n SDUs inside the ATM network (bit error rates are sup- posed to be very low in ATM networks). However, some applications depend on reliable SDU transfer. Error detection and correction must in that case, be solved by cooperation between ATMUs. ATM is connection-oriented. Some applications work connection-less. To simu- late connection-less communication must also be handled between ATMUs. A foreseen common use of ATM is to connect many hosts on a LAN to an IP router over a single VC connection. Therefore, a multiplexing function must exist between the IP router an the gateway to the LAN, SO that data streams from different hosts can be identified. The general solution to the problem of adapting requirements from different kinds of tele and data communication services to the characteristics of the ATM connectivity service, is to place a protocol layer between the ATM-CSP and the applications, to simulate the transport characteristics that the application should expect from its (normal) transport connections. This kind of adaption is often called emulation. ITU-T calls this layer ATM Adaption Layer (AAL), and tries to standardize a number of AAL protocols to that regard. These protocols consider 3.4 Modelling Telecom Resource Systems if compensation for variable time delays is needed, if information is sent with variable or constant bitrate, if error handling is needed, if multiplexing is needed, and if the application needs a connection-oriented or connection-less service. The reader is referred to the recommendations ITU-T 1.363, de Prycker (1993), or some other textbook for detailed information on AAL services and protocols. It is sufficient here to show in Fig. 3.71 how the AAL can be used to emulate circuit- switched synchronous connections, like the CN connections (B-channels) of the ISDN. synchronous stream of 1 octet SDUs AAL1-UDT AAL1-UDT Type 1 CS protocol Type 1 SAR protocol vpi {vci vpi., {vci } continuous stream of 48 octet SDUs The ATM-CSP AAL ATM Adaption Layer CS Convergence sublayer ATMU ATM-user 