Changes between Version 79 and Version 80 of 802.11/wlan_exp/app_notes/dcf_with_multiple_flows

Timestamp:

Oct 23, 2014, 7:58:08 AM (10 years ago)

Author:

chunter

Comment:

--

802.11/wlan_exp/app_notes/dcf_with_multiple_flows

@@ -97 +97 @@
 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.
 
-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.
+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 µsec.
 
 When physical carrier sensing works 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 physical carrier sensing is disabled, nodes will still collide when they choose the same slot count. But other nodes, with larger backoff slot values, will fail to sense the collision and will initiate their own transmission in a future slot that may overlap the ongling colliding transmissions.
 
-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 common events, such as AP beacon transmissions and STA beacon receptions. Once the time drift between nodes is corrected, the Tx timestamps and Tx durations are extracted 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.
+We can test these expectations by analyzing the timestamps of PHY Tx events in the node logs by extracting the Tx timestamps and Tx durations 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 (specifically, within a single 9 µs slot) when carrier sensing allows nodes to defer to ongoing collisions.
 
 The plots below show the results of this analysis for all 4 traffic flows with physical carrier sensing enabled (left) and disabled (right).
@@ -110 +110 @@
 
 These plots clearly demonstrate that physical carrier sensing leads to very small differences in Tx start times of colliding packets, consistent with our expectation of collisions occurring only when nodes select the same random backoff period. However, when carrier sensing is disabled, there are both more collisions and the collisions occur with a much larger range of Tx time offsets. This behavior increases the cost of a collision, reducing effective medium utilization and overall throughput.
+
+There are several other behaviors to notice in the above figures. The x-axis in each figure is an index to the number of collisions. The maximum extent of the x-axis is the total number of collisions between the two nodes specified in the title of the figure. The reason the top-left subplot within each figure is empty is because Flow 1 and Flow 2 cannot collide -- both flow originate from the same node. The AP is not going to interrupt its own ongoing transmission. Also notice that the size of the x-axis for the CS-enabled case is generally smaller than the CS-disabled case. This is consistent with our earlier observation that disabling physical carrier sensing increases the probability of collisions.
 
 ----