st signaling service ATM adaptation stratum element AAL protocols, stratum SVCI=2, 5 VPI=any Management protocols element ITU-T I.311, 371, 610 ATM cell-switching stratum element ATM transport protocol ITU-T 1.361 Other layer elements used for access Optical fibres Fig. 6.9 The B-ISDN stratum element. ...=ATM adaptation layers; P-t-p=point-to-point; SAAL=signalling AAL; SVC=signalling VC; VC=virtual channel; SVCI=SVC identifier; VP=virtual path; VPI=VP identifier The fact that B-ISDN is an evolution from N-ISDN can also be seen by the sim- ilarities and differences of this model and the N-ISDN model we discussed previ- ously (see, e.g., Fig. 2.54). In principle, the B-ISDN network system is created out of the N-ISDN system by replacing its circuit-switching stratum with an ATM cell-switching stratum. No special packet-handling stratum as in N-ISDN is needed, since the cell switching layer is already a suitable base for packet services. The B-ISDN media services stratum is fairly similar to the one for N-ISDN: it has an access call-handling agent layer, called DSS2 L3, which is a modified version of DSS1 L3; it has a corresponding network agent layer, called B-ISUP, a modified version of ISUP. The modifications take into consideration that a cell-switching stratum has to be used instead of a circuit-switching stratum, and that B-ISDN offers a wider set of end-user services than N-ISDN. The structure of connectivity strata used by these agents looks different to N- ISDN. A special stratum, called "SS7 adaptations" in the model, is used by B- ISUP. This stratum relies on the SAAL services ("signaling AAL") of the AAL stratum. DSS2 L3 relies directly on SAAL services. 6.4 ATM Cell Switching The "SS7 adaptations" stratum replaces part of the SS7 stratum. It allows the B- ISUP agent to run on typical SS7 connectivity interfaces (such as those provided by MTP SCCP, and TCAP). A relevant question is why the necessary interface adaptations are not provided by the AALs in the ATM adaptation stratum. Obviously the inventors