AM Frequency Hinders ADSL Capacity

hajmola writes "a recent study has shown that AM radio may be causing problems for ADSL. According to this story at Network World, interference from AM stations can slash high-speed bandwidth by 40% on approximately 15% of ADSL connections. While AM interferes with download speeds, it does not affect upload speeds. AM frequency only affects ADSL and its subsets (not SDSL), including rate adaptive DSL and G.lite. "

Comments

The upload link is still good, so why worry?

[]Ace[]

When you start with something like 1.5 Mbit/s download, a 40% drop in speed should not make service infeasible. It is a good thing that the upload speed is not affect as it begins lower(?)

I wonder how this will affect AOL's strategy to push the ADSL technology as opposed to bust into the closed cable rings.

Please visit FreeDonation.com - You can donate Food and Medicine for FREE to Save Children (You can donate once EVERY DAY). The donation is fully paid by corporate sponsors with the money they would have spent anyway on advertising. There is no charge to you.

Now really.....

[OnyxRaven]

People are finally figuring out they should have their phonelines SHIELDED! what a concept! :-)
(I've got my phones running through CAT5 now and have never had better connect speeds)

I guess the problem now is what to get rid of - AM radio or ADSL? let's see - AM Radio... gives us - talk radio and spanish stations over really bad mono audio. ADSL gives us nice bandwidth, neat lookin router thingies (I like the little cisco/netspeed router thingy), and more bandwidth. :-)

Re:Now really.....

[paxil]

let's see - AM Radio... gives us - talk radio and spanish stations over really bad mono audio.

AM Radio also allows people who are not in line of sight of an FM transmitter to receive radio. Think of the worker who put the lettuce in your salad tonite; it may be his only contact with the world outside of the salinas valley. (think NPR--only in Spanish)

Be careful what you choose to trivialize.

PS yes, I know the issue is carrier frequency, not modulation scheme.

Well this bites

[Abstruse]

So thanks to the guy giving play-by-play of the local junior high football game in panish on AM560, I lose somewhere near 2MBps? I'd sacrifice the bandwidth only if people would play decent crap on the radio...

Re:FCC rules

[turg]

The rule forbid "causing harmful interference" to radio stations, by appliances, not vice versa. The ADSL line is "accepting interference gracefully" -- by allowing its own performance to degrade rather than fight back against the interference (which would involve causing harmful interference to the radio signal). This is precisely what is required by the regulations to which you refer.

Hope this helps

-
<SIG>
"I am not trying to prove that I am right... I am only trying to find out whether." -Bertolt Brecht

AM interference should be accounted for

[gargle]

I happened to attend a talk last week by Professor Cioffi of Stanford University, the inventor of the DMT technique widely used in ADSL. The issue of AM interference with ADSL transmission came up during his presentation. However, what DMT does is to optimize the transmission by reducing the strength of the signal sent on frequency bands where the noise is high (or where FCC regulations do not permit transmission due to emissions produced by the DSL line), and increase the signal strength on bands where the noise is low. If I recall correctly, AM interference is accounted for and the actual ADSL specs do not transmit a significant amount of energy in the AM frequency bands, so it's strange that this article identifies AM interference as a problem.

Re:AM interference should be accounted for

[RISCy Business]

Not necessarily.

Bear in mind, AM signal strength is directly relative to several key things; distance from transmitter, power of transmitter, and frequency.

I'm guessing (I am TOO tired to read the article and make total sense of it right now) that they have already isolated which frequencies effect ADSL. That leaves it to a simple matter of determining a way to filter out the noise from those frequencies at each end. That, unfortunately, is not easy.

However, this surprises me not at all. Cable modems have similar problems. Like Digital Cable. I'm sorry, but two *VERY* busy digital signals over a single coaxial line just will NOT work. Granted, the cablemodem should be operating around 6MHz and your audio/video signals are around 40MHz IIRC, you still run into the problem that those are *baselines*. They go a certain amount either way, with another bit thrown in for tolerance. And end up crowding eachother out. Then at the cable plant, you have things like RF interference from your wireless reception/transmission equipment, and such crap. Things like ingress and dB loss. That's why there's the Cisco uBR7246 (Universal Broadband Router). It's designed with all that stuff in mind, at the cable plant. Once you leave there, you're on your own.

There's really not much that can be done but to compensate. The chances of the FCC ruling ADSL more important than AM radio, which is older than ENIAC by probably a quarter century or more, are very slim. So it's going to be up to the designers and implementers to compensate.

Re:AM interference should be accounted for

[gargle]

I'm guessing (I am TOO tired to read the article and make total sense of it right now) that they have already isolated which frequencies effect ADSL. That leaves it to a simple matter of determining a way to filter out the noise from those frequencies at each end. That, unfortunately, is not easy.

No. The primary way noise is compensated for is by sending *less* signal energy on frequency bands where the noise level is high (e.g. AM frequency bands), and *more* signal energy on bands where the noise is low.

e.g. If you imagine a band pass channel which only passes frequencies between F-df to F+df, then you would want to send *all* your signal energy in that band (since the response of the channel elsewhere is 0!) So in general, the amount of signal you send on a particular band is something like (I'm not being precise here) proportional to the SNR in that band.

