Slashdot Mirror

← Back to Stories (view on slashdot.org)

Pushing Wi-Fi's Limits: Problems and Solutions

Posted by CmdrTaco on from the bits-and-bytes-wizzing-around-your-noggin dept.

securitas writes "Forbes technology columnist Arik Hesseldahl discusses the problems with 802.11x Wi-Fi - speed and range - and how to push its limits in a pair of his Ten O'Clock Tech columns. He discusses the alphabet soup of Wi-Fi standards, so-called 'Super G' dual channel bonding that allows two of 11 channels to act as one (and the interference problems that ensue), and the multiple input/multiple output (MIMO) method 'using multiple antennas to break a single, high-rate signal into several lower-rate signals' that could be a solution. Pushing Wi-Fi's Limits, Part Two focuses on repeaters, Wi-Fi mesh networks, WiMax and a company called BelAir Networks that has deployed several Wi-Fi mesh networks."

22 of 80 comments (clear)

  1. Real issue by kneecarrot · · Score: 5, Informative

    While standards and spectrum sharing are definitely factors, hardware must move quite a bit forward if it is going to become more useful than small home networks and looking cool at a Starbucks. The real problem right now is the quality of the radio chips coming out of Taiwan. They are typically way under specified range and allow for alot of bleeding between channels. The average home user won't notice it, but when you are rigging up multi-antenna setups or relying on precise timing for a repeater, it matters to a HUGE extent.

    --

    I always save my last mod point to mod up a good troll. You people are too serious.

    1. Re:Real issue by Jeff+DeMaagd · · Score: 4, Insightful

      One thing that scares me is the multi-channel radios. There are effectively only three non-overlapping channels, and some APs are starting to take up two of them? I know the spectrum is unlicenced for low power, but I think that's usually just rude and mean for one person to just take two channels. Right now, I experiment with three APs being on, all three channels, but being in a rural area and having checked with all the neighbors, no one else is using the spectrum near me.

  2. I wonder how healthy it is by 2057 · · Score: 3, Interesting

    I wonder how healthy it is to be surrounded day in and day out by all these microwaves and such....

    --
    For The Best Jazz/Hip-hop fusion > COlD DUCK
    1. Re:I wonder how healthy it is by IWantMyNickBack · · Score: 2, Insightful

      Bah... We already have AM, FM, TV, and all other sorts of signals going through us... we arent seeing any adverse side-effects, are we?

    2. Re:I wonder how healthy it is by MikeXpop · · Score: 3, Insightful

      How would we know?

      --
      Etiquette is etiquette. He kills his mother but he can't wear grey trousers.
    3. Re:I wonder how healthy it is by mOoZik · · Score: 5, Interesting

      All the popular notions of microwaves being harmful are pretty unfounded. You must remember, they're like other waves, like radio, UV, IR, radar, and so forth. It seems to me most misconception arises from the fact that we use microwaves to cook food and that stray waves from cell phones and other such things could be harmful to us, but again, this is untrue for most purposes.

      You see, microwaves excite water molecules - they make them move back and forth really fast - thus heating them and increasing their temperature; this is how a microwave oven works. The fear with cell phones (which have a very weak transmitter) is that they may increase the temperature of brain cells or other, critical cells above a normal temperature, thus cause an unfavorable outcome. However, studies have shown that the increase of temperature from a cell phone antenna - when put against one's ear - is less than 1/10th of a degree Centigrade. As you can imagine, this is insignificant; our bodies are able to remain undamaged at temperatures MUCH higher than this.

      The point is that cellphones, while not the topic of this article, transmit much more powerful microwaves, much closer to the head. This means most WiFi waves are not at all powerful enough to have an impact.

      Of course, I do not suggest you stick your head near a multi-megawatt microwave transmitter.

      --
      A blog like any other.
    4. Re:I wonder how healthy it is by jdhutchins · · Score: 3, Insightful

      The rate of cancer that we found went up 64% between 1970 and 1997. That doens't necessarily mean there was 64% more cancer, it just means we know how to find cancer better. I'm not saying that cancer hasn't gone up any, but it's probably not that dramatic.

    5. Re:I wonder how healthy it is by PurpleFloyd · · Score: 2, Informative
      If you think that correlation implies causation, then I have an Eternal Life Braclet to sell to you.

      The only way to determine the cause of an effect is via scientific experimentation - a double-blind study with experimental and control groups. We could, for example, take 100 mice, leave 50 in "normal" cages, and put the other 50 in cages near to a broadband EM source. Then, we examine the rate of cancer in each group - if it goes up, then and only then can we say that "electromagnetic radiation (at a certain level) causes cancer in mice, which are similar enough to humans that we can safely say to stay away from EM sources."

      There could well be many other reasons for the higher cancer rates reported in the 20th century - in addition to all the reasons you have mentioned, people have started to live much longer; if you die from cholera or typhoid fever at 25, you won't live long enough to die from cancer at 65. Also, the broad penetration of medical care into the working class, as compared to the 19th century, means that deaths which previously had no known cause might now be easily diagnosed as cancer. There are many reasons why reported cancer rates might go up - an actual rise in the cancer rate is only one of them.

