signal strength, link quality; interference

Have a question or want to start a discussion? Post it! No Registration Necessary.  Now with pictures!

Threaded View
my pcmcia card software reports "signal strength" and "link quality" but
I think this does not take into account interference and other variables
that effect overall throughput. It also shows transfer speed, in
mb/sec?, but this is highly variable and changes constantly. What can I
use to better test each antenna I make and to properly aim it for
optimal throughput? I know, I know, someone is going to say you're
asking the wrong/dumb question or not providing enough info, right? OS
is 98se. card is, I think orinoco.

Re: signal strength, link quality; interference

Quoted text here. Click to load it

try Netstumbler - if it supports your cards chipset -
has a nice moving graph for each SSID that it detects.

Re: signal strength, link quality; interference
On Thu, 15 Jan 2009 18:20:01 -0600, "ps56k"

Quoted text here. Click to load it

Agreed.  However, the current version of Netstumbler (0.4.0) does not
exactly work with Windoze 98se.  It works only if you have the
original Hermes chipset based Orinoco Gold or Silver PCMCIA card, also
known as Orinoco "classic".  However, the previous version (0.3.30)
works just fine but has a few bugs that were fixed in 0.4.0.  Avoid

Since you're into antique hardware and operating systems, this might
be of interest:

You can use signal strength to aim the antenna.  The response time on
most wireless manager graphs aren't really fast enough to do it
properly, but with patience, it can be done.  The problem is that the
wide variations in signal strength you're observing is for real.
2.4GHz bounces around quite a bit.  You're seeing cancelations and
reinforcements of multiple paths between endpoints.  Just pointing the
antenna in some random direction, and walking around nearby, will
yield substantial variations.  In an uncontrolled environment, this is
what you'll get.  If you had an RF anechoic chamber, it will be far
more stable.

Just to make life difficult, you'll find that the RF output of a Wi-Fi
xmitter various somewhat between 1Mbits/sec 802.11b thru 54Mbits/sec
802.11g.  Additional modes will also have different power levels.  You
might want to lock your xmit mode to some favorite speed before the
variations drive you nuts.

Interference definately shows up in "link quality".  The problem is
that chipsets have different ways to measure link quality.  Some use
the noise level between transmission to calculate SNR (signal to noise
ratio).  Others use the bit error rate to estimate the SNR.  I think
there are at least two other methods in use.  All of them will show
the effects of interference.

For testing antenna, you need a (drum roll) antenna test range, a
signal source, and a reference antenna with a known gain.  It doesn't
have to be fancy, but it should make an effort to avoid interference
and reflections.  I have a convenient WISP system located on a
mountain top approximately 5 miles away.  I'm on a hill side,
surrounded by trees.  The trees block most reflections and sources of
interference.  The valley in between prevents ground bounce from
becoming a problem.  I can point a directional antenna at the WISP
system, and get a fairly stable signal.  Even so, I like to plot the
signal levels with a data logger PC, throw out the extremes, and use
the average values.  I always use the reference antenna to remove the
effects of calibration, drift, and coax cables.

Jeff Liebermann
150 Felker St #D
Santa Cruz CA 95060
Skype: JeffLiebermann     AE6KS    831-336-2558

Re: signal strength, link quality; interference
Quoted text here. Click to load it

I *enormously* respect Jeff's views and this is not intended
to contradict them.

One approach that I have used successfully to
engineer a few wireless networks is to test the
link with fping.exe.

This allows you to sent pings at the fastest rate possible
(i.e. send the next one as soon as you have a reply to
the last one) and provides a sensitive indicator of
good communications. It is best in my view to specify
long (say 1400 byte) packets. I cannot at the moment
recall the exact parameters that I like but post back
if you require further assistance with it.

Re: signal strength, link quality; interference

Quoted text here. Click to load it

Thanks.  However, don't over rate my competence.  I do make mistakes.

Please note that the original question was for testing antennas, not
testing the link.  There's a difference.  With antenna testing, you're
really interested in:
1.  The antenna pattern, both vertical and horizontal.  Example:
2.  VSWR plot versus frequency (or usable bandwidth).  Same example.
Ping isn't going to show those.

Quoted text here. Click to load it

Yep.  fping is good for testing links.  Under idea conditions, what
you're looking for is a stable and consistent latency for every ping,
with no timeouts.  If you've got any manner of link impairment, you're
going to see variations and timeouts.  These represent
retransmissions, retries, and in extreme cases, packet loss.  In
effect, you're measuring the quality and stability of the link with
fping.  Unfortunately, fping does not yield a quantified measure of
quality and stability.

Quoted text here. Click to load it

For link testing, I prefer iperf and jperf.  Jperf (iperf with a Java
front end) is generally easier to use and operate:
< (tutorial)

These program flood the wireless pipe with as much traffic as it can
handle.  Wireless slows down when it hits errors, interference,
reflection, impairments, etc, which will be indicated by the thruput
benchmarks.  If thruput is slow, there's something wrong.  It can also
be used to test maximum thruput through a router, which is sometimes a
limiting factor on cable modem and FIOS high speed connections.

Jeff Liebermann
150 Felker St #D
Santa Cruz CA 95060
Skype: JeffLiebermann     AE6KS    831-336-2558

Site Timeline