1. Selected Data Rate Packet Loss Channel-error Loss Collision Loss Reduced Packet Probing (RPP) Multirate Adaptation For Multihop Ad Hoc Wireless Networks Jun Cheol Park, Sneha Kumar Kasera School of Computing, University of Utah References Introduction Multirate support in the IEEE 802.11 physical layer [1] allows a wireless network interface card to select different data rates at the physical layer so that it can dynamically adapt to diverse channel conditions. Developing an effective multirate adaptation scheme in multihop ad hoc wireless networks is challenging because accurate assessment of the instantaneous channel conditions is difficult due to multiple collision domain. We propose an efficient multirate adaptation scheme, Reduced Packet Probing (RPP), that enables a sender node to effectively approximate channel-error loss in the presence of collisions. Evaluation Reduced Packet Probing What if guarantee no change in collision loss, then analyze channel-error loss? What if guarantee no change in collision loss, then analyze channel-error loss? 1. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, ANSI/IEEE Std., 1999. 2. Emulab, http://www.emulab.net. 3. Roofnet, http://pdos.csail.mit.edu/roofnet/doku.php http://pdos.csail.mit.edu/roofnet/doku.php 4. 4. DSR, http://www.monarch.cs.rice.edu/ Motivation Fig. 1 Two causes of packet loss Cause of packet loss is unknown! Which data rate? Packet Loss Anomaly Currently, wireless nodes have no effective way of distinguishing between packet loss due to channel errors from those due to collisions. So multirate algorithms reduce the data rate even when they observe loss due to collisions. Interestingly, the reduction in the data rate increases the frame transmission time and thus increases the medium occupancy time. This increase in the medium occupancy time actually increases the probability of collision loss. The relationship between channel-error loss and collision loss as a function of a selected data rate is shown in Fig 2. When a data rate increases, the collision loss decreases whereas the channel-error loss increases. Fig 2. Packet loss anomaly !!!Hmm…interesting… We have implemented our preliminary scheme in the Emulab wireless testbed [2]. We use our own modified version of the Madwifi-0.9.3 IEEE 802.11 driver on Linux-2.6.20.6 to make it compatible with Roofnet software [3] that provides the Dynamic Source Routing (DSR) [4]. We have implemented our preliminary scheme in the Emulab wireless testbed [2]. We use our own modified version of the Madwifi-0.9.3 IEEE 802.11 driver on Linux-2.6.20.6 to make it compatible with Roofnet software [3] that provides the Dynamic Source Routing (DSR) [4]. → Our preliminary implementation shows a 20% ~ 50% improvement in throughput on a 3-hop ad hoc path compared to the existing multirate adaptation schemes. A packet loss event is considered to be an indication of the data rate at which the packet is transmitted being too high for the given instantaneous channel condition, and used to downgrade the rate of transmission. However, in multihop ad hoc wireless networks, the two different causes (i.e., collision and channel-error) of packet loss make decision on suitable data rates challenging. In our RPP scheme, a sender node periodically invokes “a probing phase." In the probing phase, the sender node examines from the highest data rate to the lowest data rate to find the suitable data rates. While downgrading the data rate, it reduces the packet size to be transmitted so that the actual transmission time of the reduced packet at the lowered data rate is same as the one of the packet at the previously higher data rate. The rationale behind this approach is the observation that the collision loss mainly depends on the actual medium occupancy time regardless of the selected data rate. By keeping the same medium occupancy time for different data rates (i.e., keeping the same collision loss), any changes in the packet loss at the sender node can be inferred as changes in the channel-error loss. ? Collision Channel-error Packet Loss Avoiding Contention Reducing Channel-error Case A Case B ?