Traditional telephones, answering machines, Group 3 facsimile equipment and modems are analogue end-devices and cannot be operated directly on the ISDN connection. In these cases, an ISDN terminal adapter with an a/b interface is the solution: it allows the available analogue end-devices to be continued to be used even with ISDN. Small ISDN PBX systems offering up to 16 analogue PBX lines are, in principle, equivalent to a 16-port terminal adapter. Quality and usable transmission speed are limited by the inferior performance of the analogue lines.