Does WiFi intermittent for a few seconds and then fixes itself? Don't blame the broadband, it's probably the correlation layer secretly disconnecting and reconnecting. I dug out the logs.

There was a strange problem at home last week: When I watch videos at night, the screen freezes every few minutes, spins around in circles for two or three seconds, and then comes back to life. The broadband speed test is full, the mode light is all green, the router is hot but far away from the alarm. I had restarted it before, but at that time I was messing with the router logs, and I dug out the records of the few minutes when the accident occurred, and found that it was not a "stuck" at all, but that the phone completely fell off the WiFi, and climbed back up after two or three seconds.

There is a very accurate word for this state, which is called flash break. It is two different things from lag: The lag is caused by the slow network. The package is still moving, but it’s just taking a lot of effort to move. ； Flash disconnection means that the entire connection is disconnected and then reconnected. During the few seconds of disconnection, not a single packet can be passed through. What's more annoying than lag is that there is no way to check it - you call customer service and say "My home network is intermittent", and the customer service checks your line and everything is normal, because the broken line is not the broadband line at all, but the invisible wireless connection between the mobile phone and the router. Today I wrote down this set of investigation ideas, and ranked the five real culprits according to the probability that I would trap them.

First distinguish: Is it the broadband that is missing, or is it the WiFi wireless connection?

If you do this step backwards, all the rest will be in vain. There are only two ways I can judge.: At the moment of the accident, check whether other devices are disconnected. ； Take another network cable and plug it into the router to access the web page.

When my video froze that night, there was nothing wrong with the desktop computer in the living room (which was plugged into the network cable), the web page was opened, and the other mobile phone was also browsing normally. Only my phone keeps dropping. If the broadband is disconnected and the whole house has to lie down together, I will check the method. Sudden disconnection troubleshooting sequence I wrote in the book, first look at the three lanterns of the optical modem. But there are only one or two devices in the whole house that are disconnected repeatedly, and everything else is fine. The problem is basically locked in the section "from this device to the router". Broadband and optical modems can be put aside first.

Make another network cable test: If the laptop has a network port, plug it in and surf the Internet for half an hour. During the plug-in period, it is as stable as an old dog. When you unplug the line and change the WiFi, it will get sick. That is more true - the line is not wrong, the problem is in the wireless section. That night at my house, I plugged in my desktop computer and watched videos all night, and my phone dropped four times next to it.

By the way, there is another kind of "fake flash" that should be eliminated first.: Only one specific device has a problem, and that device has a VPN or proxy turned on. What was actually broken was the VPN tunnel. The WiFi was good from beginning to end. I fell into this trap. After struggling for half an hour, I turned off the VPN and immediately reached full health. Check this first, it’s the lowest cost.

Check router logs: The evidence of the flash break looks like this

The most annoying thing about flash breaks is that "it doesn't come when you look for it." When you take out your phone and check the status, the connection has been restored, the icons are full, and there is no problem at all. So don't look at the icon, look at the log - disconnections and reconnections will be recorded in the background of the router.

Mine is OpenWrt. logread | grep -i deauth With one order, all the records of the few minutes after the accident were released. Every time the phone goes offline, there is a line of deauth (deauthentication) in the log, followed by a line of sta reconnect or assoc (reassociation), and the timestamp is clear.: It dropped at 21:07:44 and came back at 21:07:46. It dropped at 21:13:02 and came back at 21:13:05. I counted more than a dozen times in one night, all from my mobile phone. Turning to this is equivalent to getting a real hammer - it's not a metaphysical lag, it's because the phone is repeatedly disconnecting and reconnecting.

The original firmware is not so convenient, but there is a way. In the system logs or device management pages of most brands' backends, you can find "device offline/online" records, but the granularity is a bit coarser. There is really no log to read, so I resort to stupid methods.: Install an app on your phone that can remember WiFi status and keep an eye on it overnight, or simply put another phone on and check it at the same time - one phone keeps falling off and the other is stable, and the one that falls off may have something wrong with it. ； If both devices fail at the same time, the problem lies with the router or the channel. Later, my iPad kept disconnecting and it was caused by itself.

There is another word in the diary worth knowing: beacon, beacon. The router broadcasts a beacon every about 0.1 seconds, which is equivalent to a roll call, "I am still here, everyone with me is checking in." If a mobile phone loses dozens of beacons in a row, it will assume that the router is gone and actively disconnect and reconnect. How to find WiFi on mobile phone I have talked about this broadcast mechanism, but if you read it conversely, it is the offline mechanism.: If the broadcast cannot be heard, the connection cannot be maintained. Therefore, the essence of flash interruption is often not "the router kicked the phone", but "the phone can't hear the router".

The real murderer: Battery saving mode on mobile phones, the number one habitual offender

The one with the highest probability and the easiest to ignore. In order to save power, the mobile phone will lower the wireless network card to a power-saving state when the screen is locked or low activity, and the ear for receiving packets will be turned on and off. If you save too much power or the firmware is poorly written, you may miss beacons and data notifications from the router. If you save enough times, the connection will be determined to be dead and the connection will be blocked and reconnected. Acting particularly confusing: There is nothing wrong with unlocking the screen. Once the screen is locked and left there, when I pick it up again, WeChat messages lag and push notifications arrive late.

Verification cost is zero: Turn off the WiFi power saving of this phone in the settings (some call it "smart power saving" and "keep connected during sleep", the path varies from manufacturer to manufacturer, iOS calls it the low data mode set near Wi-Fi Assistant, and Android calls it in the WLAN advanced settings), or simply charge it overnight without locking the screen and hanging up the phone to compare. If you don't take it off for one night after turning it off, you are basically convicted.

