ATM Adaptation Layer
The ATM Adaptation Layer (AAL) is responsible for providing specific transport services to the higher layer protocols of the B-ISDN protocol stack. In effect, the AAL provides a convergence function for the ATM Layer; the AAL makes the ATM Layer appear to the higher layers as the higher layers want the underlying cell relay function to appear. The AAL has two sublayers. The Convergence Sublayer (CS) formats higher layer information so the Segmentation and Reassembly (SAR) Sublayer can segment it for transport in the Payload field of a cell.
However, networking is more than just the transfer of information (i.e., user plane). The entire process must be controlled (i.e., control plane) by network administrators and users (i.e., signaling protocol for connections, and so on). In addition, the network must be managed (i.e., management plane). Both planes and layers must be managed (i.e., plane management and layer management).
The user plane transports user information. The AAL supports four generic types of services in the user plane: constant bit rate (CBR) services, connection-oriented data services, connectionless data services, and other variable bit rate (VBR) services. CBR services generate a constant number of bits per unit time, while VBR services vary the number of bits generated per unit time.
The control plane is responsible for call control and connection management. These signaling procedures will also be supported by a special signaling AAL function. The control plane operates over the same layered architecture as the user plane.
ATM Adaptation Layer Functions
AAL is the highest layer in the ATM protocol stack (under the B-ISDN umbrella) and provides the interface between the generic cell switching capability of the ATM Layer and the specific requirements of higher layer applications, protocols, and services. The ATM Layer provides a common platform for all applications riding over the ATM network. The AAL maps specific higher layer services and protocols to the ATM transport service. While the ATM Layer is employed at ATM end-user stations and network nodes, normally the AAL is present only in ATM end-user stations.
When ISDN terminal equipment requests services, the request is not described in human terms such as a telephone, data, or video call. Rather, it is described as a set of characteristics defining the connection in terms of the connection type, bit rate or throughput, information flow, and data unit integrity. B-ISDN and ATM services use additional terms to describe higher layer applications and services, including the timing relationship between source and destination, type of end-to-end connection, and the required level of error detection/correction.
In other words, although the ATM network provides the same cell transport for all services and mixed traffic types, this does not mean that there are no differences between the higher layer services themselves. A stream of cells representing a voice call must be treated differently than a stream of cells representing an email transfer. The ATM switches can provide some preferential handling, but the AAL must establish these priorities, convey them to the ATM Layer, and then monitor performance.
Therefore, the AAL must preserve the characteristics of the higher layer service’s information flow, while accommodating the underlying cell switching scheme. Some of the service-dependent functions the AAL might provide are listed below.
- Clock synchronization and recovery between end stations
- Error detection and correction
- User data segmentation and reassembly
- Multiplexing and demultiplexing user data on virtual channels
In this context, error correction means only that the ATM network could provide some form of assured delivery through the network at the AAL. To accomplish this, the AAL will correct or resend cell payloads as required. Currently, ATM networks offer non-assured delivery, mainly to keep AAL complexity and speeds to a minimum. However, non-assured delivery requires the higher layers to recover from cell losses and payload bit errors, if this error correction and recovery is done by the user applications at all.
The AAL might provide some or all of the functions above for specific services, but they are not mandatory. Different services will have different requirements. A voice connection, for example, probably will not require error correction, while variable bit rate data probably will not require precise timing synchronization.
AAL Service Classes and Types (ITU-T)
Rather than define a different AAL protocol for every possible higher layer service, the ITU-T has defined different categories of service classes based on the three characteristics outlined below.
- Timing relationship between source and destination: Required (time sensitive) or not required (time insensitive). In other words, does the service require a low and stable delay over the time of the connection to function properly (e.g., a voice conversation)?
- Bit rate: Constant or variable
- Connection mode: Connection-oriented or connectionless
The ITU-T has defined four AAL service classes based on combinations of these three characteristics. Class A is a constant bit rate (CBR), delay-sensitive, connection-oriented service—or a circuit emulation service. Class B is a variable bit rate (VBR) service requiring time synchronization between sender and receiver (e.g., real-time compressed audio and video). Classes C and D are delay-insensitive VBR services.
Four AAL protocol types were defined to support the four service classes. Each type describes the format of the SAR-PDU (or the cell Payload field) and related operational procedures. The visual shows the relationship between the AAL types and the service classes.
Normally the user, not the network, chooses the AAL. But the network, not the user, provides the guarantee for service classes. The ATM Forum recognized this problem and separated service classes from AAL types. They follow the alignments recommended by the ITU-T, but any AAL can be assigned any service class.
The functions provided by the AAL to support different classes of service are split across two sublayers: the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) Sublayer. Both of these sublayers process higher layer information for handling by the ATM Layer. The amount of processing performed by each sublayer is specific to the type of service.
The higher of the two AAL sublayers, the Convergence Sublayer mainly provides service-specific functions to higher layers. It is important to note that these functions would not typically be used by all services, and some functions might not even be available to some services in many ATM network configurations. What the AAL CS boils down to is a system of headers and trailers that provide the necessary information and functions to allow higher layer services to operate properly over the ATM network. These functions are described below.
- Clock recovery: Clock information can be implicitly obtained by monitoring the buffer lengths. If the buffer length shrinks, cells are arriving more slowly. If the buffer length increases, cells are arriving more quickly. This buffer monitoring could be necessary for some services where end-to-end timing is important, such as voice and video.
- Recovery from cell loss or non-sequential delivery: The SAR detects cells that are lost or delivered out of sequence. The CS sublayer, however, provides the mechanism for the higher layer to recover from these errors.
- Explicit time indication: Some applications need explicit timing marks, achieved by attaching a timestamp to time-sensitive traffic such as voice and video. This is particularly important in variable bit rate services so that the information the destination equipment receives can be reproduced accurately, ignoring the timing variations (jitter) introduced by the network.
Segmentation and Reassembly Sublayer
The Segmentation and Reassembly Sublayer is responsible for creating the payload to transport in an ATM cell. The functions of the SAR are described below.
- Creation of the cell payload units: The SAR is responsible for formatting the cell’s Payload field. The Payload field is 48 octets in length. Some octets might contain header and trailer fields instead of user data; for example, a 1-octet header might precede the remaining 47 octets of user information.
- Detection of lost SAR data units: As alluded to above, the SAR might be able to detect lost or out-of-sequence cells.
- Detection of bit errors: In some applications, the SAR might be able to detect bit errors in the cell payload. In some circumstances, the SAR can correct a single-bit error.
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