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.
- 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.
- 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
- 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
- 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
- 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
- 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
- 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.
- 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
- E.164 - Public
network addressing standardized by ITU-T. Used by both N-ISDN, SMDS and public
- 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.
- 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
- 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
- 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.
- GCAC - Generic Connection
Admission Control - A form of CAC used by PNNI when routing a connection
- 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.
- 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
- 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
- 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.
- 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
- 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.
- 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
- 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
- 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)
- 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.
- 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.
- 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
- 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.
- 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
- 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.
- 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
- 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.
- 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
- 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
- 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.
- 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.
- 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
- 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
- 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
- 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
- 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,
- VPC - Virtual Path
Connection - An end to end connection consisting of two more Virtual Path
- 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