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Beware Of 2.4 GHz Interference 191

Posted by Hemos
from the i'm-a-living-testimony dept.
RobinX writes: "If you have any combination of cordless phones, wireless ethernet, wireless video, or Bluetooth you could be having problems. I've got two different 2.4 GHz phone brands that are interfering with each other and with my home 802.11b wireless ethernet network. It seems that the 2.4 GHz range isn't licensed so companies are free to do their own thing. Check out this article for more." I've been noticing problems recently as well, between phones from the same manufacturer and the WaveLan cards.
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Beware Of 2.4 GHz Interference

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  • This isn't quite accurate. There are two flavors of 802.11 -- Frequency Hopping and Direct Sequence. I believe the author specified 802.11b which is the Direct Sequence flavor used by Lucent's WaveLAN and the Airport. (I may be wrong about which letter is which.) Frequency hopping equipment will indeed cause interference with the direct sequence equipment, since it's hopping through the ranges used for direct sequence pretty frequently. In the lab we've seen about 30% packet loss on WaveLAN when streaming full speed over 802.11a using the Raytheon 802.11 chipset used in WebGear Aviators. The Aviators seem to be pretty resilient though, and aren't affected significantly the other way.
  • Microwave ovens operate in the 2.4 GHz band, and are poorly shielded high power transmitters. Wireless lans and other ISM band applications are low power transmitters, but many microwave ovens (especially older ones) radiate (read: interfere) more power than these low power transmitters are allowed to do, because of their poor shielding. I would be very sceptical to use equipment running in the 2.4 GHz for "mission critical" work. 5.7 GHz ISM band equipment would be a better choice.
  • Regarding X10, my microwave oven interferes with the audio (mostly) on their ScanCAM product. Then again, the same microwave oven interferes with 60-66 MHz between the DSS box and the TV...
  • It is sad to see this. I've been am ham since 1989. And, I find it alarming how the consumer electronics market pays so little regard for amatuer radio operators.

    • 40-meters has a lot of commercial broadcast that makes 7.251 unusable toward evening. Also, "touch activated" lamps forced me to install a phased antenna noise canceller (MFJ makes one [])
    • Part of the 220-MHz band had already been acquired, and an attempt was made to grab 440-MHz bands for the "land mobile service".
    • And now phones and other equipment using 902-Mhz and above.

    I think it is somewhat dangerous to allow the consumer electronics market to gain inroads, even at low power. The complaints filed against hams are only likely to increase because we may overload the cheap receiver front-ends on phones, LANs, wireless video transmitters, etc. I fear that the complaint of a large sector of population, as well as the consumer electronics market, would result in them winning the battle to acquire our dwindling RF spectrum.

    Definately something to think about.

    73 de KU4ZK

  • I used to play this star trek-ish game on my school's commadore PET, (I think it had a Motorola 680x0 CPU of some kind?). The manual stated that for sound effects, you tuned an AM radio to a certain frequency, and it picked up RF from the CPU. It worked. You'd get all kinds of buzzing and whirring noises as the ship fired and maneuvered on the screen.

    If it ain't broke, fix it 'til it is!
  • It's perfectly legal if you have an amateur radio license. I could legally setup a 500 watt ATV (amateur TV) transmitter on 2.4 GHz. If it wiped out your wireless LAN, that would be your tough luck.
  • by OceanWave (192467) on Friday June 16, 2000 @04:47AM (#998505)

    Another often overlooked culprit around this frequency band is the microwave oven. These run in the neighborhood of 2.45 GHz (give or take). Even a small leakage from the RF shielding can produce a detectable signal on or about this frequency.

    Also, the the band from 2.3 GHz to 2.45 GHz [] is (and had been for quite sometime) used by amatuer radio operators. A higher powered ham tramsmitter could also be a source of interfere with this equipment. Technically, low-power consumer equipment should have been located on another band.

    Poorly designed equipment can "mix" signals on different bands and hear interference on their operating frequency, also.

  • So your argument is that just because we don't have conclusive proof that there is indeed a cause and effect relationship, we shouldn't care?

    As time goes on we are relying more and more on RF equipment. We are bathing ourselves in EM radiation. Have long term exposure studies been done?

    What about non-cancer effects? The brain is nothing more than a large, wet, electochemical device. It is suceptible to EM feilds. We have known for more than 50 years that EM feilds in the microwave range can cause psycological effects.

    So...these are low power...I understand that, but what about long term effects. We are fast aproaching a world where we will be exposed to this stuff to chronic low levels.

    I simply mean to point out that we have evidence that there may be health issues involved and that this really needs to be considered. People should be informed so that they can ecide for themselves about the possibility of risk involved.
  • 440Mhz is in the amateur radio service band. A warehouse full of lights putting out that freq would probably draw some vigilante action.
  • My brother in law was a Marine, and he operated Hawk missile batteries. They could train the illuminator on seagulls a half-mile down the beach, and knock them out of the sky, basically cooking them in their own juices in seconds.

    There are stories about burning people at range (without their knowledge), as a joke. But I think that falls under "urban legend".

    If it ain't broke, fix it 'til it is!
  • Back in '85 when I was in college cordless phones were something of a novelty, but were inexpensive enough that upper-income college student could afford them. The best part was the fact that they were all analog (43Mhz) and used the same 10 "channels".

    The across the hall from me had a scanner and a cordless handset with jacked up power. We used to LOVE to use the scanner to find cordless phones and then "hijack" their base stations to do stuff like call the dorm office (owing to the University's PBX, it had caller id) and make all kinds of obnoxious requests, knowing that if we made enough of them the guy would get busted, as well as free long distance calls, and other phone obnoxiousness.

    Pornographic and semi-pornographic conversations we heard all the time, and if we would have had a DTMF decoder and mag stripe writer we could have emptied a lot of bank accounts as bank-by-phone gives away all the info you need to make unauthorized withdrawals.

    To this day I only use digital cordless products for casual conversations, and corded products for financial transactions.
  • Well - I'm working on my 100 Watt transmitter
    for 2.4 Ghz to talk to the guy in the next town over. Hmm - maybe I'll build a spread-spectrum repeater on that band. That's the ticket - oh yeah - a amateur radio ticket ;-)

    The point of the story is - all this stuff is un-licensed (as has been pointed out before) and ANY licensed service has priority(in the US at least.) So if I DID build such a repeater, it would have priority, and all those neato devices that work at 2.4Ghz would have to accept/deal-with any interference caused. This whole story isn't news. The same thing happened to the 900Mhz band a couple years back. In my area the Richochet service pretty much trashes the entire band for any other use. Hmmm - Maybe a 900Mhz frequency hopping spread spectrum repeater...
  • While neurons carry current, using chemical electrolytes, I don't believe that you can induce a current in a neuron via EM radiation. If that were true than everytime you were subjected to EM radiation or a high magnetic field you would start shooting lightning bolts around, and I haven't seen this yet.

