elements aspects Internetwork interfaces 1,400 Series User-network Note - Models, reference interface aspects configurations, tools, methods, are contained in the appropriate I-Series Recommendations. Figure 44.1 Structure of CCITT I. Series Recommendations munication services are the products which are supported by the The following circuit-mode bearer services have been defined in ISDN as such and there are various categories of telecommunica- the 1988 recommendations tion services supported by the ISDN. The CCITT Recommendations on telecommunication services to 1. 64kbit/s unrestricted, 8kHz structured. be offered by an ISDN appear in the 1.200 series of Recommenda- 2. 64kbit/s, 8kHz structured, usable for speech information trans- tions, and the first of these, Recommendation 1.210 defines the general principles and categorisation of services. The CCITT has 3. 64kbit/s, 8kHz structured, usable for 3.1kHz audio information developed a hierarchical structure for the definition of services in transfer. the ISDN because of the wide range of services likely to be offered. 4. Alternate speech, 64kbit/s unrestricted, 8kHz structured. There are two main categories of telecommunication services, 5. 2x 64kbit/s unrestricted, 8 kHz structured. bearer and teleservices, to both of which can be added a third 6. 384 kbit/s unrestricted, 8 kHz structured. category of supplementary services. 7. 1536 kbit/s unrestricted, 8 kHz structured. 8. 1920 kbit/s unrestricted, 8 kHz structured. 44.3.1 Bearer services The following packet-mode bearer services have been identified: Bearer Services supported by an ISDN provide the capability for transferring information between ISDN user-network interfaces and 1, Virtual call and permanent virtual circuit. involve only low-layer functions (layers 1 to 3 in the protocol stack). 2. Connectionless (for further study). Customers may choose any set of high-layer protocols for their 3. User signalling (for further study). communications and the ISDN does not necessarily ascertain com- patibility