| | 103 | The 802.11 DCF attempts to avoid collisions by requiring every node wait (i.e. backoff) a random period and sense the medium before transmitting. The random wait is designed to stagger transmission attempts by nodes that are not otherwise coordinated. Collisions could be made less likely by using longer random backoff periods. But long backoff periods reduce overall medium utilization, negatively impacting performance. Shorter backoff periods increase medium utilization, but also increase the probability that nodes select the same random value and collide. The 802.11 DCF balances this tradeoff with its contention window definition, discussed above. |
| | 104 | |
| | 105 | The DCF backoff procedure divides each waiting period into discrete slots. Nodes wait an integral number of slots before transmitting. The slot duration is 9 usec. |
| | 106 | |
| | 107 | When carrier sensing is working correctly, we expect that collisions will occur when two nodes select the same backoff slot count and initiate their transmissions in the same slot. When carrier sensing is disabled, nodes will still collide when they choose the same slot count. But other nodes, which larger backoff slot values, will fail to sense the collision and will initiate their own transmission in a future slot. |
| | 108 | |
| | 109 | We can test these expectations by analyzing the timestamps of PHY Tx events in the node logs. The absolute time difference between nodes is first resolved by identifying log entries of a common event, such as beacon transmissions and receptions. Once the time drift between nodes is corrected, the Tx timestamps can be extract and analyzed for overlapping Tx events. When overlapping Tx events are located, the timestamps of the Tx events are compared to compute the difference in Tx start times. We expect this difference to be small when carrier sensing allows nodes to defer to ongoing collisions. |
| | 110 | |
| | 111 | The plots below show the results of this analysis for all 4 traffic flows with carrier sensing enabled (left) and disabled (right). |
| | 112 | |
