 wanted the ATM adaptation stratum to be general enough to be able to support different kinds of signaling systems. They therefore restricted the SAAL to handling sig- naling links only, i.e., the correspondence to the MTP L2 level in SS7 and the LAPD level in DSS1 (these adaptations are called SSCF-UNI for use by DSS2 L3 and SSCF-NNI for use by the SS7 adaptation stratum). A principle difference is how media support is realized. In the N-ISDN model, media resources that are needed for end-user services rely directly on connec- tions offered by its circuit-switching stratum, since such connections are well suited for isochronous media services, such as voice and video. The story for media services in B-ISDN is very different: cell-switched virtual circuits are fast and reliable, but are not suited for any particular media service. It was therefore obvious from the start that, in order to make an ATM stratum a platform for all kinds of services, one had to add some adaptation layers (the AALs) on top of the ATM switching stratum, that could offer connectivity serv- ices suited for different types of media layers. For example, one such connectiv- ity service was obviously a service that was similar to a circuit-switched connection, often called "circuit emulation." The AAL developed for that pur- pose was called AAL1. Other standardized AALs are the AAL2, AAL5, and the SAAL (read more about the AAL stratum in Sect. 6.4.4). 6.4.3 The ATM Stratum We will now turn more specifically to the ATM stratum in Fig. 6.9. In N-ISDN we had to define the N-ISDN switching stratum as interfacing physical media strata since N-ISDN does not define a single or any particular agent layer for data trans- port (except in accesses). ATM is different: it defines a single, network-wide agent layer that can run on any type of stratum for physical transport. Primarily an SDH cross-connecting stratum was considered, but the development of other technolo- gies (e.g., ATM directly over an optical fiber network) are already in the pipeline. Thus