and is not specific enough in the MPOA work. Refer to Internetwork Address Sub-Group, Direct Set, Host Apparent Address Sub-Group and One Hop Set for more specific definitions.
Subnetwork: A collection of managed entities grouped together from a connectivity perspective, according to their ability to transport ATM cells.
subNMS: Subnetwork Management System: A Network Management System that is managing one or more subnetworks and that is managed by one or more Network Management Systems.
Summary Address: An address prefix that tells a node how to summarize reachability information.
SUT: System Under Test: The real open system in which the Implementation Under Test (IUT) resides.
SVC: Switched Virtual Circuit: A connection established via signaling. The user defines the endpoints when the call is initiated.
SVCC: Switched Virtual Channel Connection: A Switched VCC is one which is established and taken down dynamically through control signaling. A Virtual Channel Connection (VCC) is an ATM connection where switching is performed on the VPI/VCI fields of each cell.
SVE: SAP Vector Element: The SAP address may be expressed as a vector, (ATM_addr, ATM_selector, BLLI_id2, BLLI_id3, BHLI_id), where:
SVPC: Switched Virtual Path Connection: A Switched Virtual Path Connection is one which is established and taken down dynamically through control signaling. A Virtual Path Connection (VPC) is an ATM connection where switching is performed on the VPI field only of each cell.
Switched Connection: A connection established via signaling.
Switching System: A set of one or more systems that act together and appear as a single switch for the purposes of PNNI routing.
Symmetric Connection: A connection with the same bandwidth value specified for both directions.

