s for AAL2, since frame boundaries must be preserved). Thus, the SAR can be modeled as a layer in the case of AAL2 but not in the case of AAL1. The use of the term "sublayer" in AAL standards has forced some writers (see Tanenbaum (1996)) to statements such as "in the AAL1, the SAR layer has no protocol but in the AAL2 it has," which sounds rather strange, considering that a "layer" that defines no protocol is not a layer (by defini- tion). Nevertheless, assuming that a generic structure for AALs was possible and meaningful to define, the AAL committee defined an internal generic structure based on four generic "sublayers" (also called "protocol" or "functions") and seven structural concepts (see Fig. 6.15). The intention was to use this structure for defin- ing specializations for different user-layer requirements, resulting in a large set of standard documents. 9 Note, however, that the present status of the generic AAL structure has become unclear, since the overview documents have been withdrawn. The depicted model is a best effort model, based on information available to the author. 9 The present set of AAL standards comprises 11 documents in the ITU-T I-series (I.363 through 1.366), and an additional number in the Q.2000-series (the latter primarily covering SSCOP variants and SSCF functions). 6 Applying AMLn Media resources Convergence sublayer element Service-specific convergence sublayer element Service-specific coordination function Service-specific AAL protocol connection-oriented protocol Common part convergence sublayer element Segmentation and reassembly sublayer element ATM actors Fig. 6.15 The generic architecture of an AAL element, as described by the AAL standard Much has been said about this structure and its specializations since it was defined. One of the more moderate comments are given by Tanenbaum (1996): "If the full AAL committee had turned its work in as a class project, the professor would probably have given it back with instructions to fix it and turn it in again when it was finished