ffects the information rates and service quality in TE-SAPs. Fur- ther differentiation depends heavily on the type of user-data terminations, and on the type of resources that exist in the resource system. A short description of the ISDN teleservices follows: User-to-user messages is a simple message service that can be used either in parallel with another circuit-switched call (in-call messages) or in isolation (out- side call). In both cases, it relies on transport of message data over the ATC in accesses and over the connection-oriented SCCP services4 of the NTC in the core network. From a protocol point of view, messages are part of the teleservice access protocol ITU-T Q.931 and of the teleservice distribution protocol ISUP5 in the core network. Telephony is actually two different services. One offers 3.1-kHz telephony, the other a higher-quality (7-kHz telephony). ISDN also supports facsimile and pic- ture phone services, although not depicted here. The telephony service requires a B-channel from the ATD. The resources needed in terminal systems are A/D converters. In the core network, 64K CS switches and A-u converters are needed. Audio is used for low-rate circuit-switched data communication. The service requires a B-channel from the ATD. The resources needed on the terminal side are modems and A/D converters. In the core network, 64K CS switches and rate adapters are needed. Rate adapters are resources that allow users who use modems with different rates to communicate. Unrestricted data provides 64 kbps-2 Mbps transparent data communication service. The service requires 1-30 B-channels from the ATD. The resources needed on the terminal side are suitable B-channel data terminations for circuit- switched data (X.21). In the core network, 64K CS switches are needed. Packet access (B) provides a 64-kbps access to a packet-handling network (X.25 or IP) outside the ISDN. The access service requires a B-channel from the ATN. In the core network, 64K switches are used to access the external network. To make use