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T1E1: An ANSI standards sub-committee dealing with Network Interfaces.
T1M1: An ANSI standards sub-committee dealing with Inter-Network Operations, Administration and Maintenance.
T1Q1: An ANSI standards sub-committee dealing with performance.
T1S1: An ANSI standards sub-committee dealing with services, architecture and signaling.
T1X1: An ANSI standards sub-committee dealing with digital hierarchy and synchronization.
TB: Transparent Bridging: An IETF bridging standard where bridge behavior is transparent to the data traffic. To avoid ambiguous routes or loops, a Spanning Tree algorithm is utilized.
TBE: Transient Buffer Exposure: This is a negotiated number of cells that the network would like to limit the source to sending during startup periods, before the first RM-cell returns.
TC: Transaction Capabilities: TCAP (see below) plus supporting Presentation, Session and Transport protocol layers.
TC: Transmission Convergence: The TC sublayer transforms the flow of cells into a steady flow of bits and bytes for transmission over the physical medium. On transmit, the TC sublayer maps the cells to the frame format, generates the Header Error Check (HEC), sends idle cells when the ATM layer has none to send. On reception, the TC sublayer delineates individual cells in the received bit stream, and uses the HEC to detect and correct received errors.
TCAP: Transaction Capabilities Applications Part: A connectionless SS7 protocol for the exchange of information outside the context of a call or connection. It typically runs over SCCP and MTP 3.
TCP: Transmission Control Protocol: Originally developed by the Department of Defense to support interworking of dissimilar computers across a network. A protocol which provides end-to-end, connection-oriented, reliable transport layer (layer 4) functions over IP controlled networks. TCP performs the following functions: flow control between two systems, acknowledgements of packets received and end-to-end sequencing of packets.
TCP: Test Coordination Procedure: A set of rules to coordinate the test process between the lower tester and the upper tester. The purpose is to enable the lower tester to control the operation of the upper tester. These procedures may, or may not, be specified in an abstract test suite.
TCR: Tagged Cell Rate: An ABR service parameter, TCR limits the rate at which a source may send out-of-rate forward RM-cells. TCR is a constant fixed at 10 cells/second.
TCS: Transmission Convergence Sublayer: This is part of the ATM physical layer that defines how cells will be transmitted by the actual physical layer.
TDF: An ABR service parameter, TDF controls the decrease in ACR associated with TOF. TDF is signaled as TDFF, where TDF = TDFF/RDF times the smallest power of 2 greater or equal to PCR. TDF is in units of 1/seconds.
TDFF: Refer to TDF. TDFF is either zero or a power of two in the range 1/64 to 1 in units of 1/cells.
TDM: Time Division Multiplexing: A method in which a transmission facility is multiplexed among a number of channels by allocating the facility to the channels on the basis of time slots.
TE: Terminal Equipment: Terminal equipment represents the endpoint of ATM connection(s) and termination of the various protocols within the connection(s).
TLV: Type / Length / Value: A coding methodology which provides a flexible and extensible means of coding parameters within a frame. Type indicates parameter type. Length indicates parameter's value length. Value indicates the actual parameter value.
TM: Traffic Management: Traffic Management is the aspect of the traffic control and congestion control procedures for ATM. ATM layer traffic control refers to the set of actions taken by the network to avoid congestion conditions. ATM layer congestion control refers to the set of actions taken by the network to minimize the intensity, spread and duration of congestion. The following functions form a framework for managing and controlling traffic and congestion in ATM networks and may be used in appropriate combinations.
TMP: Test Management Protocol: A protocol which is used in the test coordination procedures for a particular test suite.
TNS: Transit Network Selection: A signaling element that identifies a public carrier to which a connection setup should be routed.
TOF: Time Out Factor: An ABR service parameter, TOF controls the maximum time permitted between sending forward RM-cells before a rate decrease is required. It is signaled as TOFF where TOF=TOFF+1. TOFF is a power of 2 in the range: 1/8 to 4,096.
TOFF: Time Out Factor: Refer to TOF.
Topology Aggregation: The process of summarizing and compressing topology information at a hierarchical level to be advertised at the level above.
Topology Attribute: A generic term that refers to either a link attribute or a nodal attribute.
Topology Constraint: A topology constraint is a generic term that refers to either a link constraint or a nodal constraint.
Topology Database: The database that describes the topology of the entire PNNI routing domain as seen by a node.
Topology Metric: A generic term that refers to either a link metric or a nodal metric.
Topology State Parameter: A generic term that refers to either a link parameter or a nodal parameter.
TP-MIC: Twisted-Pair Media Interface Connector: This refers to the connector jack at the end user or network equipment that receives the twisted pair plug.
TPCC: Third Party Call Control: A connection setup and management function that is executed from a third party that is not involved in the data flow.
Trail: An entity that transfers information provided by a client layer network between access points in a server layer network. The transported information is monitored at the termination points.
Trailer: Protocol control information located at the end of a PDU.
Transit Delay: The time difference between the instant at which the first bit of a PDU crosses one designated boundary and the instant at which the last bit of the same PDU crosses a second designated boundary.
Trm: An ABR service parameter that provides an upper bound on the time between forward RM-cells for an active source. It is 100 times a power of two with a range of 100*2-7 to 100*20
TS: Transport Stream: One of two types of streams produced by the MPEG-2 Systems layer. The Transport Stream consists of 188 byte packets and can contain multiple programs.
TS: Traffic Shaping: Traffic Shaping is a mechanism that alters the traffic characteristics of a stream of cells on a connection to achieve better network efficiency, while meeting the QoS objectives, or to ensure conformance at a subsequent interface. Traffic shaping must maintain cell sequence integrity on a connection. Shaping modifies traffic characteristics of a cell flow with the consequence of increasing the mean Cell Transfer Delay.
TS: Time Stamp: Time Stamping is used on OAM cells to compare time of entry of cell to time of exit of cell to be used to determine the cell transfer delay of the connection.
TTCN: Tree and Tabular Combined Notation: The internationally standardized test script notation for specifying abstract test suites. TTCN provides a notation which is independent of test methods, layers and protocol.

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UBR: Unspecified Bit Rate: UBR is an ATM service category which does not specify traffic related service guarantees. Specifically, UBR does not include the notion of a per-connection negotiated bandwidth. No numerical commitments are made with respect to the cell loss ratio experienced by a UBR connection, or as to the cell transfer delay experienced by cells on the connection.
UDP: User Datagram Protocol: This protocol is part of the TCP/IP protocol suite and provides a means for applications to access the connectionless features of IP. UDP operates at layer 4 of the OSI reference model and provides for the exchange of datagrams without acknowledgements or guaranteed delivery.
UME: UNI Management Entity: The software residing in the ATM devices at each end of the UNI circuit that implements the management interface to the ATM network.
Unassigned Cells: A cell identified by a standardized virtual path identifier (VPI) and virtual channel identifier (VCI) value, which has been generated and does not carry information from an application using the ATM Layer service.
UNI: User-Network Interface: An interface point between ATM end users and a private ATM switch, or between a private ATM switch and the public carrier ATM network; defined by physical and protocol specifications per ATM Forum UNI documents. The standard adopted by the ATM Forum to define connections between users or end stations and a local switch.
Unicasting: The transmit operation of a single PDU by a source interface where the PDU reaches a single destination.
UPC: Usage Parameter Control: Usage Parameter Control is defined as the set of actions taken by the network to monitor and control traffic, in terms of traffic offered and validity of the ATM connection, at the end-system access. Its main purpose is to protect network resources from malicious as well as unintentional misbehavior, which can affect the QoS of other already established connections, by detecting violations of negotiated parameters and taking appropriate actions.
Uplink: Represents the connectivity from a border node to an upnode.
Upnode: The node that represents a border node's outside neighbor in the common peer group. The upnode must be a neighboring peer of one of the border node's ancestors.
UT: Upper Tester: The representation in ISO/IEC 9646 of the means of providing, during test execution, control and observation of the upper service boundary of the IUT, as defined by the chosen Abstract Test Method.
UTOPIA: Universal Test & Operations Interface for ATM: Refers to an electrical interface between the TC and PMD sublayers of the PHY layer.
UTP: Unshielded Twisted Pair: A cable having one or more twisted pairs, but with no shield per pair.

