Impact of interference and capture effects in 802.11 wireless networks on TCP

摘要

As broadband wireless channels become common, the performance of TCP over end-to-end paths containing such links is important. TCP SACK suffers substantially when residual packet error rates increase beyond a value of about 1%--5%(especially for longer RTTs). Recently we have proposed improvements to TCP (called LT-TCP) to make TCP loss-tolerant in heavy and bursty erasure environments. However, real world wireless systems do not just present bursty randomloss patterns to the transport layer. The PHY, MAC and transport layers all respond to errors, interacting in myriad ways. In this paper, we focus on one underlying source of packet erasure (non-congestion loss), namely interference in 802.11 environments (from Bluetooth and co-channel interferers), and the resulting interaction between the MAC and transport layer mechanisms. MAC layer mechanisms cannot fully mitigate the interference problem and tend to misinterpret interference as noise and aggressively respond with techniques like rateadaptation. Such aggressive responses lead to poor scheduling performance at the MAC layer (e.g., well-known unfairness and capture effects) and limit mitigation opportunities at the transport layer. We argue that aggressive rate adaptation is undesirable in these situations and show how a combination of reconfiguration of MAC layer mitigation options and increased buffering leads to significantly improved end-to-end performance.