ATM Glossary

AAL - ATM Adaptation Layer - A collection of standardized protocols that adapt user traffic to the cell format. The AAL is subdivided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) sublayer. There are currently 4 types of AALs: AAL1, AAL2, AAL3/4 and AAL5 to support the various AAL service classes

AAL1 - AAL type 1 - Protocol standard used for the transport of time-dependent Constant Bit Rate (CBR) traffic (e.g. audio and video) and the emulation of TDM-based circuits (i.e. DS1, E1). Timing information is required to be exchanged between the source and the destination. AAL1 supports QoS Class A.

AAL2 - AAL type 2 - Protocol standard for supporting time-dependent slow or Variable Bit Rate (VBR-RT) connection-oriented traffic (e.g. packetized and compressed audio and video). Timing information is required to be exchanged between the source and the destination. AAL2 supports QoS Class B.

AAL 3/4 - AAL type 3 and 4 - Protocol standard for supporting both connectionless and connection-oriented Variable Bit Rate (VBR-NRT) traffic. AAL3 supports QoS class C while AAL4 supports QoS class D. They are currently combined into one type. AAL3/4 also performs resequencing and cell identification operations. AAL3/4 services are suitable for supporting interworking with Frame Relay, SMDS and X.25.

AAL5 - AAL type 5 - Protocol standard for supporting connection-oriented Variable Bit Rate (VBR-NRT) data traffic and signaling messages. AAL5 supports QoS Class X. AAL5 services are suitable for supporting interworking with most data networking protocols, such as Frame Relay, SMDS, Ethernet and IP.

ABR - Available Bit Rate - One of five ATM forum defined service categories. In this service type, the network makes the best effort to pass maximum cells but without an absolute guarantee for the cells delivery. Supports variable bit rate data traffic with flow control, a minimum guaranteed data transmission rate and specified performance parameters. In turn for regulating user traffic flow, the network offers minimal cell loss of accepted traffic. Traffic parameters are PCR and MCR. QoS parameters are CLR and CER

AIS - Alarm Indication Signal - One of the OAM function types used for fault management (see also CC, RDI).

AMS - Audiovisual Multimedia Services - ATM forum work group focusing on development of specifications for video-on-demand, voice and video conferencing, broadcast video and other multimedia services over ATM.

ATM - Asynchronous Transfer Mode - A connection-oriented transmission protocol based on a fixed length cell of 53 bytes (including a 5-byte header) in length. It is used for transmission of integrated services, broadband switching and multiplexing with high-performance and cost-effectiveness under certain QoS guarantees.

ATM-ARP - ATM Address Resolution Protocol - An address resolution protocol for mapping ATM and IP addresses (each host is assigned a unique IP address). It can be used for discovering LAN hosts attached to an ATM network or in classical IP over ATM.

ATM-SAP - ATM Service Access Point - The physical interface at the boundary between the AAL and the ATM layer.

AToMMIB - ATM MIB. IETF-defined Management Information Base (MIB) for managing VP/VC links and ATM PVC supported interfaces and services.

ATM Glossary

BEC - Backward Error Correction - An error correction scheme where the sender retransmits any data to be found in error, based on the feedback from the receiver.

BECN - Backward Explicit Congestion Notification - An indicator bit in the Frame Relay header to notify the source of traffic that the virtual circuit is passing through a congested switch. It is set on any traffic flowing from the destination back to the source that passes through the congested switch. There is no equivalent to BECN in ATM.

Best effort - A QoS class in which no specific traffic parameters and no absolute guarantee are provided. Best effort includes UBR and ABR.

B-ICI - Broadband Inter-Carrier Interface - An interface that supports service connections (such as in CRS, CES, SMDS, FR) across public ATM networks.

B-ISDN - Broadband Integrated Services Digital Network - A protocol platform, introduced by ITU-T, to support the integrated, high-speed transmission of data, audio and video in a seamless fashion. ATM emerged as a suitable transport standard. The B-ISDN architecture model has been adopted by ATM, which is referred as the ATM reference model.

ATM Glossary

CAC - Connection Admission Control - An ATM control function which determines whether a virtual connection (VC) request should be accepted or rejected. The decision is based on the QoS required, the network resources available and the availability of the connection over the network.

