More questions on tuning my wireless network...

Discussion in 'Wireless Internet' started by lrtherond, May 24, 2007.

  1. lrtherond

    lrtherond Guest

    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

    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:

    2 possible models are:

    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

    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

    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.

    lrtherond, May 24, 2007
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  2. hath wroth:
    Or, those that know are too busy to answer questions. I have time
    today, so draw your own conclusions.
    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.
    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 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
    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
    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
    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.
    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.
    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.
    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:
    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.
    My various DD-WRT v23 SP2 boxes seems to report signal strengths on
    the low side. It's like that on all my boxes.
    Yeah, that's the result of all that interference which creates errors,
    which causes the AP to slow down in order to compensate.
    A few paragraphs ago, it was 35. You must live in one of those fast
    growing areas.
    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.
    Make and model of game console? What type of wireless client? Same
    as before for the apartment? Range, construction, signal strength?
    Make and model of laptop? Type of wireless card? Operating system
    version? Which wireless card in the Playstation?
    You may have signal strength but I'll bet your thruput sucks. Download
    and install 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.
    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.
    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:
    Jeff Liebermann, May 24, 2007
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  3. lrtherond

    lrtherond Guest

    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...
    lrtherond, May 25, 2007
  4. lrtherond

    lrtherond Guest

    Google apparently misplaced that answer I spend 1 hour typing!

    Oh boy...
    lrtherond, May 25, 2007
  5. lrtherond

    lrtherond Guest

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

    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.
    lrtherond, May 25, 2007
  6. hath wroth:
    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.
    My DSL is 1500/256Kbits/sec. I'm jealous.
    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.
    <> (free)
    <> (expensive)
    Try it:
    Lots of others (but I can't find the free local tester I was thinking
    of). Note the IPerf can also report jitter.
    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).
    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.
    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.
    Actually, DD-WRT v23 sp2 is a transparent bridge. There are some
    articles on the subject in the DD-WRT wiki and blog.
    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.
    It's not going to work for gaming or VoIP. Web access is not time
    critical. Gaming and VoIP are.
    That will help, but I think doing something to prevent external and
    self-generated intereference would have a bigger effect.
    Ye're welcome.
    Jeff Liebermann, May 25, 2007
  7. lrtherond

    lrtherond Guest

    I'll check this out.

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

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

    Thanks, I just upraded to that version.

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

    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.
    lrtherond, May 26, 2007
  8. hath wroth:
    That's the down side of license-free technology.
    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.
    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
    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:
    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
    4. Wrap the house in sheet metal and foil.
    Jeff Liebermann, May 26, 2007
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