More questions on tuning my wireless network...

I compiled some other questions I can't get any answer to. (either I
am not asking them right, or no one really knows)

---

I noticed my APs often change Transmission Speed...
It varies widely, usually between 11 Mbps and 54 Mbps.

If I configure dd-wrt to use 24 Mbps by default, will it improve
stability?

Can this frequent switching between transmission speeds cause jitter
in terms of latency?

---

Wireless parameters are often undocumented by vendors of cheap
wireless equipment. (Examples: Linksys, Netgear)

Is there a good reference on how/when to tweak these parameters?

I am particularly interested in anything that can improve signal
strength and/or reduce latency.

---

Please help me clarify which antenna I should prefer:

http://www.hyperlinktech.com/web/antennas_2400_in.php

2 possible models are:

http://www.hyperlinktech.com/web/re11dp.php
http://www.hyperlinktech.com/web/re11ds.php

The context is an apartment with some wall and kitchen appliances
between 2 WRT54GL AP in WDS mode. Both APs are on the same floor.

Should I prefer "spatial diversity" or "polarization diversity"?

There are about 35 APs in my immediate surroundings.

According to the dd-wrt interface, signal strength (as reported by
both my APs) is between 40% and 50%.
I thought I would improve this a little.

I also noticed the Transmission Speed between my 2 APs changes a lot,
between 11 Mbps and 54 Mbps.

There are probably 100+ APs in a 4 acres radius around me.

Otherwise, my situation is like that:

1 AP handles my DSL, 1 workstation and 1 server.

The other AP is in another room, at the other end of the apartment. It
used to be set up in Client Mode, with 2 game consoles connected to
it.

In that other room, I also use 2 laptops and 1 PSP, which need
Internet access.
When the second AP was set up in Client Mode, the 2 laptops and the
PSP got wireless access from the first AP.
Now, with WDS, they hook up to the second AP with excellent signal
strength.

So, based on my current set-up, I only need to make sure the link
between my 2 APs is rock solid and all my clients will get excellent
coverage with little jitter.

In the middle of Silicon Valley, it seems everyone and their dog runs
a wireless network.

---

(E-Mail Removed) hath wroth:

>I compiled some other questions I can't get any answer to. (either I
>am not asking them right, or no one really knows)

Or, those that know are too busy to answer questions. I have time
today, so draw your own conclusions.

>I noticed my APs often change Transmission Speed...
>It varies widely, usually between 11 Mbps and 54 Mbps.

Yep. It's a feature. As the error rate increases, the access point
will reduce the speed in order to improve the receiver sensitivity,
which automagically improves the error rate.

>If I configure dd-wrt to use 24 Mbps by default, will it improve
>stability?

No. Your WRT54G(???) is not unstable. Having it change speed is
perfectly normal. You'll get a 54Mbit/sec connection up to a range of
about 10ft using the stock antenna. Any farther, and it will slow
down. I can did some graphs and curves out of SmallNetBuilder.com if
you want details.

Fixing the wireless speed at 24Mbits/sec will result in a different
behavior. You'll have a usable 24Mbit/sec link out to about 25ft at
which point it will just quit working due to a sky high BER (bit error
rate). However, there are places where you might want a fixed data
rate. I do it often, but at much slower speeds, such as 6 or
12Mbits/sec.

>Can this frequent switching between transmission speeds cause jitter
>in terms of latency?

Sure it can if you mean variations in latency. For example, if you
lose a packet due to a noise hit or collision, your latency will be
double (or more) of what it appears normally. That's the way I detect
collisions, errors, noise, interference, and sometimes reflections.
Anything that interferes with packet delivery will cause a change in
latency.

>Wireless parameters are often undocumented by vendors of cheap
>wireless equipment. (Examples: Linksys, Netgear)

Actually, they're a well guarded secret in the tradition of "You don't
need to know". Besides, too much information tends to confuse the
consumer.

>Is there a good reference on how/when to tweak these parameters?