So for the problem of AM intereference, they can simply elect not to send much signal energy in those bands, and this is already accounted for in the ADSL specs.

Frequencies here?

[DoomHaven]

Wait a minute, I thought certain frequencies were reserved, and AM radio happened to be one of them. So why now is this being noted? You would imagine that, sitting in a lab Engineer Bob would think, "Hey, this frequency is between 400-1400KHz, I wonder what else is in that frequecy range".

Actually, wait another minute. How can you pipe 1.5Mb/s over AM frequencies, anyways? Don't you need 2x frequecy to send x b/s? Doesn't Nyquist theorem state this?

Ah, here it is:
"The Nyquist-Shannon Theorem states that an analogue signal of bandwidth B can be completely recreated from its sampled form provided it is sampled at a rate s equal to at least twice its bandwidth."

Found at: (no, I am not going to piss around with HTML tags)
http://www-dept.cs.ucl.ac.uk/staff/S.Bhatti/D51- notes/node4.html#equNyquistShannon

Sorry, just a bit of communication theory from a sleep derprived engineer-in-training.

Re:Frequencies here?

[wowbagger]

Actually, the capacity of a channel is proportional only to the signal to noise ratio of the channel. Were your application of Nyquist's theory valid, you could go no higher than 7200 bps over POTS (plain old telephone service). Now, look real close at your modem. I'll bet it's going higher than 7200bps.

The trick is that a phone line has a very high signal to noise ratio, and you can make use of this to move more bits than the line has hertz of bandwidth.

Now, over a radio link, your signal to noise ratio drops, and you have to use more bandwidth to raise the S/N up. Over wires (like DSL) you are able to maintain a better S/N ratio. However, when your local AM station bleeds in, you lower the S/N ratio and thus reduce the bearing capacity of the channel.

However, part of DSL is avoiding the frequencies that have interference, and using the ones that are clear. Yes, if the line were completely clean you could get more throughput, but crosstalk from the other lines in the bundle, noise from AM and CB radio, and attenuation from the fact the wires were not designed for RF frequencies all reduce throughput and were allowed for in the spec.

A link with more detail . . .

[layne]

I remember this mention in a study done by avalon.net two years ago. This piece gives some more detail regarding frequencies at cross purposes.

As it mentions, a T-1 loop running in parallel to your POTS pair can also cause drop-out in your DSL frequency spectrum.

No, that's STP.

[Wakko Warner]

CAT5 cable can be either unshielded or shielded. Most installs I've seen use cheap UTP cable.

-A.P.
--

"One World, one Web, one Program" - Microsoft promotional ad

Amateur Radio is also affected.

[Rick Evans]

One thing that often gets missed in this discussion is that amateur (ham) radio is also degraded by ADSL.

Two prime frequency bands: 75/80 meters in the 1.8-2.0 Mhz range, and 40 meters in the 3.5-4.0 Mhz range. In both cases, the amateur radio operator is often trying to receive a signal from a 100 watt (typical) transmitter in a foreign country. The ADSL 'white noise' overpowers the signal.

Amateur radio shares many traits with open source software. Cooperation and sharing of information are respected qualities in an operator. A non-trivial number of hams still build their own equipment or modify the design to improve performance.

I think ADSL is pretty cool, and wish both of my hobbies could co-exist.

Rick Evans, KG4FER
Orlando, FL

Re:Amateur Radio is also affected.

[revnight]

adsl isn't even going to be the largest factor affecting hf. as i recall, idsl broadcasts across the entire u.s. hf amatuer spectrum...

more details at http://www.hamradio-online.com

Re:Amateur Radio is also affected.

[revnight]

my apologies...vdsl is the *dsl technology i was referring to.

if you want to read a bit more about this, check out this link:
http://www.hamradio-online.com/library99.html#ju l1999

Lotta greedy bastards here...

[Wakko Warner]

"I can't download pr0n as fast as I should be able to, so let's ditch AM radio!" Now, let's see which came first, has more users (listeners), and is generally more useful to a greater amount of people. That would be AM radio. Maybe ADSL should be the one to have to change, wouldn't you say? Or, should I and millions of other people give up our baseball games, news reports, and weather so you can play Quake a little faster? It seems awfully ignorant to want to get rid of an entire medium just because another, poorly-designed, newer medium chokes because of it.

- A.P.
--

"One World, one Web, one Program" - Microsoft promotional ad

Re:Lotta greedy bastards here...

[WNight]

Actually, that wouldn't be a bad idea. AM radio, as many have said, transmits very well, and is easy. So we should keep it, but do we need as wide a space for it (not that AM radio is too wide, but...). Ditto with FM and TV...

Maybe space for some of AM, and a TV channel or two. Everything else would be better served, in metropolitan areas, by using copper or fiber to transmit that and freeing the airwaves for things that have to be wireless.

And with enough airspace, you could simply listen to streaming news reports via a cell-phone -> car stereo link, if you needed that. I don't think music stations, as the sole players of new music, will be around much longer with the ability to stream a custom choice of music, etc.

