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Glossary of ATM Terms and Acronyms

ATM Terms

AAL---See ATM Adaptation Layer.

AAL 1---Supports connection-oriented services that require constant bit rates and have isochronous-like timing and delay requirements. Example: circuit emulation service.

AAL 3/4---Intended for both connectionless and connection-oriented variable rate services. Originally two distinct layers, they have merged into this single AAL.

AAL 5---Supports connection-oriented variable bit rate data services such as the typical "bursty" data traffic found in today''s LANs.

ABR---See Available Bit Rate.

Asynchronous---Signals that are sourced from independent clocks. These signals generally have no relation to each other and have different frequencies and phase relationships.

Asynchronous Transfer Mode (ATM)---A form of digitized data transmission based on fixed-length cells that can carry data, voice, and video at high speeds.

ATM---See Asynchronous Transfer Mode.

ATM Adaptation Layer (AAL)---One of the three layers of the ATM protocol reference model. It performs Segmentation and Reassembly (SAR); translates higher-layer data into ATM cell payloads, and translates incoming cells into a format readable by the higher layers.

ATM Endpoint---The point in an ATM network where an ATM connection is terminated or initiated. This includes ATM-attached workstations, ATM-attached servers, ATM-to- LAN switches, or ATM routers.

Available Bit Rate (ABR)---An ATM service type in which the ATM network makes a "best effort" to meet the transmitter's bandwidth requirements. ABR differs from other "best effort" service types by employing a congestion feedback mechanism that the ATM network uses to notify the transmitters that they should reduce their rate of data transmission until the congestion situation ceases. Thus, ABR does offer a qualitative guarantee that the transmitter's data should get to the intended receivers without experiencing unwanted cell loss. (see UBR)

Broadband ISDN (B-IDSN)---A set of services, capabilities, and interfaces supporting an integrated network and user interface at speeds greater than that of ISDN. The ITU-T initially decided to develop ATM for B-ISDN networks in 1988.

Broadcast and Unknown Server (BUS)---A LAN Emulation component that receives all broadcast and multicast MAC packets as well as MAC packets with an unknown ATM address and transmits these messages to every member of an emulated LAN.

CAC---See Call Admission Control.

Cell Loss Priority (CLP)---A one-bit descriptor found in ATM cell headers, indicating the relative importance of a cell. If set to 0, the cell should not be discarded. If set to 1, the cell can be discarded if necessary.

Circuit Steering---A mechanism that some ATM switches may employ to eavesdrop on a selected virtual connection and copy its cells to another port where an ATM analyzer is attached. (see Port Snooping)

CLP---See Cell Loss Priority

Congestion Collapse---A condition where re-transmission of frames are compounding an already congested ATM network or switch experiencing even small amounts of cell loss. Congestion collapse ultimately results in little or no traffic "goodput." ATM networks that do not employ ATM switches with either adequate and effective buffering mechanisms complimented with intelligent packet discard or ABR congestion feedback mechanisms frequently suffer congestion collapse.

Constant Bit Rate (CBR)---A data transmission that can be represented by a nonvarying, or continuous, stream of bits or cell payloads. Applications such as voice circuits generate CBR traffic patterns. CBR is an ATM service type in which the ATM network guarantees to meet the transmitter's bandwidth and Quality of Service (QoS) requirements. (see ABR, UBR, VBR)

Connection Admission Control (CAC)---The set of actions taken by the network during the connection setup phase in order to determine whether a connection's requested QoS can be accepted or should be rejected. (Generic) CAC is also used when routing a connection request through an ATM network (see PNNI).

Connection-Oriented---A type of communication in which a connection must be established between senders and receivers before data transmission can occur.

Data Exchange Interface (DXI)---A frame-based interface between a packet-based router and an ATM-capable DSU. One of the ATM Forum standard specifications.

DXI---See Data Exchange Interface.

Dynamic IISP---Interim-Interswitch Signaling Protocol (IISP) is a basic call routing scheme agreed to while waiting for PNNI Phase 1 completion. IISP does not provide a mechanism for routing connections automatically around link failures; Dynamic IISP does route connections automatically around link failures. (see Interim-Interswitch Signaling Protocol)

Early Packet Discard (EPD)---A mechanism employed by some ATM switches that allows a complete AAL 5 frame to be discarded when a threshold condition is meet, such as one indicating that congestion is imminent. Useful for avoiding unwanted congestion situations that may jeopardize the switch's ability to properly support existing connections with a guaranteed service. (see Trail Packet Discard)