CAS - Channel Associated Signaling - Voice signaling based on bits taken from voice time slots, used by many PABXs.

CAT-3 - Category 3 UTP - Unshielded Twisted Pair standard, commonly used with ATM for cell transmission at low speeds up to 25 or 51 Mbps at very short distances (few hundred meters).

CAT-5 - Category 5 UTP - Unshielded Twisted Pair standard, commonly used with ATM interfaces for higher-speed cell transmission (more than 50 Mbps).

CBR - Constant Bit Rate - One of the five ATM classes of service, which support the transmission of a continuous bit-stream of information where traffic such as voice and video, require a constant amount of bandwidth allocated to a connection for its duration.

CC - Continuity Cell - A cell used periodically to check whether a connection is idle or has failed. Continuity checking is one of the OAM function types for fault management

CCR - Current Cell Rate - The currently acceptable transmission rate for an end-system as defined by RM cells within ABR. The field in the RM cell indicates the current complying cell rate (i.e. ACR) a user can transmit over a virtual connection (VC).

CDV- Cell Delay Variation - A QoS parameter that measures the difference between a single cell's transfer delay (CTD) and the expected transfer delay. This parameter is important for time sensitive virtual circuits such as CBR and VBR-RT.

CDVT - Cell Delay Variation Tolerance - Used in CBR traffic, it specifies the acceptable tolerance of the CDV (jitter).

Cell - The 53-byte basic information unit within an ATM network. The user traffic is segmented into cells at the source and reassembled at the destination. An ATM cell consists of a 5-byte ATM header and a 48-byte ATM payload, which contains the user data.

CER - Cell Error Rate - A QoS parameter that measures the fraction of transmitted cells that are erroneous over a specific period of time (i.e. those that contain errors when they arrive at the destination).

CES - Circuit Emulation Service - ATM Forum-defined service that provides a virtual circuit connection, which emulates the characteristics of a real, constant-bit-rate, dedicated-bandwidth circuit. 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.

CI - Congestion Indicator - A field in the RM-cell that indicates congestion in the network which can ultimately lead to the source reducing its allow cell rate (ACR).

CIF - Cell Information Field - The payload (48 bytes) of the ATM cell.

CIR - Committed Information Rate - A term used in Frame Relay, which defines the information rate the network is committed to provide the user with, under any network conditions.

Classical IP and ARP over ATM - A standardized model where ATM acts as a high-quality link layer transport for higher-level protocols. The classical model is defined in RFCs 1577 (Laubach, 1993) and 1483 (Heinanen, 1993), and was first deployed ATM architecture.

CLP - Cell Loss Priority - A 1-bit field in the ATM cell header specifying whether a cell is more or less likely to be discarded by an ATM network experiencing congestion.

CLR - Cell Loss Ratio - A QoS parameter that gives the ratio of the lost cells to the total number of transmitted cells.

Congestion control - A resource and traffic management mechanism to avoid and/or prevent excessive situations (buffer overflow, insufficient bandwidth) that can cause the network to collapse. Congestion control schemes may be based on fields within the ATM cell header (CLP, EFCI within the PTI) or may be based on a more sophisticated mechanism between the ATM end-system and ATM switches. The ATM forum has developed a mechanism based on rate control for ABR-type traffic.

CRM - Cell Rate Margin - A measure of the residual useful bandwidth for a given QoS class, after taking into account the SCR.

CRS - Cell Relay Service - A bearer service offered by an ATM network, to the end users that delivers ATM cells directly.

CS - Convergence Sublayer - The sublayer of the ATM AAL where traffic is adapted based on its type before undergoing segmentation into cells (SAR process). The CS includes the CPCS and the SSCS.

CTD - Cell Transfer Delay - A QoS parameter that measures the average time for a cell to be transferred from its source to its destination over a virtual connection (VC). It is the sum of any coding, decoding, segmentation, reassembly, propagation, processing and queuing delays.

ATM Glossary

DCC - Data Country Code - Part of an ATM address, DCC contains the code for the country in which the ATM address is registered.

DI - Direction Indicator - A 1-bit field in the RM-cell to indicate whether it is an RM-cell traveling from the source to the destination or returning to the destination.

DTL - Designated Transmit List - PNNI generated list of ATM switches to be transmitted across an ATM network.