  • No, instead it's made out of electrolytes and ions that serve to send electrical pulses along the surface of partially conducting neurons.

    It ain't just copper that carries current.

    Anomalous: inconsistent with or deviating from what is usual, normal, or expected
  • Resonance of various frequencies with various materials is due to the length of a specific wave with material that consists of particles of the same size as the wave length. Let's say the wave length is 1 millimeter than dust with diameter of 1 millimeter will resonate with this frequency. The power of the wave also matters, since more powerful waves will hit the particles at higher speed and more waves are produced with higher power for equal time periods. 2.4GHz is a frequency and does not tell you what the wave length is, so we are missing the wave length to find out exactly whether this wave length is the same 'size' or length as a water molecule diameter. Now if a wave length of size of a water molecule is directed at some water container with frequency of 2.4GHz that water may boil in seconds because of the resonance, the wave will not go through the molecule (as would have happened if the wave was much shorter) and it will not go around the molecule (if the wave was much greater) but it will 'fit' the molecule size and will cause it to move much faster and faster moving molecules give out heat.
  • This sounds similar to every NEXTEL phone that I've ever seen -- it has the power on the bursts of digital data that can actually put a stereo or simple computer speakers into fits whenever it bursts.....


  • Cisco bought Aironet, but the terms of the agreement mean that Telxon [] are still the easiest route to go through.
    Symbol also do an 11Mbit DS system, but aren't pushing it 'cos they're waiting for the 25Mbit kit, which could be out this Autumn.

    Lucent do a good one too.


  • "There's a power limit? Damn! I wanted to stick a 300 GigaWatt transmitter on top of my house."

    The generic term for that is Hiroshima-level EMP device. Plan for replacement of the device after a single brief use.

  • There is a UK TV program called watchdog - they bitch about everything to everyone - its an ok show, there was an epp recently about some company in the UK the put up a nice big antenna on their site for all the radios and whatnot they use, its perfectly legal, has correct license and what not, BUT no one in the town can watch TV anymore because the TV signal boosters that everyone requires because the Terrestrial TV signal sux also amplifies the signals from the companys radio network, the company is trying to contact the manufactures of the booster equip so they make them just boost the TV signals, - I fought it was funny :) - would tell you the name of the place and company, but I didnt hear that bit - damn brother bitching about something!

  • 2.45Ghz to be exact.
    Although they are shielded they still produce a lot of noise (because a typical microwave can have up to 1000 Watts of output power). Most wavelan devices should besides a bandwith drop not have any real problems with it. Video transmitters do have problems with it!


  • and I'm pretty sure my cellphone isn't operating on the same wavelength as my radio and my TV.

    Actually its neither one. The wavelength of a Cellphone is closer to the microwave then radio and TV. I don't have a technical explenatition but it has indeed everything to do with interference; one signal influencing the other.

  • Bluetooth frequency hops at 1600 times a second. But from what I recall, IEEE802.11 hops at a much lower rate, just a few hundred times per second.

    I also remember that if you installed a bluetooth device and a wireless LAN card in the same PC it would kill the LAN. But then I haven't heard anything more about that problem for quite a while.

  • Could this just be a case of poor receiver design? I've seen this is VHF/UHF receivers. A strong signal will produce intermodulation distortion in the front end and wipe out any weak signals. A good receiver will have a linear front end with a wide dynamic range. This costs money, which probably means that the receivers in consumer grade equipment are a major cause of the problem.
  • by Adrian Harvey (6578) on Friday June 16, 2000 @12:47AM (#998522)
    This article gives very little detail on whats going on here. They don't even say if the author even tried adjusting the frequency settings of the interfering components. 802.11 has 11 (in the US) -non Frequency hop mode frequency settings (most of which overlap) and selecting another one might have helped a bit!

    Some background for the curious: 802.11 sends 'chirps' - the same bit is sent simultaniously on a range of frequencies, with some bits reversed. this is done to prevent interference, or blocking on one band from interfering with the transmition.

    The channels look somthing like this (in ASCII anyway) (if this looks wrong, paste it into an xterm, or notepad, or somthing with a fixed-width font)

    frequency ----->

    Note that the vertical axis doesn't represent anything, it's just used to stop everything going on top of everything else. The dots are there because slashdot slashes spaces, but leaves dots! Nor is this diagram accurate, or to scale or anything, it's just ment to give you the gist.....

    Here each ---x--- is a range of frequencies over which the bits of the chirp are spread.

    There only one set of 3 channels which don't overlap, so if you need more than 3 802.11 networks in the same place, you're our of luck.

    If you run your network in frequency hopping mode, you only transmit one bit on one frequency at a time (chirps send about 12 bits), but change frequencies often, across the whole range (no channels) This means that interference on one range will only kill some of your data. You obviously than need to retransmit failed sends (by the time a retransmit happens you will have switched to a different frequency.

    The quality of the hardware you use can also make a big difference. The better equipment uses two aerials, spaced apart, to prevent reflected signals and some other kinds of interference from silencing the signal. The idea is that if a signal and it's reflection interfere to create a minimum (no signal) at one point, there will be signal just a short distance sway.

    Most devices just ship set to a channel, and it's nearly always the same one - surprise surprise - 1! I don't know about the phones, but they would probably be similar.

    I guess no detail, or background research is about what we expect from ZDNet..... :-(
  • I'd suggest you go over to the residential zones surrounding Lopik and talk to the people there. After you did come back here and we'll talk again.
  • I'll buy that. Thanks!

    But what is to be done about this? Surely those designing the technologies realized this would happen at the time? Isn't all this spare current running around inside my electronics going to damage it?

    Most importantly -- who should I sue? :-)

  • And very limited spread. IIRC, 6 bits of real data for every 8 chips sent over the air. Consider yourself lucky if it works at all with another 2.4Ghz device.
  • That would have been a Zenith TV. The remotes (way before IR or RF remotes) used ultrasound. The remote - also called a "clicker," would strike a metal bar inside to produce a tone. The TV had a mic and some filter arrangement to detct these clicks.

