." The author looks at this structure as the result of the AAL standard committee giving priority to the need of dividing the work of specifying AALs, ahead of defining a useful architecture. In the light of AMLn, we can analyse AAL structures and con- clude that: Although ITU-T denotes AALs "layer," they are only SO in some cases (e.g., AAL1). In other cases (e.g., the SAAL) they are strata, i.e., consist of several layers. In AMLn we do not recognize "sublayers." Whenever this concept appears in standards, one can suspect that either they are agent layers, they define actor layer functions, or they are layers of an implementation model, i.e., they are not network layers at all. The number of structural elements in the internal AAL structures varies. The fact that elements in these structures are characterized differently ("protocol," "function," "sublayer," "sublayer_in_sublayer_in_sublayer"), indicates some hid- den semantic differences (most likely the fact that when specializing from this architecture, an element may or may not define a protocol, depending on the actual specialization). 6.4 ATM Cell Switching Due to the confused definition of the AAL generic architecture, it is impossible to define a single mapping on an AMLn model. Instead, a specific AMLn model must be defined for each specialized AAL. For AAL1 and AAL2, the correspond- ent AMLn model defines a single layer, as depicted in Figs. 6.13 and 6.14. Neither the space available in this book nor the AAL standard inspires the author for an exhaustive analysis of all AALs. We will, however, take the "signal- ing AAL" (SAAL) as a mapping example of an AAL that defines more than one layer. There are variants of SAALs as well. The variant depicted in Fig. 6.16 is used for supporting the B-ISDN service control stratum. The figure compares the standard's description of the B-ISDN SAAL architecture with a corresponding AMLn model. Note that the standard describes five sublayers although there are only two layers that define any protocol. B-ISDN SAAL stand