Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

On-demand Routing in MANETs 39random variable with zero mean and standard deviation σ, which is called shadowingdeviation. β and σ are obtained by measurement. For example, β is 2 for free space, 2to 3 for obstruction inside factories [10]; σ ranges from 4 to 12 for outdoor environments.3 Mobile Ad Hoc Network with Shadowing ModelWhen applying a shadowing model in our simulations of DSR and AODV, we noticesevere performance degradation compared to the use of a two-ray ground model. Wewill first discuss the behavior of DSR and AODV with a shadowing model, then presentour proposal.3.1 The Network Simulator (NS2)All the modifications and simulations in this paper are based on The Network Simulator(NS2). NS2 is a discrete event simulator developed by the University of Californiaat Berkeley and the VINT project [12]. The Monarch research group at Carnegie-Mellon University developed support for simulation of multihop wireless networks inNS2. It provides tools for generating data traffic and mobile node mobility scenariopatterns and NS2 comes with implementations of DSR and AODV. In NS2, the DistributedCoordination Function (DCF) of IEEE 802.11 for wireless LANs is used asthe MAC layer protocol. The radio model uses characteristics similar to a commercialradio interface, Lucent’s WaveLAN, which is modeled as a shared-media radio withnominal bit rate of 2Mb/s and a nominal radio range of 250 meters with omnidirectionalantenna. We choose the traffic sources to be constant bit rate (CBR) sources.Transmission rate is 4 packets per second. Each packet is 64 bytes long.3.2 Impact of Shadowing Model on DSR and AODVIn an ideal environment, for a transmitter and receiver pair, the received signal poweronly depends on the distance between them. So if the receiver receives a packet with asignal strength is above the reception threshold, it can be sure it is in the transmissionrange of the transmitter, and also this link will last for a while, depending on the twonodes’ mobility pattern. With a shadowing model, the received signal power has aGaussian distribution fluctuation. When a packet transmitted successfully on a link, itis not guaranteed that the next packet can be delivered even if the time between thesetwo packets is short. Figure 1 shows how the signal strength for packet receptionchanges for a pair of nodes over the same period of time (i.e., the distance betweentwo nodes is the same), using two propagation models. Notice that these graphs recordall the packets sent by both nodes, including all control packets at the MAClayer, and the two nodes are moving apart. All the data are obtained from NS2, theshadowing model sets β=2 and δ=4, corresponding to a free space environment.From Figure 1(a) we can see that in a shadowing model, the signal strength fluctuatesover at least 2 orders of magnitude. This will cause many active links to be con-