    Jingling your keys would make the TV very loud and turn the picture green. Lots of fun.

  • I am a grad student at CMU and over the past few months the Multimedia Wireless Group has conducted a series of outdoor tests to quantify the interference between 802.11 DSSS and 20dB Bluetooth trancievers. The results are curiously interesting since at close distances the packets lost are max (highest SNR), at medium distances (30 yards) the interference is minimum and at far distances +80 yards) the packet loss increases afgain (falling SNR). Slashdotters are welcome to take a look at the MatLab plats and send me their comments. The data and plots are available via email (
  • by austinij (139193) on Friday June 16, 2000 @05:13AM (#998528) Homepage
    First off guys, there are some things you can do to help trim down problems you may have with interference from other 2.4 devices.

    1. If you are experiencing problems with your cordless phone, try adjusting the operating frequency of your access point. Any AP worth thier weight will allow you to choose different frequencys to operate on, all within the 2.4 GHz band. Most commonly: 2412, 2417, 2422, 2427, 2432, 2437, 2442, 2447, 2452, 2457, and 2462.

    2. Access Point placement: Make sure to place your AP in a strategic location at your home/office. Central locations work best, and make sure your orientation is correct for the kind of radio you use. A bad place to set one of these things is next to your microwave (for obvious reasons)

    3. Cordless Phone base placement. Minimize multipath transmissions by keeping your base station away from corners. Multipath transmissions from your cordless base station can and will take down your wireless network as it confuses your client radios on your other PCs due to all the 2.4 GHz traffic in the air.

    4. The new Lucent 6.0 driver for their ORINICO WaveLAN cards has a new feature called "Microwave Oven Robustness." When this feature is enabled it prevents the radio from falling back to less than 2 MBit/sec when it thinks it is in poor coverage. This should only be enable in environments that will not experience fringe/poor coverage, however it should help your problems with interference if it is enabled in a good coverage area.

    Numbers two and three are probably the most vital in getting multiple 2.4GHz devices to co-exist, so try many different placements! Try not to get discouraged, it does work!

    -- Ian

  • Time for me to dig out the old 27Mhz phones... Nobody's on that freq anymore - they all dem newfangled jigahurts stuff. And I won't nuke my brain!

    My remote control cars ran on 27 and 45 Mhz - played hell with the phone sometimes... now I just get this popping noise in my 900Mhz phone... only sometimes, but when it does it's about a pop every three or four seconds. Rather annoying.
  • I am a grad student at CMU and over the past few months the Multimedia Wireless Group has conducted a series of outdoor tests to quantify the interference between 802.11 DSSS and 20dB Bluetooth trancievers. The results are curiously interesting since at close distances the packets lost are max (highest SNR), at medium distances (30 yards) the interference is minimum and at far distances +80 yards) the packet loss increases afgain (falling SNR). Slashdotters are welcome to take a look at the MatLab plats and send me their comments. The data and plots are available via email (
  • Read meat doesn't cause brain tumors... I'd like a pointer to any valid studies that you know about that actually show that read meat causes brain tumors.
  • This reminds me of RC car days, when you carried 3+ crystal sets on you so you could change freqs that your servo/transmitter used. If someone else had your same freq at a race, and neither of you had spare cyrstals, SOL to one of you..

    Same thing here, pretty much. A (semi-)open band, and people are gonna use it for pretty much everything they can. History repeats itself as always, go figure. Wish I knew more regs on this so I could say how things are supposed to be.

    bash: ispell: command not found
  • I saw a while back on Ars-Technica that some pagers and cell phones share the same freq as the master oscilator in AMD boards. When the pager went off, the computer would die! I think the guy fixed it by building a grounded box around the chip.

  • My microwave zaps my 2.4Ghz Phone, only thing that does it. But because it's DSS, then it only pops in and out a little, when it's frequency hopping happens to land on the one of the noisy freqs of the mircowave.

    Now, in theory, a large number of DSS devices can share the same range of freqs because they spread out and "randomly" hop from carrier to carrier, supposedly fast enough not to cause an issue. It's a bit more complex than that, as the bits are encoded, then a DSP does some mangling to broadcast some info on one carrier, and other info on another carrier...

    However, if you've got an analog 2.4Ghz phone, your screwed, or even a digital, if it's narrow-band and not DSS. Narrow-band will work if it can select a "channel".

    Company I work for does interesting stuff with power-line communications using both DSS, and dual-channel Narrow-band communications. Both types of communication beat the hell out of anything else I've seen for reliability, but they have different strengths.

    If you're interested: http://www. ations.htm [] contains presentations about this kind of stuff. The technology update presentation [] contains information about the different strengths of DSS and narrow-band info. It covers power-line communications, but the same applies to air-waves, just that there is much, much less distortion []... Which is why DSS works so well over the air.

  • My point excactly. How will these radiation affect our nevral nets, how will it affect the inside of our nevrons? Of course we have a much higher tolerance of fault in our brain than electronic equipment, but that doesn't mean we're invulnerable.

    But, anyhow, if we just ignore the problem it will probably go away huh? :-)

    - Steeltoe
  • In practice you are right, but an em field is distorted by a receiver. For example try to use a satelite dish behind another one, it won't receive much unless you move further back were the distortion is less.

    You can't have unlimited receivers, but since each is only getting a very small piece of the available energy it looks unlimited. Unlimited power is also impossible because of some stupid law that says that the total energy is constant. (you can't get more out than you put in)


  • Well, that prolly means that I'd have problems at college with suitemates having interfering devices. And I was so hoping to have that wireless ethernet surfing the net out in the courtyard...
  • First, I'm not a raido expert at all, but has anyone done real testing to see how each of these devices are causing each other problems. 2.4ghz has been un-lic for a while without problems, (yes it has gotten a lot more popular). I think some "real" testing should be conducted. I'd hate to see wireless devices take even longer to implement ...
  • Ok, let me rephrase. MY phone that operates at that frequency has no encryption, and experiences interference
  • ... the band range the licenses for which the German government is about to auction off with an expected profit of $50,000,000,000 ?
  • It's the cancer-causing meglawaves emitting from the phone interferring with the noted devices.
  • It probably would help if the various devices used frequency-hopping spread-spectrum techniques. The interference would happen randomly, but not everything would be lost. As the article says, it gets "fuzzy" as the noise level increases.

    Notice he was using a video transmitter, which uses a lot of the frequency spectrum. It's probably simply covering most of the band continuously.

