agement functions are modeled as separate layers (in particular when connectivity for layer management is provided by an APi of the very layer). One of many examples of this is LAPD that we discussed in Sect. 2.1.1. This means that many network functions that are defined as a single layer/protocolo in stand- ards, should dissolve into layers that belong to different AMLn planes (the man- agement plane and the traffic plane respectively). In the present chapter we will only take a look at how strata can be identified in layer structures. The ISDN network system, for example, is not known as being stratified. Nevertheless we can model it as such. For example, the model of Fig. 2.51 indicates a possibility to define its layer structure into a media-services stra- tum and three connectivity-service strata (a circuit-switching stratum, a packet- handling stratum and diverse X. and V. strata), as depicted in Fig. 2.54. 2.4 Stratum Levels ISDN services ISDN services users requesters Media-services stratum DSS1 L3 Call-handling- agent layer actor layer Packet services agent layer users T-ROs Packet- common agent layer handling stratum Media Media agent actor MTP L3 stratum Other resources MTP L2 agent layer, X. and V. Alternative ways Switching- circuit-switching to provide actor layer agent layer, connectivity stratum for MTP L2 multiplexing layer stratum Fig. 2.54 A basic stratification of the ISDN network system The model now indicates that the ISDN circuit-switching stratum may be replaced with another stratum (e.g., a packet-switching stratum), the ISDN still being able to offer the same (and possibly improved) end-user services. Obviously, however, if we would pursue such a replacement, we would have to adapt to other types of layer interfaces. Such adaptations can be very costly, unless existing layer interfaces between strata are preserved. This implies that, for a stratum to be open to replacement of internal layers, it must include adaptation layers (such as the ATM adaptation layers). To qualify for being a re