Glossary of Terms

– A –

ABR – Available Bit Rate

An ATM service class where the network makes a ‘best effort’ to meet the application’s bandwidth requirements. It differs from UBR in that it uses a feedback mechanism whereby the network informs a source of an acceptable transmission rate. If the source complies, the network guarantees a low CLR.

ATM – Asynchronous Transfer Mode

A switching and multiplexing technology based on the segmentation of voice, video, and data traffic into equal length cells which are then interleaved over a physical connection in a time-asynchronous manner. This contrasts with TDM where different traffic sources are assigned fixed timeslots.

ATM address

An end-system identifier based either on the ISDN-format E.164 address for public ATM networks or on an NSAP-format address for private ATM.

ATM Cell

The basic 53 octet unit used in ATM switching and de-multiplexing. The ATM UNI cell contains a 5 octet header and 48 octets of data payload.

ATM Cell Stream

Continuous stream of ATM cells based upon the line rate of the link.

ATM Forum

A multivendor organisation with the task of adapting international standards into implementation agreements, or developing standards where none exist. The organisation consists of equipment vendors, service providers, research organisations, and end users.

ATM Layer

Layer in the ATM model that supports the switching of ATM cells within a network.

AAL – ATM Adaptation Layer

Layer within the ATM model that supports segmentation and reassembly of services into/out of ATM cells.

– B –

B-ISDN – Broadband Integrated Services Digital Network

Broadband ISDN. A high -speed network providing voice, data, and video services which evolved from ISDN. ATM was chosen by the ITU-T as the technology to deliver B-ISDN.

BRI – Basic Rate Interface

Basic Rate interface for Narrowband-ISDN which is 192 kb/s (2B+D).

– C –

CAS – Channel Associated Signalling

Voice signalling based on bits taken from voice timeslots, used by many PABXs.

CBR – Constant Bit Rate

An ATM service class providing for the support of constant bit streams such as those generated by PABXs. The CBR class specifies a PCR, CDV, and CLR, thus guaranteeing bandwidth and QoS.

CDV – Cell Delay Variation

A QoS parameter specifying the variance in delay between ATM cells. The maximum acceptable CDV is referred to as the Cell Delay Variation Tolerance, or CDVT.

CES – Circuit Emulation Service

ATM Forum service supporting VCCs which emulate a CBR dedicated circuit.

Circuit Transfer Mode

Transfer mode which uses a permanent allocation of a link to a connection.

CLP – Cell Loss Priority

Indication of the priority of a specific cell. It is used during times of network congestion to decide which cells are lost.

CMIP – Common Management Information Protocol

Standard for the message formats and procedures used to exchange management information within the OSI environment. In most internetworks, SNMP is used in place of CMIP.

Congestion Control

A method by which congestion across the ATM network is reduced. Congestion control schemes may be based on fields within the ATM cell header (CLIP, EFCI within the PTI) or may be based on more sophisticated mechanisms between ATM end-systems and ATM switches. The ATM Forum has developed a mechanism based on rate control of ABR-type traffic.

Connectionless Service

Service that does not require a connection to be established prior to use.

Connection Oriented Service

Service that requires a connection to be established prior to use.

CPCS – Common Part Convergence Sublayer

A sublayer within the ATM AAL providing for packet framing and error detection. Used by all services which use the specific AAL.

CPE – Customer Premise Equipment

A term for the networking equipment located at the customer’s site.

CRC – Cyclic Redundancy Check

A checksum appended to CS-PDU’s to ensure that the upper-layer data has not been corrupted.

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.

CUG – Closed User Group

A definable closed group of users that are independent of physical location. CUGs are established for traffic segmentation and security requirements. Users can be assigned different priority levels and can be granted access to more than one CUG.

– D –

DHCP – Dynamic Host Configuration Protocol

Allows end systems to automatically set parameters such as IP addresses from a server.

DS-1 – Digital Signal Level 1

1.544 Mbps level of North American Digital Hierarchy, supporting 24 DS-0 signals. Sometimes called T1.

DXI – Data Exchange Interface

An HDLC-based ATM interface defined between a packet-based ATM end-system and an external ATM DSU supporting the ATM UNI.

– E –

E1 – European Signal Level 1

2.048 Mbps transport rate within the European PDH hierarchy, supporting 32×64 Kbps channels.

E2 – European Signal Level 2

8.192 Mbps transport rate within the European PDH hierarchy, supporting 4 E1 channels. E-2 is not widely supported.

E3 – European Signal Level 3

34.368 Mbps transport rate within the European PDH hierarchy, supporting 16 E1 channels or 4 E2 channels. At higher data rates, E4 at 140 Mbps is also defined.

ETSI – European Telecommunications Standards Institute

Develops standards for use by service providers within Europe.

