, the ATM switching stratum has no direct interface to physical media (similar to IP). It interfaces other strata, e.g., SDH, over layer interfaces that are connectiv- ity service points. Note that no specific access strata, such as the BRA and PRA in N--ISDN are defined for B-ISDN. Let's now take a deeper look inside the ATM cell-switching stratum (see Fig. 6.10). 6 Applying AMLn Media resources B-ISDN control stratum P-t-p Broadcast protocols! adaptation strata VCI=2, 5 1.610 VC-OAM ITU-T 1.371 VP-OAM control Adjacent ITU-TI.610 VCI=3, 4 node maint Source/dest ITU-TI.610 1.311 node maint. cell-switching VCI=1 stratum element ATM actor ITU-T I.361 Control 00..11 ATM agent store ATM stratum element Other layer elements SDH T. used for access Optical fibers Fig. 6.10 The ATM stratum element. AAL...=ATM adaptation layers; ABR=available bitrate; OAM=operation and maintenance; PTI=payload type identification; SAAL=signalling AAL; SVC=signalling VC; VC=virtual channel; VCI=VC identifier; VP=virtual path; VPI=VP identi- fier; VC-OAM=operation and management of VCs; VP-OAM=operation and management of In one sense, the internal structure of the switching stratum is more like the X.25 VC switching model than the N-ISDN circuit-switching model. There is a single agent layer, defined by the ITU-T I.361 protocol, that handles cell transport between ATM terminals and ATM access nodes, and between network nodes. In another sense it is more like N-ISDN in that the switching actor is used for termi- nating streams for several types of users (the X.25 switching actor supports a single user only). Similar to N-ISDN, layer management (SVC-OAM, VP-OAM, and VC-OAM, see Fig. 6.10 and following page) is also carefully specified. Virtual channels are offered by the switching stratum. A VC is identified by a virtual channel identifier (VCI). Every VCI must belong to a virtual path, identified by a virtual path identifier (VPI). A VPI can contain up to 65,536 VCs, and the stra- tum can switch both whole paths and single VCs. Service prope