A Practical Method for Verifying the Uniformity of the Backoff Distribution in 802.11 Network Cards

Recent studies have revealed that many vendors implement nonuniform backoff distributions in their commercial 802.11 Network Interface Cards (NICs) in order to obtain a better performance in particular scenarios. Therefore, in many cases, the backoff behavior of the cards can be not adherent to the 802.11 standard specifications. By consequence, the theoretical and the experimental performance of an 802.11 distributed network, including throughput and packet delay, may not coincide because the real backoff behavior of the adopted cards is different from the expected one. This paper presents a practical method for verifying if the backoff generated by a given 802.11 NIC is uniformly distributed, as dictated by the 802.11 standard, or not. The proposed method is based on the comparison between the theoretical and the experimental statistics of the time between two successfully transmitted packets. Thus it does not require intrusive measurements inside the chipset and can be performed using a simple open source software platform.