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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."

8 of 80 comments (clear)

  1. 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....

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    For The Best Jazz/Hip-hop fusion > COlD DUCK
    1. 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.

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    2. 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.

    3. 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.

  2. 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...

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      If the road to hell is paved with good intentions, where does the road paved with evil intentions lead to?
  3. 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.

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    FLR
  4. 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.