nto a common protocol specification per reference point (i.e. between logical nodes) is not that the protocol-network protocols have some functional dependence in a ref- erence point. On the contrary, protocol-network protocols in the same reference point represent independent functions, since the relations between protocol net- works are not defined in protocols, but in CSIs (or SAPs) inside logical nodes. Instead, a common integration function is used over the reference point, see Fig 5.11. 5 Protocol Network Modelling Integrator Integrator userData MTid Message type identifier PNid Protocol network identifier userData UserData An SDU fragment Fig. 5.11 Discrimination in the protocol between the MT and the BSC logical nodes Figure 5.11 shows the reference point between the logical nodes MT and BSC in the GSM-PCS, and two alternatives to how discrimination could be implemented for this case. The discrimination function is part of the reference-point protocol, and consists of a permanently defined separation of messages that belongs to dif- ferent service elements. The fact that the endpoints are static means that if another protocol network needs to use the same reference point, the reference-point proto- col must be updated. Figure 5.11 shows two methods for discrimination: the refer- ence-point protocol can base discrimination on a protocol network identifier (PNid), or on a message type identifier (MTid). In the former case, the discriminator needs only to know which service elements exist in the node; in the latter, it needs to know which message type is used by which service element. The latter case is, of course, less flexible. Other methods exists as well, see Sect. 5.3. Since the GSM standard does not define protocol networks, discrimination is mostly based on different kinds of mes- sage type identifiers. An exception is the BSSAP parameter that discriminates between DTAP messages and BSSMAP messages (bit 7 of the DISCRIMINA- TION parameter in the userData field of BSSAP messages). This is a kind of pro