The ATM Cell Header
Figure 20.5 shows the format of an ATM cell header for a cell that is sent between an endpoint system and its adjacent ATM switch.
Figure 20.5. Cell header sent between an endpoint system and an ATM switch.
Currently, the first 4 bits are unused. Recall that a virtual circuit is identified by its Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI). The VPI occupies the next 8 bits. When expressed as a decimal number, the VPI value can range from 0 to 255.
The VCI occupies 16 bits. Hence, when expressed as a decimal number, the VCI value can range from 0 to 65,535. In all, there are 16,777,216 possible (VPI, ...