3.4.3.1 Introduction The variation of switching techniques and route types in this area ranges from cir- cuit-switched (CS) to packet-switched (PS) routes, from connection-oriented (CON) to connection-less (CNL) routes, from routes that are addressed by network addresses (NA) only, over routes that are identified by both an NA and an access point-identifier (APi), and from switching layers/strata that realize a single level of switching to layers/strata that realize several levels of switching. A main difference between circuit and packet switching (as route identification is concerned) is that, in the CS case, the user of the route need not identify the 3 Node Structures route in any way. In the case of PS, some type of identifier is always needed, either an NA or some type of virtual circuit identifier (VCi). An NA is used also in the CS case, but only by the control layer in the set up phase. CON switching is one mode of operation of X.25. Other CON-switching net- works are FR and ATM. CNL switching (or "datagram switching," as it is more often called) is another mode of X.25, and is also the mode in which IP and MTP L3 work. In CS networks and in X.25, FR and ATM, NA is the only means to identify a route. In MTP L3 and IP, routes are identified both by an NA and an APi. The user of the route is not bothered by the APi, however, since the layer APi method is used by these layers, which implies that the APi is the same for all layer ele- ments of a using layer, and is set at installation time by the management system. All PS network systems except ATM include a single level of switching. The ATM/AAL stratum includes three levels of switching. At the basic level, virtual paths (VP) are switched. This is a level that can be used as a cross connecting level for a large set of virtual circuits (VC). VCs can also be switched individu- ally, which is the second level of switching. The third level is realized by the layer AAL2 that can switch parts of user data of cells ("cell" is the packet type that is switched on V