ATM adaptation layer. The standards layer that allows multiple applications to have data converted to and from the ATM call. A protocol used that translates higher layer services into the size and format of an ATM cell. ATM Adaptation Layer sits above ATM and converts non-ATM bit streams - end user data - into ATM cells. The AAL accepts data from different applications and presents it to the ATM layer in the form of 48-byte ATM payload segments. AALs consist of two sublayers: CS and SAR. The AAL is the protocol used on top of ATM to support higher-layer service requirements. For data communications services, the AAL defines a segmentation/re-assembly protocol for mapping large data packets into the 48-octet payload field of an ATM cell. AALs differ on the basis of the source-destination timing used, whether they use CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transfer. At present, the four types of AAL recommended by the ITU-T are AAL1, AAL2, AAL3/4, and AAL5.
--ATM adaptation layer 2. One of four AALs recommended by the ITU-T. AAL2 is used for connection-oriented services that support a variable bit rate, such as some isochronous video and voice traffic. See also AAL and ATM.
--ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5 supports connection-oriented VBR services and is used predominantly for the transfer of classical IP over ATM and LANE traffic. AAL5 uses SEAL and is the least complex of the current AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability. See also AAL, ATM, and SEAL.
1. actual cell rate. The rate at which the source is transmitting cells in an asynchronous transfer mode (ATM) available bit rate (ABR) connection. 2. allowed cell rate: An ABR service parameter, ACR is the current rate, in cells/sec, at which the source is allowed to transmit cells in an asynchronous transfer mode (ATM) available bit rate (ABR) connection. In other words, the available bandwidth, in cells per seconds, for a given quality of service (QoS) class, which is dynamically controlled by the network.
--Asynchronous Transfer Mode. International standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E2, SONET, and T3.
CBR--constant bit rate. QoS class defined by the ATM forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery.
--committed information rate. In a Frame Relay network, each PVC is assigned a Committed Information Rate, measured in bits per second. The CIR represents the average capacity that the Port Connection should allocate to the PVC. This rate should be consistent with the expected average traffic volume between the two sites that the PVC connects. The CIR that is assigned to a PVC cannot exceed the speed of either the originating or terminating Port Connection. The rate is averaged over a minimum increment of time.
--class of service. Telephone service distinctions that include rate differences between individual and party lines, flat rate and message rate, and restricted and extended area service.
CS--convergence sublayer. One of the two sublayers of the AAL common part convergence sublayer (CPCS), which is responsible for padding and error-checking. PDUs passed from the SSCS are appended with an 8-byte trailer (for error checking and other control information) and padded, if necessary so that the protocol data unit (PDU) is divisible by 48. These PDUs are then passed to the SAR sublayer of the CPCS for further processing. See also AAL, and SAR.
ELAN--emulated LAN. ATM network in which an Ethernet or Token ring LAN is emulated using a client-server model. Multiple ELANs can exist simultaneously on a single ATM network. ELANS are defined by the LANE specification.
LAN--local area network.
LANE--LAN emulation. Technology that allows an ATM network to function as a LAN backbone. The ATM network must provide multicast and broadcast support, address mapping (MAC-to-ATM), SVC management, and a usable packet format. LANE also defines Ethernet and Token Ring ELANs.
--Specifies the largest burst of data above the insured rate that is allowed temporarily on an ATM PVC, but is not dropped at the edge by the traffic policing function--even if it exceeds the maximum rate. This amount of traffic is allowed only temporarily; in general, the traffic source needs to be within the maximum rate. Specified in bytes or cells.
--maximum burst size. See maximum burst.
--minimum cell rate. ATM term for an ABR (Available Bit Rate) service traffic descriptor, in cells/sec, that is the rate at which the source is always allowed to send. This parameter is defined by the ATM Forum for ATM traffic management. MCR is defined only for ABR transmissions, and specifies the minimum value for the ACR. See ACR.
--peak cell rate. Parameter defined by The ATM Forum for ATM traffic management. In CBR transmissions, PCR determines how often data samples are sent. In ABR transmissions, PCR determines the maximum value of the ACR. See ACR.
--permanent virtual circuit. A circuit or channel through an ATM network provisioned by a carrier between two endpoints; used for dedicated long-term information transport between locations. Also virtual connection (VPC/ VCC) provisioned for indefinite use in an ATM network, established by the network management system (NMS). This is a link with static route defined in advance, usually by manual setup. Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time. Called a permanent virtual connection in ATM terminology. Compare with SVC.
SEAL--simple and efficient AAL. Scheme used by AAL5 in which the SAR sublayer segments CS protocol data units without adding additional fields. See AAL, AAL5, CS, and SAR.
--switched virtual circuit. Virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic.
--unspecified bit rate. Traffic class defined by the ATM Forum. UBR is an ATM service category that does not specify traffic related service guarantees. UBR allows any amount of data up to a specified maximum to be sent across the network, but there are no guarantees in terms of cell loss rate and delay. Specifically, UBR does not include the notion of a per-connection negotiated bandwidth. No numerical commitments are made with respect to the cell loss ratio experienced by a UBR connection, or as to the cell transfer delay experienced by cells on the connection.
--UBR with early packet discard (EPD) or partial packet discard (PPD).
--virtual channel connection. As an ATM term, it is a concatenation of VCLs that extends between the points where the ATM service users access the ATM layer. The points at which the ATM cell payload is passed to, or received from, the users of the ATM Layer (that is, a higher layer or ATM entity) for processing signify the endpoints of a VCC. VCCs are unidirectional. ATM VCC can have one of two services types: 1) connection-oriented-path established before data is sent or 2) Connectionless data sent as datagrams. The connection-oriented path is typically used for AAL 1, 2, 3, 5 circuits. The connectionless VCC is for AAL4 is usually associated with Switched Multimegabit Data Service (SMDS).
--virtual channel link. A means of unidirectional transport of ATM cells between the point where a VCI value is assigned and the point where that value is translated or removed.