This is a theoretical question about router+modem units, not a troubleshooting question per se.

I have repeated the following observation several times now, at different times of day, between computer and router restarts, and a pattern has emerged:

  1. Enable laptop WiFi and connect to the router
  2. Laptop may have internet access for up to a minute, or may connect to router and not have internet, or may not see router at all
  3. Within minutes, the router+modem combo box's "Broadband" and "Internet" lights are no longer green like they normally are, they are entirely off instead
  4. (No other WiFi device is able to connect to the router, or if it is, it is not able to get internet access) -- not interested in this aspect particularly, but just FYI

Now, I understand how a haywire WiFi device could pollute the wireless environment (disallowing other WiFi devices from connecting), or at least confuse the router enough that its WiFi stopped working (point 4 above.) Fair enough. My question is, how is it possible that the router itself no longer connects to the internet in this situation? How does its DSL connection have anything to do with its WiFi interface?

This particular router+modem box is a Thomson B102B8, but I am asking in general.

  • Could it be an IP conflict possibly? – Questionmark Apr 29 '15 at 14:34
  • @Questionmark, on the LAN or the WAN? Either way my question would still be the same, I think you would need to flesh out how exactly that would cause the modem to disconnect. – Kev Apr 29 '15 at 14:40
  • On the LAN. An IP conflict where your laptop had the same IP as your router would make it so that all the other devices on your network are unable to communicate correctly with your router when that laptop is on the network. I am just suggesting this as a possibility... – Questionmark Apr 29 '15 at 14:51
  • @Questionmark, please read the question again: it's not point 4 that I'm interested in (i.e. that other devices are not able to connect, either to the router, or to the internet via the router.) It's the router+modem box, itself--its apparently-resulting disconnect from the internet that I want to know more about. How can an IP conflict, or anything else for that matter, on the LAN side possibly cause a disconnect on the WAN side? – Kev Apr 29 '15 at 14:55
  • What does it take to get the router's full functionality back? Does it come back automatically after a minute? Do you have to reboot it? Do you have to disconnect the problem client? It's entirely possible it's triggering a bug in the router firmware and causing some form of hang/crash/reboot. Or maybe pegging the CPU with bad traffic, causing a kind of DoS that takes down other interfaces. – Spiff Apr 30 '15 at 0:40

There are several likely scenarios as to why the problematic wifi connection is crashing your modem. Either the router has completely crashed and hardware reset itself (hence the lights go off completely) and on restart continues to reset as the laptop re-establishes wifi connection. The internet lights will not come back on green as it takes a bit of time to establish an ADSL connection and before the modem has a chance to connect, the laptop sees the wifi network and tries to reconnect, crashing it again. You may have intermittent limited connectivity available to other devices before the laptop reconnects, but no internet access as you have observed.

However if you can see other lights on the router that remain on, even when the broadband/internet lights go out, that would imply it is not resetting completely, however the modem is no longer working. This points towards a firmware (internal software) issue such as the CPU entering some sort of infinite loop (getting stuck in an untested code path) - perhaps the developers introduced code to deal with a particular wifi connection issue, but being uncommon it wasn't tested extensively and has a bug in it - this is all pure speculation of course.

The other possibility is as Spiff suggested in his comments - the particular wifi problem may be causing the wifi interface on the router to be overloaded with traffic. Depending on how this interface is serviced (eg by interrupts etc) then I guess it's possible servicing these requests may be given CPU priority over (interrupt) other router processing activities which could result in a loss of internet connection if the modem/WAN interface is neglected.

I wasn't able to find any information on that model number to check this in more detail. Either way my assumption would be it's an issue with your router. I'd look for a firmware upgrade and also see if there are any logs in your router that might shed some light on exactly what is happening. There is possibly also an uptime statistic somewhere in the router config too which could confirm if the entire thing is resetting.

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