s it is connected, see Fig. 5.14. 5.3 Discrimination Logical node Parameters defined by Proxy Proxy Parameters defined by PRc Over- Over- SE1id SE2id Level 1+2 integrator SE1id SE2id Did Discrimination identifier X,Y SE1s Proxy Protocol handled by X B,C SE2s Fig. 5.14 Discrimination based on service element names The integrator in Fig. 5.14 perform the SAH function and two levels of discrim- ination. The overhead parameter in the discrimination PDUs represent the fact that an integrator may also perform functions other than just pure discrimination (see Sects. 5.4 and 5.5). 5.3.3 Discrimination Qualifiers The service elements and levels in the previous models were identified by nam- ing elements. Element names are just one of the four types of discrimination quali- fiers that are used in different types of protocols. For creating level discrimination, service access points or connection endpoint discrimination can be used. No level 1 discrimination protocol is then required, see Fig. 5.15. Logical node Logical node ICI SDU Fig. 5.15 Level 1 discrimination based on SAPI and CEI The model to the left shows discrimination based on a logical node subscribing to several addresses (SAPI). This method is used in the GSM core network to address the logical nodes of the GSM-PCS, as is described in Sect. 5.2.1. It is an appropriate method when a CNL-CSP is used. The service elements used on each address may be of different types, since it is of no concern to the CSP which serv- ice element is associated with a particular address. This technique is also used over LAPD links to differentiate between teleservice-handling elements (SAPIO) and 5 Protocol Network Modelling packet service elements (SAPI16). It may also be used over IP networks to separate different types of IP applications in the same logical node. SAPIs are the means for routing. From a routing point of view, there is no differ- ence between serving a logical node that has several SAPIs, and a set of logical nodes that have a single SAPI each. Thus, to implement