an ATM stratum consists of an ATM cell-switching stratum and an AAL stratum, the latter compris- ing many different types of AALs. 6.4.2 ATM in B-ISDN An ATM stratum can be used as the switching stratum in any other network system that offers end-user services. One of these could be the B-ISDN 6 ("broadband ISDN") network system. Figure 6.9 depicts the B-ISDN network system, here as a generic derived node. The B-ISDN was intended to substitute the existing ISDN in the long run. Consequently the latter was renamed "narrowband ISDN," N-ISDN. B-ISDN must therefore be able to offer exactly the same services as N-ISDN does. Thus, a B-ISDN operator network with ATM adapted terminals must be able to B-ISDN is an extremely complex system to understand, at least if you try to understand it via its standards. The author made a rough estimate of the number of standard pages that describe just one of its layers (the control layer DSS2 L3 for access signaling). There are in all 54 docu- ments that describe different aspects of this layer. The basic protocol standard, ITU-T Q.2931, comprises 255 pages, so the total number for DSS2 L3 should lie in the range of 2000-3000 pages. The number for B-ISDN as a whole, including ATM standards, is much larger. Due to the amount of information given by the standards, and to the rather unfriendly way in which it is presented, the author cannot guarantee that the model discussed in this chapter is 100% accurate. This should not disturb the reader too much. After all, the intention here is not to provide a tutorial on B-ISDN/ATM, but to show that the AMLn principles for modeling switching layers are applicable to ATM as well. 6 Applying AMLn interwork with a number of surrounding PSTN and N-ISDN networks when it is put into operation. B-ISDN services B-ISDN services end-users requesters B-ISDN media services stratum element DSS2 L3 ITU-T Q.2931+ Control AC (access) T-ROs B-ISUP ITU-T Q.2762+ Media (network) resources e.g. MTP-3b SS7 adaptations AAL5 AALI AAL2 P-t-p signaling service Broadca