 at these layers between customers. An example of a bearer service is a switched 64kbit/s circuit mode unrestricted service. 44.3.2 Teleservices Bearer services are characterised by a set of low-layer attributes and these are classified into three categories: Teleservices provide the full capability for communication by means of terminals, network functions and possibly functions pro- 1. Information transfer attributes. vided by dedicated centres. They involve the standardisation of the 2. Access attributes. higher layer function (layer 4 to 7 in the protocol stack). Examples 3. General attributes, including operation and commercial at- of teleservices are telephony, teletex, videotex and message hand- tributes. ling. Teleservices are characterised by a set of low layer attributes, Network aspects 44/5 a set of high layer attributes and operational and commercial at- tributes. CONNECTION 44.3.3 Supplementary services ACCESS TRANSIT ACCESS The two categories of services that have been described above are CONNECTION CONNECTION CONNECTION referred to as "basic services". The basic service is what the cus- ELEMENT ELEMENT ELEMENT tomer gets when he asks for a communication capability. A sup- plementary service modifies or supplements a basic Figure 44.2 Example of connection elements forming an telecommunication service, for example redirection of calls to an- ISDN connection other number. Consequently the supplementary service cannot be offered to a customer as a standalone service, and must be offered in association with a basic telecommunication service. It is possible for the same supplementary service to be common to a number of Recommendation 1.320 describes a protocol reference model for telecommunication services. The technique for describing sup- ISDN. It is based on the general principles of layering given in the plementary services also uses the attribute method. However, be- X.200 series of Recommendations and has been developed to model cause of the complexity of these supplementary services, the the inf