General Recommendation I.120 provides the conceptual principles on which an ISDN is based. The main feature of an ISDN is the support of a wide range of service capabilities@ including voice and non-voice applications@ in the same network by offering end-to-end digital connectivity. The ISDN concept supports a wide range of service capabilities within the same network such as: packetswitched and circuit-switched connections up to and including 64 kbits/sec on a bearer (B) channel. A digital PLMN network supports the same wide range of service capabilities@ including reliable end-to-end digital connectivity and contains specialized supporting functions@ e.g. speech bit rate reduction processing. However@ due to the limited resources associated with radio channels nominal circuit-switched connections may have to be limited to rates lower than 64 kbit/s (e.g. connections up to and including 16 kbits/sec). The key element of service integration for a digital PLMN is the provision of a set of standard@ multipurpose user network interfaces. These interfaces present a focal point for the development of digital PLMN components and configurations that result in PLMN ISDN-like terminal equipment and applications. A digital PLMN is recognized by the service characteristics available through the user-network interfaces@ rather than by its internal architecture@ configuration and technology. This concept plays a key role in permitting user and network technologies and configurations to evolve separately. The service definitions are provided in separate Recommendations. The digital PLMN user-network interface point should provide: a) flexibility of user terminals with respect to digital PLMN network interconnection; b) flexibility of separate user terminal evolution and PLMN technology evolution; c) flexibility of definition and provisioning for PLMN basic bearer services@ teleservices and supplementary services; and d) operation and maintenance capabilities. Interface applications Figure 1/Q.1061 shows some examples of digital PLMN interfaces. The following cases are identified: 1) access of individual digital PLMN terminals (i.e.@ hand-portable or vehicle-mounted terminals); 2) access of multiple digital PLMN terminal installation; 3) access of a mobile PABX or LAN; 4) dedicated service networks; and 5) land-line ISDN networks. A number of digital PLMN signalling reference points are specified and any one of these reference points may be used as the user to network interface point. Figure 2/Q.1061 illustrates the major signalling reference points in a digital PLMN. a) Xm - TEm to mobility management reference point; b) Sm - mobility management to RF transmission management reference point; and c) Um - mobile station (MS) to base station (BS) reference point. The description of the functional entities is given in ??. The MS-BS interface for the digital PLMN mobile station includes both the logical part of the communication between the MS and the BS@ and the control and physical requirements of the radio channel. It should be noted that the Um reference point is the traditional radio-based network interface and is very much RF technology dependent. The detailed Um reference point will be specified in separate Recommendations.