Don’t get confused if you have a close relative with symptoms: The root cause of the diagnosis of "default gateway unavailable" on the computer is that the network card is saving power. In the Power tab of the network card properties in Windows Device Manager, there is a check mark "Allow the computer to turn off this device to save power." I wrote a separate article about this Troubleshooting that the default gateway is unavailable , if your computer disconnects repeatedly, read that article first. The idea is the same, but the location is different.

The real murderer two: The signal jumps repeatedly across the critical line

The second reason for high incidence: Where you are standing, the signal is just on the edge of "connectable but barely". In the article where the mobile phone signal dropped by one bar and the internet speed dropped by half, I copied a gear chart ( here ), the rate begins to shrink significantly when going down to -65dBm, and retransmissions are queued for errors below -75. If it goes further, it will no longer be slow - there are so many errors that the link cannot be maintained, and the mobile phone will be dropped directly. For people whose signals happen to fluctuate in the critical range of -72 to -78, their performance is intermittent.: Every time a person takes two steps, a door opens, or a microwave oven turns, the connection is disconnected.

The judgment method is still to read dBm, don't look at the grid number. Install a signal measuring tool on your mobile phone (there are a lot of Android phones, and the Mi series also has an entrance), and stand at a commonly affected spot to take readings. If it's lower than -75, don't check anything else first, get the signal up first.: Move the router, don't put it in a weak power box, or add a node. To put it bluntly, quite a few of these outages can be cured by simply moving the router, without spending a penny.

Don't trust the signal amplifiers on the Internet. As I said in the dBm article, those worth tens of dollars are basically illegal and useless. The proper way is to either position yourself or make up for coverage.

The third murderer: The channel is too crowded and the evening rush hour breaks out regularly.

Very distinctive features: It was fine during the day, but started to flash after 7 or 8 pm. All the neighbors in the building came home from get off work, and the three non-overlapping 2.4G channels were squeezed into a pot of porridge again. Beacons and normal packets were drowned in noise, and edge devices were dropped first. Just take a look at the surrounding signals and count the number of neighbors sharing the same channel with you, and you can basically count them. The channel rules of 2.4G and the selection methods of 1, 6 and 11 are in The article about disconnection and slowing down I wrote in it that if you scan and change lanes, you will be much more comfortable during the evening rush hour.

There is also a deeply hidden version: Old equipment is a drag. The old WiFi4 router at the uncle's house next door occupies the channel and transmits slowly. It eats up most of the airtime, and your mobile phone's queue times out. I'm here for this account Old equipment slows down the entire network After careful calculation, all devices that can move 5G have been moved over, leaving 2.4G for those who are not in a hurry in the smart home, and the interruptions and lags can be solved at the same time.

The real murderer four: Roaming is secretly switching

If you have Mesh or dual-band enabled at home, check this out. When the mobile phone goes from the living room to the bedroom, it should switch to the nearby sub-router or 5G, but the moment of switching is a "disconnect first and then connect" - the rough firmware, the disconnection for one or two seconds is particularly obvious. If the route you are walking back and forth happens to be on the signal intersection between two nodes, and the phone switches back and forth repeatedly, it will seem intermittent.

There are also ways to verify: Compare the number of disconnections between standing still for half an hour and walking around the room for half an hour. Whether you fall while standing or fall while walking, 80% of it is a matter of wandering. I am here to troubleshoot the sticky set of Xiaomi Mesh The article does not switch when roaming As written in full, the core is to make the overlap of coverage so thick and to move the placement of sub-router. In addition, the TWT power-saving mechanism of WiFi6 will occasionally delay the notification of small devices such as door locks and bracelets. It looks like a flash interruption but is actually a power-saving schedule. I will write about that The TWT article It was actually tested during the test, so don’t blame the router unfairly.

The real five: The pitfalls of DHCP leases and address pools

The probability is not high, but it will be particularly weird if it happens: The rhythm of the interruption is very regular, and it will come back every once in a while. When it is interrupted, an exclamation mark will briefly appear on the WiFi icon, or "Connected but unable to access the network" will be displayed. This is probably because something went wrong with the lease renewal - the mobile phone was not renewed halfway through the lease, and the address was taken away. It was restored after reconnecting and asking for the address again. The lease mechanism itself is The DHCP article The complete process has been drawn here. Normally it is continued quietly, so you don’t have to worry about it. ； It only becomes a flash when it is abnormal.

A common way to step into this pitfall is that the address pool is too small, or there is a second device at home that is also issuing numbers (one DHCP is not turned off for the dual router, and the two machines each issue their own number, and the mobile phone is confused when holding two house numbers). Check the law: Go into the router to see if the DHCP address pool range and the number of devices in your home are almost full, and then check to see if there is a second number source. Enlarge the pool and turn off redundant DHCP, and regular outages will be eliminated.

ending: Use this table to get your seats, and don’t buy equipment blindly.

Arrange the five lines into a checking order and follow them:

End with a splash of cold water: If you look through the logs and find that it is not the mobile phone that is disconnected but the router itself that is restarting (the log timestamp is cleared, and then a series of power-on messages), or the entire house is disconnected, then it is not within the scope of this article. Go back to the optical modem and broadband lines to check. This problem of flashing can be found 80% of the time without spending a penny. Check the log first, then the wallet. Don’t reverse the order.