Tweak which parameters? What are you optimizing for? Range? Thruput?
Packet loss? Interference? I'll see what I can find. There are
documents covering various timing parameters and settings, but no
single document that covers everything.

>I am particularly interested in anything that can improve signal
>strength and/or reduce latency.

Notice that my list did not include signal strength. That's handled
by antenna design and is generally unaffected by anything you can set
in your router (except tx power). Latency is affected by literally
everything. If you want to minimize latency, you basically have to
have a good strong signal, with execellent signal to noise ratio, and
few reflections, or the retransmissions will increase latency
spectacularly. What numbers for latency were you expecting? For what
application? Video, games, etc? Also realize that latency is most
commonly affected by your broadband connection. If you're measuring
your latency to include what's added by the internet, you fighting the
wrong battle.

>Please help me clarify which antenna I should prefer:
>http://www.hyperlinktech.com/web/antennas_2400_in.php
>2 possible models are:
>http://www.hyperlinktech.com/web/re11dp.php
>http://www.hyperlinktech.com/web/re11ds.php
>
>The context is an apartment with some wall and kitchen appliances
>between 2 WRT54GL AP in WDS mode. Both APs are on the same floor.

Are the number of walls variable or does "some walls" mean that you
can't count? How far apart? How many walls? What are they made
from? How transparent do they appear to your existing hardware
(signal levels)? You're trying to solve a specific problem, with a
specific location, using a specific number of walls. Being
intentionally vague doesn't help.

I have no idea what you have to work with and therefore cannot
recommend anything specific in the way of antennas. Since this is an
indoor affair, a large yagi or dish are out due to size
considerations. Same with a monster panel antenna. Basically, you're
limited to small panel, patch, and biquad antennas.

If wireless can't be made to work well, there are wired altenatives
that use the phone lines, power lines, CATV coax, CAT5, and fiber that
might work.

>Should I prefer "spatial diversity" or "polarization diversity"?

Sigh. Neither are particularly applicable or useful for an indoor
system. Your existing WRT54GL diversity system is currently spacial
diversity. However, when converting it to a point to point link, I
usually switch to no diversity receive. If you are also using either
WRT54GL as an access point (for wireless client connections), you will
have to deal with a compromise antenna system. Diversity reception
does not work with two different antennas. Be sure to read the Golf
Course article in the Cisco URL below:
<http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml>
<http://www.eetimes.com/showArticle.jhtml;jsessionid=DUOSM45NEGFBCQSNDLOSK H0CJUNN2JVN?articleID=16501888>
<http://www.commsdesign.com/showArticle.jhtml;jsessionid=DUOSM45NEGFBCQSNDLOSK H0CJUNN2JVN?articleID=16500279>

>There are about 35 APs in my immediate surroundings.

Give up now, while you're still sane. You won't find an empty channel
and you will get lots of interference. You're best bet is to move to
5.8GHz (802.11a) or minimize your dependence on wireless by using
wired networking.

Incidentally, WDS and other forms of store and forward repeaters do
not work well in the presense of interference. No wonder you're
having a latency problem. Every time some other station transmits,
your WDS system loses a packet. If you're going through the remote
WDS end as an access point, you lose two packets.

>According to the dd-wrt interface, signal strength (as reported by
>both my APs) is between 40% and 50%.
>I thought I would improve this a little.

My various DD-WRT v23 SP2 boxes seems to report signal strengths on
the low side. It's like that on all my boxes.
<https://home.learnbydestroying.com:8080>
Now, where does it show signal strength in percent?

>I also noticed the Transmission Speed between my 2 APs changes a lot,
>between 11 Mbps and 54 Mbps.

Yeah, that's the result of all that interference which creates errors,
which causes the AP to slow down in order to compensate.

>There are probably 100+ APs in a 4 acres radius around me.

A few paragraphs ago, it was 35. You must live in one of those fast
growing areas.

>Otherwise, my situation is like that:
>1 AP handles my DSL, 1 workstation and 1 server.

Speed of DSL? Make and model of workstation? Make and model of
server? Operating system and versions? You may know all these but
anyone offering to help will need to guess.

