ecifying the internal view of an exter- nal interface and by mapping network service interfaces in one level to network interfaces in a lower level. This transform is applicable in the separation between mobile termination (MT) and terminals (TE) in the GSM/GPRS physical network model, see Fig. 4.21. gsm&gprs PDN applications Host, plmn.0 pdn.0 fxn.0 plmn. 1.1 applications ms. 1.1 Other- GGSN - 4.1 applications Fig. 4.21 Network service interface transformation in a GSM-MS 4.5 Implementing Network Service Interfaces The internal network interfaces k1.1 and k2.1 in the MS are invisible from the GSM/GPRS-network side, since the functional spaces of ms1.1 and plmni 1.1 are completely independent. In the GPRS standard, the network interface k1.1 is called reference point R and specified in GSM 07.60. To specify the mapping of a network service interface on a network interface comprises: Defining a generic message type that can carry the information that is expressed by abstract service primitives of the network service interface. Further refining the generic message type and adding other message types, con- sidering the transfer properties of the CSP that is used. Developing the message protocol while considering the behaviour defined by the abstract protocol and the constraints of the CSP. Describing the behaviour of the MEH. The first three points belong to the protocol specification problem and are dealt with in more detail in Chap. 5. To describe the behaviour of the MEH implies that the service primitives that are defined in the original network service interface must be mapped on the service primitives and operations that are offered by the new CSP. This problem is discussed in Sect. 4.5.2, using a SAP interface as an example. 4.5.2 Implementing Service Access Points Based on the understanding we have about the transformation of network serv- ice interfaces, in this section we will analyse the implications of such transforma- tions on the operations of network service interfaces. We will used the model of Fig. 