al stratum, all layer interfaces that a stratum provides must be standardized and defined in details. In the ISDN case, connectivity service points of the switching stratum are standardized as to their properties, but the con- trol points over which switching and media actors are controlled are not. Every supplier implements its own solutions to these interfaces. This is also part of the explanation to why ISDN is not characterized as "stratified." The ISDN structure is also an example of how strata can be defined recursively. Figure 2.55 depicts four internal strata that exist in the ISDN media-services stra- tum. All substrata depend on the same switching stratum. In addition the SS7 con- 2 Layer Structures nectivity stratum can run on other strata as well, such as V.- and X. networks. Each stratum can be specified and designed separately, i.e., defines its own internal layer structure. ISDN services ISDN services users requesters Media-services stratum ISDN call-handling layer DSS1 L3 Call-control- agent layer actor layer agent layer T-ROs Other common resources agent layer Media stratum Media Media connectiv. agent actor MTP L3 stratum stratum MTP L2 ISDN circuit-switching stratum X. and V. Fig. 2.55 Internal strata in the ISDN media-services stratum This structure reveals a stratum substructure of four types of strata: 1. A media stratum consisting of diverse media agents and actors. 2. At the top, the ISDN call-handling layer, consisting of call-handling actors and three types of agent layers, relying on two other internal strata in the ISDN media-services stratum. 3. The SS7 connectivity stratum, consisting of SCCP, MTP L3 and MTP L2 layers. 4. A number of LAPD strata. The SS7 connectivity stratum relies on services (called "signaling data links") of primarily the ISDN switching stratum, which is part of the explanation of why it is not regarded as a common packet-switching stratum (such as TCP/IP or ATM). However, as all strata, the internal structure of the SS7 connectivity stratum can be modified, provi