>The other AP is in another room, at the other end of the apartment. It
>used to be set up in Client Mode, with 2 game consoles connected to
>it.

Make and model of game console? What type of wireless client? Same
as before for the apartment? Range, construction, signal strength?

>In that other room, I also use 2 laptops and 1 PSP, which need
>Internet access.

Make and model of laptop? Type of wireless card? Operating system
version? Which wireless card in the Playstation?

>When the second AP was set up in Client Mode, the 2 laptops and the
>PSP got wireless access from the first AP.
>Now, with WDS, they hook up to the second AP with excellent signal
>strength.

You may have signal strength but I'll bet your thruput sucks. Download
and install IPerf:
<http://dast.nlanr.net/Projects/Iperf/>
Install it one a desktop that is connected to your main wireless
router via CAT5. Run the server as:
iperf -s
On various clients, run:
iperf -c ip_address_of_the_server
and get TCP speed statistics. Note that this test does NOT use any
internet benchmarking sites or apps. I suspect you're going to see
some rather miserable thruput.

>So, based on my current set-up, I only need to make sure the link
>between my 2 APs is rock solid and all my clients will get excellent
>coverage with little jitter.

If you plug into your remote WRT54GL (the one that does NOT have the
DSL modem connected), you'll get full speed, as if you were using it
as an ethernet client radio. However, if you connect to it via
wireless, you're using it as a store and forward repeater. Your
maximum thruput will be cut in half because in store and forward, only
one transmitter can be on at a time.

>In the middle of Silicon Valley, it seems everyone and their dog runs
>a wireless network.

The surest sign of success is pollution. Wireless is successful.

You might want to look at this list of interference sources that
Netstrumbler and such cannot see:
<http://wireless.wikia.com/wiki/Wi-Fi#Interference>

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff, you do well to correct all my sloppy questions.

I need to take as much time to answer your various questions as you
took to provide your initial answers.

In other words, it's going to take some time...

Google apparently misplaced that answer I spend 1 hour typing!

Oh boy...

After the blow Google gave me yesterday, I will summarize the
situation...

I don't do any file transfers over my wireless network.
So, my throughput requirements are limited to the capacity of my DSL.
(3000-6000 Kbps / 512-768 Kbps)

Latency is a bigger concern to me, since I use Skype-like applications
and online gaming.
I certainly do not want my wireless laptops (connected to the WDS
client AP) to increase overall latency.

I ran Iperf and I confirmed I was getting 2.3 Mbps of effective
throughput. My eyes bled.
During these tests, the same pattern could be observed each time:
Transmission speed would rapidly degrade from 54 Mbps to 11 Mbps.

So, I converted the WDS client AP to Wireless Bridge mode. Probably,
it's not a transparent bridge, but it's no issue in my case.

Since this change, my latency has improved significantly when all
clients are active. (6.2 Mbps, which is > the capacity of my DSL)

The drawback is that my wireless laptops and PSP must now struggle to
get a good link from the remaining AP.
It works well enough for Web access.

I will now go on and see if I can improve the strength of the signal
between the AP and the Wireless Bridge.

Thanks for all your good advice.