    The wireless LAN is most likely to use frequency-hopping and retransmission, so it is most likely to be the least affected. But it's also possible that the engineers just have it sit on a single frequency because that's cheaper; we don't have enough information.

    The telephone is only a voice device and should be using the least of the radio spectrum. But it's probably a simple device which is more prone to interference than if it were using FH SS digital. Although perhaps it was using FH, as the article mentioned it would "fritz out" rather than that the audio got fuzzy, and it also mentioned that it will "interfere with anything".

    Perhaps the phone would interfere less than the video -- but he disconnected the phone and didn't test the combinations, much less do an RF spectrum analysis of what was going on. For all we know, the school two blocks away might have recently installed a video microwave beam running over his house.

  • Ofcourse they are not licensed, the whole idea behind this frequency band is that you don't need to have a license as long as you are using approaved low power equipment.


  • Heh - someone noticed :-)

    It's even funnier with cheese.
  • There's a power limit? Damn! I wanted to stick a 300 GigaWatt transmitter on top of my house.
  • by Anonymous Coward .html

    leads directly to the interesting sentence:

    The frequency used in microwave ovens (2,450,000,000 cycles per second or 2.45 GHz) is a sensible but not unique choice. Waves of that frequency penetrate well into foods of reasonable size so that the heating is relatively uniform throughout the foods. Since leakage from these ovens makes the radio spectrum near 2.45 GHz unusable for communications, the frequency was chosen in part because it would not interfere with existing communication systems.

    So GSM bandwidths (900, 1800 and 1900 MHz) used with 2W emitters are already suspected of being able to generate a brain cancer or whatever unproven or undemonstrated organic disorder, ok ok ok

    Now just tell me why o why known nocive frequencies (2.4+GHz) are to be used domestically, for whatever purpose, even at 150mW?

    This looks to me like a major health problem in the feature, especially as this is to be taken at least as a little dose effect. Whenever some geeks around will assemble and put up such devices around their heads (very cooooool wearable networked computers) I cannot guarantee they won't be cooked at all. I'm looking forward to get some litterature regarding possible brain damages incured by weird/inapropriate consumer usage.

    This case is simply interesting and deserves some follow up on the health point of view. I'll wait another 2-3 years prior trying them out.

  • You're right. Note also that direct sequence 'chirps' and Frequency hopping *shouldn't* interfere, as the FH signal should only be able to kill one bit of the chirp at a time... leaving the other 11(?) or so to get through intact.
  • You are right about radio and tv equipment being disturbed, but trains moving on there own is bullshit.


  • by cshotton (46965) on Friday June 16, 2000 @01:15AM (#998549) Homepage
    I can definitely confirm the problems referenced in the article. I had almost the same set-up, with a Sony 2.4 gHz phone, a BreezeCom wireless LAN, and the X10 Video Sender.

    The short answer is that the X10 Video Sender is a piece of ca-ca and was the source of all of the problems. The other 2 devices do frequency hopping and spread spectrum transmissions to avoid (and compensate for) interference. The cheesy X10 device just blasts away on a fixed frequency with a very low quality transmitter that spills all over adjacent frequencies.

    The best answer I found was to stick to 900 mHz phones and run a wire for video. I boxed up the Video Sender and gave it to my Dad. It was just a bad idea all around.

  • Digital phones send bursts of high power signal so audio electronics picks it up a lot easier.

    It's important to note that this is actually limited to digital phones based on TDMA (like GSM and IS-136), where the transmitter is switched on and off rapidly several times a second, since there are many phones (8 for GSM, 3 for IS-136) on the same channel, time multiplexed. Each phone is allocated a timeslot during which it transmits, and remains silent the rest of the time. CDMA based digital phones don't exhibit the same behavior... Put a CDMA phone next to a speaker and you won't get any interference, because the transmition is continuous (and it's also spread over a wider channel, this might have something to do with it too).

  • There's been some problems with France having to suddenly free up this freqency in order to comply with EU regs, as it was used by some (?)military kit.

    Also in the US some old Life Support system used on 2.4Ghz for something. They were supposed to be phased out before y2k hit, but they weren't and test transmissions killed them (not to mention the poor humans attached to them at the time!).
  • Notice he was using a video transmitter, which uses a lot of the frequency spectrum. It's probably simply covering most of the band continuously.

    Indeed it was. I had the exact same video transmitter he did -- the X10 one -- and was very happy with it until I set up a Proxim Symphony wireless network. Symphony frequency-hops and several of the frequencies it hopped through were used by the X10 transmitter. So every few seconds, fft-fft-fft-fft-fft went the TV audio as a bar of noise walked down the screen. The wireless network didn't seem to be affected by the A/V transmitter, though.

    I ended up hard-wiring the A/V connection, which was more fun anyway (and a good excuse to upgrade a component or two) but it certainly made me think harder about 2.4GHz as a "general" wireless solution.

  • How did you get a standard Aviator 2.4 card to talk between Windows and Linux? I had translation/encapsulation issues that could only be resolved by going to the better firmware of the Aviator Pro's.

    By the way, Aviator tech support was some of the nicest people I've ever dealt with to get this resolved.

    Tim Gaastra
  • That's right - those devices aren't licensed, but I am! My ham radio ticket has spectrum available in the 2.4Ghz range...and I can push a 100Watts no problem LEGALLY...hehehehehe..

    Not that I'm going to bother..but it's the thought that counts.
  • by 4x4camper (166687)
    Now the digits are having to learn about the black world of RF. go to the the web site and brouse through the many books on RF including the do a search on RFI and some articles come up regarding FCC regulations and current interference laws. One comment, doing wireless communications will not be as easy as many think. Just dust off that old CB radio (if you have one) and just listen in. There is so much interference going on you cannot understand anyone. If you have access to a spectrum analyzer that you can connect an antenna to you will be able to see all the RF signals being transmitted in the band of interest. Maybe now the schools will start to teach RF engineering again.
  • If I'm right, microwaves, since they are a form of electromagnetic radiation (EMR), travel at the speed of light (through air), c. Thus, to find the wavelength, you just take the velocity and divide by frequency (c/f). My point is that the frequency DOES tell you the wavelength.
  • There IS an alternative. FHSS systems are not plagued by these problems.

    802.11 is more than the 11Mbps radios.

    I say, "Hey! direct sequence has colocation defecits", and they say "but it's cheap!", and now everyone complains...

    DSSS systems are flawed as they are not capable of coexisiting with more than one or two other devices. ISPs are rolling out service based on DSSS and when Mabel lights up her cordless phone, it shuts down the service... and I say, "Hey! direct sequence has colocation defecits", and they say "but it's cheap!", and now everyone complains...