ATM Glossary

E.164 - Public network addressing standardized by ITU-T. Used by both N-ISDN, SMDS and public ATM networks.

EDFG - Edge Device Functional Group - Multilayer LAN switch with MPOA.

EFCI - Explicit Forward Congestion Indication - A 1-bit field in the PTI that contains information whether congestion at an intermediate node has been experienced. The EFCI bit is set when a threshold (e.g. buffer contains) has been exceeded.

ELAN - Emulated LAN - The ATM segment of a virtual LAN (VLAN) based on the ATM forum LANE standard. A VLAN consists of an ELAN segment along with traditional LAN segment.

ENR - Enterprise Network Roundtable - An ATM forum associated group of ATM users to provide feedback on ATM-related issues and also present users with completed interoperable capabilities and functionality.

EPD - Early Packet Discard - A procedure for discarding cells related to one user frame to minimize the impact of congestion. Can be implemented on any virtual circuit using AAL5 since the PTI field indicates last cell.

EPRCA - Explicit Proportional Rate Control Algorithm - One of the flow control algorithms specified for ABR service.

ER - Explicit Rate - A field in the RM cell header specifying the cell rate a user should use for transmission over a virtual connection (VC), as dictated by the RM.

ES - End System - An end system resides in an end device (i.e. CPE at the source or destination of an B-ISDN (e.g. ATM) connection. An end system is responsible for initiating and terminating a connection and for generating and sending back RM-cells.

ESI - End System Identifier - The 6-octet field within the ATM NSAP format address, which identifies an end system. Equivalent to the MAC address of the device.

ATM Glossary

FC - Feedback Control - An end-to-end network control mechanism to regulate the rate at which a sender can inject cells into an ATM network, based on feedback received from the RM-cells.

FD - Frame Discard - A procedure for discarding cells related to one user frame to minimize the impact of congestion. Can be implemented on any virtual circuit using AAL5 since the PTI field indicates last cell.

FECN - Forward Explicit Congestion Notification - An indicator bit in the Frame Relay header to notify the destination of traffic that the virtual circuit is passing through a congested switch. The EFCI bit in ATM is equivalent to FECN.

Flow control - A congestion control mechanism that results in an ATM system implementing flow control.

Frame Relay - An efficient packet switching technology to provide reliable packet delivery over virtual circuits. The link layer handles much of the network layer functionality. Some of the concepts used in frame relay have been incorporate in ATM networks.

FUNI - Frame User Network Interface - Frame-based interface to ATM supporting signaling and QoS. To interoperate with a Frame Relay end system, the ATM switch should support FRF.8 which is the Frame Relay/ATM Service Internetworking specification. Replaces the ATM-DXI.

ATM Glossary

GCAC - Generic Connection Admission Control - A form of CAC used by PNNI when routing a connection request.

GCRA - Generic Cell Rate Algorithm - A reference model proposed by the ATM forum for defining cell rate conformance in terms of certain traffic parameters for VBR virtual circuits. Its implementation is known as the Leaky Bucket algorithm.

GFC - Generic Flow Control - A four-bit field within the ATM cell header which may be used to identify whether or not an ATM system implements congestion control.

ATM Glossary

HEC - Header Error Check - A one-byte field within the ATM cell header providing for error detection. If an error is detected, the cell will be discarded before undergoing reassembly.

ATM Glossary

ICR - Initial Cell Rate - The rate that a source is allowed to start up at following an idle period. It is established at connection set-up and is between the MCR and the PCR.

IETF - Internet Engineering Task Force - A body, which was initially responsible for developing specification, required for the interoperable implementation of IP. One of the issues IETF has been focusing on is the implementation of classical IP over ATM.

IISP - Interim Interswitch Signaling Protocol - A protocol that uses UNI-based signaling (i.e. UNI 3.0/3.1) and pre-fix routing for switch-to-switch communication. Formally known as PNNI Phase 0.

ILMI - Interim Local Management Interface - ATM forum SNMP-based network management interface between an end-system and an ATM switch for status and configuration reporting as well as registering/de-registering ATM addresses.

IMA - Inverse Multiplexing over ATM - A method to pass ATM traffic over multiple E1/T1 links while keeping the ATM's quality of service and optimize the bandwidth usage.