I see some great uses for widecasts, namely in the interior of British Columbia where I grew up, or the even less populated northern edge of the prarie provinces. But, in cities where cell phones do more good, I think we should be trying to reallocate space, move things aroung, and make more room for digital cells.

Why, AM radio, of course.

[Wakko Warner]

In 10 years, people will laugh at what we consider "fast". Do you honestly think people will only be able to get by with a mere megabit per second of bandwidth? On the other hand, I anticipate I'll still be able to listen to a baseball game by the pool or at the beach on my little cheap AM radio.

Oh, and until FM radio can be broadcast over hundreds of miles, which AM can do easily, there will still be a need for AM radio. There's a *reason* AM is still around, and it's not simply because stations own the bandwidth still.

Wake up, indeed.

- A.P.
--

"One World, one Web, one Program" - Microsoft promotional ad

Re:ditch the am

[SEWilco]

Broadcasting noise will just add more noise. The "may" in your statement is simply false.

The "collision" which you are hoping for can only happen if you transmit exactly the opposite of the signal which an antenna will receive, such that the original signal and your opposite signal arrive at that antenna at exactly the same time. However, you have to send your opposite signal before the target antenna receives it, you have to send your opposite signal from the proper distance and time for it to reach the target antenna at the proper time, you have to send exactly the opposite signal so you have to listen to the original signal before the target antenna listens to it, and your cancellation signal only works at that single target antenna -- every other location just hears more noise. That's why when some government jam shortwave signals what the radios hear are various mixtures of noise. You can hear the same thing on a crowded AM/Shortwave frequency.

Re:ditch the am

[AndroSyn]

Why would we ditch AM radio, when ADSL will probably only be available to maybe 20% of the entire US population, whilst AM radio is available to damn near every person in the country, regardless if they live in a really big ditch or not.

Of course if the FCC did decide that they wanted to junk the AM band, you'd then need for the whole International Telecommunications Union to agree to ditch it too. Fat chance in hell if you ask me. There is lots of money that the AM broadcasting industry has to toss about to get there way.

Perhaps a better idea would have been to implement ADSL using spread-spectrum technology.

But in the mean time, ADSL users don't have much of a choice but to accept the interference that drops there packets.

Amateur radio operators have been dealing with cable companies on similiar issues for quite a few years now. It just so happens that cable channel 19(I think its 19) video signal falls on 145.25MHz. Right towards the low-end of the 2 meter band. This of course causes all sorts of problems with amateur radio communications on this frequency because of shotty coaxial cable. But the neighbors of course, bitch a storm when channel 19 gets washed out by the interference from the crazy guy with all of the antennas on his roof down the street. Well guess what if you don't like it bitch at the cable company. I have the right to be there in the ether, they don't.

Re: Come on, AM is the poorly-designed one

[tzanger]

That's why we have FM now !! wake up !
What do you think will be more important 10 years from now ? The old low-quality mono AM radio or high speed net access ??

AM will be around for a very long time to come. AM transmitters and receivers are far simpler to design and use far less components than FM.

Yes FM is higher fidelity but AM wins hands-down in the "simple to set up" category, which is important in any kind of emergency (I'm thinking TEOTWAWKI) situation.

Just call the FCC and get the ADSL cut off.

[Velox_SwiftFox]

Amateur Radio is authorized to radiate in the bands assigned to it.

ADSL, cable, et cetra are not authorized to radiate at all in those bands, the AM ones, or any other frequency; if it does, it is violating FCC regs. Simple as that.

CAT 5 - UTP or STP?

[ultra1]

C'mon now - you're experiencing the benefits of higher grade copper and tighter twist, not shielding. STP cabling has different signaling characteristics, for one. For the other, unless the link is shielded end-to-end, it isn't truly shielded. For a third, if you don't have a shielded jack to plug the shielded connector for your shielded CAT 5 into, you are going to run into problems with grounding loops, at which point you would be much better off with UTP.

In the end, CAT 5 UTP has more than enough twist in it to compensate for all but the most noisy environments. After that, you're running STP cabling, which more than likely means you are running your network over Token Ring (see 'signaling characteristics' above). Even nasty cheap-ass CAT 5 is going to be much better than phone cabling, hence your observations. The 'shielding' you think you are seeing is just a protective wrap for the wires, that way when people walk on them and what not the copper doesn't break between the jacks. This is also why you see stranded core used for hooking up desktops to the wall and what not, and solid core run through the walls (stranded core is less prone to breakage than solid when it comes to tight turns, people stepping on it constantly, etc. but it is more expensive - you can save money on your total plant cost by taking advantage of the fact that nobody is going to be stepping on and monkeying around with the cables in the walls that much).

Anyway, I agree, if you are having a house built or rewired, run CAT 5 UTP for both data and phone, or at least CAT 3 for phone. It does make a difference.

Re: Come on, AM is the poorly-designed one

[WNight]

Sure, higher fidelity, but the signal degrades more easily. Does anyone use AM Stereo btw?

And, if we designed them today, we'd use compression and massive error correction, probably reducing the bandwidth both need as well as boosting the fidelity.