ment planes of the underlying ATM network. The general protocol structure for the provision of connectionless (CL) data service is illustrated in Figure 20.11. Figure 20.12 shows the protocol architecture for support- ing connectionless layer service. The CLNAP (connectionless network access protocol) Customer ATM switched terminal capabilities equipment plus CLSF CLNAP CLNAP AAL 3/4 AAL 3/4 Physical Physical Physical Physical Figure 20.11 General protocol structure for the provision of CL data service in B-ISDN. CLNAP user layer CLNAP Type 3/4 AAL Physical Figure 20.12 Protocol architecture for supporting connectionless service. CLNAP stands for connec- tionless network access protocol. ASYNCHRONOUS TRANSFER MODE layer uses the type 3/4 AAL, employs unassured service, and includes the necessary functionality for the connectionless layer service. The CL service layer provides for transparent transfer of variable-sized data units from a source to one or more destinations in a manner such that lost or corrupted data units are not retransmitted. This transfer is performed using a CL technique, including embedding destination and source addresses into each data unit. 20.8 B-ISDN/ATM ROUTING AND SWITCHING An ATM transmission path supports virtual paths (VPs) and inside those virtual paths are virtual channels (VCs) as illustrated in Figure 20.13. In Section 20.4.1 we described the ATM cell header. Each cell header contains a label that explicitly identifies the VC to which the cell belongs. This label consists of two parts: a virtual channel identifier (VCI) and a virtual path identifier (VPI). 20.8.1 The Virtual Channel Level Virtual channel (VC) is a generic term used to describe a unidirectional communication capability for the transport of ATM cells. A VCI identifies a particular VC link for a given virtual path connection (VPC). A specific value of VCI is assigned each time a VC is switched in the network. A VC link is a unidirectional capability for the transport of ATM cells between two consecutive ATM entities