Wireless Waffle - A whole spectrum of radio related rubbish
ADSL: A Daily Service Losssignal strength
Monday 19 November, 2007, 11:53 - Much Ado About Nothing
Posted by Administrator
For the past few months, every evening around sunset, my (ADSL) broadband connection at home has been 'drying up'. By this, I mean that the throughput has got smaller and smaller until eventually there has been no incoming or outcoming bandwidth available at all. If left alone, after an hour or two, the situation tends to correct itself but with sunset currently occuring during the working day, it's annoying to lose internet connectivity at these times.

At first I thought the problem might be to do with my ISP, Sky Broadband, but a quick scan of the rather useful, but unofficial, Sky User Forums didn't seem to indicate that others were suffering the same problems as me (though there were one or two veiled comments about how the connection sometimes dropped at night).

bt adsl filterMy next thought turned to my home installation. I'm quite a way from the local exhange and as such only get a connection speed of approximately 4 Mbps downstream and more like 0.4 Mbps upstream. According to my router, the available signal to noise ratio on the downstream link is only 8dB, that is to say that the ADSL signal on my phone line is only approximately 2.5 times higher than the noise on the line which isn't a great deal. But fiddling around with filters and connections makes next to no difference and the connection remains resolutely poor. During the 'dry periods' the signal to noise falls to 3dB or less (hence the loss of the connection), but re-booting the modem at these times oddly yields a return to 8dB.

Being a radio engineer type, and knowing that ADSL uses radio frequencies, I began to wonder whether the problem might be to do with increased radio interference on the line around sunset. ADSL uses frequencies from approximately 26 to 138 kHz for the upstream connection (i.e. from the home to the exchange) and in the range 138 kHz to 1104 kHz for the downstream connection (from the exchange to the home), though this frequency range is extended to just over 2200 kHz for the faster ADSL2+. Given my distange from the exchange, my connection is resolutely ADSL only (and not ADSL2+) and therefore if there was an increase in interference it would need to be in the frequency range 138 to 1104 kHz. However, as most ADSL modems are capable of running ADSL2+ even if the line is not capable of supporting it, it is likely that the receivers in them are not filtering out unwanted or unused frequencies making them susceptible to interference on frequencies up to 2200 kHz and quite possibly even higher even if those frequencies are not in use.

pole dancerNow as it happens, this frequency range is home, in Europe, to both long wave and medium wave radio transmitters (in the frequency range 148.5 to 285.5 kHz and 526.5 to 1606.5 kHz respectively) and there are many high powered radio transmitters in this frequency range. Could it be that the propagation that exists around sunset causes such a significant rise in the level of signals in this frequency range that it was knocking out my ADSL connection? The telephone connections in my area are via flown cables between telegraph poles (as they are still called despite telegraph having died out eons ago) and as such probably make rather good aerials, so it is quite possible that my modem is susceptible to incoming interference from high power long and medium wave broadcast transmitters.

When first booted up, the ADSL modems at both the exchange and the home end 'train', that is to say that they check all the available frequencies to see which have the best signal to noise and then use just these frequencies for the connection. This would perhaps explain why re-booting the modem during a dry period restores the signal to noise: clearly the frequency on which the interference occurs is one which is clean in the mornings when the modem is first switched on.

microfilter circuit diagramAs yet I have not been able to confirm my theory that interference from high power broadcast transmitters is knocking out my broadband connection, but it does seem eminently reasonable. Next steps are to do some tests (maybe connect a receiver to the phone line - bearing in mind it has 50 Volts on it!) Another option might be to add some filtering to the incoming line: surprisingly ADSL microfilters do not touch the ADSL frequency range they just stop the ADSL signal getting into/out of any phones connected (see the example circuit diagram on the right), which means, incidentally, that if you have nothing other than an ADSL modem connected to any phone socket in your house, you don't need a microfilter on it: you only need microfilters on the sockets where phones are connected. Perhaps, therefore, there's room for an improved microfilter that cleans up the incoming ADSL connection as well as keeping it out of the phones. I'll keep you informed of any progress - and if you experience these problems yourself leave a comment and I'll update you personally if I make any breakthroughs!

Update: 18 Dec 2007 Forcing the modem to use ADSL instead of ADSL2+ whilst reducing the connection speed by less than about 5% has almost eradicated the problem. It seems that purposefully avoiding the use of frequencies above 1.1 MHz does make a worthwhile difference.

