18.4 NOTE 1 begins by stating: "A conformant implementation using the mandatory to implement default MPPDU encapsulation algorithm (20.10.1, Figure 20-3) encodes at most two fragments in any given MPPDU as follows: a final fragment (Express or Preemptable), followed by zero or more complete user data frames, followed by an initial fragment (Express or Preemptable) followed by zero or more octets of pad." However, based on my understanding of the standard, the default MPPDU encapsulation algorithm can encode up to three fragments in an MPPDU as follows: a final Express fragment, a final Preemptable fragment, and an initial Preemptable fragment. This can occur when a single Privacy Channel is enabled that transmits both Express and Preemptable frames. If Express Frames become available for transmission while the encapsulation process holds the remainder of a Preemptable Frame, the encapsulation process can end up holding the remainder of an Express Frame and the remainder of a Preemptable frame. If another Preemptable Frame becomes available for transmission at that point, the encapsulation algorithm can create an MPPDU that encodes three frame fragments as described previously. For example, two Preemptable Frames are available for transmission on the Privacy Channel. The default encapsulation algorithm state machine shown in Figure 20-3 is in the NEXT state. It transitions to PFRAME, encodes the first Preemptable Frame, transitions to NEXT and then to INITIALPFRAME, encodes the initial octets of the second Preemptable Frame, transitions to TX and back to NEXT. The encapsulation process now holds the remainder of the second Preemptable Frame. At this point, two Express Frames become available for transmission on the Privacy Channel. The state machine transitions to EXPRESSFRAME, encodes the first Express Frame, transitions to NEXT and then to INITIALEXPRESS, encodes the initial octets of the second Express Frame, transitions to TX, and back to NEXT. The encapsulation process now holds the remainder of the second Express Frame and the remainder of the second Preemptable Frame. At this point, a third Preemptable Frame becomes available for transmission on the Privacy Channel. The state machine transitions to FINALEXPRESS, encodes the final octets of the second Express Frame, transitions to NEXT and then to FINALPFRAME, encodes the final octets of the second Preemptable Frame, transitions to NEXT and then to INITIALPFRAME, encodes the initial octets of the third Preemptable Frame, and then transitions to TX. The MPPDU that is ready for transmission contains three fragments.