[Techtalk] DSL modem

[Maria Blackmore]

On Mon, 16 Sep 2002, Carla Schroder wrote:
> On Monday 16 September 2002 12:24 pm, Dave North wrote:
> > Hmm. We have the Westell with the swooshy red paintbrushy W logo, and see
> > something similar. What happens is, slowly but surely the connection will
> > degrade, eventually dying if we wait long enough.
> 
> This is what happened with my original modem, which some people say is 
> actually a bridge- it's not even a real router, and it certainly is not a 
> modem.

hmm, ADSL modems really are modems in the conventional sense, in the way
they work.  They just use a wider range of frequencies on the wires to
achieve higher throughput, and use a different method of modulation to
achieve this.

One of the main problems with ADSL is that the line level protocol is ATM,
which by itself isn't much use to anyone, you have to carry something over
it to achieve an actual connection.  In the UK we use PPP over ATM, mostly
from a convenience point of view, because PPP is a universally available
protocol, and it doesn't have to run over a dialup connection, it doesn't
care what it runs over.  So there's a connection that runs through the
modem, over the ATM (ATM25 to be exact) on the phone line, through the
DSLAM at the exchange, and then through the telcos network to a router
that terminates the connection, in the UK this typically being a Cisco
7206 VXR or 3662 for smaller ISPs.  It's all rather complicated, plus
because it's ATM you have an enormous overhead from the fact that
everything is transmitted in 60 byte cells.

Of course the upshot of using ATM is that you can pass whatever you want
over it, and it will just do it.  So you can pass ethernet over ATM over
it, some countries do this .. the original BT trial in the UK did this,
but they decided to switch to PPP.  You can also pass voice calls over the
data part of a DSL line.

In the countries where you do get ethernet over ATM on your DSL line, you
won't actually get a bridged ethernet connection that you can use for
something useful, you have to run PPP over ethernet to make a connection
to a router which then routes your traffic to the internet.  So that
actually makes it PPP over Ethernet over ATM  *boggle*

> The connection would fade, then die.

In rate adaptive ADSL this is supposed to happen, it degrades gracefully
rather than just failing to work.  It means you can get a DSL connection
over what would otherwise be an unsuitable connection, either due to
quality or due to the phone line being too long.  Of course, it can only
degrade so far before it gives up, but it's pretty good really.

The real question is if it's fading, what is making it degrade?

Is it the phone line?

Is your DSL modem dying in some way?

Is your DSL modem overheating?

Is there a small rodent nibbling at the wire? (no.  I'm not joking)

Is there a fault on your line card in the DSLAM?

> Power cycling did not always work, it often took several tries.

When you power cycle the modem the line has to resynchronise.  It does
sometimes take a short time for the DSLAM to notice your modem is gone,
and restart synchronisation.  Sometimes the DSLAM will get itself wedged
and you have to wait for it to reset.  Sometimes the DSLAM will get really
really wedged and you have to poke at the telco to do something about it.

eg. in the UK asking for a analogue line test to be done will reset your
DSLAM line card too, and has the benefit of being completely automated.  
This doesn't always fix it though and is by no means official

If this is a chronic problem, continuous complaints to the telco may
induce them to switch your DSL connection to a diffrent DSLAM line
card.  They may be unwilling to do this since it requires physical
intervention by a human bean, and they don't actually make any profit out
of it.  They usually do this simply to stop you from complaining so much
:)

Also, if it's caused by something in the DSL modem overheating and
going out of tolerance, it may need a little more time to cool down to the
point where it's happy again.  If this is the case then the only solution
would be a new DSL modem.  It might just be a problem with yours, or it
might be a design flaw in that model.

> > 	I don't think this is the modem at all. I traced the situation a
> > couple of times, and what it boiled down to was an overloaded router on
> > the other end.
> > 	<snip>
> 
> That's quite possible.

yes, that's true, some ISPs in the UK have had chronic problems with this,
since here you have to buy something called a BT Central
(http://www.serviceview.bt.com/list/current/docs/Broadband/1205.htm)
which is a connection from BTs network to the ISPs network, and may or may
not include a router to terminate the ADSL connections and turn them into
something that can access the internet.  In almost all cases the BT
Central becomes overloaded a long way before the router does.

In almost all DSL networks the router that terminates your connection will
be one of the cheapest parts of the whole network, it being possible to
pick up a second hand 7204 VXR for less than 7000 UK pounds nowadays.

The only way for an overloaded router to cause complete service failure is
if the poor thing can't cope with what it's doing, and just reload itself
to see if it feels any better then.  In most cases the overloaded router
will just cause things to get slower when downloading/uploading

> > 	The workaround is simple though: we put a timer on the modem that
> > turns it off for three minutes in the middle of every night.
> > 	Since then, no problems. The theory might be right or wrong, but
> > the patient has been cured.
> 
> An easy cheap fix to try- I like it.

Well .. hum .. this seems like a rather windows solution ;-)

In all seriousness this does seem rather like masking the problem, rather
than fixing it.

> After reading all the responses, I'm more inclined to think the problem lies 
> somewhere upstream. 

I wouldn't rule anything out, just yet, there are simply too many places
where there could be a problem and produce similar effects.


Good luck

Maria