 we call it stratum. The network system, or network of 2 Layer Structures which the stratum is a part, is then said to be stratified. To be able to treat such groupings as black boxes, the layer interfaces on which users of the stratum rely must be thoroughly specified (e.g., by using AMLs, see Appendix D). Note that by "user" we do not necessarily refer to end-users, but any other layer or stratum that relies on the stratum. Once these layer interfaces are specified, the internal layer structure of the stratum becomes invisible to using layers and strata. There can be many reasons for defining parts of a layer structure as a stratum: A group of layers plays a certain role in a network. The SS7 connectivity stra- tum, being the nervous system in the core of PSTN and ISDN networks, plays such a role. The set of adaptation layers in ATM also plays a specific role in broadband ISDN systems, which is to adapt the raw cell transport performed by the ATM switching stratum to connectivity services that different types of media service are used to (this kind of adaptation is sometimes called "emulation"). Parts of a layer structure that can be foreseen to be affected by technology changes are candidates for being hidden in strata. This is a very important rea- son, considering that networks have been and are in constant evolution, both as regards low level functions, such as physical media, transmission and transport technologies, and high-level service-oriented functions. This was also the reason for the only effort to define a stratum concept that ITU- T has published: the "generic functional architecture of transport networks" (which we call GTA), applied to create a functional model of the SDH stratum. In Sect. 2.1.1 we discussed the importance (for network system management in a context of continuous evolution) of keeping layer definitions simple. A layer should offer a single service SO that the use and design of a layer and its proto- cols becomes easy to understand. One aspect of this is that in AMLn, layer man- 