Packet Processing Delay (PPD) is the amount of time required for each side to process the maximum amount of data buffered in their receive packet sliding window. The PPD is the value exchanged between the PAC and PNS when a call is established. For the PNS, this number should be small. For a PAC making modem connections, this number could be significant.
Sample is the actual amount of time incurred receiving an acknowledgment for a packet. The Sample is measured, not calculated.
Round-Trip Time (RTT) is the estimated round-trip time for an Acknowledgment to be received for a given transmitted packet. When the network link is a local network, this delay will be minimal (if not zero). When the network link is the Internet, this delay could be substantial and vary widely. RTT is adaptive: it will adjust to include the PPD and whatever shifting network delays contribute to the time between a packet being transmitted and receiving its acknowledgment.
Adaptive Time-Out (ATO) is the time that must elapse before an acknowledgment is considered lost. After a time-out, the sliding window is partially closed and the ATO is backed off.