On May 24, 3:37 pm, Jeff Liebermann <j...@comix.santa-cruz.ca.us>
wrote:
>
> >Please help me clarify which antenna I should prefer:
> >http://www.hyperlinktech.com/web/antennas_2400_in.php
> >2 possible models are:
> >http://www.hyperlinktech.com/web/re11dp.php
> >http://www.hyperlinktech.com/web/re11ds.php
>
> >The context is an apartment with some wall and kitchen appliances
> >between 2 WRT54GL AP in WDS mode. Both APs are on the same floor.
>
> Are the number of walls variable or does "some walls" mean that you
> can't count? How far apart? How many walls? What are they made
> from? How transparent do they appear to your existing hardware
> (signal levels)? You're trying to solve a specific problem, with a
> specific location, using a specific number of walls. Being
> intentionally vague doesn't help.
>
> I have no idea what you have to work with and therefore cannot
> recommend anything specific in the way of antennas. Since this is an
> indoor affair, a large yagi or dish are out due to size
> considerations. Same with a monster panel antenna. Basically, you're
> limited to small panel, patch, and biquad antennas.
>
> If wireless can't be made to work well, there are wired altenatives
> that use the phone lines, power lines, CATV coax, CAT5, and fiber that
> might work.
>
> >Should I prefer "spatial diversity" or "polarization diversity"?
>
> Sigh. Neither are particularly applicable or useful for an indoor
> system. Your existing WRT54GL diversity system is currently spacial
> diversity. However, when converting it to a point to point link, I
> usually switch to no diversity receive. If you are also using either
> WRT54GL as an access point (for wireless client connections), you will
> have to deal with a compromise antenna system. Diversity reception
> does not work with two different antennas. Be sure to read the Golf
> Course article in the Cisco URL below:
> <http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note091...>
> <http://www.eetimes.com/showArticle.jhtml;jsessionid=DUOSM45NEGFBCQSND...>
> <http://www.commsdesign.com/showArticle.jhtml;jsessionid=DUOSM45NEGFBC...>
>
> >There are about 35 APs in my immediate surroundings.
>
> Give up now, while you're still sane. You won't find an empty channel
> and you will get lots of interference. You're best bet is to move to
> 5.8GHz (802.11a) or minimize your dependence on wireless by using
> wired networking.
>
> Incidentally, WDS and other forms of store and forward repeaters do
> not work well in the presense of interference. No wonder you're
> having a latency problem. Every time some other station transmits,
> your WDS system loses a packet. If you're going through the remote
> WDS end as an access point, you lose two packets.
>
> >According to the dd-wrt interface, signal strength (as reported by
> >both my APs) is between 40% and 50%.
> >I thought I would improve this a little.
>
> My various DD-WRT v23 SP2 boxes seems to report signal strengths on
> the low side. It's like that on all my boxes.
> <https://home.learnbydestroying.com:8080>
> Now, where does it show signal strength in percent?
>
> >I also noticed the Transmission Speed between my 2 APs changes a lot,
> >between 11 Mbps and 54 Mbps.
>
> Yeah, that's the result of all that interference which creates errors,
> which causes the AP to slow down in order to compensate.
>
> >There are probably 100+ APs in a 4 acres radius around me.
>
> A few paragraphs ago, it was 35. You must live in one of those fast
> growing areas.
>
> >Otherwise, my situation is like that:
> >1 AP handles my DSL, 1 workstation and 1 server.
>
> Speed of DSL? Make and model of workstation? Make and model of
> server? Operating system and versions? You may know all these but
> anyone offering to help will need to guess.
>
> >The other AP is in another room, at the other end of the apartment. It
> >used to be set up in Client Mode, with 2 game consoles connected to
> >it.
>
> Make and model of game console? What type of wireless client? Same
> as before for the apartment? Range, construction, signal strength?
>
> >In that other room, I also use 2 laptops and 1 PSP, which need
> >Internet access.
>
> Make and model of laptop? Type of wireless card? Operating system
> version? Which wireless card in the Playstation?
>
> >When the second AP was set up in Client Mode, the 2 laptops and the
> >PSP got wireless access from the first AP.
> >Now, with WDS, they hook up to the second AP with excellent signal
> >strength.
>
> You may have signal strength but I'll bet your thruput sucks. Download
> and install IPerf:
> <http://dast.nlanr.net/Projects/Iperf/>
> Install it one a desktop that is connected to your main wireless
> router via CAT5. Run the server as:
> iperf -s
> On various clients, run:
> iperf -c ip_address_of_the_server
> and get TCP speed statistics. Note that this test does NOT use any
> internet benchmarking sites or apps. I suspect you're going to see
> some rather miserable thruput.
>
> >So, based on my current set-up, I only need to make sure the link
> >between my 2 APs is rock solid and all my clients will get excellent
> >coverage with little jitter.
>
> If you plug into your remote WRT54GL (the one that does NOT have the
> DSL modem connected), you'll get full speed, as if you were using it
> as an ethernet client radio. However, if you connect to it via
> wireless, you're using it as a store and forward repeater. Your
> maximum thruput will be cut in half because in store and forward, only
> one transmitter can be on at a time.
>
> >In the middle of Silicon Valley, it seems everyone and their dog runs
> >a wireless network.
>
> The surest sign of success is pollution. Wireless is successful.
>
> You might want to look at this list of interference sources that
> Netstrumbler and such cannot see:
> <http://wireless.wikia.com/wiki/Wi-Fi#Interference>
>
> --
> Jeff Liebermann j...@comix.santa-cruz.ca.us
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558

