Ordered packet scheduling in wireless ad hoc networks: Mechanisms and performance analysis

Wireless ad hoc networks based on the IEEE 802.11 protocol can incur severe unfairness even in simple topologies. In particular, two topological properties that we define in a graph-theoretic framework and refer to as information asymmetry and perceived collisions result in significant performance degradations and unfairness. In this paper, we present the design and analysis of Distributed Wireless Ordering Protocol (DWOP), a distributed scheduling and media access algorithm targeted towards ensuring that packets access the medium in an order defined by an ideal reference scheduler such as FIFO, Virtual Clock, or Earliest Deadline First. In this way, DWOP enables QoS differentiation as well as fairness when combined with TCP. Our key technique is piggy-backing head-of-line packet priorities in IEEE 802.11 control messages so that nodes can assess the relative priority of their own queued packets. With a graph-theoretic problem formulation, we design DWOP to achieve the exact reference ordering in fully connected graphs, and to have well-characterized deviations from the reference order in more complex topologies. A simple theoretical model indicates that the scheme attains rapid convergence for newly arriving nodes, and extensive simulations indicate that nearly exact reference ordering can be achieved, even in complex asymmetric and perceived-collision topologies.