Explicit Forward Congestion Identification (EFCI)---One of the congestion feedback modes allowed in the Available Bit Rate (ABR) service. In the EFCI marking mode, switches can set a bit in the headers of forward cells to indicate congestion; these are then turned around at the destination end system and sent back to the source end system. (see ABR)

Explicit Rate Mode (ER)---One of the congestion feedback modes allowed in the Available Bit Rate (ABR) service. In the Explicit Rate Marking mode, switches can indicate within either forward and backward Resource Management (RM) cells (or both) exactly which rate they are willing to receive along a particular connection. (see ABR)

Generic Call Admission Control (GCAC)---see Call Admission Control and PNNI

Goodput---Generally referring to the measurement of actual data successfully transmitted from the sender(s) to receiver(s). This is often a more useful measurement than the number of ATM cells per second throughput of an ATM switch if that switch is experiencing cell loss that results in many incomplete, and therefore unusable, frames arriving at the recipient.

ILMI---See Interim Local Management Interface.

Interim-Interswitch Signaling Protocol (IISP)---Formerly know as PNNI Phase 0 as originally proposed by Cisco Systems. IISP is a basic call routing scheme agreed to while waiting for PNNI Phase 1 completion. IISP relies on static routing tables established by the network administrator. (see Dynamic IISP)

Interim Local Management Interface (ILMI)---An SNMP-based management protocol defined by the ATM Forum that provides interface status and configuration information and is used to register and/or acquire ATM addresses.

Isochronous---Signals that are dependent on some uniform timing or carry their own timing information embedded as part of the signal.

LAN Emulation (LANE)---Defines how an ATM network emulates enough of the MAC protocol of an existing IEEE LAN, specifically Ethernet and Token Ring. LAN Emulation allows existing higher-layer protocols and applications to be used transparently over an ATM network interconnecting Ethernet and Token Ring LANs. This subworking group in the ATM Forum is chaired by Cisco Systems.

LAN Emulation Client (LEC)---Software that resides in every LAN device attached to an ATM network, such as workstations, routers, and LAN switches. The LEC keeps learned address translation information and establishes the direct connection necessary for communication with other devices over the ATM network.

LAN Emulation Configuration Server (LECS)---The LAN Emulation component that maintains configuration information about the ATM network and enables network administrators to control which physical LANs are combined to form VLANs.

LAN Emulation Server (LES)---The LAN Emulation component that provides MAC address-to-ATM address resolution services for LAN Emulation over an ATM network.

LAN Emulation User-to-Network Interface (L-UNI)---The ATM Forum standard for LAN Emulation on ATM networks; defines the interface between the LAN Emulation Client (LEC) and the LAN Emulation Server components. (see BUS, LES, LECS)

Media Access Control (MAC)---The lower half of the data-link layer that governs access to the available IEEE and ANSI LAN media.

Multiprotocol over ATM (MPOA)---A relatively new standardization effort in the ATM Forum that plans to specify how existing and future network-layer protocols such as IP, IPv6, Appletalk or IPX running over an ATM network with directly attached hosts, routers, and multilayer LAN switches will exploit the unique benefits of ATM. These benefits include quality of service (QoS) and direct connections between different VLANs. This subworking group in the ATM Forum is chaired by Cisco Systems.

NNI---See Private Network-to-Network Interface

OAM---See Operations and Maintenance

Operations and Maintenance (OAM)---A number of preventative maintenance principals recommended by the ITU for B-ISDN networks. In an ATM network, OAM information is generally passed around in specified cells.

Payload Type Identifier (PTI)---A 3-bit descriptor found in ATM cell headers, indicating what type of payload the cell contains. Payload types include user and management cells; one combination indicates that this cell is the last cell of an AAL5 frame.

PDU---See Protocol Data Unit.

Permanent Virtual Connection (PVC)---A logical (rather than physical) connection between endpoints established by an administrator that stays intact until manually torn down.

PNNI---See Private Network-to-Network Interface.

Point-to Multipoint---A unidirectional connection with one root endpoint and several leaves. See Point-to-Point.

Point-to-Point---A unidirectional or bidirectional connection with only two endpoints. See Point-to-Multipoint.

Port Snooping---A mechanism that some ATM switches may employ to eavesdrop on a selected ATM port and copy all those cells to another port where an ATM analyzer is attached. (see Circuit Steering)

Private Network-to-Network Interface (NNI)---Also known as a Private Network-to-Node Interface. The interface between ATM switches or ATM switches and an entire switching system in a private network. The PNNI exchanges much more topological and complex quality of service (Q0S) routing information than the UNI.