IP - Internet Protocol - A networking protocol for providing a connectionless service to the higher transport protocol. It is responsible for discovering and maintaining topology information and for routing packets across homogeneous networks. Combined with TCP, it is commonly known as the TCP/IP platform.

I-PNNI - Integrated Private Network-Network Interface - Protocol used to exchange reachability information between routers that augment or replace protocols such as OSPF and IPX and is compatible with PNNI. This enables the integration of existing router-based connectionless networks with ATM networks.

ISDN - Integrated Service Digital Networks - An early, CCITT-adopted protocol reference model intended for providing a ubiquitous, end-to-end, interactive, digital service for data, audio and video. ISDN is available as BRI, PRI and B-ISDN.

ATM Glossary

ATM Glossary

ATM Glossary

LANE - LAN Emulation - An ATM forum standard providing for the support of native LAN protocols across an ATM network by emulating the MAC protocol. LANE defines a single Virtual LAN (VLAN) consisting of traditional LAN segments and an emulated LAN (ELAN) segment across the ATM network. Routers will connect multiple VLANs. LANE provides for ATM attached systems acting as LECs communicating with a LECS, LES and BUS across an ELAN.

LAP-B - Link Access Procedure-Balanced - A data link layer protocol, defined by ITU-T, used in X.25. It is very similar to HDLC.

LAP-D - Link Access Procedure for the D-channel - A data link layer protocol, defined by ITU-T, for use in ISDN. The LAP-D protocol is based on HDLC.

Leaky Bucket - A flow control algorithm, where cells are monitored to check whether they comply with the established connection parameters. Non-conforming cells are either tagged or dropped from the network. The analogy is taken from a bucket with a hole in its bottom that allows the fluid to flow out at a certain rate.

LEC - Line Emulation Client - Typically located in the ATM end system, its task in LAN Emulation is to maintain address resolution tables and to forward data traffic. It is uniquely associated with an ATM address.

LECS - LAN Emulation Configuration Server - A server, used in LAN emulation, whose main function is to provide configuration information to an LEC.

LES - LAN Emulation Server - A server function within LANE providing for LEC registration, MAC to ATM address resolution and security management of a VLAN.

LIS - Logical IP Subnetwork - An IP subnetwork is a single network on which all devices have a direct communication path to all other devices. Examples would be a LAN or a point-to-point circuit. An LIS is a network in which the IP protocol can operate as if all devices are directly connected, even if they are not, such as a virtual circuit-based network.

LME - Layer Management Entity - An entity to support and facilitate ATM management layer (OAM) services and functions.

LMI - Local Management Interface - An ITU-T defined interface to provide an ATM end system user with network management information.

LNNI - LAN Emulation Network Node Interface - the interface between two LANE domains.

LUNI - LAN emulation User Network Interface - specifies the UNI between a LEC and the network providing the LAN emulation.

ATM Glossary

MAC - Media Access Control - A protocol that defines the way workstations gain access to transmission media, most widely used in reference to LANs. For IEEE LANs, the MAC layer is the lower sublayer of the data link layer protocol.

MAN - Metropolitan Area Network - A network that provides regional connectivity within a metropolitan area (such as city). MANs are categorized between LANs and WANs.

MARS - Multicast Address Resolution Server - A mechanism to resolve IP and ATM multicast address.

MBS - Maximum Burst Size - A traffic parameter that specifies the maximum number of cells in a burst that can be transmitted at the peak rate (PCR).

MCR - Maximum Cell Rate - An ABR traffic parameter (in cells per second) that gives the slowest rate that the network controls the flow of the source on an ABR virtual connection (VC).

MCS - Multi Cast Server - Device for efficient forwarding of multicast data within the classical model and MPOA.

MCTD - Mean Cell Transfer Delay - The average delay for ATM cells across an ATM connection.

MIB - Management Information Base - A database of device configuration and performance information which is acted upon by SNMP or CMIP.

MPC - MPOA Client - Group of functions that typically reside in an edge device or host and implement the internetworking capabilities (no routing) of the MPOA protocols.

MPOA - Multi-Protocol Over ATM - IETF defined specifications and procedures that enable Network Layer protocols to operate directly on top of ATM and provide end-to-end internetworking between hosts in an ATM and non-ATM environment.

