hings, primarily because it is a key con- cept in stratified systems, which almost all network concepts are. Also, in SM&AML it is somewhat ambiguous, since it is used both for denoting switched or non-switched transport network concepts, like PDH and SDH, as well as a set of links or routes that interconnect switches in switched networks. The latter are, however, unique for the type of CSP or logical network and are denoted by the prefix that denotes the particular CSP. Be aware also of the use of "transport" to denote the OSI RM layer 4, the TCP/UDP layer in IP, etc. From the SM&AML point of view, these are not transport layers at all, but link layers in a recursive structure of logical networks. CN (for connection) for units in the resource system that provides circuit switched connections on demand, primarily to be used for teleservice provision- ing (sometimes called the connection level of such networks). The reason we do not use the prefix CS (for circuit switching) is that switched transport networks are also circuit switching networks. If there is no risk for confusion, CS is, how- ever, probably a more accepted prefix in the telecom context. PS (for packet switching) for units in packet-switching networks. Since these exist either in a connection-oriented (CON) or connection-less (CNL) form, the prefixes CON and CNL are normally used instead. TE (for teleservice) for units in teleservice networks (sometimes called the call- level in teleservice networks that are based on CN connections, as ISDN and PSTN). These prefixes correspond to the logical network structure that was discussed in relation to Fig. 3.12. Most physical networks exhibit, however, a deeper structure than that, which means that the AML user must invent additional prefixes. For example, the layered structure in ATM (or B-ISDN) defines five layers (CS, SAR, ATM, TC and PM, see ITU.T 1.321), which, from an ATM-user point of view, represent a CSP that provides an ATM connectivity service (VP and VC). OSI RM would regard this as a network ser