Impact of Error Characteristics of an Indoor 802.11g WLAN on TCP Retransmissions

In this paper we present the results of extensive measurements made over an experimental wired-to-wireless testbed, which consisted of a TCP protocol combined with a real-world indoor IEEE 802.11g WLAN. We investigated the effects of signal attenuation due to client distance from the AP on the 802.11 frame error rates (FER) and consequently on the segment loss rates and retransmission behavior of TCP at the sender in the fixed network. Specifically, we experimented with different modulation schemes belonging to the OFDM 802.11g PHY in order to gauge differences in performance between them, comparing real-world FERs calculated from actual frame captures against SNR, for both the forward and reverse WLAN channel directions. We also present real-world distributions of frame retransmissions made over the WLAN by the AP, with useful findings. Our results confirm that the reverse channel of a WLAN possesses a higher FER than the forward channel, and poses a greater threat to TCP´s retransmission mechanism.