Changes between Version 19 and Version 20 of Internal/RunningDemos

Timestamp:

Dec 23, 2005, 6:27:01 PM (18 years ago)

Author:

kishore

Comment:

—

Internal/RunningDemos

@@ -11 +11 @@
 The objective of this demo is to show the failure of current implementations of IEEE 802.11 infrastructure networks in supporting a large number (O(10)) of clients. The cumulative throughput at the access point (AP) is the measured and it is shown that beyond a certain number of clients, there is a sharp drop rather than graceful degradation. It is hypothesized that the 802.11 bit-rate adaptation algorithm is the primary culprit for this behavior - the algorithm does not differentiate between collision-based packet loss and packet loss due to poor channel conditions. It incorrectly infers collision-based loss as an indication to drop the bit-rate, resulting in the sharp drop in cumulative throughput.
 
-The demo itself consists of one receiver and 25 senders -- the start times of the senders are staggered (3-4 senders join the experiment in 10 second intervals). Traffic generated is UDP-CBR (constant bit-rate) at 1.5Mbps emulating high-quality video streaming from the clients to the AP.
+The demo itself consists of one receiver and 25 senders -- the start times of the senders are staggered (3-4 senders join the experiment in 10 second intervals). Traffic generated is UDP-CBR (constant bit-rate) at 1.5Mbps emulating high-quality video streaming from the clients to the AP. All the clients and the AP use 802.11a (channel 36 (5.18Ghz)) with negligible interference from other sources.
 
 === Steps to Execute Demo ===
@@ -34 +34 @@
 On the display machine, point your Internet Explorer or Mozilla Firefox browser to http://sb9.orbit-lab.org. What you expect to see is a page with three frames - one showing the topology of the smaller grid, one showing a control interface to change the packet size and offered load and the third with a link to plot the cumulative throughput at the access point (AP). 
 
-The frame showing the topology should initially display 64 yellow boxes representing the nodes on the grid. Once the nodes in the experiment turn ON, the corresponding yellow boxes will turn green. Once the applications start on the nodes, the senders will turn blue and the receiver will turn red.
+The frame showing the topology should initially display 64 yellow boxes representing the nodes on the grid. Once the nodes in the experiment turn ON, the corresponding yellow boxes will turn green. Once the sending and receiving applications start, the senders will turn blue and the receiver will turn red.
   
-Click the link for the cumulative throughput in the third frame -- the expected behavior for the curve is that initially, the throughput will rise as senders are added but beyond a certain number of senders, throughput will fall drastically.
+Click the link for the cumulative throughput in the third frame -- the expected behavior for the curve is that initially, the throughput will rise as senders are added but beyond a certain number of senders, throughput will fall drastically. The explanation for this drop is that 
+a. the current implementation of CSMA/CA fails to prevent losses from occurring in a congested environment 
+b. the 802.11 bit-rate adaptation algorithm, which believes CSMA/CA to be perfect, infers these losses to be due to poor channel (SNR) conditions and drops the bit-rate accordingly. This in turn makes the situation worse given that the same frame will now take a much longer time on the medium -- all senders will drop their rate to 6Mbps and the cumulative throughput will reflect this (it should be equal to the saturation throughput at 6Mbps).
 
 == Streaming video demo ==