IEEE802.11 wireless LAN systems are being widely used and multiple wireless LAN coexistence systems like 802.11b/g are available at the same time. However, in such environments, network performance is likely to degrade because the CAMA/CA mechanism does not always work well. Especially in the 802.11b/g systems, issues on performance degradation caused by the CTS-protection mechanism come to surface. To manage and maintain the QoS in such environments, it is required to detect and estimate the QoS degradation effectively. We found that the so-called retry ratio (i.e., the retry ratio of the numbers of transmitted packets to retransmitted ones) is strongly correlated with IP-packet quality metrics through our IP packet experiments. It is thus considered that retry ratio is applicable to managing the quality of some applications over wireless LAN.