    Cheap is good, but if it fails to function, it is wasted money.
  • by Glamatron (158833)
    Telephone broken?
    Waves lie heavy in the air.
    Where's the damn tin-foil?
  • Tis true, and sorry that I obviously did not make myself more clear.

    As you say, the problem is not what frequency it is on. The problem is that everything seems to be moving up here.

    However, it does not matter whether it is spread spectrum or not. Yes, that is going to help, but it is not the total solution, either.

    First, there are a limited number of frequencies there at 2.4 that can be used. Add to this that not all devices work on each of these frequencies. They may use 1, 2, 5, 10, or 20 of the frequencies. Next, even if they are frequency hopping there is a limit to the number of things that can be on a given frequency at the same time.

    You mentioned that some devices don't frequency hop. True. So, if a device is in use and it does no hopping, that frequency is pretty much useless for devices that do hop.

    We are seeing so many things using RF nowadays. And we are going to see many many more. Hell, I love wireless things. But I also realize that the more things I own, the more the possibility that they are going to start colliding. That is why I try to spread out between the different bands.

  • You don't need a license for the 2.4Ghz ISM band.
    This is the band that your microwave oven works in (though that is supposed to be shielded)

    Really, it's only licensed for spread spectrum use, either FHSS or DSSS.

    And yes, it is possible for the two to smash each other. fhss devicess will tend to not smash each other.. and dsss won't either, as long as power levels are reasonable, but if you mix them.......
  • You just described more or less direct-sequence spread spectrum.
    802.11 can also, Ibelieve, work over frequency-hopping spread spectrum, which works differently.
  • Boosting your own power would just make the problem worse for everyone else.

    The article only dealt with interference between the gadgets of one person. What would have happened if the author had a neighbor who also had a bunch of wireless stuff?

    This is really disheartening about the near-future of wireless. If a few wireless things can't co-exist in one person's house, then cities are doomed. So wireless will be great, as long as you are the first one with it and only until your neighbors get it too.
  • Also, the the band from 2.3 GHz to 2.45 GHz is (and had been for quite sometime) used by amatuer radio operators. A higher powered ham tramsmitter could also be a source of interfere with this equipment. Technically, low-power consumer equipment should have been located on another band.

    Yeah, guess who has priority there, too? The hams. Guess who has priority on 900MHz? The hams.
    On these bands, hams are perfectly allowed to interfere with consumer electronics, and said electronics may not interfere with amateur radio operations.

    In practice, however, it's damned hard to track down the guy who's screwing up your radio, so generally hams have to vacate the band once the consumers move in. Seeing as nobody would be using microwaves if it weren't for ham experimentation, it's sad that we have to leave.

    Oh well, ever upward, to other useless pieces of spectrum, which we'll discover a use for and then get kicked off again. Maybe we'll get some HF bands back in the future

    73 de N9RUJ

  • Hmm. It it's so bad that stuff like stoves can go crazy, what happens to people with bionic ears and pacemakers?

  • I know that cellphones broadcast unusually powerful signals in the UK (3 or 5 times greater than the rest of Europe)

    Unless you have a pre-GSM analog phone, which I know nothing about, then no, cellular phones in the UK don't transmit a 3-5 times the power used in the rest of Europe. Handheld units for GSM 900 output up to 2 watts, and GSM 1800 up to 1 watt. In reality the power is usually less than that, and it's determined by the network based on the strength of your signal when it's received at the tower (ie it's going to be really low if you're really close to the tower, and much higher if you're far from the tower and/or there is something blocking your signal).

  • This is very interesting.

    Now I know the power output is nothing like that of a microwave oven but, it does make you think. There have been links made between putting cell phones next to your head and brain tumors...

    Beyond that... it is well documented that electromagnetic waves in the microwave range can cause psycological effects in humans (in fact, I believe this was one of the many things that certain government agencies did some research in to see if they could use it to modify behavior)

    I have to wonder what the long term effects of "imersing" yourself (for lack of a better term) in a "bath" of low power microwave rane EM radiation is.

    Interestingly, health risks are exactly the objection that a co-worker of mine cited when we talked about deploying wireless ethernet in some places around campus...his background? Well he is a HAM radio operator and said "I know the risks and I voluntarily use the equipment with that knowledge. What about people who just happen to be nearby and don't even know its there - they don't get a choice" (or some similar sting of words to that effect...was a few weeks ago)

    Food for thought.
  • Back in the 70's some friends of the family had a remote control TV. Their little boy had a fire engine toy that rang a bell when you push it along the floor. Everytime the bell rang in the same room as the TV, the TV changed channels!
    You can see why ultrasonic remotes went the way of the dodo.


    Freedom is Slavery! Ignorance is Strength! Monopolies offer Choice!
  • Most countries in the world limit the 2.4GHz band - although it is unlicensed - by power output. The US gets 1 Watt max, UK and Europe get 0.1 Watts (France keeps changing its regs) and the Middle East still hasn't fully complied:)


  • Unfortunately, the 802.11 Frequency Hopping standard does not allow the device to learn which parts of the frequency range are interfered with. It would be nice, but it just can't happen, as it would screw up so many other parts of the standard.

    Sorry to disappoint


  • Yes but you dont need to induce the power into the enginge. Mess up the controls and you could get a freghttrain to run amok. It has been done with planes ( cellphones, you know the rest ) Perhaps you should enlighten us? How exactly _did_ such a low-power radio such as a cell phone mess up the _controls_ of an aircraft? No, you are horribly and woefully misinformed. (or you completely lack understanding in this area, in which case you shouldn't say anything) What happens with cell phones and aircraft has nothing to do with the controls, but the navigational equipment that recieves radio signals. Since these devices are likely to overlap in the frequencies they use, it's not such a huge leap of logic as to why such navigational equipment might not be very accurate under these circumstances. It's also worth noting that freight trains do not need radio navigation equipment to find their way. You shouldent underestimate the strangeness of things that can happen when dealing with HFEM radiation. Riiiiiiight. Seen any funny lights in the sky lately?
  • And on most wireless phones and LANs there's only ONE preset frequency, and since it's neatly set at EXACTLY 2.4 GHz, ofcourse they'll interfere...

    I don't know about wireless phones, but wireless LANs (at least 802.11b) usually have the option of 11 frequencies in the 2.4GHz band in the US. Other areas of the world have different restrictions. Usually a 5 channel separation will eliminate interference from other 803.11b devices, so if you are using channel 6 and your neighbor is using channel 1 and your other neighbor is using channel 11 their shouldn't be a problem.