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UBR
VBR: Variable Bit Rate: An ATM Forum defined service category which supports variable bit rate data traffic with average and peak traffic parameters.
VC: A communications channel that provides for the sequential unidirectional transport of ATM cells.
VCC: Virtual Channel Connection: A concatenation of VCLs that extends between the points where the ATM service users access the ATM layer. The points at which the ATM cell payload is passed to, or received from, the users of the ATM Layer (i.e., a higher layer or ATM-entity) for processing signify the endpoints of a VCC. VCCs are unidirectional.
VCI: Virtual Channel Identifier: A unique numerical tag as defined by a 16 bit field in the ATM cell header that identifies a virtual channel, over which the cell is to travel.
VCL: Virtual Channel Link: A means of unidirectional transport of ATM cells between the point where a VCI value is assigned and the point where that value is translated or removed.
VCO: Voltage Controlled Oscillator: An oscillator whose clock frequency is determined by the magnitude of the voltage presented at its input. The frequency changes when the voltage changes.
VD: Virtual Destination. Refer to VS/VD.
VF: Variance Factor: VF is a relative measure of cell rate margin normalized by the variance of the aggregate cell rate on the link
Virtual Channel Switch: A network element that connects VCLs. It terminates VPCs and translates VCI values. It is directed by Control Plane functions and relays the cells of a VC.
Virtual Path Switch: A network element that connects VPLs. It translates VPI (not VCI) values and is directed by Control Plane functions. It relays the cell of the VP.
VLAN: Virtual Local Area Network: Work stations connected to an intelligent device which provides the capabilities to define LAN membership.
VP: Virtual Path: A unidirectional logical association or bundle of VCs.
VPC: Virtual Path Connection: A concatenation of VPLs between Virtual Path Terminators (VPTs). VPCs are unidirectional.
VPI: Virtual Path Identifier: An eight bit field in the ATM cell header which indicates the virtual path over which the cell should be routed.
VPL: Virtual Path Link: A means of unidirectional transport of ATM cells between the point where a VPI value is assigned and the point where that value is translated or removed.
VPT: Virtual Path Terminator: A system that unbundles the Vcs of a VP for independent processing of each VC.
VS: Virtual Scheduling: Virtual Scheduling is a method to determine the conformance of an arriving cell. The virtual scheduling algorithm updates a Theoretical Arrival Time (TAT), which is the "nominal" arrival time of the cell assuming that the active source sends equally spaced cells. If the actual arrival time of a cell is not "too" early relative to the TAT, then the cell is conforming. Otherwise the cell is non-conforming.
VS: Virtual Source. Refer to VS/VD.
VS/VD: Virtual Source/Virtual Destination: An ABR connection may be divided into two or more separately controlled ABR segments. Each ABR control segment, except the first, is sourced by a virtual source. A virtual source implements the behavior of an ABR source endpoint. Backwards RM-cells received by a virtual source are removed from the connection. Each ABR control segment, except the last, is terminated by a virtual destination. A virtual destination assumes the behavior of an ABR destination endpoint. Forward RM-cells received by a virtual destination are turned around and not forwarded to the next segment of the connection.
VTOA: Voice and Telephony Over ATM: The ATM Forum voice and telephony over ATM service interoperability specifications address three applications for carrying voice over ATM networks; desktop (or LAN services), trunking (or WAN services), and mobile services.

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WAN: Wide Area Network: This is a network which spans a large geographic area relative to office and campus environment of LAN (Local Area Network). WAN is characterized by having much greater transfer delays due to laws of physics.

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XDF: Xrm Decrease Factor: An ABR service parameter, XDF controls the decrease in ACR associated with Xrm. It is a power of two in range: [0, 1].
Xrm

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