[gentoo-user] Slow local network - how to debug?

[gentoo-user] Slow local network - how to debug?

[Helmut Jarausch]

Hi,

I have no experiences in debugging a slow network.

At home, I have a router which is connected to my telephone line via  
VDSL2.
I have 2 PCs one of which is connected by an ethernet cable (i.e.  
wired) while the other
one uses a wireless connection which is specified as 56 Mbit/s.

When  copying data from one machine to the other one I see varying  
speeds from only 0.5 Mbits/s up to
more than 20 Mbits/s. I have no idea why it is so slow some times.
There are no other wireless devices nearby.

What tools and techniques can I use to debug this situation?

Many thanks for a hint,
Helmut.

Re: [gentoo-user] Slow local network - how to debug?

[Nilesh Govindrajan]

On Fri, Feb 22, 2013 at 1:34 PM, Helmut Jarausch
<jarausch@igpm.rwth-aachen.de> wrote:
> Hi,
>
> I have no experiences in debugging a slow network.
>
> At home, I have a router which is connected to my telephone line via VDSL2.
> I have 2 PCs one of which is connected by an ethernet cable (i.e. wired)
> while the other
> one uses a wireless connection which is specified as 56 Mbit/s.
>
> When  copying data from one machine to the other one I see varying speeds
> from only 0.5 Mbits/s up to
> more than 20 Mbits/s. I have no idea why it is so slow some times.
> There are no other wireless devices nearby.
>
> What tools and techniques can I use to debug this situation?
>
> Many thanks for a hint,
> Helmut.
>

Check for packet loss. This is the first and most common reason.

--
Nilesh Govindrajan
http://nileshgr.com

Re: [gentoo-user] Slow local network - how to debug?

[Mick]

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On Friday 22 Feb 2013 08:04:38 Helmut Jarausch wrote:
> Hi,
> 
> I have no experiences in debugging a slow network.
> 
> At home, I have a router which is connected to my telephone line via
> VDSL2.
> I have 2 PCs one of which is connected by an ethernet cable (i.e.
> wired) while the other
> one uses a wireless connection which is specified as 56 Mbit/s.
> 
> When  copying data from one machine to the other one I see varying
> speeds from only 0.5 Mbits/s up to
> more than 20 Mbits/s. I have no idea why it is so slow some times.
> There are no other wireless devices nearby.

Where do you see these transfer speeds?  On the wired machine, or on the 
wireless machine?

What do you use to check the transfer speed?

Do the speeds go up when you used a Cat5e cable at full duplex?

Have you scanned for access points to be absolutely sure that there are no 
other wireless devices, or APs?

Do you use encryption?

What frequency and channel are you on and have you tried to change 
channel/frequency?  (some domestic devices like cordless phones, wireless 
earphones, bluetooth, microwaves, perimeter sensors, etc. can cause co-channel 
and adjacent channel interference and/or force sharing of the bandwidth).

Are both machines and the router's LAN on the same MTU?


> What tools and techniques can I use to debug this situation?
> 
> Many thanks for a hint,

First use ifconfig to see how many dropped packets you get on both machines.  
Then use iwconfig, or cat /proc/net/wireless on the PC running the wireless to 
see how many discarded packets you get, fragmentation, etc. as well as nwid 
packets from different networks on the same frequency.

Then check with e.g. iptraf-ng what network connections are running, their 
rate(s) and if any of these should not be there.

The router stats may also be revealing in this respect, as well as dropped 
packets on the WiFi.

I hope the above will give something to work with, others may have better 
ideas and more specialised tools for troubleshooting this.

PS.  I haven't mentioned spectrum analysers to keep this thread within the 
realm of quick fixes, but if you do have EM interference and the source of it 
is obscure, you may need more professional equipment to nail this problem.
-- 
Regards,
Mick

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Re: [gentoo-user] Slow local network - how to debug?

[Helmut Jarausch]

On 02/22/2013 02:55:07 PM, Mick wrote:
> On Friday 22 Feb 2013 08:04:38 Helmut Jarausch wrote:
> > Hi,
> >
> > I have no experiences in debugging a slow network.
> >
> > At home, I have a router which is connected to my telephone line via
> > VDSL2.
> > I have 2 PCs one of which is connected by an ethernet cable (i.e.
> > wired) while the other
> > one uses a wireless connection which is specified as 56 Mbit/s.
> >
> > When  copying data from one machine to the other one I see varying
> > speeds from only 0.5 Mbits/s up to
> > more than 20 Mbits/s. I have no idea why it is so slow some times.
> > There are no other wireless devices nearby.
> 
> Where do you see these transfer speeds?  On the wired machine, or on  
> the
> wireless machine?
> 
> What do you use to check the transfer speed?