Protocol Data Unit (PDU)---A unit of data consisting of control information and user data exchanged between peer layers.

PTI---See Payload Type Identifier.

PVC---See Permanent Virtual Connection.

Relative Rate (RR) Mode---One of the congestion feedback modes allowed in the Available Bit Rate (ABR) service. In the Relative Rate Marking mode, switches can set a bit within either forward and backward Resource Management (RM) cells (or both) to indicate congestion. (see ABR)

SVC---See Switched Virtual Connection.

Switched Virtual Connection (SVC)---A logical (not physical) connection between endpoints established by the ATM network on demand after receiving a connection request from the source/root. It is defined in the ATM Forum UNI specification and transmitted using the Q.2931 signaling protocols.

Synchronous---Signals that are sourced from the same timing reference and have the same frequency.

Traffic Policing---A mechanism used to detect and discard or modify cells/traffic that violate the traffic contract agreed to at connection setup. Although applicable to both public and private networks, traffic policing will most likely be used by public ATM service providers where tariffing can be based on guaranteed service.

Traffic Shaping---A mechanism used to shape or modify bursty traffic characteristics in order to match a desired traffic contract.

Trail Packet Discard (TPD)---A mechanism employed by some ATM switches that allows the remaining cells supporting an AAL 5 frame to be discarded when a one or more cells of that AAL 5 frame have been dropped/lost. This avoids the unwanted waste associated with sending partial AAL 5 frames through the ATM network when they will likely have to be retransmitted by the sender. (see Early Packet Discard)

UBR---See Unspecified Bit Rate

UBR+---Sometime used to refer to Unspecified Bit Rate (UBR) service complimented by ATM switches using intelligent packet discard mechanisms such as Early Packet Discard or Trail Packet Discard. (see Unspecified Bit Rate)

UNI---See User-to Network Interface.

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

Usage Parameter Control (UPC)---The set of actions taken by the network to monitor and control traffic. Its main purpose is to protect network resources from malicious as well as unintentional misbehavior, which may adversely affect the guaranteed service of other already established connections by detecting violations of negotiated traffic contract parameters and taking policing actions.

User-to Network Interface (UNI)---The ATM Forum specification that defines the interface that directly links a user's device to an ATM network through an ATM switch. (See PNNI)

Variable Bit Rate (VBR)---A data transmission that can be represented by irregular grouping of bits or cell payloads followed by unused bits or cell payloads. Most applications other than voice circuits generate VBR traffic patterns. VBR is an ATM service type and is divided into Real-Time VBR and non-Real-Time VBR, in which the ATM network guarantees to meet the transmitter's bandwidth and Quality of Service (QoS) requirements. (see ABR, UBR, CBR)

VC---See Virtual Connection.

VCC---See Virtual Channel Connection.

VCI---See Virtual channel Identifier

Virtual Channel Connection (VCC)---A concatenation of virtual channel links between to endpoints where higher-layer protocols are accessed.

Virtual Channel Identifier (VCI)---A connection-identifying value found in the header of each ATM connection cell.

Virtual Connection--- A connection between end users that has a defined route and endpoints. (see PVC, SVC).

Virtual Local Area Network (VLAN)---A collection of users or ports grouped together in secure, autonomous broadcast and multicast domain. Membership to a VLAN is not restricted by physical location and can be defined across multiple LAN switches and ATM devices in the enterprise. A VLAN in an ATM network is built upon an emulated LAN segment. (See LAN Emulation)

Virtual Path---An ATM network can be preconfigured with some number of virtual paths. Each virtual path can support many channels. Virtual paths can be used to construct end-to-end connections called VPCs.

Virtual Path Connection (VPC)---A concatenation of virtual path links between to points where the VCI values are either reassigned or terminated. Several different VCCs can be bundled into one VPC.

Virtual Path Identifier (VPI)---A connection identifying value found in the header of each ATM cell.

VP---See Virtual Path

VLAN---See Virtual Local Area Network.

VPC---See Virtual Path Connection.

VPI---See Virtual Path Identifier.

