PD-layer element, while B channels are used primarily for creating media services, and are therefore terminated in diverse media layer elements (e.g., a voice A/D, which is an halog<>digital voice con- verter that requires 64 kbps (B1 or B2 channel)). Since a transmission rate of 128 kbps is available, media services that require more than 64 kbps can be supported as well. The discrimination function in the layer element is therefore dynamically controlled from the call-handling layer over a resource control point (rCP). Over this control point, the call-handling layer tells the BRA if 64 or 128 kbps is required, and over which media layer interface to ter- minate the bits. If 128 kbps is required, the layer element multiplexes both B chan- nels over the actual layer interface. The same principle is used for discrimination in ISDN terminals of the PRA type. The difference is that there are 30 B channels in that case, which can support media services that require up to 2 Mbps by multiplexing in the PRA layer element. 2.2.6 Summary The following five forms of discrimination have been discussed: 1. Discrimination by CEi and Ci, which (in the OSI RM) supports instantiation on the network level and requires no extra protocol. 2. Discrimination by APi, which supports concurrent layers but requires an extra protocol. Two ways of using APi for discrimination have been identified: the layer APi method and the layer-element APi method. 3. Discrimination by NA, which is feasible but rare and does not support instanti- ation. Discrimination by sockets, which (in case of TCP) supports both CON-service properties and concurrent layers, but not instantiation. 2.3 Agents and Actors 5. Discrimination by position, applied primarily in circuit-switching layers. Be aware that, in standardized network systems, the distinction between these forms of discrimination is not always explicitly defined. Furthermore, terminating elements may handle other forms as well. Sometimes a single, very complex iden- tifier is used for several forms o