On-demand Routing in
MANETs:
The Impact of a Realistic Physical Layer Model
| Liang Qin, Thomas Kunz | |
| Department of Systems and Computer Engineering | |
| Carleton University, Canada | |
| Mobile Ad Hoc Network Routing Protocols: | ||
| Limited radio transmission range and node mobility | ||
| Problems: | ||
| Most existing performance comparisons based on ideal propagation model (free space model, two-ray ground model) | ||
| Our simulation results with shadowing model show severe performance degradation | ||
| Ideal Models | ||
| Free space model and two-ray ground reflection model | ||
| Shadowing Model | ||
| b: loss exponent, corresponding to mean transmission range | ||
| s:shadowing deviation | ||
| Signal strength fluctuates in shadowing model | ||
| Ideal model (left) and shadowing model (right) over the same distance between two nodes | ||
| Fluctuation at least 2 orders of magnitude | ||
| Signal strength fluctuation causes active links to “break” | ||
| Simulations of AODV & DSR show that | ||
| Packet Delivery Ratio (PDR) decreases significantly | ||
| Broken links introduce more Route Discovery processes | ||
| Cause more control messages and longer packet delay | ||
| Transmitter did not receive an ACK from the receiver and salvaged the packet | |
| Result in duplicated packets and increased traffic | |
| Based on AODV and DSR | ||
| Apply new signal strength threshold during Route Discovery Process | ||
| Higher than the threshold used in NS2 | ||
| In order to select more stable links to resist to signal strength fluctuation | ||
| DSR: Route Request & Route Reply, AODV: Route Request only | ||
| Different from other proposals (e. g, SSA): | ||
| Power strength only applying on Route Discovery | ||
| Reduce unnecessary control messages in DSR | ||
| Turn the “shortening” option off to prevent “weak link” joining the route again | ||
| No Route Request for undeliverable Route Error and Route Reply messages | ||
| No salvage for Route Error and Route Reply messages | ||
| The Network Simulator (NS2) | ||
| Simulation metrics: | ||
| Packet Delivery Ratio | ||
| Total number of control messages | ||
| Average hop count | ||
| Average packet latency | ||
| Simulation parameters | ||
| 1500x300 m area, 4 data packets/s, 20 sources, 64 Bytes/packet, 20m/s max node speed | ||
| Equal node coverage (Equals to 50 mobile nodes in ideal model) | ||
| Performance Comparisons with Different b values |
| Performance Comparisons with Different Mobility Patterns | |
| Selecting reliable links by using higher signal strength threshold can significantly improve performance | ||
| PDR increases | ||
| Total number of control messages and average packet delay decrease | ||
| Future work | ||
| Route maintenance | ||
| Appropriate signal strengths threshold for different node density and b values | ||