On Tue, Apr 17, 2012 at 7:02 PM, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote: > (...) > Keep in mind that TSF == start of TSF field, while rx_status->timestamp > == start of first symbol, which I didn't even remember when you guys did > all the Toffset things, I'm guessing they're all wrong :-) I think you guessed wrong :) http://www.mail-archive.com/devel@xxxxxxxxxxxxxxxxxxxx/msg01552.html + if (rx_status->flag & RX_FLAG_MACTIME_MPDU && rx_status->mactime) { + /* + * The mactime is defined as the time the first data symbol + * of the frame hits the PHY, and the timestamp of the beacon + * is defined as "the time that the data symbol containing the + * first bit of the timestamp is transmitted to the PHY plus + * the transmitting STA's delays through its local PHY from the + * MAC-PHY interface to its interface with the WM" (802.11 + * 11.1.2) + * + * T_r, in 13.13.2.2.2, is just defined as "the frame reception + * time" but we unless we interpret that time to be the same + * time of the beacon timestamp, the offset calculation will be + * off. Below we adjust t_r to be "the time at which the first + * symbol of the timestamp element in the beacon is received". + * This correction depends on the rate. + * + * Based on similar code in ibss.c + */ + int rate; + + if (rx_status->flag & RX_FLAG_HT) { + /* TODO: + * In principle there could be HT-beacons (Dual Beacon + * HT Operation options), but for now ignore them and + * just use the primary (i.e. non-HT) beacons for + * synchronization. + * */ + goto no_sync; + } else + rate = local->hw.wiphy->bands[rx_status->band]-> + bitrates[rx_status->rate_idx].bitrate; + + /* 24 bytes of header * 8 bits/byte * + * 10*(100 Kbps)/Mbps / rate (100 Kbps)*/ + t_r = rx_status->mactime + (24 * 8 * 10 / rate); + } -- Javier Cardona cozybit Inc. http://www.cozybit.com -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
