-plane/control-plane" separation, defined in the ISDN standard). This is a major architectural difference between circuit- switching and packet-switching network systems (such as the SS7 or TCP/IP). In the latter case, there is no such distinction between control and service points. For datagram systems (such as IP and MTP), layer interfaces realize service points only, while in connection-oriented systems (such as TCP and SCCP) a single layer interface realizes both a control point and a service point. Note also that the separation of an OSI layer structure into control and connec- tivity structures can be recursively applied. For example, SDH is separated into a control structure, consisting of cross-connecting and path-controlling actors (that setup and manage 2 Mbps SDH paths between the ISDN switching actors), and SDH agent layers. The SCCP layer also indicates that it can be separated into sepa- rate control and layer structures, since it exhibits a service control point (that is 2.3 Agents and Actors used by the ISUP agent layer when requesting connection-oriented SCCP serv- ices). Media layers consist of resources that create media connections (or "calls") to their users, e.g., ordinary telephony connections (transports 3.1 kHz voice signals). A media-agent layer element manipulates media signals that are transported through the switching layer. Example of media-agent-layer elements are: Transcoders for translating between different types PCM coding, or between compressed and uncompressed formats. Conference bridges and mixers for multimedia calls. Echo suppressors and echo cancellers. Different kinds of interworking units (IWU) for circuit-switched data between ISDN and PSTN (analog) terminals. Since the same switching layer is used both for analog and digital accesses, ana- log-digital converters are also parts of media layers. The call-handling actor layer also makes use of media layers as a means of com- munication with end-users. To that regard it uses a number of media actors, such as digit receptor