(E-Mail Removed) hath wroth:

>After the blow Google gave me yesterday,

Some of my postings in other groups are not appearing on Google
Groups. Methinks something might be wrong with Google, but I don't
want to burn the time finding out.

>I don't do any file transfers over my wireless network.
>So, my throughput requirements are limited to the capacity of my DSL.
>(3000-6000 Kbps / 512-768 Kbps)

My DSL is 1500/256Kbits/sec. I'm jealous.

>Latency is a bigger concern to me, since I use Skype-like applications
>and online gaming.

Have you tried any of the online VoIP testers? Unlike the more
conventional bandwidth testers, they show jitter. You can also
download the Java applet and run it locally to identify jitter, which
is one manifestation of packet loss.
<http://www.myspeed.com/pe/index.html> (free)
<http://www.myvoipspeed.com/pc/index.html> (expensive)
Try it:
<http://www.voipreview.org/voipspeedtester.aspx>
Lots of others (but I can't find the free local tester I was thinking
of). Note the IPerf can also report jitter.

>I certainly do not want my wireless laptops (connected to the WDS
>client AP) to increase overall latency.

They will increase latency. The question is by how much. A wireless
direct link should add 2-3 msec additional delays. However, if you
have packet loss from interference, it will climb quickly. WDS, when
used as a store and forward repeater will make any delays 2 or more
times worse (as you just discovered).

>I ran Iperf and I confirmed I was getting 2.3 Mbps of effective
>throughput. My eyes bled.

Yep. That sucks. You should be doing much better. However, the
interference is probably the culprit. What you might find is that the
performance won't be much better if you move in close to the access
points. That's the because the interference will appear in the 802.11
delay spread (between packets) causing just as much interference as
with a weak signal. However, that's still no guarantee of success.
One of the fun demonstrations that I've done is to place a repeater or
WDS bridge in a closed room along with the main WDS router and a few
wireless laptop clients. With all these radios in a small area, there
is quite a bit of mutual inteference. Using IPerf, thruput sucks and
interference effects are obvious. So are reflections and multipath.
Basically repeater and WDS do not work if the end points can hear each
other. If they can, they simply add to the interference. It's
possible that your WDS system is creating its own interference. Try
it in an RF isolated environment such as a basement.

>During these tests, the same pattern could be observed each time:
>Transmission speed would rapidly degrade from 54 Mbps to 11 Mbps.

That's normal. When moving no traffic, the SNR is good, so the speed
creeps back up to 54Mbits/sec. As soon as traffic starts, the errors
start to appear along with the traffic. The AP tries to compensate by
slowing down. The delay in changing speed varies with manufactory,
but my guess is about 30 seconds of no traffic.

>So, I converted the WDS client AP to Wireless Bridge mode. Probably,
>it's not a transparent bridge, but it's no issue in my case.

Actually, DD-WRT v23 sp2 is a transparent bridge. There are some
articles on the subject in the DD-WRT wiki and blog.

>Since this change, my latency has improved significantly when all
>clients are active. (6.2 Mbps, which is > the capacity of my DSL)

What do you mean "all clients are active"? Are you moving traffic
simultaneously with other wireless clients? That will just split the
bandwidth in half. Try to setup an ideal test, with one AP and one
client, turning everything off. Then add clients to see what breaks.