      --

      That's it. I'm no longer part of Team Sanity.
    6. Re:I wonder how healthy it is by uncommonlygood · · Score: 2, Interesting

      Put a piece of raw meat in your microwave. Set it for 15 minutes. Look at it when the 15 minutes are up and you can answer your own question.

      Yeah, do it on "defrost" and it you won't be so scared, and the oven would still be pumping out about 1000 times more power than a wi-fi card.

      Someone once told me (with a completely straight face) that a mobile phone generates enough microwaves in an hour to boil an egg. Well, I get a lot of free minutes per month with my phone that I don't normally use, so we put the phone right next to an egg and rang my house for an hour. Guess what, it didn't boil.

    7. Re:I wonder how healthy it is by aXis100 · · Score: 2, Interesting

      The effect of microwaving a piece of meat is due to the power level, not the overall exposure.

      1000 Watts of a microwave heats the meat enough to cause cooking. The rate at which energy is added is higher than the rate it dissipates, so the equilibrium temperature goes up.

      30mW from a wifi device does not cause enough of a temperature increase to cause any harm.

  3. A clever concession to state of the market. by ofdm · · Score: 5, Interesting
    This solution seems to be quite a clever approach. A fundamental problem with 802.11b is the lack of spectrum. Although the channels are labelled 1 to 11 (in the USA), the numbers refer to a spacing of 5MHz chunks over 50MHz in the 2.4GHz ISM band. The problem is that the 802.11b signal uses almost 4 of those channels to actually transmit data. As a result, in order to have systems on different frequencies which are not interfering with each other, you end up with three effective channels - 1, 6, and 11. (If you have a WiFi AP accessible, check what channel it's on - most likely it will be one of those). Due to the low number of channels, it's impossible to do much in the way of channel planning. The result is that adjacent APs have to share the spectrum. The net outcome is that the data rate that users get between their client and the AP is reduced.

    802.11a at 5GHz was supposed to solve this. The 5GHz band is notable because of the extra spectrum it has. Compared to the 3 effective channels at 2.4GHz, the 5GHz UNII band has (again, it depends on your country) at least 8 usable channels of 20MHz. Additionally, the link rate is between 6 and 54 Mbps (as compared to 1 to 11Mbps for 11b, although this is somewhat moot given the growing preponderance of 11g solutions at 2.4Ghz). However, the 802.11a market never really took off and killed the 11b market the way we (engineers) expected it to. Mostly due to good (if slippery) marketing of 11g. As a result, there's a lot of unused 11a spectrum begging to be used. There are a lot of people with 2.4GHz equipment who want more range without losing data throughput. Using the 11a spectrum to extend the 11b/g range is what these guys have done. Neat - they get to use a superior technology with cheap chips available, to leverage a large market (albeit of dullards wed to an inferior solution).

    1. Re:A clever concession to state of the market. by ttsalo · · Score: 2, Interesting
      The result is that adjacent APs have to share the spectrum.

      I once thought up a solution for this. The APs could have active antennas with a grid of elements, much like modern military radars. This sort of antenna is directional and the beam is electronically steerable. As long as there was a different band for downstream and upstream, interference would be virtually eliminated. Finding the client's direction and the schedule for listening to the clients would have to be somehow solved.

      This sort of AP wouldn't be exactly cheap, though...

      --

      --
      If the road to hell is paved with good intentions, where does the road paved with evil intentions lead to?
  4. Problems on a more fundamental level by BelugaParty · · Score: 5, Insightful

    I can sit in a college library and browse people's laptops as if they are on a trusted network. People don't realize how public WiFi is in these environments. I think the main cause for this is the connection wizard (microsoft specifically). When you first connect the computer for wireless access it automagically, without a lot of warning, shares folders, printers ... etc, because it is assuming you are at your house with your linksys router; not at the library, coffee shop, or hijacking i-net from an apartment complex across the street.

    1. Re:Problems on a more fundamental level by JUSTONEMORELATTE · · Score: 2, Funny

      I can sit in a college library and browse people's laptops as if they are on a trusted network.

      Cool!
      So, what kind of MP3 collection does the average college student have on their laptop these days? This "library" you speak of sounds like iTunes, but without the credit-card part!

      --

  5. There could be a problem... by Cytlid · · Score: 2, Interesting

    Concerning the second article, 802.11a seemed pretty clever to use for the uplink. A mesh within a mesh. But isn't 802.11a unencrypted? What's to stop me from pulling over along the side of the road with my trusty 802.11a nic and sniffing cleartext (uplink) traffic? That's a lot of pop3 passwords, my friends.

    --
    FLR
  6. use more power by max+born · · Score: 2, Informative

    Many of these problems can be easily solved with more power. The FCC has imposed severe power limitations on 802.11 of about 100-200mW per channel.

    If the FCC would allow us amatuers to use, say, half the power that cell phone companies do, we'd be able to Wi-Fi the whole country.