MPS - MPOA Server - Group of functions to provide internetworking support to an MPOA system components.

ATM Glossary

NHRP - Next Hop Resolution Protocol - An IETF protocol for ATM address resolution between MPOA servers.

NIC - Network Interface Card - The hardware communication interface (circuit board) required for the DTE (workstation, PC) to access the network.

NMS - Network Management System - The hardware and software supporting network management (OAM&P) functions.

NNI - Network Node Interface (or Network-to-Network Interface) - ITU-T specified standard interface between nodes within the same network. The ATM forum distinguishes between two standards, one for private networks called P-NNI and one for public networks known as public NNI.

ATM Glossary

OAM - Operation Administration and Maintenance Management framework defined by the ITU. OAM cells are special purpose ATM cells exchanged between two ATM entities for network fault and performance management, analysis and fault isolation.

ATM Glossary

Payload - the 48-byte segment of the ATM cell containing user data. Any adaptation of user data via the AAL will take place within the payload.

PCR - Peak Cell Rate - A traffic parameter (in cells per second) that characterized the source and gives the maximum rate at which cells can be transmitted. It is calculated as the reciprocal of the minimum intercell interval (the time between two cells) over a given virtual connection (VC).

PGL - Peer Group Leader - ATM switch within a PNNI peer group, which summarizes reachability within the group and sends this information up the routing hierarchy.

PHY - Physical layer - The bottom layer of the ATM protocol reference model, it is subdivided into two sublayers: Transmission Convergence (TC) and Physical Medium (PM). It provides ATM cell transmission over the physical interfaces that interconnect the ATM devices.

PNNI - Private Network-Network Interface - The interswitch interface within a private ATM domain. The PNNI trunking protocol providing for hierarchical ATM-layer routing and QoS support.

Policing - A method to verify that the incoming VC comply with the user's service contract.

PTI - Payload Type Identifier - A 3 bit field within the ATM cell header indicating the AAL used, whether a congestion has been experienced (EFCI), and whether or not the cell contains OAM information. When an AAL5 frame passes through SAR, the PTI within the last cell identifies the end of this AAL5 frame.

PVC - Permanent Virtual Circuit - A virtual connection established by the network management between an origin and a destination that can be left up permanently.

PVP - Permanent Virtual Path - A term to describe a set of grouped together permanent virtual channels (PVCs) that exist between two cross points.

ATM Glossary

QoS - Quality Of Service - A group of service classes defined by the ATM forum in terms of different QoS parameters:

Class 0 refers to the best effort service (UBR).

Class 1 specifies the parameters for circuit emulation, and the transport of CBR uncompressed video and for VPNs. AAL1 supports this kind of delay sensitive connection oriented service.

Class 2 specifies the parameters for the transport of VBR (low speed or compressed packetized) audio and video. AAL2 supports this delay sensitive, connection oriented class.

Class 3 specifies the parameters for connection oriented data transfer. AAL3/4 and mostly AAL5 supports this delay tolerant class which is intended to provide interoperability with SMDS and IP.

Class 4 specifies the parameters for connectionless data transfer. AAL3/4 or AAL5 can be used to support this delay tolerant class which is also intended to provide interoperability with SMDS and IP.

Class X refers to the connection oriented transport service where the traffic type (CBR or VBR) and timing requirements (delay sensitive or non-sensitive) are defined by the user. It is known as an unrestricted service class and which is supported by AAL5.

ATM Glossary

RDI - Remote Defect Indication - One of the OAM function types used for fault management.

RDF - Rate Decrease Factor - An ABR service factor by which a source should decrease its transmission rate if there is congestion.

RFC 1483 - Specifies the encapsulation of multiprotocol data for transmission over an ATM network. RFC 1483 make use of AAL5 in the support of PVCs and SVCs. The two methods defined in this RFC are VC muxing and LLC/SNAP encapsulation.

RFC 1490 - Specifies the encapsulation of multiprotocol data for transmission over Frame Relay. Frame Relay over ATM uses this encapsulation in combination with AAL5.

RFC 1577 - Specifies an architecture for the support of IP over ATM, also defining an ATM ARP server. RFC 1577 relies on RFCs 1483, 1626, and 1755.

