s of SAPs are the characteristics of SAPs, often called the quality of service (QoS) of the connectivity service. QoS properties have a profound impact on the communi- cation between CSP users, and on how the protocols between users must be designed. QoS properties are also required for simulation of characteristics, based on logical network models. One set of QoS properties refers the rate, capacity and quality of data transfer over SAPs. Typical parameters used to describe these properties are transfer mode, thoughput (measured as average or maximum number of bits per seconds, bps), transit delay (the time it takes for an SDU sent in one SAP to be receivable in another SAP) and error rates. For CSIs, properties such as establishment delay (average or maximum time to establish a connection or a teleservice) are of interest. Efforts to standardize QoS properties have been made by ITU-T several times, see, e.g. ITU-T X.213 and ITU-T 1.340, 350 and 352. This subject is, however, outside the scope of this book. In SM&AML there is also presently no formal defi- nition of QoS properties available. As a temporary solution to characteristics simu- lation, AML allows any set of user-defined attributes to be associated with any unit and communication relation in a model fragment. This information can then be used by an external simulation tool, and simulation results can be fed back to the AML model. The transfer mode (circuit or packet switching) of the CSP has a profound impact on the use of connectivity services, particularly with respect to costs and quality: The resources dedicated to a circuit-switched connection are considerably more expensive than the corresponding resources in packet switching. This speaks for packet switching. Good QoS properties are considerably easier to achieve with circuit switching. In particular, the problem of achieving good quality with packet switching for all types of non-discrete signals (isochronous services), like voice, audio and video signals, is recognized. ATM is SO far an excep