An Idea of Linux Implementation of Fixed Backoff-time Switching Method for Wireless Mesh Networks Sritrusta Sukaridhoto 1 Nobuo Funabiki 1 Toru Nakanishi 1 Kan Watanabe 1 Shigeto Tajima 2 Graduate School of Natural Science and Technology, Okayama University 1 Graduate School of Engineering Science, Osaka University 2 Abstract As a flexible and cost-efficient scalable Internet ac- cess network, we have studied architectures, proto- cols, and design optimizations of the Wireless Internet- access Mesh NETwork (WIMNET). WIMNET is com- posed of wireless connected access points. Previously, we proposed the Fixed Backoff-time Switching (FBS) method for the CSMA/CA protocol to improve the real- time traffic performance in WIMNET by giving the necessary activation chances to each link, and verified the effectiveness using the QualNet simulator. In this paper, we present an idea of the FBS method imple- mentation at Linux kernel after quickly reviewing it. 1 Overview of FBS Method The FBS[1] method selects either of the two backoff- times, namely, the shorter active backoff-time and the longer passive backoff-time, for each link transmission by comparing the target link activation rate and the actual link activation rate, so that the link can be acti- vated to handle the required traffic. Any backoff-time is assigned a different fixed value so that no pair of the conflicting links may be activated simultaneously. Besides, the backoff-time for a link with larger traffic is assigned a smaller value than that for a link with smaller one, so that congested links can be activated preferentially. During communications, every time a node holding packets detects that the channel becomes free, it up- dates both the target activation rate and the actual activation rate. If the actual one is smaller than the target one, it selects the active backoff-time to let the link be activated, because the current activation rate of the link is not sufficient to handle its traffic. On the other hand, if it is larger, it selects the passive backoff- time to let other links with active backoff-times be ac- tivated with higher priorities. A link with the passive backoff-time can be activated only if any conflicting link with the active backoff-time does not hold pack- ets. The following sections describe how to calculate the parameters in the FBS method. 2 Target Link Activation Rate For a wireless link l ij transmitting packets from AP i to AP j for i =1, ··· ,N and j =1, ··· ,N in WIMNET with N APs, the target link activation rate rt ij can be calculated by: rt ij = tn ij an ij (1) where tn ij represents the target number of activating link l ij per second, and an ij does the average number of link activations per second. tn ij can be given from the requested bit rate by: tn ij = rb ij fb ij × (1 + fe ij ) (2) where rb ij represents the number of bits per second that link l ij needs to be transmitted, fb ij does the average number of bits in one transmitted frame, and fe ij does the rate of causing the frame transmission error. an ij can be given by: an ij = 1 ft ij (3) where ft ij represents the average duration time of one frame transmission. Among the parameters for the target link activation rate, rb ij should be calculated by taking the summa- tion of the bit rates requested by the applications using link l ij in the routing path of WIMNET. The others, fb ij , fe ij , and ft ij , should be updated during commu- nications by the following equations: fb ij = sb ij sf ij (4) fe ij = ff ij sf ij + ff ij (5) ft ij = t sf ij + ff ij + of ij (6) where sb ij , sf ij , ff ij , and of ij represent the total num- ber of successfully transmitted bits by link l ij , the to- tal number of successfully transmitted frames, the total number of failed frames, and the total number of trans- mitted frames of the interfered links with link l ij , when t seconds have passed since the communication started in WIMNET, respectively. Copyright © 2012 IEICE 2012/9/11 〜 14 富山市 S-82 BS-5-29 2012 年電子情報通信学会通信ソ サイ エ テ ィ 大会 （ 通信講演論文集 2 ）