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I have a WiFi 801.11n/g network from the ADSL box of my provider (Freebox V6, should someone be from France). While streaming video to mobiles (iPhone 4S, iOS 6.0.1 or Galaxy S3/Note 2, Android 4.0.1) I notice that after some time (say, 20 minutes) the synchronization speed drops to about 19 Mbps (from an initial 54 Mbps). This happens both for n and g mode, fairly regularly (though this is not a rule) - I notice the drop when the video stops being fluid. My laptop does not have this issue.

I can clear this off by disconnecting the WiFi and then reconnecting. Rebooting the box also helps. I had exactly the same issue with the previous box (older generation - Freebox V5, a completely different box). There is no particular traffic when the sync speed drops, I am the only one using it at that time. The neighbouring WiFi traffic is quite dense (about 20 AP discovered) but it is stable.

I mostly would like to understand if this is a "normal" behaviour of mobiles (from experience of by design) or if there is a problem somewhere.

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closed as off topic by Ƭᴇcʜιᴇ007, Simon Sheehan, Dennis, slhck Dec 29 '12 at 22:11

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Is it the same if there's only one mobile connected to the WiFi (and no other devices)? Have you tried different channels (etc.)? – Ƭᴇcʜιᴇ007 Dec 28 '12 at 17:29
@techie007: yes, no matter if there is one or more devices. I tried switching channels, switching from n to g etc. This is why I belive this is not a network congestion:interference issue – WoJ Dec 28 '12 at 19:50

Possible cause: enabling power-saving mode for WiFi (which may be set from the manufacturer).

A mobile device using Power Saving Mode flips its wireless radio between fully powered and a sleep setting, for periods lasting between seconds and tens of milliseconds, to conserve energy. For example, after sending a request for a file from the Web, a phone might sleep if it doesn’t receive the file after half a second. While sleeping, the device listens for a beacon message that indicates its data is ready, after which it switches to full power and asks the access point to send it.

Found here:

So it may be that, while buffering, the phone flips into power-saving mode which is where you see the speed reduction.

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