Update: 11 Feb 2012 After much messing about Wireless Waffle has designed a filter which cures the ADSL drop-out problem that is described above. It costs next to nothing and can be made in minutes!
14 comments ( 2411 views )   |  1 trackbacks   |  permalink   |   ( 2.9 / 5980 )

Essentials of Modern Spectrum Managementsignal strength
Wednesday 31 October, 2007, 05:35 - Spectrum Management
Posted by Administrator
Spectrum management has traditionally been about the use of technical criteria developed through long-winded compatibility studies to determine what can (and can not) be allowed access without causing harmful interference to other users. essentialsofmodernspectrummIncreasingly, however, regulators are using forward looking market-based spectrum management techniques such as administrative incentive pricing, auctions and comparative selection (beauty parades), trading and property rights in order to be able to adapt to the rapidly changing and liberalised markets which use the radio spectrum.

Mssrs. Cave, Webb and Doyle have documented the current state of play in these forward looking techniques (often lumped together under the banner of 'Spectrum Pricing') in their new publication Essentials of Modern Spectrum Management. They describe developments in both the technical and economic tools used to manage the radio spectrum as well as looking at other related issues such as the need for and benefits of spectrum commons. There's also a good discussion of why Ultra Wide Band (UWB) has made many regulators reflect on whether existing spectrum management techniques are appropriate or have sufficient longevity and flexibility to cope with such new approaches to spectrum use.http   www assoc amazon co uk e ir t sheffieldrock 21 amp l ur2 amp o 2

cave webb doyle

The book uses a number of international case studies together with the practical experience of the authors to illustrate many of the concepts involved. Whilst the book would not be suitable for someone wishing to gain an overall understanding of spectrum management, it provides a useful and inciteful reference on these more advanced techniques and would suit anyone who has experience of spectrum regulation and who wanted to understand better how such regulation is being transformed to encourage more efficient spectrum use. Certainly a book that should be on the shelves of any self-respecting modern spectrum manager!
add comment ( 1377 views )   |  0 trackbacks   |  permalink   |   ( 3 / 4101 )

Trains, Boats, Planes, God, Kylie Minogue, and more Planessignal strength
Saturday 22 September, 2007, 07:08 - Spectrum Management
Posted by Administrator
Years ago I used to know the frequency¹ for the downlink of Capital Radio in London's 'Flying Eye', the aircraft used to scout about for travel blackspots. It was a useful frequency to have as there was no better place to get the latest travel news. I also remembered that there was an uplink from the studio on around 455 MHz too.

minaret antennaOne quiet afternoon I thought I'd have a tune around to see whether or not the old up and downlink frequencies were still active. Listening around 467 MHz, there seemed to be no sign of the downlink (though being around 25 miles outside of central London and with a downlink power of less than a Watt, this wasn't perhaps, that surprising). The uplink, however, is still active on 455.075 MHz.

What was rather odd, however, was to hear dozens of Imam's calling their congregation to prayer on frequencies just below this at around 454.5 MHz. Was this some freak long-distance propagation carrying signals from arabic speaking countries in North Africa or the Middle East? Was it a freak spurious response on my receiver, allowing reception of, perhaps, satellite radio? Were these the link frequencies to the numerous 'Radio Ramadan' stations that appear on the FM band during the festival?

No. It was none of these. Instead it seems that there is a radio service, established in around 2000, called 'On-Site Religious Observation' or OSRO. This a a licensed radio service which allows any religious body to use the old Wide Area Paging (WAP) channels to deliver voice communications to pagers. What a good use for these otherwise quiet channels: to allow Muslim's their right to hear the Imam's call without the need to build noisy minarets, just build unsightly radio towers instead!