RFC 1626 - Defines a 9180 octet Maximum Transmission Unit (MTU) over ATM.

RFC 1755 - Specifies the method of signaling SVCs within the classical model.

RIF - Rate Interface Factor - An ABR factor by which a source can increase its transmission rate if the RM cell indicates no congestion.

RM - Resource Management cell - A cell used for carry network resource management information, such as available buffer space, residual bandwidth and indicate the presence of any congestion in the network. RM cells are injected in regular data cells streams in order to monitor and report the network conditions. A value of 6 is reserved in the PTI field of an ATM cell to indicate an RM cell.

ROLC - Routing Over Large Clouds - Protocol specifications defined by the IETF for supporting efficient IP routing over large networks.

Routing - Layer 3 forwarding of datagrams based on a calculated routing table.

RSVP - Resource reSerVation Protocol - A protocol developed for supporting different QoS classes for IP applications.

RTT - Round Trip Time - The round trip time it takes for a packet to travel between a source and a network device. In ATM, RTT is usually measured in numbers of cells.

ATM Glossary

SAAL - Signaling AAL - Service specific parts of the AAL protocol responsible for signaling. Its specifications, were adopted from N-ISDN.

SAP - Service Access Point - Reference point between the networking stack and applications within an end system.

SAR - Segmentation And Reassembly - Segments the information frames into cells at the source and reassembled these cells back into information frames at the destination. These activities occur at the lower part of the AAL. Each AAL type has its own SAR format.

SAR-PDU - Segmentation And Reassembly Protocol Data Unit - The 48 octet PDU that the SAR sublayer exchanged with the ATM layer. It is comprised of the SAR-PDU payload and any control information that the SAR sublayer might add.

SCR - Sustainable Cell Rate - A traffic parameter that characterized a bursty source and specified the maximum average rate at which cells can be sent over a given virtual connection (VC). It can be defined as the ratio of the MBS to the minimum burst interarrival time.

SEAL - Simple Efficient Adaptation Layer - ATM AAL5 was originally defined as SEAL, providing adaptation with no additional cell payload overhead for data traffic.

Service types - There are four service types CBR, VBR, UBR and ABR. CBR and VBR are guaranteed service while UBR and ABR are described as best effort services.

SIR - Sustained Information Rate - A flow control mechanism used in SMDS.

SLIP - Serial Link Internet Protocol - An Internet protocol for host dial-up connection. SLIP frames are encapsulated IP datagrames in which SLIP adds just a few bytes of control data.

SMDS - Switched Multimegabit Data Service - A connectionless, fast packet switching B-ISDN service, based on 53 byte packets. It originally targeted the interconnection of different LANs into a switched public network. It is used also to interconnect WANs and MANs at speeds up to 45 Mbps.

SNMP - Simple Network Management Protocol - Management protocol for the OAM&P of internetworks. Originally designed for TCP/IP devices but now extended to other products and functions.

SPVC - Soft Permanent Virtual Connection - A PVC type connection where SVCs services are used for virtual circuits establishment and routing within the network. It is also called Smart PVC.

STP - Shielded Twisted Pair - Cable consisting of a pair of twisted wires surrounded by a metallic shield.

SVC - Switched Virtual Circuit - A logical ATM connection established via signaling. End systems transmit their UNI 3.1 or 4.0 signaling request via the Q.2931 signaling protocol.

SVPC - Switched Virtual Path Connection - A virtual path connection that is set up and turn down dynamically through signaling procedures.

ATM Glossary

TC - Transmission Convergence layer - A sublayer within the physical layer where cells are prepared for transmission by the PMD.

TCP/IP - Transmission Control Protocol/ Internet Protocol - A protocol platform, known also as the Internet protocol suite, that combines both TCP and IP. Widely used applications, such as Telnet, FTP and SMTP, interface to TCP/IP.

TM - Traffic Management - Set of actions and operations performed by the network to guarantee the operability of the network. TM is exercised in the form of traffic control and flow control. The ATM traffic management includes the following: CAC, FRM, NRM, Priority Control (PC), Traffic Shaping (TS) and UPC , which the goal is to maintain the required QoS.

Traffic contract - An agreement between the user and the network management agent regarding the expected QoS provided by the network and the user's compliance with the pre-determined traffic parameters (i.e. PCR, MBS, burstiness, average cell rate).

