e time. With the basic interface work stations are examples of equipment or combinations of equip- structure, two B channels and one D channel are always present at ment which provide TE functions. the ISDN user/network interface, but one or both B channels, Two types of TE have been categorised: however, may not be supported by the network. The primary rate interface structures correspond to the primary 1. TE1 is an ISDN terminal equipment with an interface that rate of 1544kbit/s and 2048kbit/s. At the 2048khit/s primary rate, complies with the ISDN user/network interface. the interface structure is 30B + D, although one or more of the B 2. TE2 is a terminal equipment with an interface that complies channels may not be supported by the network. Channel types at with non-ISDN interface Recommendations (for example, the rates higher than 64kbit/s have been defined such as 384kbit/s, CCITT X-series and V-series interfaces) or interfaces not in- 1536kbit/s and 1920kbit/s, and these will also be supported by this cluded in CCITT Recommendations. interface. The CCITT 'brand' name for the basic access interface is 1.420 The terminal adapter (TA) functional grouping includes functions and for the primary rate interface 1.421. These recommendations equivalent to layer 1 and higher layers of the OSI Reference Model then call up 1430 and 1431 respectively for the layer 1 descriptions, that allow a TE2 terminal to be served by an ISDN user/network and common recommendations for layer 2 in 1440/1 and layer 3 in interface. 1450/1. The relationship between these recommendations is illus- trated in Figure 44.6. The characteristics of these recommendations 44.5.2 Interface structures will now be described in a more detail since they represent an important aspect of the ISDN. Structures and access capabilities of the ISDN interface are de- scribed in Recommendation 1.412. A set of channel types has been 44.5.3 Layer Basic rate access defined for the ISDN user/network interface. The main types of channels recommended by CCITT are