 that is intended for com- pressed interactive services, such as video conferencing. ABR ("available bit rate"). This is a channel that guarantees good properties for a certain minimum level of transfer rate, and a best effort for higher rates. If the ATM switching stratum cannot deliver, the source is informed about the conges- tion situation. UBR ("unspecified bit rate"). This is a channel that guarantees only best effort transfer delivery, and gives no information about congestion state. It is the chan- nel type that would be used by an IP network, since its properties are similar to the IP service. When a virtual channel is ordered and established, it represents a contract between the using-operator network (e.g., an B-ISDN) and the ATM network. In this contract, the ATM network guarantees negotiated properties and the using net- work accepts the costs (if any). The configuration of layer elements that is defined by the model in Fig. 6.10 indicates that the stratum provides connectivity for three purposes. Two mecha- nism are used for that: one is to dedicate VCIs in a specific way; the other is to use a parameter in the ITU-T I.361 protocol, called "payload-type identification" (PTI). 1. Support for AAL1, AAL2, and AAL5. These strata are supported by regular VCs and VPs that are established by the B-ISDN service control actor over its resource control point to the ATM actor (similar as in N-ISDN and X.25 VC). The VCI for these VCs must lie in the range VCI= 32 through 65,536 and the VPI must not be 0. 2. Support for the SAAL. Two VCIs in every VPI are dedicated for that purpose: VCI=2 defines a point-to-point VC; VCI=5 defines a broadcast VC. These VCs are also called "signaling VC" (SVC). 6 Applying AMLn 3. Support for management. There are three separate management areas defined: operation and management of SVC (SVC-OAM); operation and management of VPs (VP-OAM), and operation and management of VCs (VC-OAM). Both ded- icated VCs and the PTI are used as discriminating mechanisms: The three VC- OAM layers ar