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ATM Glossary

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01 Feb 1997

AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) A standards organization that participated in developing the BroadbandIntegrated Services Digital Network (B-ISDN) standards, which were the firstAsynchronous Transfer Mode (ATM) standards.

ASYNCHRONOUS TRANSFER MODE (ATM) A high-speed, connection-oriented, data transmission technology thattransmits fixed-size cells rather than variable-length packets.

ATM ADAPTATION LAYER The highest layer of the ATM model, which roughly corresponds to thenetwork layer of the Open Systems Interconnection (OSI) model. The ATM adaptationlayer consists of four AAL protocols, each of which formats packets differentlyto support different types of transmissions, such as audio, video, and data.

ATM-ATTACHED DEVICE A device, such as an end station, file server, router, or bridge, thatis directly connected to the ATM network.

ATM CLOUD The ATM network.

ATM FABRIC The ATM network.

ATM FORUM A consortium of ATM vendors that develops ATM standards.

ATM LAYER The middle layer of the ATM model that sits between the physical layerand the ATM adaptation layer. The ATM layer roughly corresponds to the data-linklayer of the OSI model. Standards at the ATM layer specify how to establish,clear, and maintain ATM virtual circuits.

AVAILABLE BIT RATE (ABR) An ATM service category that is used for data traffic. The ABR categorycan tolerate delays. For each data transmission, ABR negotiates a rangeof acceptable bandwidths and an acceptable cell loss ratio (the number ofcells that can be lost in any transmission).

BROADBAND INTEGRATED SERVICES DIGITAL NETWORK (B-ISDN) The first ATM specifications, ratified by ANSI and by the InternationalTelegraph and Telephone Consultative Committee (CCITT). B-ISDN specifiesstandards for a high-speed network using ATM as its transport mechanism.

CELL A small, fixed-length packet. ATM cells are 53 bytes long, comprisedof a five-byte header and 48 bytes of data.

CELL-RELAY A type of network that transmits data in cells rather than in packets.

CLASSICAL IP OVER ATM An Internet Engineering Task Force (IETF) standard that enables you toroute Internet Protocol (IP) packets over an ATM backbone by encapsulatingIP packets inside ATM cells.

CONSTANT BIT RATE (CBR) An ATM service category that is used for time-sensitive traffic suchas audio and video. CBR guarantees that audio and video cells arrive ontime, with a minimal variation in the spacing between cells, by reservingbandwidth for a virtual circuit.

EMULATED LAN In a LAN emulation (LANE) environment, an emulated LAN is a virtual networkmade up of end stations on more than one physical LAN. These workstationscommunicate as if they were located on the same physical LAN.

INTEGRATED PRIVATE NETWORK-TO-NETWORK INTERFACE (IPNNI) A routing protocol for ATM that enables IP and ATM routing devices toshare information about the network topology.

INTERNATIONAL TELECOMMUNICATIONS UNION (ITU) A standards organization that grew out of the CCITT, which participatedin the development of the B-ISDN standards. All previous CCITT standardsare now ITU standards.

INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE (CCITT) A standards organization that participated in developing the first ATMstandards. The CCITT has since become the ITU.

INTERNET ENGINEERING TASK FORCE (IETF) A standards organization for TCP/IP networking. The IETF developed ClassicalIP Over ATM.

IP SWITCHING A proprietary method of enabling IP users to send IP packets over anIP backbone. ATM switches in an IP switching environment do not use theATM Forum standards.

LAN EMULATION (LANE) A set of ATM Forum specifications that enables you to bridge LANs overan ATM backbone, creating a single virtual network or ELAN. The ATM Forumhas defined LANE 1.0 and will finalize LANE 2.0 in March 1997.

LAN EMULATION NETWORK-TO-NETWORK INTERFACE (LNNI) A LANE 2.0 standard that will define the interface between two LANE serverswithin the same virtual network, or ELAN.

LAN EMULATION USER-TO-NETWORK INTERFACE (LUNI) A LANE standard that specifies the components needed to connect two LANsusing an ATM backbone, to create one virtual network, or ELAN.

LATENCY The time that lapses between the point at which an end station seeksaccess to the transmission medium and the point at which the end stationreceives that access.

MULTIPROTOCOL OVER ATM (MPOA) An ATM Forum standard that enables you to route protocols from traditionalLANs over an ATM backbone. MPOA contrasts with LANE, which enables you tobridge protocols but does not provide routing.

