ML term service access point with the OSI term service access point. Substitute the SM&AML term logical node with the OSI term user. Substitute the SM&AML term communication service with the OSI term data communication service. In that case, there is no difference between the OSI view on service provider and the concept communication service provider (CSP) in SM&AML. However, in SM&AML most concepts that are based on OSI RM concepts have been general- ized in order to make SM&AML applicable to a wider set of communication sys- tems than just packet-switched data communication systems. For example, we have already seen that we can also apply a service provider view on systems that provide teleservices in circuit-switched systems, like the ISDN. Figure 3.7 (upper part) depicts ISDN as a CSP and compares this model with the OSI model of a service provider (lower part). 3.2 The Communication Service Provider SM&AML: Telephony user Telephony user User protocol TE-SAP The ISDN-CSP (a TE-CSP) In-call Outside call Operator Audio Unrestricted Access to packet service User-to-user messages CSP control over D- or B-channel User protocol Service provider Fig. 3.7 The ISDN-CSP compared to the service provider in OSI RM A number of differences are visualized by this simple comparison: OSI RM defines only one service interface per user, an OSI-SAP. In the AML- CSP, there can exist two interfaces per user, one for sending and receiving (the SAP), another for requesting services (the control service interface, CSI6. An OSI user is also the one who handles the user protocols. In AML, only the service user (the TEU in the model, which is a logical node) handles protocols. The OSI service provider is simple from a service point of view: it provides only one type of service. An AML-CSP can offer many types of services. In the OSI model, every SAP has an address, the service access point identifier, SAPI. In AML, only some types of SAPs have addresses. Although not obvious just by looking at the model in Fig. 3.7, the OSI service prov