 converter that encodes analog voice signals in the TE- SAP to a 64-kbps data flow, and vice versa. This is called pulse code modula- tion, PCM. The teleservice protocol, in this case, defines the encoding method. The operator network will most likely support protocol conversion in this case, since there exist two different protocol standards for PCM encoding in public networks (called A- and u-law encoding). Using an A/ju protocol converter means that terminal systems that are equipped with TE-UDTs for A-law voice encoding can talk to other terminal systems that are equipped for u-law coding. Figure 3.40 provides an overview of the type of resources that are used in cir- cuit-switched networks for creating CN connections between terminal systems, annotated with the type of symbols that are used. The classification system of Fig. 3.40 is based on the following criteria: Communication service provider (CSPs) and relaying units (R) differ from user- data terminations (UDTs) in that they do not terminate protocols. The internal structure of CSPs and Rs may be based on protocol relations, but that is invisible for the users of these types of units. CSPs and Rs transfer and relay service data units (SDU) between SAPs. SDUs are data that are neither interpreted nor stored by the units, only transferred. 3.4 Modelling Telecom Resource Systems The function of CSPs and Rs is defined by the type, the QoS, the direction and the number of SAPs on the unit's boundary. The transfer function of some units (switches and switched CSPs) can be dynamically changed by controllers. In that case, there will exist one or several CSIs on the boundary of the unit symbol as well. As Fig. 3.40 indicates, CSPs take the provider role in all SAPs. Relaying units take the user role in all SAPs. Resources in circuit switched networks Communication service Relaying units (R) User data terminations (UDT) providers (CSP) Multiplexer Transmission Links Digital cross connect (DCC) Transport Connection Circuit switch Transmission link Transport link 