  • This generally shouldn't become an issue, as the 802.11 standard allows for intelligent loadsharing (assuming rf coverage by more than one AP).
    Certainly, we like to overlap quite heavily in industrial areas - continuous coverage by 3 AP's for any mobile device pretty much guarantees robustness (for hw fail or congestion) - of course using Voice over IP over 802.11 rf does mean we need nice fast routers in there as well:)


  • Well, the phones have a whole stack of problems in their own right. First, any idiot can tune in to them (no encryption). Second, they interfere with each other a lot. You're not likely to notice the second, unless you live in between a college dorm and a college sorority, like I do (thank you God). At any rate, I'll keep the 802.11, and I'll keep the phone, since nobody really cares what I am getting on my pizza.
  • Man, first its kids getting all hopped up on "Placebo", now this "Wireless" stuff!? When will the madness end?!
  • I have a rack of machines with FM tuner cards that recieve and stream FM radio for local radio stations on the net. The things are all hooked up to a cybex KVM switch (AutoView 200) with a "longview" extension that runs the video, kbd, mouse, etc over a piece of Cat5 200 feet away.

    For some bizzare reason, the Longview box spews all over 107.9MHz making it impossible to recieve that radio station inside of the office (I had to go with a roof-mounted antenna)... The other funny problem with 107.9MHz was that whenever I tuned one of the radio cards in the rack to the station, one of the computers would crash! I taped a piece of lead foil around the chassis. I think it was screwing up the SDRAM (whose oscillator was probably a little bit funky and going at 107.9MHz!)

  • Yes, I realised after posting that I had not actually used the words "direct sequence" in the description. Doh!

    I did, however mention frequency hopping, and describe it briefly.
  • Actually all frequencies can interfere and resonate with your various body parts. It has being established that in experiments with rats and mice frequencies on which US cell phones operate cause the rodents to develop disorientation and worsens their further learning abilities. Of-course rats and mice are not people but in some respect mice are the closest human cousines, they have the DNA closest to the humans (after the primates of-course). So it's not only your wireless network that is suffering, it could well be your wireless brain too.

    'nuf said.
  • by Lion-O (81320) on Thursday June 15, 2000 @11:51PM (#998626)
    And I'm not just talking about one device influencing the other here. I'm also very concerned where human health is concerned. There are so many different frequencies being used and we hardly know anything about the real effect it has. And to be honost; this article is only the top of the iceberg.

    I'm from Holland and like in every country we also got out television and radio stations, next to a line up of GSM networks. All of these have transmitters. The GSM's have small antenna's which are spreak among the country but the television and radio have one big antenna which allmost covers the entire country. And here the fun part begins.... People living there are having extremely difficult times in buying electrical equipment. Why? Because it hardly works and or acts extremely funny. And I'm not talking about weirdness like we all know from Windows. No; this is serious stuff. Like electronic stoves going crazy (hot / cold), microwaves which act crazy or not at all for no reason what so ever, electric trains which run out of their own; a copper wire is more then efficient. Things are so bad that most people just can't use any electrical devices such as computers; they don't work as it should. Things are so extreme that local re-sellers are refusing to sell these people electrical equipment since they keep claiming due to problems.

    So basicly this article doesn't come as a surprise to me. There is more going on then people know, and all the radio waves out there are doing something. IMHO even more then most people realize.

  • I chose an ad hoc configuration, so there was no problem; the non-pro version of the driver should work to connect windows to Linux (in fact I'm using the standard aviator driver on one of my home machines, and the raytheon version of the pro driver -- for the same card. The only difference is the pro driver supports communication to access points used to bridge the wireless network to the ethernet. Instead, I created a separate subnet and use the Linux box as a router. You could, i guess, bridge if you wanted to set it up in your Linux kernel. I haven't tried it.
  • There have been some past articles in slashdot about adapting Apple AirPorts (try a search) to a parabolic dish for extended distances. The aviators cards cannot be so adapted because their antenna is integrated.

    I'm not sure, but think you may be using the term "bridging" somewhat loosely. If by "bridging", you mean to maintain a single broadcast domain but to segregate out unicast packets, I believe it can be done by configuring your kernels for bridging. However, I don't see much point in cluttering the airways with broadcast packets.

    If you want to connect two networks, I would have a Linux box on each end acting as a router. The aerial link becomes another subnet. I wouldn't try running the laptops off the same subnet; I'd get an additional card on each end to handle the local subnet (because you replaced the antenna on the link card with a directional one).
  • 2.4 Ghz , in respect to 802.11b and such, is actually a band from 2.4 to 2.49 or something.. I forget exactly.
    Microwave ovens operate right in the middle of this.. and there is a reason....
    that's why ISM is the Industry, Science, and Medical band..... and the reason it is unlicensed is mainly because of this.
    2.4 Ghz has all kinds of industrial uses, so it's 'dirty'.

    So.. first off, your microwave oven is 100% shielded basically... there should be absolutely no leakage.. (not that it would be a big deal if there was a bit).

    I can operate a 2.4 Ghz DSSS router right next to 2 microwave ovens and it continues to pump out 11Mbps.
  • by frog51 (51816) on Thursday June 15, 2000 @11:56PM (#998635) Homepage Journal
    I install 802.11 networks by Symbol, Lucent and Telxon (Aironet/Cisco) and this is something I come into contact with more and more.
    Frequency Hopping (FH) devices tend to kill the reception by Direct Sequence (DS) devices, mainly due to the differences in signal strength. Multiple DS networks can happily coexist, and run at 1,2,5,11 or 25 MBit/s while keeping the actual signal at below ambient noise strength (nice - security-wise)
    FH networks just tend to upset all other 802.11 networks, and they only go up to 2 Mbit/s at the moment. The reason people use them is that they are very stable and solid. They just work, without tweaking!

    With todays bandwidth demands, you have to go for the 25 Mbit/s gear (which gives you throughput roughly equivalent to a 40Mbit/s ethernet type protocol - due to use of CSMA/CA not CSMA/CD) so things should get better as more people use DS not FH:)


  • I'd be interested to know if anybody can figure out why my cellphone causes extremely loud, audio interference on a variety of appliances -- my TV, my radio, and even my laptop speakers. I live in the UK, and I'm pretty sure my cellphone isn't operating on the same wavelength as my radio and my TV. I know that cellphones broadcast unusually powerful signals in the UK (3 or 5 times greater than the rest of Europe) -- could the sheer strength of the signal be causing resonance with all these speakers? The signal is a very strange, repetitive clacking noise that sounds like it's searching -- it gets louder, then my cellphone registers a call or a message, then it fades away again.

