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Wireless TX power

Discussion in 'Tomato Firmware' started by Dragon2611, Mar 11, 2008.

  1. Dragon2611

    Dragon2611 LI Guru Member

    Want to try upping the Wireless TX power slightly on my WRT54GL to see if it helps with one of the computers that's having the odd dropout.

    Thing is I've heard if I raise it to high I could end up destorying the wi-fi transmitter in the router.. Being as its a brand new WRT54GL I don't really want to fry it just yet.

    What sort of level should I take as the Maximum I can use without cooking it.
    It's on the default atm.
     
  2. kevanj

    kevanj LI Guru Member

    Take a look here:

    http://www.extremetech.com/article2/0,1697,1934576,00.asp

    You might find that the device that is having the problem may be the source of the issue, rather than the strength of the signal the router is transmitting. Increasing the router's transmit power doesn't help if the client device doesn't transmit at high enough power to maintain a stable connection. You may also be seing interference in the signal, rather than a low signal issue. I have a number of devices that show low signal strength on the router, but they don't drop out....I'd start by looking for sources of interference close to the affected device (2.4GHz cordless phones, microwave ovens) or try moving it so the signal may have to go through fewer walls/floors, etc.
     
  3. Dragon2611

    Dragon2611 LI Guru Member

    Ok thanks for the info... I'll do some channel hopping later and play about it with it a bit.
     
  4. HennieM

    HennieM Network Guru Member

    Very good advice from kevanj. However, to answer your original question, the general consensus is that up to 100mW is OK. Higher than that don't really get you anything, as the noise goes up.

    I don't know if you can fry the radio though..., but I would think there might be some truth in that.
     
  5. kevanj

    kevanj LI Guru Member

    I'm not sure it's a question of limiting damage to the radio so much as generating so much heat that it damages the circuit board in the router and kills it completely...The link I posted does reference A 70mW limit, but I believe I have seen that 100mW is a safe upper limit too. It may vary depending on the model of router, case design, heat dissipation characteristics, etc....but I think I have also read that the effects of boosting xmit power are really minimal. Looking for sources of interference, cleaner channels, etc might be the better way to go initially...
     
  6. wtm

    wtm Network Guru Member

    Improving the antenna has more benefit.

    The problem with tinkering with the output power is that the WRT devices have been de-contented as much as possible and there isn't much design margin left. Obviously, it wouldn't be any benefit to Linksys to sell the device to put out any less power than the maximum of which it is capable. The exception would be operting inside various government regulatory domains that would cap the allowable power output - though I am not aware of any.

    The power output is designed to get the most usable output without driving the ouput stage into an operating point that increases the non-linear distortion of the transmitted signal. Increasing the noise floor could actually make receiving the signal more difficult despite having more output power. For the receiver, the signal-to-noise ratio determines the likelyhood of receiving the data error-free.

    Installing a better antenna is going to benefit both receiving and transmitting by the WRT. This is good. The reciprocity theorem says that a good transmitting for a given applicaiton antenna is also a good receiving antenna for the same application. That means the WRT is more likely to be able to receive the weak signal coming from the remote computer as well. Merely increasing the transmit power, even if it added nodistortion, would not do anything to improve receiving the remote computer's signal.

    Moving the WRT to a higher and/or less physically obstructed location and installing a better antenna are best things you can do. Also, keep the coaxial cable from the WRT as short as possible and use high quality calbe. A cheap skinny cable even just a few meters long can have enough loss to negate the benefit of having a better antenna.

    There are bi-lateral amplifiers that may be installed between the router and antenna, that improve signal level both receiving and transmitting. Again, the amplifier amplifies noise as well as signal, so it isn't as effective as a better antenna at improving the signal-to-noise ratio. It is true the antenna will add noise to the system, but antenna thermal noise is miniscule compared to that of any active amplifier.

    The last suggestion is to keep the WRT in a location that is at least somewhat temperature regulated. The radio is made from consumer grade parts and presumed to be operating in a household environment. If you install the router in an attic or similar location, the radio could drift off fequency at temperature extremes.

    Here is a web site I have found useful for antennas, coaxial cable, amplifiers and pats:
    http://www.fab-corp.com/ I am sure there are many others; this is just one site. Don't be shy about searching on Google.
     
  7. HennieM

    HennieM Network Guru Member

    @wtm: You are spot-on with "a better antenna usually beats anything else", and of course the "minimize interference/obstacles". However, you are, IMO, assuming 2 things that are not so:

    1) There are several instances which requires or allows limiting or increasing the Tx power, especially when you start changing antannae in an AP/router - ETSI, US, directional, omni-directional, etc. The older Linksys routers/APs all have detachable antennae...

    2) The use of the radio, and specifically the Tx power, in routers/WAPs is not necessarily optimal - mainly because the radio was not designed to be used exclusively in APs. I.e. the radio was not designed by Linksys to fit the APs, but is a generic radio (Broadcom/Atheros) to be used for various purposes such as wireless cards, APs, etc. Many cards are set to 18dBi Txpower (63mW) with the same radio as an AP/router that comes at 28 or 42mW.

    The default settings in APs, and this is, IMO, true for most, if not all, OEMs, are probably more a marketing/sales thing than a technical thing - make it strong enough to give medium coverage, but not so strong that we can't sell another one to the same customer, or that it would interfere with the neighbor's...
    Many people have tried to find out why v1-4 WRT54G(S)s are set to 42mW, and v5s to 28mW with basically the same radio - no clear answer yet to the best of my knowledge.

    As mentioned, my 2c opinion only.
     
  8. Odin-60

    Odin-60 LI Guru Member

    My Buffalo WHR-G54S is usually operating at 64 mW. Recently I tried to change this
    to 128 mW, just to see what would happen. Well, the results were disappointing:
    The WLAN connection to my Apple iBook in the same room (!) became slow and
    unstable, and my wireless mouse (approx. 70 cm away from the router) was heavily
    jammed. After going back to 64 mW, everything was alright again.
     
  9. HennieM

    HennieM Network Guru Member

    @Odin-60:
    When your transmitter and receiver are very close to one another, the increased signal strength sort of overwhelms the receiver, which in turn sees most of the signal as noise, and in turn your speed goes down.

    It's like if you shout in somebody's ear when it's quiet - he won't hear what you are saying because his ear drum bottoms out.

    If your iBook was, say, some 40m or 2 or 3 walls away from the AP, and you now increase the Tx power, you should see 3dB gain on the iBook's received signal (because you doubled the Tx power of the AP), and have a better signal.

    However, wireless is a 2-way street, and the receiver must talk back and become the transmitter - if you do not also double the Tx power on the iBook, you would gain nothing to very little.

    If you shout at me from two rooms away, I can hear what you are saying, but I must also shout, or you won't hear what I'm saying.

    For "same room" applications, assuming normal dipole or worse antennas, I would not take the Tx strength higher than 28mW or about 14dBi. If your 64mW works, and you sometimes carry the iBook to the other side of the house, stick with it.

    Also, some Buffalo APs have build-in amps. Don't know if the G54S has, but if it does, your 64mW could actually be much higher by the time it leaves the antenna of the Buffalo.
     

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