GSM was conceived in accordance with the guidelines of ISDN. Therefore, a reference con®guration is also de®ned for GSM systems, similar to the one used in ISDN systems. Using the reference con®guration, one gets an impression of the range of services and the kinds of interfaces to be provided by mobile stations. Furthermore, the reference con®guration indicates at which interface which protocols or functions terminate and where adaptation functions may have to be provided. The GSM reference con