>The drawback is that my wireless laptops and PSP must now struggle to
>get a good link from the remaining AP.
>It works well enough for Web access.

It's not going to work for gaming or VoIP. Web access is not time
critical. Gaming and VoIP are.

>I will now go on and see if I can improve the strength of the signal
>between the AP and the Wireless Bridge.

That will help, but I think doing something to prevent external and
self-generated intereference would have a bigger effect.

>Thanks for all your good advice.

Ye're welcome.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On May 25, 11:21 am, Jeff Liebermann <j...@comix.santa-cruz.ca.us>
wrote:
> Have you tried any of the online VoIP testers? Unlike the more
> conventional bandwidth testers, they show jitter. You can also
> download the Java applet and run it locally to identify jitter, which
> is one manifestation of packet loss.
> <http://www.myspeed.com/pe/index.html> (free)
> <http://www.myvoipspeed.com/pc/index.html> (expensive)
> Try it:
> <http://www.voipreview.org/voipspeedtester.aspx>
> Lots of others (but I can't find the free local tester I was thinking
> of). Note the IPerf can also report jitter.

I'll check this out.


> They will increase latency. The question is by how much. A wireless
> direct link should add 2-3 msec additional delays. However, if you
> have packet loss from interference, it will climb quickly. WDS, when
> used as a store and forward repeater will make any delays 2 or more
> times worse (as you just discovered).

WDS is gone. I'll avoid keeping the wireless laptops on while gaming
or using Skype.


> >I ran Iperf and I confirmed I was getting 2.3 Mbps of effective
> >throughput. My eyes bled.
>
> Yep. That sucks. You should be doing much better. However, the
> interference is probably the culprit.

Yes, I must say that in my conditions, this technology barely works.


> Actually, DD-WRT v23 sp2 is a transparent bridge. There are some
> articles on the subject in the DD-WRT wiki and blog.

Thanks, I just upraded to that version.


> >Since this change, my latency has improved significantly when all
> >clients are active. (6.2 Mbps, which is > the capacity of my DSL)
>
> What do you mean "all clients are active"?

I meant "powered up". If I have them produce traffic, things get
worse, as expected.


> That will help, but I think doing something to prevent external and
> self-generated intereference would have a bigger effect.

I don't believe there is any solution to that when you live across
from Cisco's campus and 80% of your neighbors are Cisco engineers
running wireless networks.

(E-Mail Removed) hath wroth:

>Yes, I must say that in my conditions, this technology barely works.

That's the down side of license-free technology.

>> Actually, DD-WRT v23 sp2 is a transparent bridge. There are some
>> articles on the subject in the DD-WRT wiki and blog.
>
>Thanks, I just upraded to that version.

I've been having problems with v23 sp2 only on my WRT54GS installs.
The remote admin access page seems to hang after about a days
operation. It doesn't happen with WRT54G or various Buffalo products.
Weird.

>I meant "powered up". If I have them produce traffic, things get
>worse, as expected.

It varies. The small amount of traffic generated by an idle wireless
computer should not affect your connection very much. It's only when
the idle laptop starts to move traffic, that problems begin. However,
that assumes that most of automagic update, polling features, and "ET
call home" features of Windoze XP are disarmed or blocked. Even an
idle Windoze box generates quite a bit of traffic allegedly doing
nothing.

>I don't believe there is any solution to that when you live across
>from Cisco's campus and 80% of your neighbors are Cisco engineers
>running wireless networks.

Sure, there are solutions.
1. Switch to 802.11a at 5.8GHz. I'm sure Cisco has some systems
working on 802.11a, but they will have less range, and don't penetrate
as badly. In your circumstances, I would borrow a spectrum analyzer
and check first.
2. 900MHz OFDM is coming. See:
<http://www.ubnt.com/super_range9.php4>
Finding commerical products is rough, but it's possible to throw
together something based solely on their MiniPCI products. Note that
this is currently FCC type certified.
3. Switch to power line, phone line, coax cable, or fiber optic wired
networking.
4. Wrap the house in sheet metal and foil.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558