Speed is fluctuating to less than half

[#1]

This seems to be happening a little too often to be random. My speed, which is around 6mbps, and was fine until 9.30, has slowed right down - PN speed test between 2.5 and 4.5 mbps, BT speedtest at the time I took it - 3.8mbps. A drop from my usual 720kbps to between 350 and 500, consistently dropping down is quite noticeable. My usage limit is fine, it's mid morning so very off-peak, and it isn't to do with any website as I'm using several. Nobody else is using my network. Most of the time there isn't a problem, then I go through these periods where it's like this. The fact that it never fluctuates back up to my line speed during this time is what worries me. If it was simply a matter of contention then I'd expect to see a wider range of throughput. But when it's like this it never goes higher than 500 odd. Can anyone shed some light on what could be causing this?

[#2]

Hiya,
You've been connected to the internet solidly for over a month.
I'd initially suggest rebooting your router

[#3]

Hi James
Thanks. That seems to have done the trick!

[#4]

Quote from: Jameseh

I'd initially suggest rebooting your router

Why does this work? I try to leave my router up 24/7 for as long as possible, to keep the profile at a reasonable value for my line (currently 1500). More often than not the router stats show a SNRM of 3-4dB, which is below the target of 6dB and therefore I would expect a reboot to give me a lower sync rate. What effect does a long time between reboots have on the router which can result in speed (throughput) fluctuations?

[#5]

I believe that the router's firewall tables can become saturated over a period of time, making them somewhjat sluggish.
A reboot is always the best first action.

[#6]

Thanks James, I didn't know routers had firewall tables, now that I do I can look up what they are for and how they get saturated. Or I can simply accept that routers do what routers do and sometimes need me to give them a kick. 
Its now just a question of finding the best time in terms of noise margin.
Incidentally, VMBU hasn't updated for over 3 hours, so maybe the connection has dropped of its own accord (I'm at work so can't tell at the moment).
Edited @ 15:10 - VMBU now updated, seems that someone may have given it a kick.

[#7]

Look it up and post about it - education is a fine thing

[#8]

It might not of been the router. The Plusnet network is badly out of balance at the moment and as you were connected for a long time it is highly probable you were on one of the overloaded gateways. Reconnecting has probably put you on one of the lighter loaded gateways. See http://www.plus.net/support/network_performance/broadband_packets_drop.shtml for the current status of the gateways.

[#9]

Quote from: Anotherone

Look it up and post about it - education is a fine thing

OK Chris, I've taken up the challenge, and now my head is spinning. I think maybe I should stick to my day jobs, polishing cars and writing safety assessments for nuclear power stations.
Anyway, I think what Jameseh was hinting at was the NAT (network address translation) table. The NAT table is part of the NAT operation, which takes place within the router (or any NAT-enabled device) . Each connection from the internal network (ethernet side of the router) to the external network, and vice versa, is tracked and a special table is created to help the router determine what to do with all incoming packets on all of its interfaces This table is populated gradually as connections are created across the router. It appears that deleting entries is a less efficient process than creating them, so depending on the capacity of the router this table can become congested such that browsing becomes slower. Rebooting the router clears the entries in the table.
It appears that I have answered my own question, at least to the level of detail I can comprehend. If anyone wants to add to, correct or contradict what I have posted, I wouldn't be at all offended (or surprised).

[#10]

Another bit of "degradation" that appears to be going on in some routers relates to the connection speed, due to the action of bitswapping over time and loss of bits on individual "bins" that doesn't seem to recover.
I don't know if this applies universally, but on my DG834GT you can definitely see it, though I guess it depends on your noise margin and the type of interference you get and how it varies over time.
The effects are small and a regular (every week or so) reboot restores the speed to optimum.

[#11]

This effect was very noticeable on my Speedtouch 585v6 where a resynch would bring a lot more bins into play. Then again bit swapping on the Speedtouch routers always used to be fairly dire.
With my Netgear DG834G with the Broadcom chip as in the GT it is much less obvious except if I push the speed up to the point where the errors overnight get quite high. Under these conditions the noise margin doesn't recover fully during the day without a resync.

[#12]

Quote from: maranello

OK Chris, I've taken up the challenge, and now my head is spinning. I think maybe I should stick to my day jobs, polishing cars and writing safety assessments for nuclear power stations.

Nah, you've become a line stats expert so I knew you could do it. That looks a reasonable explanation that most can understand

[#13]

Quote from: Anotherone

Nah, you've become a line stats expert

Hardly Chris, but I'm getting better.
I decided that this morning would be a good time to check noise margin and do a reboot, since I believed I had kept sync for about a month continuously. It turns out that a resync occurred two days ago and sync rate was halved (896kbps). Router was reporting SNR margin of 6.5dB d/s so I decided a reboot was worth going for. My D-Link dsl-2640B has a 'Reboot' button in the interface, which I clicked, but connection was not restored. Router was apparently in sync with exchange (DSL light green) but Internet light not lit. I did notice that the wireless lan was enabled, even though previously I had disabled wireless. Started to wonder if somehow the router had been hacked, then I heard the toaster pop up so decided to power down and leave it alone as I don't like cold toast . Thinking about it on the drive to work it occured to me that maybe the router had undergone a factory reset such that the connection/authentication details were reset. This would account for the symptoms but I don't understand how the reset could have happened as this shouldn't occur unless the reset button on the back of the router is pushed in. Anyone else had this happen?
Can anyone at Plusnet tell me if the resync on Monday at approx 2pm was deliberate as a result of load balancing reconnections, or is it more likely to be random?

[#14]

Probably random.
We didn't have any drops on our network around that time.

[#15]

That would have been around the time that everything was coming back up after 56570, but wouldn't have affected sync to the exchange in any event. More likely a local power dip? (or BT fiddling at exchange , cab or dp) but that doesn't explain why your WLAN was enabled unless this happens on that router after a powerdown, a bit naughty if it does. Did you have to re-enter your user details or were they ok in the end?