host) The IP-CSP Routing Analyser switch protocol AAL3/4U protocol ISDN packet, AAL3/4-UDT access service ATM-CSP IP router IP-TS (host) AAL3/4U user of AAL3/4 service IPU IP user ATMU ATM VP/VC user LAPB Link access procedure, version B CNU ISDN connection service user PH ISDN packet handler IP Internet protocol PHU ISDN packet service user Fig. 3.72 Part of an IP logical network, showing how hosts and routers may be interconnected Thus, there is no layer for connection handling in IP networks (and in CNL net- works in general). The layer structure of different classes of CSPs are therefore dif- ferent. By keeping relaying and routing analysis out of the layer definition, these two concepts become reusable for all classes of CSPs. For example, in CNL net- works the routing analyser becomes directly connected to the packet switch, while in CS and CON networks it becomes connected to the network process controller. The layer structure that is associated with a particular type of CSP is defined only by the type of links that are defined for the CSP (i.e. the protocols that are ter- minated by its UDTs). As Fig. 3.72 shows, in IP networks there is only one layer, defined by the Inter- net protocol (IP). As a matter of fact, if we regard the IP switch and the routing analyser as part of that layer (which we do not), we have the OSI-definition of network layer. In X.25 there are two layers in access links (defined by the LAPB protocol and the X.25 service access protocol). Transport links in X.25-CSPs have another set of layers. In ISDN, there are three layers in access links with complicated structural rela- tions: the physical layer (defined by ITU-T 1.430 and 1.431), the LAPD link layer (defined by ITU-T Q.920 and protocol Q.921) and the process control 3 Logical Network Modelling layer (defined by the teleservice access protocol ITU-T Q.931). Transport links in ISDN-CSPs have another set of layers. Thus, the perception of a "layer" is further complicated by the fact that there are often different layer structures 