Could you please provide a detailed explanation of the meanings of the neighbor factor and the node factor, and clarify their respective impacts on the beacon period? In addition, what do the parameters a and b in the equation represent, and how should they be selected in practice?
In the APID protocol, each underwater node dynamically adjusts how often it sends out beacon signals (its beacon period) based on two main factors: its neighbor factor and its node factor. The neighbor factor, described by Equation (1), considers the number of nearby nodes and how fast they are moving relative to itself. If a node has many neighbors or if those neighbors are moving very differently from it, this factor increases. The node factor, from Equation (2), reflects the nodes own condition—specifically how much battery energy it has left and how much its own speed has recently changed. More energy or a recent sharp change in speed increases this factor.
The combined value of these two factors directly controls the beacon timing. A higher total value means the network conditions around the node are changing rapidly or the node itself is very active and energy-rich. In such cases, the protocol shortens the beacon period to update neighbor information more frequently. Conversely, a lower value leads to a longer, more energy-saving beacon period.
To prevent the beacon period from becoming impractically too short or too long, the protocol uses two control parameters, a and b, in the final calculation (Equation 3). Parameter b sets an upper limit on the adjustment, ensuring beacons are never sent too frequently even in the most chaotic scenarios. Parameter a sets a lower limit, preventing beacons from becoming too infrequent even in very calm conditions. In practice, these parameters (a and b) are selected through simulation or testing to balance between having fresh routing information and conserving the limited energy of the UUVs, typically with a being a small positive number and b being a larger number greater than one.