    I'm really mystified by the cause, I'd appreciate anybody who know what the 'cause is because I really do worry about my brain getting fried by these things.

  • by Sethb (9355)
    When I lived in the dorms at Iowa State University, I used to pick up people on my 900mhz phone all the time. One night my friends and I listend to a girl who lived 3 floors below me talk to her boyfriend at Purdue for 2 hours. We could hear both of them, but they couldn't hear us for some reason. It was rather funny, he kept asking her if she was masturbating because she missed him, because he was, "because he missed her so much".

    It made me wary of ever saying anything too personal over my cordless phone! Are there any encrypted consumer phones available?
  • Actually, the DS receiver design is pretty good - DS uses a large chipping algorithm which effectively smears the signal from a high, narrow-band to a low, wide-band. A receiver using the wrong chipping algorithm just further smears the signal, but one on the same algorithm rebuilds the signal back to a peak and flattens all noise/extraneous signals - as best it can.
    What seems to be happening is that the FH transmissions are just far too high a noise level for the DS receiver to cope with!

    In the UK the signal strengths are so low (100mW) that this is very localised - adjacent buildings act as individual cells, but the US has more of a problem as they are allowed 1W output power.
    Don't fry your brain cells, guys!


  • by Kishar (83244)
    Well, he gives us the answer in his article.
    In the 802.11b (the b is important) standard, he's using DSSS (Direct Sequence Spread Spectrum) radios as opposed to frequency hopping. This means he's able to get up to 11Mbps per device, but, it also means only 3 non-overlapping channels. While there are many channels (11 or 12, IIRC) in the 2.4GHz range, the DSSS 11Mbps radios lump multiple channels together (802.11 radios were 2Mbps because they only used one channel instead of groups of channels). Now, toss the Seimens Gigaset (which is also Spread Spectrum, I believe) and other wireless devices into the mix and ... well, one can understand why there's just not enough of the bandwidth left. Just like with the 900MHz range, 2.4GHz is in the ISM band (Industry, Science, and Medical), the two options at this point are: change the 802.11b devices to 5.7GHz devices, or stick with 900MHz for the appliances to free up the 2.4 for the NICs.

    Just my thoughts.
  • Speaking of making light bulbs glow, we used to have this problem in our cars when CB radio was popular among my peer group. The car of a friend would suddenly die when keying down the microphone. Mine had lights that weren't even connected to the battery anymore, such as what went to old fog lights, would light up when keying down.

    OK, so I had a 150 watt amplifier. It also caused problems with phones and television in the vicinity, but it was nothing compared to the problems a friend of mine who had a 500 watt Tram amplifier connected to a beam antenna. He demonstrated how his radio got into cable television broadcasts.

    So those were a bit more than one watt, but we had a crazy idea of making our own 10GHz 1KW coffee heater installed in our front bumpers. Most people who have radar detectors would know the other use of that one.
  • by sde1000 (10806) <> on Friday June 16, 2000 @04:28AM (#998658) Homepage

    Well, the phones have a whole stack of problems in their own right. First, any idiot can tune in to them (no encryption). Second, they interfere with each other a lot.

    Not true, at least for DECT phones like the Siemens phone mentioned in the article. First of all, DECT is a frequency-agile standard (the handset continuously monitors the signal from the base station, and can initiate a handoff to another frequency/time slot or basestation if the quality drops too low). Second, the DECT standard defines authentication and encryption algorithms which are supposed to be supported by all DECT-compatible equipment.

    Unfortunately, like all other ETSI encryption standards these are not publicly available, and as far as I know there has been no effort put into cryptanalysing them. If they are anything like other ETSI-sourced algorithms (for example those in use in the GSM system) they are probably full of holes.

    Interestingly, Siemens claim to have implemented their own encryption algorithm in their GigaSet range of products, but since they don't publish the details they have exactly the same problem...

  • For home use, you can save major dough by skipping the access point and using peer to peer ("ad hoc"). Cost is about $80 per node for 2MB access using the webGear aviator 2.4Ghz card, including two ISA PCMCIA adapters. The webgear aviator card is a relabelled raytheon raylink card, so you can get the "professional" features (mainly the ability to access an access point) by downloading the raytheon drivers for Windows.

    The raylink drivers are also included in the all the recent Linux PCMCIA subsystems, as well as the Lucent WaveLan cards (much faster, shorter range); don't download any "drivers" from the WebGear site, they're stale. Under Linux, the card works like a charm, far better than under windows; I can pop the card in or out as necessary without my system hiccupping, whereas Windows tends to hang.

    In any case, I can roam around the house or out into the backyard with no problem. I used it to share an Internet connection with our kitchen computer because for $160 bucks for a pair, it was well worth avoiding the hassle of pulling cable, and 2Mb/sec is plenty fast.

  • by Anonymous Coward
    Ofcourse this is expected to cause problems... What's concerning me is, as the article proposes, that the manufacturers do not seem concerned about this. In the old days when You bought an open fequency "commodity" (anything really), You could usually "tune" the set to a slightly lower or higher fequency (as You changed Your crystals). This option seems to be gone from most equipment You buy today. I tested a Wireless Video/Audio transmitter set, and it only had TWO settings for frequencies ! Suppose both my neighbors had the same set ? And on most wireless phones and LANs there's only ONE preset frequency, and since it's neatly set at EXACTLY 2.4 GHz, ofcourse they'll interfere... So what are we supposed to do ? Shield the transmitter and reciever in a cage ? Might as well go back to wires then.
    No this is DEFINATELY up to the manufacturers to work out. if they want to sell their crap to us. They better make sure it works.
  • by Jon Peterson (1443) <> on Friday June 16, 2000 @12:10AM (#998662) Homepage
    If you have any combination of cordless phones, wireless ethernet, wireless video, or Bluetooth you could be having problems. Not only will your bank balance will be suffering a from the debhilitating effects of continuous expenditure on unecessary geeky networking technologies, but your health will be in sever danger.

    An article to be published in the Lancet later this month will show how people can suffer serious side effects from replacing all the cables in their house.

    "It started happening after I went to one of those underground Linux install parties" reports a young man, who we'll call 'Alf'.

