Cross-Layer Settings of Power, Data Rate, and Forwarding Range for IEEE 802.11 Based Multihop Ad Hoc Wireless Networks
Rima Khalaf and Izhak Rubin
In this paper, we study cross-layer design and dynamic selection tradeoffs between settings of the transmission data rate, the transmission power and the forwarding range in both single hop and multihop IEEE 802.11 based ad hoc wireless networks while taking the effect of capture into account. We present an approximate mathematical model for calculating throughput and packet delay performance and show that it can be effectively used to dynamically select the nodal cross-layer parameters. We present simulation results that have been used to validate the precision of our analytical model. We demonstrate the performance behavior of such networks under a multitude of settings applied to the nodal cross-layer parameters by employing data rate and power level settings that are present in commercially available IEEE 802.11b radios. Under such settings, we show that in networks with lower nodal spatial density, operations that select higher nodal transmit power values along with higher data rate level settings, using relatively longer distance forwarding ranges, tend to exhibit enhanced delay-throughput performance characteristics. Our performance analyses show that the throughput-capacity level tends to increase as the nodal transmit power level used in the network increases. Our results also show that operating at the lowest transmit power value that keeps the network connected often yields distinct degradation in the throughput and the delay performance. This is mainly due to the fact that the higher spatial re-use factors introduced by operating at lower nodal transmit power levels is typically not sufficient to offset the resulting increase in the average path length of a flow’s route. This increase in path length increases the intensity of the internal traffic process, and thus induces higher access contention and packet delays. By using a free space attenuation model, we study the effect of the attenuation factor of this propagation model on the selection of the transmit data rate and observe that, depending on the value of the underlying propagation factor, an increase in nodal transmit data rate does not always yield a consequent increase in the system’s throughput-capacity level. We demonstrate the underlying performance tradeoffs that are realized through the selection of the nodal cross layer parameters by considering illustrative multihop network topologies. Our analytical methods can be employed to determine the effective settings of the corresponding transmit power, data rate and forwarding range levels for general CSMA/CA based multihop ad hoc wireless network systems.
Keywords: Ad hoc Networks, IEEE 802.11, transmit power control, transmit data rate, throughput, delay, cross-layer settings.