call-handling function over a resource control point. For each established virtual channel, the call-handling function tells the ATM actor to which layer interface to relay the substreams defined by the CTi values (information stored in the T-CO table). The substreams can be directed to different AAL ele- ments or to the same. This concept is not defined in ITU-T 1.361. We introduce it here to be able to make the distinc- tion between the PTI and the B3 bit, This is just another example of that the B-ISDN concept has evolved from N-ISDN mainly just by replacing the N-ISDN circuit-switching layer with the ATM cell switching stratum. Most other N-ISDN functions and architectural solutions have been retained, however. 6 Applying AMLn After having read this presentation of how the four PTI functions are represented in a cell, the reader can conclude that a more functional (and thereby more intelligi- ble) encoding is preferable. The author could only agree, especially since it would cost only three extra bits in a cell (two if the CTi was excluded). 6.4.4 The Adaptation Stratum Let's end this chapter by commenting on the AAL standards: the properties of the connection type that is offered by a particular AAL is a combination of a VC-type (CBR, ABR, etc.) and the functions that are defined by the AAL. For example, the AAL1 stratum was developed to emulate circuit-switched connections, which obviously requires the CBR type of VCs. A basic function that is required in the AAL1 is segmentation and reassembling since the user of AAL1 delivers a contin- uous stream of bits without any particular message boundary, which the AAL1 must deliver to the ATM switching actor as payload in cells over a particular VC. The AALI is not allowed to perform any type of payload error handling since that would corrupt real-time properties of the AAL1 service. However, it is of some importance to detect cells that are lost in the ATM switching stratum. The AALI therefore adds two parameters (in the overhead, OH, defined by the AALI proto- c