    "At first, it was just phones. You know, people passing around some Nokia's and Ericssons, and it felt really good to be cordless. It was like I was with the in-crowd."

    "After a few weeks though, people started getting out the infra-red enabled PDAs out. I didn't think anything of it at the time."

    But, as the report shows, cordlessness is an unpleasant and addictive activity, and it's only a matter of time before the serious health implications start. 'Ben' has been in re-hab for three months now, getting used to staying in the same place when he talks on the phone, and being re-trained in Cat5 cabling.

    "I can't remember much towards the end" says Ben, "I was really out of it. There was like about 4 of us in this house in Shoreditch, you know with serious 802.11b right through. It was like a permanent trip. We used to have these wild parties at weekends with loads of girls and booze, it was pretty wild, people doing it with like Psion5's and i-mode phones, really f**cked up stuff."

    But although Ben is recovering, it's a growing problem thoughout London and the whole of the West. Dissatisfied with their parents' strict ideas of free love, home grown dope, and long skirts, the young generation are turning to hardcore wireless technologies, with street names such as Bluetooth, WAP and i-mode.

    Next: The Goverment launches "War on Wireless" to stop this disturbing trend in our young people.
  • by skion_filrod (201359) on Friday June 16, 2000 @12:12AM (#998663)
    This is interesting: although the 2.4GHz band is "unlicensed" it doesn't mean there is no regulation. For example, in Europe all 2.4 GHz equipment has to fulfill certain regulations derived by ETSI (European Telecomunications Standards Institute):

    • ETS 300 328
    • ETS 300 826
    The later standard is used for Bluetooth applications; from what I understand all equipment must abide to ETS 3090 328.

    The fact that the Siemens Gigaset and X10 are noisy could be that the ETSI standards actually allows them to do prett much what they want to; it could also be the case of bad design of the X10 or Gigaset equipment. I have seen plenty of cases where equipment from wellknown manufacturers claims to be approved according to CE emissions standards (EN55022/23), but when measured up proves to be way of.

    Regarding Bluetooth, it is my understanding (after working with it for one year from a hardware designer perspective) that BT is designed to work in "noisy" environments. BTs frequency jumping scheme is designed to make the most of the frequency band, even if there are cordless phones and wireless LANs using the spectrum also.
    Also, BT is a low power technique in contrast to IEE802.11 and possibly the cordless telephones.
    I have followed some threads regarding possible interference between IEEE802.11 and BT, and the latest information is that they do not interfere and thus can coexist.

    Standard disclaimer: I may be wrong :-)

  • by adolf (21054) <> on Friday June 16, 2000 @12:22AM (#998664) Journal
    The author of the linked article is obviously inadept at grasping the reality of what he was witnessing.

    The 802.11 cards and Siemens phone system are frequency-hopping. By switching frequencies often, they reduce overall interference at the expense of a little bandwidth (there's plenty of room at 2.4GHz for these things to co-exist). Some types of frequency-hopping "spread spectrum" devices will dynamically learn trouble-spots and avoid them, bringing bandwidth back up to a point approaching ideal (unless that entire block of spectrum is completely hosed).

    So, the phone system and wirelss LAN should work fine together. There will be a slight (measurable, but imperceptable) decrease in bandwidth for the LAN while phones are in use. The phones, if they're poorly designed and/or the CODEC is intolerant of errors, may suffer an occasional (and very brief) dropouts; due to the real-time streamed nature of the device, retransmissions aren't possible as they are with 802.11. I don't suspect these dropouts would be overly bothersome, or even noticable in most instances.

    Interestingly, the X10 video-sender box was the last thing he threw away. Oddly enough, that's the device which should have gone away *first*. It's cheap - too cheap to use any of the present-day bandwidth-reducing digital coolness of most other 2.4GHz devices. So, it spews forth broadband analog video - likely using *more* bandwidth than a TV station to avoid expensive modulation/demodulation parts - destroying the 2.4GHz for the rest of the household toys. Remember the remark above about the spectrum being completely hosed? This is probably a better example of an RF monster than anything else available to a consumer today.

    Had he turned off the bargain-bin X10 stuff first, I strongly suspect he would have had no further difficulty (and would continue to enjoy the hideously-cool phone system).

    That all said, I really don't see the need for moving to 2.4GHZ for *everything*. It offers more bandwidth for a given slice of spectrum, which is nice - and really not needed for things like telephones. I prefer to get my cancer from tobacco, standing too close to the microwave, and hanging out by 600,000 volt transmission lines - not talking on the phone.
  • I recently bought a siemens gigaset, however I have this terrible problem with a loud oscilating ring and echo noise. I was wondering what kind of interference could create an _echo_? I thought it was rather strange... I should try to turn off our microwave see if that helps.. but its built into the house so that's easier said then done.. it means flipping the circuit breaker for 1/2 the kitchen...
  • These problems are no different than when you go to Radio Shack and buy some of their radio controlled toys. The toys are plainly marked what frequency they run on. If you get two cars that are on the same frequency and try to run them at the same time, of course they are gonna go wacky!

    We are going to see more and more devices up at 2.4 GHz. What is the solution? Quit it! It is that simple.

    These same problems everyone is gritching about is how it was with the early 49 MHz phones. Someone would get one then their neighbor would get one and they would inevitably take them back to the store because "they were broken". Well, sorry, but they are not broken, it is just that someone else is on that frequency also.

    Look around your house and check out how many wireless devices you have. Do you need 3 cordless phones? Do you need wireless TV transmitters and ethernet cards too? Yeah, they are nice, but when you start getting interference between them, well, I hate to say it, but there is only one person you can blame for it.

  • I bought a Siemens Gigaset too, and I had problems with rings that wouldn't stop and random changes in voice volume. Tapping on the case affected results. Sent the thing back. I think they have a manufacturing quality problem.
  • by Beta (31442) on Friday June 16, 2000 @12:29AM (#998687) Homepage
    The radio signals induce currents in the wires inside your TV/radio/whatever. The currents are rectified by any active semiconductor (transistor, ic, whatever) and the resulting signal has frequencies all over the spectrum. So your nice 900MHz GSM signal gets transferred to audio spectrum. This "artifact" is used in primitve AM receivers to convert the filtered (just the channel you're listening to) high frequency signal to an audio frequency signal.

    The reason older analog cellular phones don't do this is that they send a continuous but relatively low power signal. Digital phones send bursts of high power signal so audio electronics picks it up a lot easier. The average power over time is about same for analog and digital phones tho.

Work is the crab grass in the lawn of life. -- Schulz