NETWORK-TO-NETWORK INTERFACE (NNI) A generic term that describes the interface between two switches on aswitched network.

NEXT HOP ROUTING PROTOCOL (NHRP) A protocol being developed by the IETF that will add routing capabilitiesto Classical IP over ATM.

OPEN SYSTEMS INTERCONNECTION (OSI) MODEL A seven-layer networking model developed by the International StandardsOrganization (ISO). The OSI model is a guideline for developing standardsto enable dissimilar network computing products to communicate.

PERMANENT VIRTUAL CIRCUITS (PVCS) Virtual circuits that the network supervisor or ATM service providersets up when configuring the ATM network. Bandwidth is always reserved forthe virtual circuit, whether the PVC is in use or not, so PVCs are alwaysavailable for immediate use.

PHYSICAL LAYER The lowest layer of the ATM model, roughly corresponding to the OSI physicallayer. Standards for the ATM model's physical layer, like those for theOSI model's physical layer, specify how to send bits over the transmissionmedium.

PRIVATE NETWORK-TO-NETWORK INTERFACE (PNNI) An ATM Forum standard that defines the interface between two switcheson an all-ATM network. It enables switches to distribute routing information.

QUALITY OF SERVICE PARAMETERS Parameters that specify how many cells can be lost during a transmission,how long it can take for cells to reach their destination, and how muchthe amount of time between cells can vary. Quality of Service parametersare used to determine the quality of service a given virtual circuit willprovide.

SERVICE CATEGORIES Classes of service that are used to provide different levels of servicefor different types of traffic. ATM service categories are CBR, VBR, UBR,and ABR.

SWITCH A device that sets up a virtual circuit and forwards cells. Switchesact like routers while setting up virtual circuits; that is, they determinethe best path for the cells to take. Once the virtual circuit has been setup, they act as bridges, simply forwarding cells.

SWITCHED VIRTUAL CIRCUITS (SVCS) Virtual circuits that ATM end stations establish on-demand, when theyneed to communicate. Different types of SVCs exist for different amountsof time.

UNSPECIFIED BIT RATE (UBR) An ATM service category that is used for data traffic such as TCP/IP,which can tolerate delays. UBR does not reserve any bandwidth for a connection.

USER-TO-NETWORK INTERFACE (UNI) The ATM Forum standard that defines the interface between an end stationand a switch on an ATM network. Also, a generic term used to describe theinterface between an end station and a switch on a switched network.

VARIABLE BIT RATE (VBR) An ATM service category that is used for time-sensitive traffic suchas frame relay. Like CBR, VBR reserves a certain amount of bandwidth forthe connection. Unlike CBR, VBR can tolerate delays.

VIRTUAL CIRCUIT (VC) The connection between two ATM end stations for the duration of the connection.

VIRTUAL SUBNET A logical LAN made up of end stations on more than one ATM-connectedLAN, communicating as if they were located on the same physical LAN.

Where to Go for More Information


  • Introduction to ATM Networkingby Walter J. Goralski (McGraw Hill, 1995) provides a good technical description of Asynchronous Transfer Mode (ATM).

  • Understanding ATMby Stan Schatt (McGraw-Hill, 1996) includes a basic explanation of ATM and discusses several ATM vendors and products.


  • Datamationalways includes several technical but readable articles about ATM and ATM interoperability standards.

  • Dobrowski, George."Standards Progress Steadies Advances."LAN Times, September 16, 1996: 84, 88. This article, written by the ATM Forum's technical chair, includes useful information about LAN Emulation (LANE), MultiProtocol Over ATM (MPOA), Private Network-to-Network Interface (PNNI), and other ATM interoperability standards.


  • The ATM Forum site ( contains useful ATM information, including the ATM Forum's technical specifications.

  • The Cell Relay Retreat ( contains a great deal of ATM information, links to other ATM pages, and drafts of International Telecommunications Union (ITU) specifications.

  • Datamation's site ( provides archives of some of its ATM articles and includes a good ATM glossary (

  • The International Telecommunication Union site ( contains ITU standards.

  • Ipsilon's home page ( provides information about IP switching and informational Requests for Comments (RFCs).

  • The RFC 1577 page ( contains the Classical IP Over ATM specification.

  • Vendors' home pages, listed in "ATM Product Vendors," usually include detailed and easy-to-understand ATM information.

* Originally published in Novell Connection Magazine


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