1. Network Capture and DCF QoS
January 2001
Network Capture and DCF QoS
Matthew Fischer
Broadcom, Corp.

2. Network Capture within 802.11 Networks
January 2001
Network Capture within Networks
Published empirical results demonstrate network capture within networks
Unfairness and Capture Behaviour in Adhoc Networks
Christopher Ware, et al., Switched Networks Research Centre, Univ. Wollongong

3. Experimental Setup Two hidden nodes 500kByte TCP flows from B & C to A
January 2001
Experimental Setup A B C
Two hidden nodes
500kByte TCP flows from B & C to A
both Adhoc and AP connections examined
(I.e. STA A as AP vs an adhoc network)
11Mbit/sec b nodes

4. Scenarios Trial 1: Trial 2: No RTS, equal SNR at receiver (STA A)
January 2001
Scenarios
Trial 1:
No RTS, equal SNR at receiver (STA A)
complete capture occurs
simulations support this result [*]
Trial 2:
RTS enabled (500B thresh), equal SNR at receiver (STA A)
sharing occurs
M Gerla, R Bagrodia, K Tang, and L Wang. TCP over wireless multi-hop protocols: Simulation and experiments, In IEEE International Conference on Communications, volume 1, Vancouver, Canada, IEEE.

5. Scenarios Trial 3: Trial 4:
January 2001
Scenarios
Trial 3:
RTS enabled, 5dB SNR difference at receiver
RTS disabled, 5dB SNR difference at receiver
either case, capture is utter and complete (contest goes to the strongest)
Trial 4:
dynamic SNR change during transfer
one STA starts at +5dB vs other STA
midway through transfer, stronger STA is reduced to -8dB relative to other STA
capture of channel moves from +5dB STA to other STA once the signal level drops
performed to substantiate near-far capture effect

6. January 2001
Results
use/disuse of RTS and SNR differences can cause capture to occur within an network when hidden nodes are present

7. January 2001
Questions/proposals
How is each of the DCF QOS proposals affected by the scenarios described?
Do any of the proposals include any mechanism to specifically deal with the described scenarios?
Do any of the proposals include any mechanism to deal with any source of network capture?
Do simulations demonstrate the robustness of any of the proposals against capture?
All DCF QOS proposal simulation results must include per-STA throughput and latency measurements
aggregate numbers do not show the effects of capture or other unfairness
any potential indication of unfair sharing among nodes of a single traffic-class must be available to be explored