nt/inter- face lies on the interpreter, since most protocol architectures show only part of the functionality in a reference point/interface. These models are said to specify the logical architecture of GPRS. Each stack is defined down to the level of transmission, which, in SM&AML terminology, classifies the units in the model as physical nodes. The standard says, however, that no conclusion of what the physical nodes may look like shall be drawn by these models. Obviously GSM 03.60 has its own definitions of the terms "phys- ical node" and "logical node", which, however, are not defined in the standard. No conclusion can be drawn from the models in Figs. 4.16 and 4.17 about how the reference points/interfaces are to be specified. This is SO because the net- work level 0 is not defined, and consequently, the distinction between external and internal views of reference points/interfaces cannot be made. For example, the reference point/interface Um, according to the models in Fig. 4.17, corre- sponds to the network interface on physical level 0 between a GPRS-MS and 4.5 Implementing Network Service Interfaces the GPRS network. If we instead look at the interface from the BSS-side, it cor- responds to the internal view of the same interface that BSS has on physical net- work level 1. Which of these is the correct interpretation of Um is not defined in GSM 03.60. It is most likely the latter, but that is contradicted by the models. The same thing is true for some other reference points/interfaces that are defined in the standard. Since no logical networks are defined, the distinction between resource system and process control system functions and protocols is not made and is also diffi- cult to extract. A similar distinction to the ISDN user and control plane is made on the access side, although called transmission plane and signalling plane. Besides the rather unconventional use of the term "transmission", this distinc- tion is not continued in the core network part, where user data, control data and transport mec