ATM Acronyms

AAL ATM Adaptation Layer
ABR Available Bit Rate
ACR Allowed Cell Rate
AIP ATM Interface Processor for the Cisco 7000 families
ANSI American National Standards Institute
API Application Program Interface
ARP Address Resolution Protocol
ATM Asynchronous Transfer Mode
BECN Backward Explicit Congestion Notification
B-ICI Broadband Inter-Carrier Interface
B-ISDN Broadband Integrated Services Digital Network
BT Burst Tolerance
BUS Broadcast and Unknown Server in LAN Emulation
CAC Connection Admission Control
CBR Constant Bit Rate
CER Cell Error Ratio
CDV Cell Delay Variation (a.k.a. jitter)
CDVT Cell Delay Variation Tolerance
Cisco IOS (TM) Cisco Internetworking Operating System
CLP Cell Loss Priority
CLR Cell Loss Ratio
CRC Cyclic Redundancy Check
CPCS Common Part Convergence Sublayer
CS Convergence Sublayer
DHCP Dynamic Host Configuration Protocol
DS-1 Digital Signal Level 1 (1.5 Mbps)
DS-3 Digital Signal Level 3 (45 Mbps)
DSU Data Service Unit
DTL Designated Transit List
DXI Data Exchange Interface
E-SIG European SMDS Interest Group
E.164 ISDN (telephone) number
E1 European Signal Level 1 (2.0 Mbps)
E3 European Signal Level 3 (34 Mbps)
EFCI Explicit Forward Congestion Indication
ELAN Emulated LAN
EOM End of Message
EPD Early Packet Discard
ER Explicit Rate Mode in ABR Service
ESI End System Identifier (usually an IEEE 802 address)
FDDI Fiber Distributed Data Interface
FECN Forward Explicit Congestion Notification
FR Frame Relay
GCAC Generic Call Admission Control
GCRA Generic Cell Rate Algorithm
HEC Header Error Control
IE Information Element used in signaling messages
IEEE Institute of Electrical and Electronic Engineers
IETF Internet Engineering Task Force
IISP Interim Inter-switch Signaling Protocol
ILMI Interim Local Management Interface
IP Internet Protocol
IPng Internet Protocol next generation (a.k.a. IPv6)
IPX Internetwork Packet Exchange used in Novell's NetWare
ITU-T International Telecommunications Union-Telecommunication Sector
JPEG Joint Photographic Experts Group
LANE LAN Emulation
LEC LAN Emulation Client
LECS LAN Emulation Configuration Server
LES LAN Emulation Server
LGN Logical Group Node
LIS Logical IP Subnetwork
LLC/SNAP Logical Link Control/Subnetwork Access Protocol
L-NNI LAN Emulation Network-to-Network Interface
LSU Link State Update
MAC Media Access Control
MBS Maximum Burst Size
MCR Minimum Cell Rate
MCTD Maximum Cell Transfer Delay (latency)
MIB Management Information Base
MID Message Identifier
MPEG Motion Picture Experts Group
MPOA Multi-Protocol over ATM---subworking group in the ATM Forum
MUX Multiplexer
NDIS Network Driver Interface Specification
NHRP Next-Hop Resolution Protocol
NHS Next-Hop Server
NNI Network-to-Node or Network-to-Network Interface
NSAP Network Service Access Point
OAM Operations, Administration, and Maintenance
OC Optical Carrier
ODI Open Datalink Interface used by Novell's NetWare
OSPF Open Shortest Path First
PCR Peak Cell Rate
PDU Protocol Data Unit
PGL Peer Group Leader
PIM Protocol Independent Multicast
PLSP PNNI Link State Packets
PNNI Private Network-to-Node Interface
PTI Payload Type Identifier
PVC Permanent Virtual Connection
PVP Permanent Virtual Path
QOS Quality of Service
Q.2931 Signaling Protocol
RFC Request for Comments
RM Resource Management
ROLC Routing over Large Clouds---working group in IETF
RR Relative Rate in ABR
RSVP Resource Reservation Protocol
SAR Segmentation and Reassembly
SCR Sustainable Cell Rate
SDH Synchronous Digital Hierarchy
SIG Special Interest Group
SMDS Switched Multimegabit Data Service
SN Sequence Number
SNA Systems Network Architecture
SNAP Sub Network Access Point
SNMP Simple Network Management Protocol
SNP Sequence Number Protection
SONET Synchronous Optical Network
SSCOP Service-Specific Convergence Protocol
STS Synchronous Transport Signal
SVC Switched Virtual Connection
TCP/IP Transmission Control Protocol/Internet Protocol
TPD Trail Packet Discard
UBR Unspecified Bit Rate
UNI User-Network Interface
UPC Usage Parameter Control
UTP Unshielded Twisted Pair
VBR Variable Bit Rate
VC Virtual Channel, Virtual Connection
VCC Virtual Channel Connection
VCI Virtual Channel Identifier
VLAN Virtual Local Area Network
VP Virtual Path
VPC Virtual Path Connection
VPI Virtual Path Identifier

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