One tool is ttcp which is very similar to netcat and this concerns me  
most.
The other tool is a remote webserver for measure internet speed

> Do the speeds go up when you used a Cat5e cable at full duplex?

The PC which is wired doesn't have those problems since I can transmit  
data from
my remote office machine at the highest possble rate which is specified  
by my
internet provider.


> 
> Have you scanned for access points to be absolutely sure that there  
> are no
> other wireless devices, or APs?
> 
> Do you use encryption?

Yes, but since I do get optimal performance "some time" I don't think  
this
matters.

> 
> What frequency and channel are you on and have you tried to change
> channel/frequency?  (some domestic devices like cordless phones,  
> wireless
> earphones, bluetooth, microwaves, perimeter sensors, etc. can cause  
> co-channel
> and adjacent channel interference and/or force sharing of the  
> bandwidth).
> 
> Are both machines and the router's LAN on the same MTU?

Sorry, what's "MTU" ?
> 
> 
> > What tools and techniques can I use to debug this situation?
> >
> > Many thanks for a hint,
> 
> First use ifconfig to see how many dropped packets you get on both  
> machines.
> Then use iwconfig, or cat /proc/net/wireless on the PC running the  
> wireless to
> see how many discarded packets you get, fragmentation, etc. as well  
> as nwid
> packets from different networks on the same frequency.
> 
> Then check with e.g. iptraf-ng what network connections are running,  
> their
> rate(s) and if any of these should not be there.
> 
> The router stats may also be revealing in this respect, as well as  
> dropped
> packets on the WiFi.
> 
> I hope the above will give something to work with, others may have  
> better
> ideas and more specialised tools for troubleshooting this.

Yes, that'll keep me busy a while.
> 
> PS.  I haven't mentioned spectrum analysers to keep this thread  
> within the
> realm of quick fixes, but if you do have EM interference and the  
> source of it
> is obscure, you may need more professional equipment to nail this  
> problem.
> --
> Regards,
> Mick

Many, many thanks Mick,
regards,
Helmut.

Re: [gentoo-user] Slow local network - how to debug?

[Mick]

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On Friday 22 Feb 2013 14:36:22 Helmut Jarausch wrote:
> On 02/22/2013 02:55:07 PM, Mick wrote:
> > On Friday 22 Feb 2013 08:04:38 Helmut Jarausch wrote:

> > > When  copying data from one machine to the other one I see varying
> > > speeds from only 0.5 Mbits/s up to
> > > more than 20 Mbits/s. I have no idea why it is so slow some times.
> > > There are no other wireless devices nearby.
> > 
> > Where do you see these transfer speeds?  On the wired machine, or on
> > the
> > wireless machine?
> > 
> > What do you use to check the transfer speed?
> 
> One tool is ttcp which is very similar to netcat and this concerns me
> most.
> The other tool is a remote webserver for measure internet speed

I suggest that you troubleshoot speeds across your LAN first, before you look 
at connections across the Internet which tend to be less consistent.


> > Do the speeds go up when you used a Cat5e cable at full duplex?
> 
> The PC which is wired doesn't have those problems since I can transmit
> data from
> my remote office machine at the highest possble rate which is specified
> by my
> internet provider.

What I'm saying is:  When you connect BOTH machines to your router with cat5e 
cables does the performance improve?

I didn't know that you were using ttcp and was trying to eliminate other 
bottlenecks, e.g. storage caching, or drive controller problems if you were 
transferring a file.


> > Have you scanned for access points to be absolutely sure that there
> > are no
> > other wireless devices, or APs?
> > 
> > Do you use encryption?
> 
> Yes, but since I do get optimal performance "some time" I don't think
> this matters.

Right, inconsistent performance could be due to interference, rather than 
someone free-riding your open WiFi network.


> > What frequency and channel are you on and have you tried to change
> > channel/frequency?  (some domestic devices like cordless phones,
> > wireless
> > earphones, bluetooth, microwaves, perimeter sensors, etc. can cause
> > co-channel
> > and adjacent channel interference and/or force sharing of the
> > bandwidth).

This may eliminate interference problems, but only if concentrated on a 
particular frequency.  Broadband transmissions will still interfere.

You could try moving your laptop around the premises to see if you can spot 
some device/appliance that may be causing this.

> > Are both machines and the router's LAN on the same MTU?
> 
> Sorry, what's "MTU" ?

  http://en.wikipedia.org/wiki/Maximum_transmission_unit

I was trying to see if there is a mismatch which may cause excessive 
fragmentation.

-- 
Regards,
Mick

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