– F –

FDDI – Fibre Distributed Data Interface

Local area network based upon dual optical ring architecture with up to 100Mb/s bandwidth.

Frame Relay

Variable packet size based protocol capable of operating up to 2 Mb/s.

Frame Relay Forum

Organisation composed of Frame Relay vendors, service providers, and users. Relevant efforts include standardising FR-ATM interfaces and services.

FUNI – Frame UNI

Frame-based interface to ATM supporting signaling and QoS. Replaces the ATM-DXI.

– G –

GCAC – Generic Connection Admission Control

A form of CAC used by PNNI when routing a connnection request.

GCRA – Generic Cell Rate Algorithm

Traffic shaping algorithm based on a VC’s traffic parameters to shape the traffic within a VC while enforcing traffic limits.

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. At present, the GFC field is unused.

– H –

HDTV – High Definition Television

Television with higher definition than conventional television, typically on par with computer workstations.

Header Field

Portion of an ATM cell allocated for the function of transferring the cell within the network.

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.

– I –

Idle Cell

Cell within an ATM cell stream that is not occupied by a service.

IEEE – Institute of Electrical and Electronic Engineers

US Electronics industry standards body. IEEE projects include the 802.x series of recommendations.

IETF – Internet Engineering Task Force

TCP/IP-focused international standards organisation.

IISP – Interim Interswitch Signalling Protocol

Formerly known as PNNI Phase 0. An ATM Forum standardised multivendor trunking protocol supporting signalling and simple rerouting. The IISP does not support topology discovery, hierarchical addressing, or QoS.

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.

Information Field

Portion of an ATM cell where user information is placed

IPX – Internetwork Packet Exchange

Novell-developed Layer 3 protocol.

ISDN – Integrated Services Digital Network

A public services network using telephone networks to carry data, voice and video information.

Isochronous Service

Constant bit rate service which must be sampled periodically.

ITU – International Telecommunications Union

International standards body including all PTT’s under UN treaty. The group responsible for communications standards is the ITU Telecommunications Standards Sector (TSS), formerly the CCITT.

– L –

LAN – Local Area Network

A network covering a relatively small geographic area.

LANE – LAN Emulation or LAN Emulation Service

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 to multiple VLANs. LANE provides for ATM-attached systems acting as LEC’s communicating with an LECS, LES, and BUS across an ELAN.

LLC – Logical Link Control

The upper half of the data link layer (Layer 2) often used to identify different Layer 3 protocols sharing a common medium.

– M –

MAN – Metropolitan Area Network

Network operating over a large geographic area

MIB – Management Information Base

A database of information, related to network statistics and operational parameters, which can be accessed via network management protocols.

MPEG – Motion Picture Experts Group

Industry organisation standardising methods of coding and transmitting video. The international standard for broadcast coding is known as MPEG-2. See JPEG.

MPOA – Multiprotocol over ATM

Standard which defines how routers, LAN switches, and hosts running multiple Layer 3 protocols may optimise forwarding paths and multicasting across ATM while taking advantage of ATM QoS capabilities.

Multimedia Service

A service that consists of more than one service type, such as text, graphics, sound and video.

– N –


Network capable of supporting Basic and Primary rate services.

NIC – Network Interface Card

Interface hardware allowing a host to connect to a network.

NMS – Network Management System

The hardware and software supporting network management (OAM) functions.

NNI – Network-Node Interface or Network-to-Network Interface

The NNI is better characterised as a switch-to-switch interface. The ATMF has defined a Private-NNI (P-NNI) to connect switches within a single management domain.

NRZ – Non-Return to Zero

Line signal encoding of 1s and 0s to avoid generating a DC component.

NSAP – Network Services Access Point

OSI standard for network addressing based on a 20-octet hierarchical address structure. ATM uses this structure for private network and some public network addressing.

– O –

OAM – Operation Administration and Maintenance

Management framework defined by the ITU. OAM cells are special purpose ATM cells exchanged between an ATM end-system and an ATM switch providing for network fault and performance management, analysis, and fault isolation.

OC – Optical Carrier

The transmission unit within SONET, based on an optical signal of nx51.84 Mbps. OC-3c (‘c’ for concatenated) is interoperable with SDH’s STM-1.

– P –

PABX – Private Automatic Branch Exchange

A telephone switchboard sited on user premises.

Packet Transfer Mode

Transfer mode based on packet oriented techniques that allows a link to be shared between multiple connections

PCR – Peak Cell Rate

The maximum cell transmission rate within a VC.

PDH – Plesiosynchronous Transmission Hierarchy

A public transmission hierarchy based on a non-synchronous alignment of octets at different levels of multiplexing. PDH is only bit-synchronous . PDH networks are in the process of being replaced with SDH networks. Examples of PDH transmission rates include E1 and E3.

