nection 7. The ATMUs can now send SDUs of 48 octets to each other, with a rate that cor- responds to the requested capacity. The initiating ATMU will (in this scenario) 3 Logical Network Modelling send and receive on vpi2/vci124, and the access switch will relay all those cells to and from vpi6/vci13 on link L2. Since all ATM switches along the path between the ATMUs work in the same way, the ATM VC connection guarantees sequential delivery of cells, which is one of the benefits with a connection-ori- ented service. ATM is designed to be the one and only wide-area connectivity service for all kinds of teleservices and data communication applications. The connectivity serv- ice that an ATM-CSP provides is, as we have seen, the ability to send streams of 48-octets SDUs over VC connections, nothing more and nothing less. The size of 48 octets has nothing to do with the requirements on SDU size that different tele- services and data communication application have. It is selected only to provide the best possible, economical transfer characteristics in the ATM network. Thus, to be able to use existing communication applications over ATM, we need to adapt the application SDU size to the ATM-SDU size. This is just one of the problems that must be solved between ATM users. Others are: The mode of sending is completely asynchronous, meaning that the users can send and receive SDUs at will. For data communication applications, SDUs will typically be sent and received in bursts, probably with different capacity require- ments in different directions. For isochronous services (like voice, video etc.), data will be sent in a synchronous stream and require the same capacity in both directions. The asynchronous mode implies that there is a small, but for some teleservices significant, variable time delay between consecutive SDUs. This could, e.g. dis- tort voice information unless some special actions are taken between ATMUs. To make ATM efficient for all kinds of services, no error control and error cor- rection is performed o