U sends and receives information from its counterpart. To be aligned with the OSI RM (which calls data to be transferred service data unit, SDU), we call information units in TE-SAPs service information units, SIU. A SIU can be a voice sample, a facsimile page or a text file. 9. The TE-UDT sends and receives userData messages. A userData may consist of only a fragment of a SIU. In ISDN basic access, there can exist two 64-kbps TE-UDTs in parallel. One could support speech communication, the other could support unrestricted 64-kbps data communication. An alternative would be to have two simultaneous speech- connections. Again, another alternative is to have one 128-kbps TE-UDT, using both B-channels. After service request from the TEC, and after negotiations with the network process control system, the TE-TPC directs the CNU on how to termi- nate the two B-channels in TE-UDTs. In ISDN, there is normally only one SAPI used for all TE-SAPs of a terminal system, which means that the SAPI value does not identify individual TE-SAPs, but the terminal system as a whole. The discrimination between TE-SAPs is guided by the service identifier in the scenarios. We call this function service dis- crimination. This is different from the OSI RM, where each SAP has a SAPI value, which implies discrimination on SAPI values, SAPI discrimination. ISDN allows terminal systems to have more than one SAPI in order to simplify identification of TEUs and TE-UDTs by the TE-TPC and the TEC. The terminal system can allo- 3.3 Network Service Interface Scenarios cate these addresses freely to terminals. This function represents a kind of local configuration activity managed by the TEC, where the TEC maps SERVid values on TE-SAPIs. Although this scenario is typical for ISDN, it looks basically the same for tradi- tional telephony network services provided by PSTNs. The major difference is that the PSTN scenario is considerably simpler, since PSTNs do not provide multiple CN connection endpoints and alternative teleservices. There is also no se