1. Quantification of Capture Effect in Near-Far Scenarios
Month 1998
doc.: IEEE /xxx
May 2001May 2001
Quantification of Capture Effect in Near-Far Scenarios
Greg Chesson
Aman Singla
Atheros Communications, Inc
A. Singla, Atheros Communications, Inc
John Doe, His Company

2. Known problems OS resource mis-management Protocol Stack adaptation
May 2001May 2001
Known problems
OS resource mis-management
one application can “capture” OS resources: buffers, ifq, …
Protocol Stack adaptation
TCP treats packet loss as congestion
TCP adapts by reducing offered load (window size, slow-start)
PHY rate adaptation
NIC or adaptive device driver reduces PHY rate to compensate for packet loss
Near-far effects
near station may block/drown transmissions from far stations
A. Singla, Atheros Communications, Inc

3. Objectives Reproduce near-far effect in simulation
May 2001May 2001
Objectives
Reproduce near-far effect in simulation
Quantify and analyze the magnitude of near-far effect
A. Singla, Atheros Communications, Inc

4. May 2001May 2001
Scenario
36Mb/s a PHY
Near/Far station determined on basis of the relative strengths of the signals originating from the station as received at the AP
All stations are visible to each other
Collisions between a far station and a near station resolve in favor of the near station
Simulation parameters adjusted to guarantee capture by near station(s)
Near Stations AP
1500 byte TCP/UDP Streams
Far Station
A. Singla, Atheros Communications, Inc

5. UDP Results May 2001May 2001 A. Singla, Atheros Communications, Inc
Near Stream
Far Stream
A. Singla, Atheros Communications, Inc

6. TCP Results May 2001May 2001 A. Singla, Atheros Communications, Inc
Near Stream
Far Stream
A. Singla, Atheros Communications, Inc

7. May 2001May 2001
Conclusions
Far station loses “constant” UDP bandwidth to near station(s)
percentage increases with number of near stations
Less degradation for TCP traffic
data sink has a moderating effect
less “loss” with increasing number of near stations
Limited experimental scope
no correlation with range/signal data
limited number of experiments
limited topology
A. Singla, Atheros Communications, Inc