A bit more digging revealed that there are all sorts of interesting frequencies in the range 454 to 458 MHz as listed below:

lego kylie454.0125 to 454.8375 MHz OSRO/WAP
454.84375 to 454.98125 MHz Cab Secure Radio (CSR)²
454.9875 to 455.475 MHz Programme Making and Special Events (PMSE)³
455.475 to 455.850 MHz Airport Security and Operations (455.5125, 455.5375, 455.5875, 455.625, 455.6625, 455.675 and 455.6875 MHz also used for CSR)
455.850 to 456.000 MHz Fire Service
456.000 to 456.9875 MHz Private Mobile Radio (PMR) Simplex and Duplex use
457.000 to 457.250 MHz Fire Service
457.250 to 457.475 MHz PMSE
457.475 to 457.500 MHz Fire Service
457.500 to 458.500 MHz Scanning Telemetry (457.525, 455.5375, 457.550, 455.5625 and 457.575 MHz also used for On-Board Ship Communications)

With Ofcom considering reconsidering its previously aborted plans to re-align the band it will be interesting to see where these services end up. And in the meantime if anyone has the frequency used for the Flying Eye's downlink, do share it with us!

¹467.6625 MHz if I remember correctly.
²Radio system used by Network Rail in areas where rail services have no conductor and thus the driver can not leave the cab to commnunicate with the signaller.
³Links and talkback for radio stations, TV studios, film sets, theatres and so on. Could be used by production team at a Kylie Minogue concert for example...
2 comments ( 1932 views )   |  0 trackbacks   |  permalink   |   ( 3 / 3418 )

Wireless Wobblysignal strength
Thursday 20 September, 2007, 10:10 - Much Ado About Nothing
Posted by Administrator
Wireless Waffle isn't one of a kind. There's another Wireless Waffle on the web. Whilst I claim no originality for the title of this blog, it does irk me that the man who runs the other Wireless Waffle is so upset that I accidentally stumbled across the same name as him that he feels the need to take a poke at this site on many occasions.

wirelesswafflecoukWhen I first launched this site, Keith, who runs the other Wireless Waffle, contacted me with a nice e-mail and suggested that we exchange links. I put up a link to his site with a nice button that I took time to make myself (see right) but when a reciprocal link failed to materialise on his site I took it down, and thought nothing more of it. But putting 'Wireless Waffle' into Google, I recently noted that the <title> of his site has been changed to:
Wireless Waffle - A fine radio site not the blog copying its title

If that's not enough, the description of his site says:
Wireless waffle is a specialist radio site ... it is not to be confused with the blog which is using the same title - this other site is more technical and whilst we do not have the copyright on the word waffle they could have thought of a different title...

Fair enough, I could have thought of a different title, but I didn't, but neither did I specifically pick the name on purpose to upset anyone. I was not aware of Mr. Knight's site until he sent me an e-mail.

But the rhetoric doesn't stop there. In a post he made on his site on 16 September he says,
I am wondering if you would miss the Wireless Waffle site if I decided to close it? ... There is another site, a blog, which insists on calling itself Wireless Waffle which is far more technical than this. The chap who publishes this has pictures of ladies in various poses and states of undress. I am one of those that favours proper websites rather than blogs. Blogs do not demand any knowledge of html and that is part of the fun of operating a site.

Now this is just downright misleading, and in some cases completely wrong. He is insinuating that:

thewirelesswaffle* My use of the occasional saucy picture demeans the content on this site. There are many pictures of men in various states of dress as well as women and anyway this kind of thing has been adorining British seaside postcards for many years. These pictures, with their associated captions are intended to add some levity to what can be rather colourless topics.

* That my use of 'blog' software to publish the articles I write devalues them. I use blog software as it makes presentation look nice and it's easier to find articles and for people to browse around.

* That I know nothing about HTML. This couldn't be further from the truth. Take a look at the other content on the host site for Wireless Waffle, such as my Javascript tools, or my Random Town Name Generator. All of these are written by me, using nothing more than a text editor, and most pages are in XHTML which is notoriously more difficult to write in than HTML. If you're going to level that kind of accusation at someone, at least make sure your own site is valid HTML!

The most serious accusation is that by starting this site, I have damaged the viewership of his site to the extent that he is, in essence, accusing me of forcing him to close his site down.

The reaction of many people to so many unfounded accusations might be to retaliate, but I'm not that kind of person. Mr. Knight's site is an interesting read with content that most of the readers of this site would no doubt find of passing or of direct interest. I suggest you take a visit (click on the button above) and have a look around. And pop back here afterwards and leave a comment on this post to let me know what you think.
5 comments ( 1931 views )   |  0 trackbacks   |  permalink   |   ( 3.1 / 63 )

<<First <Back | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | Next> Last>>