Traffic descriptors - A set of parameters that characterize the source traffic. These are PCR, MBS, CDV and SCR.

Traffic policing - Mechanism whereby any traffic, which violates the traffic contract, agreed to at connection setup is detected and discarded.

Traffic shaping - A method for smoothing the bursty traffic rate that might arrive on an access virtual circuit so as to present a more uniform traffic rate on the network and comply with the traffic contract.

ATM Glossary

UBR - Unspecified Bit Rate - One of the best effort service types. Realistically, no traffic parameters are specified by the source, so , no actual quality commitment is made by the network.

UDP - User Datagram Protocol - A connectionless transport protocol without any guarantee of packet sequence or delivery. It functions directly on top of IP.

UNI - User Network Interface - The interface, defined as a set of protocols and traffic characteristics, between the CPE and the ATM network.

UNI 2.0 - ATM forum UNI specification for the physical (PHY) and the ATM layers, the ILMI, OAM (traffic control) and PVC support.

UNI 3.0 - An upgrade of UNI 2.0 that includes traffic control for PCR and some recommendations regarding the operation over current transmission systems.

UNI 3.1 - A correct version of UNI 3.0, this specification also includes SSCOP standards.

UNI 4.0 - This UNI specification refers to signaling issues in ABR, VP and QoS negotiations.

UPC - Usage Parameter Control - A form of traffic control that checks and enforces user's conformance with the traffic contract and the QoS parameters. Commonly known as traffic policing, it is performed at the UNI level.

UTOPIA - Universal Test and Operation Physical Interface for ATM - An interface to provide connectivity at the PHY level among ATM entities.

UTP - Unshielded Twisted Pair - Cable consists of twisted pair wires which are not surrounded by shielding.

ATM Glossary

VBR - Variable Bit Rate - Traffic containing bursts but centered around an average bandwidth. VBR, divided into real-time (RT-VBR) and non-real-time (NRT-VBR) traffic requires the same service guarantees (that is delay, cell loss and timing) provided by CBR.

VBR-NRT - Variable Bit Rate Non Real Time - One of the two VBR service types for transmitting traffic where timing information is not critical. Since this service type is delay tolerant, it is well suited for bursty traffic such as data communications.

VBR-RT - Variable Bit Rate Real Time - One of the two VBR service types for transmitting traffic that depends on timing and control information. It is suitable for carrying delay sensitive traffic such as packetized video and audio.

VC - Virtual Connection - A connection established between end users, where packets are forwarded along the same path and bandwidth is not permanently allocated until it is used.

VCC - Virtual Channel Connection - An end to end connection consisting of a concatenation of two or more Virtual Channels between two end points. VCCs may be bundled into a VPC.

VCI - Virtual Channel Identifier - A 16 bit value in the ATM cell header that provides a unique identifier for the virtual channel (VC) within a virtual path (VP) that carries that particular cell.

VLAN - Virtual LAN - A network architecture which allows geographically distributed users to communicate as if they were on a single physical LAN by sharing a single broadcast and multicast domain. ATM forum LAN emulation supports VLANs.

VOA - Video Over ATM - Specifications and standards to provide enhanced quality video services carried over an ATM network.

VP - Virtual Path - A term to describe a set of Virtual Channels (VCs) between cross points, grouped together.

VPC - Virtual Path Connection - An end to end connection consisting of two more Virtual Path links (VPs)

VPI - Virtual Path Identifier - An 8-bit value in the cell header that identifies the VP - and accordingly the virtual channel - the cell belongs to.

VPN - Virtual Private Network- Public network service where a customer is provided a network which appears as if it were a private network. The advantage of VPNs, over the dedicated private networks, is that the former allow a dynamic use of the network resources and offer a very reliable, high speed and less expensive communications.

VTOA - Voice and Telephony over ATM - ATM forum group developing standards for voice trunking, signaling, and N-ISDN interworking.

ATM Glossary

WAN - Wide Area Network - A network that typically spans nationwide distances and usually utilized public telephone networks.

WATM - Wireless ATM - An emerging technology for interfacing and/or combining wireless and ATM networks.

ATM Glossary

ATM Glossary

ATM Glossary