为什么有些信标帧掉了 [英] Why are some Beacon Frames dropped

问题描述

为什么我不捕获所有信标帧？它们会被AP丢弃吗？

我正在Linux中使用libpcap捕获信标帧并解析时间戳。我使用时间戳来计算捕获的信标帧之间的时间间隔。

在大多数情况下，时间间隔应为102.4ms。但是，每5-6个数据包显示的间隔为102.4ms的倍数，可以是204、306和最多800ms。

我不知道是由于AP没有发送那些信标帧，或者我的pcap没有捕获该软件包。

我没有通过IEEE802.11规范来挖掘自己，但是有一些第三方文档（例如[1]）轻信指出

由于其他流量占用了媒体和退避延迟，因此信标可能会延迟到目标信标传输时间之外。信标不会重新传输如果由于将信标帧发送到广播地址而发生冲突，那么同步功能非常简单。

是的，非常简单！

如果我使用没有其他wifi流量或公共路由器的专用AP（我的Samsung S6 Edge）也没关系。

可以有人对此有所了解？

[1] https://books.google.se/books?id=RZ_6hPSjwfQC&pg=PA59&lpg=PA59&dq=delayed+beacon+not+retransmitted&source=bl&ots=B- HofLMubj& sig = lgEMnUlZvm2HjLqqr4DkwJ5VPtY& hl = sv& sa = X& ved = 0ahUKEwjC-JrpranJAhVIhSwKHW2CCSkQ6AEIHDAA＃v = onepage& q = delayed％20％20a =％ div class = h2_lin>解决方案

似乎信标被AP丢弃了不。

当我运行代码时，以终端模式（不同于通常的GUI模式）启动Raspberry Pi之后，我捕获了几乎所有信标，除了30分之一。

这清楚地表明，信标丢失是由于客户端CPU负载引起的。

希望这可以帮助人们遇到相同的问题。

Why am I not capturing all Beacon Frames? Are they being dropped by AP?

I'm using libpcap in Linux for capturing Beacon Frames and parsing the timestamp. I use the timestamps to compute the interval between captured Beacon Frames.

Most of the time the interval is what it should be, namely 102.4ms. However, every 5-6 packets show an interval of a multiple of 102.4ms, this can be 204, 306 and up to 800ms.

I don't know if this is due to AP not sending those Beacon Frames or my pcap not capturing the package.

I haven't dug myself through IEEE802.11 specs but several 3rd party documentations (e.g [1]) vagely state that

"The beacon may be delayed beyond the target beacon transmission time due to other traffic occupying the medium and backoff delays. The beacon is not retransmitted in case of a collision since the beacon frame is sent to broadcast address. Synchronization function is very simple"

Yes, very simple!

And it doesn't matter if I use a dedicated AP (My Samsung S6 Edge) with no other wifi traffic or a public router.

Can someone shed some light on this?

[1] https://books.google.se/books?id=RZ_6hPSjwfQC&pg=PA59&lpg=PA59&dq=delayed+beacon+not+retransmitted&source=bl&ots=B-HofLMubj&sig=lgEMnUlZvm2HjLqqr4DkwJ5VPtY&hl=sv&sa=X&ved=0ahUKEwjC-JrpranJAhVIhSwKHW2CCSkQ6AEIHDAA#v=onepage&q=delayed%20beacon%20not%20retransmitted&f=false

解决方案

Seems as if the beacons are NOT dropped by AP.

When I ran my code, after booting the Raspberry Pi in terminal mode (unlike the usual GUI mode), I caught nearly ALL beacons, except for 1 in 30 maybe.

This is a clear indication that the dropped beacons are due to client CPU load.

Hope this helps people encountering same issue.

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