SMC Router is suddenly slow (downstream only). Hardware defect? If yes, how to test? If no, what else?

I have been using my home network without any issues and SUDDENLY (yes, really ;-) no changes in config) my network is slow.

I am using a SMC router to connect to a ADSL 16 MBit network via PPPoE.

Hardware config looks like this:
Computer <-> Wireless SMC router <-> ADSL modem <-> Splitter (ISDN/ADSL) <-> Provider

Internet provider says I have a 14 MBit downstream and an 1.4 MBit upstream, which I used to be able to confirm.

Now, suddenly, my download rate has dropped, speed tests show:
Download: 108 kbit/s = 14 kByte/s which is double ISDN speed.
Upload is surprisingly still fast (1008 kbit/s).
 Internet provider can see my bad downstream rate and tested some line parameters, no difference.

I don't understand what the suddenly drop could have caused.

I tested it with two different computers, same result.
I exchanged all hardware (except the router), same result.
I connected the computers directly to the router via cable, same result (well, double the speed, but that's normal for Wireless vs. cable).
I reset the router to factory settings, same result.

When I connect the computers directly to the DSL modem, then I get fast speed.
So everything points to a hardware defect of the router.

As I do not have a second router (to test) does anyone have an idea:
- Whether a router can suddenly "break" (one direction) and why this can be?
- How to test this (except buying a new router)?
- Whether it can be anything else and how to fix it?

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moorhouselondonConnect With a Mentor Commented:
I wouldn't risk selling it - certainly not without pointing out that it is probably broken.  

Why do these things suddenly "go"?  My theories are:-

(1) The thing is on 24/7, once heat "gets to it" then you are going to have intermittent connections forever after as the circuit board/components inside expands/contracts.

(2) I have had a few routers "blow up" due to Telco-based or mains borne interference coming in.  One went definitely due to a lightning strike, and another two (within a quarter of a mile of each other - Routers of different manufacture) went down due to some "disturbance" in Telco equipment.  I have another client who's been through about five routers - I reckon there's spikes on his Telco input (it's probably not the mains cos no other equipment of theirs fails).

Once a router blows, bin it, you are passing on an unnecessary problem to a buyer.
Ring SMC.  They are reasonably helpful.  I had trouble with an SMC Barricade recently, bizarre behaviour which was one way - this was to do with Port Forwarding though.   Packets coming in were going to their correct destination, but replies to those packets weren't - SMC verified my firewall settings were correct, and suggested updating the Firmware - not relevant in your case.  At the end of the day they did not come up with an answer.  I replaced the router and everything's been fine since.
mguentherAuthor Commented:
Hi, moorhouselondon,

will try, however as I need Internet access (freelancer) I bought a new router (not-SMC) yesterday. And, it works, quick uploads and downloads.

I am still be interested as I want to sell the SMC router second hand and need to know whether that thing is broken or just misconfigured or just not working with my ISP.

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mguentherAuthor Commented:
Sure, that's what I am saying, whether I should sell it as broken or as acting funny.

My auctions on eBay have always been honest, which is not really valued by most bidders though. I sometimes have the feeling most like to be cheated. My actual buyers have all been happy though and I often allow returns within a couple of days.

Still, I'd like to know how ONLY download can get broken, if I understand Ethernet correctly, it is not separate wires from upload ;-)
Have a look at the statistics page of the Router's control panel to see whether errors are occuring.   I don't seem to remember the figures appearing unusual in the Barricade that I worked with.

My understanding from the above link is that there are three signals in ADSL installations.  These are listed in order of ascending frequency:-

(1) The POTS voice signal.  This is filtered out by the (low pass) micro-filter, so does not go into the modem.

(2) The Upload signal.  This comes out of the modem, but we're happy with this.  The circuitry for upload would be different to that for download.

(3) The Download signal.  Presumably there is a carrier which is modulated in a particular way to contain the digital element of the transmission.  So there will be a band-pass filter which accepts the carrier, plus the modulated component.  Then there is a decoder which splits out the modulated component into digital 'bits'.  I would have thought that where the problem would lie would be with the filter.  Anything wrong with that band-pass filter would surely have a radical effect on the errors appearing in the received signal?  A requirement for transmission lines (I believe) is that the impedance of the receiving end needs to be the same as the line itself, and the sending end too.  If this is not the case then you get a mismatch in the line, leading to standing waves.  Whether this leads to inefficient as opposed to garbled transmission I don't know, and if this is detected by the sender then the sender notches down the speed - again, I don't know.  I would have thought though that any "training" of this sort would appear as errors on the stats.

You mention Ethernet, but (again correct me if I am wrong) there is no Ethernet wire leaving the Router on its' way to the Telco - it may be PPPoE that you have configured the Modem for, but look at the following diagram which shows that, whatever you set your Modem to, the interface to the physical layer is not Ethernet.
mguentherAuthor Commented:
Well, I gave up. I guess it is really just broken.

thanks for your help. I'll give you the points for your kind responses.

Merry xmas,
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