PDU – Protocol Data Unit

The basic unit of information at a layer of the networking stack which is exchanged with a peer entity which contains control information and user data.

Physical Layer

Layer within the ATM model that deals with the physical transmission of ATM cell streams.

POH – Path Overhead

Header in the payload of a SONET or SDH frame which defines the structure and content of the STS or STM payload.

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. (See IISP.)


An ATM switch will police the data on an incoming VC to determine whether it is in compliance with the user’s service contract (which specifies a QoS).

PRI – Primary Rate Interface

Primary Rate Interface for Narrowband-ISDN which is 2 Mb/s (30B+D).

PSTN- Public Switched Telephone Network

The public telephone network.

PTI – Payload Type Identifier

A three-bit field within the ATM cell header indicating the AAL used, whether 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 Connection

A pre-configured logical connection between two ATM systems. This contrasts with SVCs.

– Q –


ITU-T signaling standard for B-ISDN and used by the ATM Forum’s UNI 3.1/4.0 to signal SVCs.

QoS – Quality of Service

Measure of the quality of a particular service based upon cell loss ratio and cell delay variation.

– S –

SAP – Service Access Point

Reference point between the networking stack and applications within an end-system.

SAR – Segmentation and Reassembly Sublayer

The sublayer where PDUs are segmented and rebuilt into/from ATM cells.

SAR-PDU – Segmentation and Reassembly Protocol Data Unit

Information which has been segmented by the SAR sublayer.

SCR – Sustainable Cell Rate

The maximum burst rate which a VC may maintain.

SDH – Synchronous Digital Hierarchy

A transmission hierarchy based on a synchronous alignment of bits at different levels of multiplexing. Though originally deployed in public networks as a replacement for the older PDH, SDH is now deployed within local ATM networks. An example of an SDH transmission rate is STM-1.

SDU – Service Data Unit

Information payload within a PDU.

SNMP – Simple Network Management Protocol

A network management protocol which assigns and monitors network parameters.

SONET – Synchronous Optical Network

A high speed network designed to operate over optical fibre.

STM – Synchronous Transfer Mode

Transfer mode that offers periodical connection of fixed length words.

SVC – Switched Virtual Circuit

A logical ATM connection established via signalling. End-systems transmit their UNI 3.1, or 4.0 signalling request via the Q.2931 signalling protocol.

– T –


A term used to refer to any DS-3-capable transmission system.


A North American physical transmission system consisting of two twisted wire pairs to support 1.544 Mbps. See DS-1.

TC – Transmission Convergence Sublayer

A sublayer within the physical layer where ATM cells are prepared for transmission by the PMD sublayer.

TDM – Time Division Multiplexing/Multiplexer

Method of combining muliple lower-speed signals into a higher rate by placing each channel within a known timeslot. TDMs form the core of many enterprise internetworks, and are being replaced by ATM.

Terminal Adaptor

Network element used to interface non-ISDN services to a ISDN network.

TINA – Telecommunications Information Networking Architecture

An emerging application environment to interconnect management platforms and applications from multiple vendors and service providers.

Traffic Management

Act of managing congestion across an ATM network by buffering, adjusting transmission rates, and policing connections. ABR is a part of ATM Forum traffic management.

Traffic Shaping

Control by an end-system of originating traffic parameters such as average bandwidth, peak bandwidth, burst, and CLP in order to meet the traffic contract.

– U –

UBR – Unspecified Bit Rate

Service type where user requests a PCR but has no guarantees as to throughput, latency, or CLR. UBR is ‘best effort’ service. A form of UBR where switches implement EPD or TPD is sometimes known as UBR+.

UNI – User Network Interface

The interface used to connect users to the network.

– V –

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) are provided by CBR.

VC – Virtual Channel or Virtual Connection

A connection between two ATM systems. When used for data, a single VC may support one or many network layer protocols. Multiple Virtual Channels share a Virtual Path, and may be concatenated into a VCC.

Virtual Circuit Connection

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

VCI – Virtual Channel Identifier

A label used to identify connections between two ATM stations. Together, the VCI and VPI labels establish an ATM end-station address.

Virtual Path

A grouping of virtual channels (VCs)., important between ATM switches to facilitate management. Multiple VPs form end-to-end VPCs.

Virtual Path Connection

An end-to-end connection consisting of two or more Virtual Path links (VPs).

VPI – Virtual Path Identifier

A label used to identify a group of virtual circuits carried along the same route.

Virtual Path Identifier

Used to identify each VP across the UNI or NNI. The VPI is an 8-bit field at the UNI; 12 bits at the NNI (no GFC).

VPN – Virtual Private Network

Public network service where a customer is provided a network which appears as if it were a private network.

– W –

WAN – Wide Area Network

A network which spans a wide geographical area.

This page and all its content is Copyright © 1997 by Jtec Pty Limited.
Most recent update: August, 1997.

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