I have a Comcast-supplied Cisco DPC3000 cable mobem and my own Linksys WRT160n V3 router.
My home network topology is: Modem > Router > Switches > Devices
I have a mix of both wired and wireless devices. All of my wired devices connect through switches; a single cable connects the router to one of the switches.
When I reset my router, I initially see 50-60 mbps download speeds and 10-20 mbps upload speeds on all of my wired devices (using Xfinity Speed Test). But over the next couple of days -- sometimes sooner -- that download speed gradually declines, until it's down below 1 mbps. Resetting my router immediately brings the download speeds back up to 50+ mbps. The decline is not unique to a single device; all of my wired PCs and laptops show the same reduced speeds. Upload speeds also decrease, proportional to the download speed's decrease.
This downward spiral occurs on both my wired and wireless devices, so I've ruled out the usual wi-fi problem (signal interference, someone 'stealing' my wi-fi bandwidth, etc.). I've run the speed tests using a mix of different servers and at different times of day, to rule out any anomolies caused by heavy network traffic.
I've also tried a bunch of experiments. With my download speeds below 1 mbps, I disconnected the cable between the modem and the switch and plugged a laptop directly into the modem. Download speeds returned to 50+ mbps. That suggests to me that the problem is with my router.
Again with the speeds below 1 mbps, I tried resetting just the modem, leaving the router and switches 'as is.' This time the speeds didn't change. Again, this seemed to point to a problem with my router.
My router is running the latest firmware. I'm reluctant to start playing around with my router's settings, since I am able to get 50+ mbps download speeds, if only initially.
What's strange is how 'linear' this decrease in download speeds seems to be. If I reset the router in the morning (50+ gbps), it might be down to 20-30 mbps by evening. Then, by the next morning, 10 mbps. Then 2-3 mbps by evening, The download speeds continue dropping steadily until I can't stand it anymore -- at one point I was getting 40 kpbs (0.04 mbps) download speeds, slow enough that my connections began timing out!
Before I fork out $100-$200 for a new router, I'd like to be sure that this really is a router problem and not something else. I'm suspicious that Comcast might be throttling back my speeds over time. (I'm not an online gamer, don't stream Netflix or other movies, etc.) If Comcast were throttling back my speeds, could resetting my router cause the throttling to be reset? (I think not, but I don't know enough about network protocols to be sure.)
Lots of networking forums describe situations very similar to mine. Most are related to wi-fi, not wired, connections. Most replies suggest the usual things (update your router's firmware, change wi-fi channels, change this or that router setting, call your ISP, etc.), without ever attempting to explain what might be going on.
I decided to try resetting my router to its factory settings, in case a setting somehow got changed that might have caused this 'decreasing speed' problem.
I first walked through all of the router's settings and recorded each one. I then reset the router to its factory settings.I then changed a few of the router's settings necessary to get wireless working again.
I then checked my download speed. 56 Mbps. 12 hours later, it had not decreased (generally the speed starts dropping within an hour or two of a reset).
So I'm cautiously optimistic that the problem has been solved. Time will tell.
Over the years there are a couple things I ran into when I had problems with speed dropoff's.
A) Is your modem and/or router getting overheated?
(Dirt, dander and gunk accumulate heat the cause the device to throttle down to reduce heat)
Check the Cisco website for firmware updates for your router.
Updated firmware (especially with shell shock in the wild) may help with the traffic.
C) When you do a modem reset do you disconnect all wires from the modem and wait 2 minutes.
(cable, power, and ethernet cables)
D) What has changed? You start using a device only during a certain part of the year? (roommate teacher had our internet at a crawl because her laptop would try to update the world whenever it was connected to the home network. Found out it was a bug in the school software.)
E) If all ese fail try a different DNS server (open dns for example 188.8.131.52 & 184.108.40.206)
FWIW, using third party DNS won't have any affect on the speeds you see at testing sites.
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Re overheating: I hadn't considered that. That's quite possible as the modem is mounted in the (open) ceiling of my basement (to improve wi-fi range). Air flow up there is obviously restricted, and we get lots of spider webs up there too that could be blocking vents.
The only problem with the overheating theory is that, once my modem slows down, it never speeds up again unless I reboot it. If in fact the modem throtles back due to overheating, I'd expect to see the higher speeds return once the air temperature goes down. But that's never happened as far as I know (maybe it did but I was never aware of it). Anyway, this idea is definitely worh checking out.
Re firmware: The router already has the latest firmware; it's an older model and there's been no new updates now for a couple of years.
Re unplugging cables: I reset the modem by unplugging it, waiting 30 seconds or so and then plugging it back in. I don't disconnect the WAN cable from my cable modem or the single LAN cable from a switch when I do that. How might unplugging cables matter when I reboot the modem?
Re what's changed: Nothing's really changed since the last time I saw this problem, other than the problem is back. :-) I do have another tablet connected to the router through wi-fi (in addition to an iPhone and iPad). Agin, the problem affects both wi-fi and wired device speeds. If it were a device somehow causing this slow-down, I'd expect to see it right away, not hours or days after rebooting the router. And rebooting the router always returns my speeds to their expected values (~75 Mbps download, ~12 Mbps upload).
Re DNS: I assume if it were my DNS sites somehow causing the slowdown, I wouldn't expect to see my higher speeds immediately after rebooting the router.
Mext steps: My son is sending me a different/newer router to try. Hopefully swapping in his router will either solve this problem or, if not, indicate that the problem is not the modem itself.
I was instructed by a cable installer that there is a base current in the cable line and that in order to ensure a cold boot of the modem that all wires needed to be disconnected. This was a few years ago and I didn't understand why the ethernet needed to be disconnected until I started getting more devices on the network and found POE (power over ethernet) was needed or generated by some of the devices.
if its open and ambient temp stays below 90 it shouldnt be a issue unless the venting has become obstructed. Overheating may be interpreted as some form of hardware failure and does not correct until device reset (power cycle)