e separated by the binary PTI values 100, 101, and 110. Manage- ment cells that are addressed to these functions are already extracted by the ATM agent. Cells with other PTI values (000 through 011) are switched by the ATM actor. Cells with VCI values of 1, 3, and 4 are also management cells. If the VCI value is 3 or 4, the cell is terminated in the VP-OAM layer interface. If the VCI = 1, the cell is terminated in the SVC-OAM. A channel with VCI = 1 is also called "meta-channel" because it is used to manage other channels, i.e., SVCs. VCI values 7 through 31 are reserved for future use. The use of the PTI parame- ter is quite interesting. Since the inventors wanted to keep the overhead in cells as low as possible, they managed to include four different functions in the three bits (B1, B2, and B3) of the PTI (see Fig. 6.11): Öther overhead A cell: B1 B2 B3 Payload (48 octets) parameters If B1=0, the payload is to be switched If B1=1, the payload is VC-OAM data. If B2=0, the VC experiences congestion If B2,B3=00, data for adjacent node maintenance If B2=1, the VC experiences no congestion If B2,B3=01, data for source/destination maintenance If B3=0, cell type is 0 If B2,B3=10, data for ABR control If B3=1, cell type is 1 If B2,B3=11, not defined at present Fig. 6.11 Usages made of the payload-type identification parameter, PTI This kind of smart coding (although sometimes necessary in order to save capacity) can make it very difficult to understand a standard, unless its authors have made an extra effort to describe the actual mechanism above the bit level (which rarely is the case, also not in the case of ITU-T 1.361). Let's therefore try to understand the four functions that are supported by the PTI in AMLn terms. 1. The B1 bit acts as an interface type identifier (ITi) for cells to be terminated by the ATM agent, and cells to be switched by the switching actor. 2. For cells that are terminated by the agent (B1 = 1), the second and third bit together act as an access-point identifier (APi). For example, B2B3 =10 