    Give us the tools and we'll finish job.

    1. Re:use more power by ofdm · · Score: 3, Informative
      This is, unfortunately, a common misconception. Increasing the range of 11b means that the available bandwidth has to be shared more widely - meaning that each user gets less bandwidth, which is to say data throughput.

      Imagine if you will, a world where you could hear everyone talking within a block of you. Sounds great - you can hear your stereo from a mile away (well, this already happens). Unfortunately you can also hear everyone elses stereo, and everyone else talking, and their refrigerators humming, and their dogs barking. The net result is that you can't hear anything clearly. It's very much the same as being in a very full pub towards the end of a horse race - everyone's making a tonne of noise, and you can't understand much of it at all.

      The solution is to have smaller cells, in the limit pico-cells, where an individual user gets the full channel to herself, and 10m away another user gets their own full channel. An alternative is to have large cells, but with APs having different non-overlapping frequencies - so that the cells are isolated in the same manner as for the pico-cells. Sadly, the lack of spectrum for 11b at 2.4Ghz makes this second solution unrealistic.

    2. Re:use more power by max+born · · Score: 2, Informative

      Yes, but with 802.11g and 802.16 the specs allow for up to 50+Mbps, several T1s. For surfing the net and reading email this is enough to supprort more users than may be commonly found in the limited area covered by the current power restrictions.

      The 802.X specs cleverly implement CSMA/CA, a collision avoidance system that seems to work pretty well. From my rooftop (downtown San Francisco), I can see 150+ networks yet never experience any symptoms of interference.

      Also, in setting the 802.11 limits on power the FCC seems to have overlooked the "skin depth" wavelength/2*pi attenuation factor that limits microwave ranges by a factor exponetially greater than for the longer wavelengths of many cell phones.

      I operate a couple of nodes as part of the sflan project and it would be nice if we could reduce the "line of sight" requirement just a tad by boosting the power, say from 200mW to 800mW.

      It just ain't fair than big bucks phone comapanies can "buy" an FCC license and blast away at 1500W while us folks trying to provide a free community service have to operate with our hands tied.

    3. Re:use more power by ofdm · · Score: 3, Interesting
      I ... never experience any symptoms of interference
      That's most likely because, as you said, your traffic requirements are low, and possibly the traffic on the newtworks you can see isn't particularly heavy. If you have access to the PHY layer, you will see that collisions are in fact very common. The standard provides a couple of ways for dealing with this. (I'm sorry if I'm teaching you how to suck eggs here - I don't know what you know, so I'm aiming low).

      At the base level, each data packet is acknowledged by the recipient. If it isn't acknowledged (an ACK) then the packet is re-sent. Depending on the particular manufacturers implementation, the retries continue, and if still unsuccessful, at some point the rate is dropped (the lower rates are more robust). Eventually, if there's no acknowledgement, the packet is dropped.

      At the higher level the MAC can take advantage of RTS/CTS. In this mode, before sending a data packet, a small "Request to Send" packet is transmitted - telling all the radios in range that it is about to send a packet and it will take such and such a length of time to do it - so please stay off the air. The recipient (the STA to whom the eventual data packet is addressed) responds with CTS - Clear-to-Send. Then the data transfer goes ahead. The RTS/CTS mechanism is designed to reduce collisions in heavily loaded networks. Unfortunately, it relies on all the radios which can make noise being able to hear and succesfully demod the RTS/CTS. Adjacent channels have the annoying property that they are load and interfere with you, but they are also incomprehensible. As a result RTS/CTS doesn't help with one of the more common mechanisms for collisions.

      In summary, collisions do happen, and adjacent cells/frequencies do lower throughput. Many users never notice this because of their low data requirements. As many posters have noted, even 1Mbps is more than most people need at present. However, as requirements go up (streaming video etc), this will become a much more visible problem.

  7. Pushing Wi Fi Limits by fegriffin · · Score: 3, Insightful

    Although there is never enough bandwidth, until we can solve the last mile bottleneck, 11Mbs 802.11b networks will be sufficient. With ADSL and cablemodem rates at less than 1Mbs that is where the problem needs to be solved.

  8. Use directional antennas by KC7YRN · · Score: 2, Informative

    There's a problem, though, with using more power. You increase interference with everybody else while making a small improvement with your intended recipient. A directional antenna helps you when you receive as well as when you transmit. If you need to serve an area, you can still benefit from an antenna that concentrates radiation in a pancake shape so you don't waste power transmitting straight up. High power conflicts with sharing.

  9. Wi Fi meshes help protect file sharers by microbrewer · · Score: 2, Funny
    The good thing about mesh networks and leaving things unsecured is that you can fileshare without much worry from the RIAA becuse you can always claim that it wasnt you downloading from a IP it was someone hooking into your wireless router .Justin Frankel the godfather of distributed P2P file sharing has also sugested that it should be done on his own blog .
    May 19, 2004 I've been saying this for a long time, well over a year now: http://www.salon.com/tech/feature/2004/05/18/safe_ and_insecure/index.html