Thursday 16 March, 2006, 13:44 - Radio RandomnessWireless LAN's are fabulous things. You put a box next to your cable or ADSL modem with a little aerial on the back and your broadband internet connection miraculously becomes available all over your house to any device fitted with the appropriate wireless card. Or does it...?
In some cases the position of the modem and wireless box are such that coverage does not extend to all points of your house. For example, most incoming telephone (and cable) connections tend to be on an external wall, which by default, is at one end of a property and not in the middle of it. And often they are at floor level, which is where all the modem/wireless equipment also ends up. Now although radio can at times seem magic, putting a wireless box on the floor at one end of your property probably means that upstairs at the other end, the signal is very weak. So what can be done to extend the coverage of wireless LANs?
The first, and easiest thing, is to put the wireless box in a more favourable position. Many can be wall mounted (though this still puts the device at one end of a building), but even raising the position from on the floor to head-height helps. If you have a phone socket or cable connection somewhere else in your house, i.e. in a lounge that might be more central, move the wireless box here and put it on a shelf so it's not at floor level. Putting the box centrally within a property helps even out the signal throughout the building.
If neither of these are possible and the only place where the box can be located is next to the phone socket, is there anything else that can be done to make the signal stronger? The answer is a resounding YES. By far the easiest way to do this is to replace the antenna that comes with your wireless box for one with higher gain. Standard antennas have a gain of 2 dBi - this means that the effective radiated power (erp) of your wireless box is the same as the transmitter power, so if the transmitter is 10 milliWatts (mW), the radiated power will also be 10 mW. Replacing the aerial with one with higher gain increases the radiated power for the same transmitter power and has the added benefit that it improves reception by the wireless box too extending the overall range. Without going into the maths, to double the range of a transmitter, the radiated power has to go up by a factor of 4 (which can also be expressed as 6 dB). So an antenna with a gain of 8 dBi will double the range (or quadruple the coverage) of the system compared to one with 2 dBi gain.
And such antennas are available, and they're not expensive. Visit eBay and do a search for 9dBi and you'll find that for about GBP15 (including postage) there are lots of 9 dBi gain antennas available (remember to make sure that the connector on the bottom of the antenna is the right one for your wireless box - R-SMC are the most common, followed by R-TNC). Replace your standard antenna with one of these high-gainers (which are about 40cm long) and you should find that your coverage has gone up significantly.
Is it legal to do this? Good question! In the UK, Ofcom allows powers up to 100 mW erp . The simple answer is 'it depends'. Most wireless LAN access points have a transmitter power of 100 mW already, so installing a high-gain antenna means the erp will exceed the UK limits. What are the chances of getting caught? That's a different matter...
Tuesday 28 February, 2006, 10:39 - Radio RandomnessWandering past a local bus-stop, I was intrigued by an advertising poster which shouted, "wireless broadband with no strings attached". Like many people, I have a wireless broadband network at home, however this ad seemed to be offering something different i.e. a broadband connection to the home which didn't rely on wires. Now I know that with a 3G mobile data card you can get a reasonably broadband (circa 256 kbps) connection, but the implication of the poster was that what was on offer was more akin to the services offered by wired broadband.
A bit of digging later, I discovered that the service being advertised was one offered by NOW and was indeed a true wire-free broadband to the home connection (so called Broadband Fixed Wireless Access or BFWA). NOW is a trade name for UK Broadband which in turn is a division of Pacific Century CyberWorks (PCCW), a Hong Kong based company who also offer BFWA services over the whole of the island of Hong Kong too. UK Broadband won 13 of the 15 regional 3.4 GHz licences available through Ofcom's auction during 2003, securing them 2 x 20 MHz of spectrum (3480 - 3500 MHz paired with 3580 - 3600 MHz). They later bought up one of the other bidding companies giving them 14 of the 15 licences and virtually nationwide coverage.
UK Broadband uses TD-CDMA technology from IPWireless. TD-CDMA is a 3G technology and is part of the UMTS stable. This is in contrast to PCCW's network in Hong Kong which is based on WiMAX. In some ways the choice of this technology is odd, partly from a commercial stand-point as there would have been obvious economies of scale if the company had used the same technology in all its markets; but also because the spectrum they won is paired, making it suitable for frequency division duplex (FDD) technologies, rather than the time division duplex (TDD) technology they have settled on. At the end of the day, financial decisions usually outweigh technology ones so there must have been a commercial imperative for this choice. An alternative line of thought is that as TDD technology can dynamically assign capacity to uplink or downlink as required, and given the asymetric use of bandwidth which internet browsing generates, TDD is actually more spectrally efficient (and economically efficient with it).
It will be interesting to see how successful NOW becomes. Various companies have tried to deliver wireless broadband to the home in the past (including cable giant ntl:) but have always found that the cost of the equipment and specialist installation was prohibitive to competition with fixed line solutions. The NOW service doesn't seem to require specialist installation which is a plus, however presumably the equipment is still expensive compared to the now almost ubiquitous ADSL modem.
Wednesday 15 February, 2006, 13:26 - Radio RandomnessI read an article in The Times newspaper yesterday entitled 'Coastguard scrambled as set-top box sends SOS'. It would appear that, on two separate occasions, radio emissions from a Freeview set-top box had raised an alert with the coastguard who had sent search and rescue helicopters out to find the source, apparently costing GBP20,000 or so on each occasion (special 'centrifrugal' helicopter fuel is very expensive you see).
The alert concerned was simply a transmission on a frequency of 121.5 MHz, one of the internationally recognised distress frequencies. Devices called Emergency Position-Indicating Rescue Beacons (EPIRBs) use this, and other frequencies (406.1 MHz for civil and 243.0 MHz for military emergencies) to alert an international network of Search and Rescue satellites (SAR-SATs) when a vessel is in distress. The various satellites which monitor this frequency can pin-point to within an accuracy of about 20km, the location of any transmission, over an area comprising about 60% of the surface of the earth. The unintentional radiation from the set-top box would therefore have triggered the satellites (which believed it was a distress beacon) to alert ground stations to the signal. The fact that the set-top box concerned was located in Plymouth, a busy naval town as opposed to in the middle of the English countryside would have further alarmed the coastguard (as boats in distress surrounded by miles of farmland would be rather suspicious).
The story continues to say that officials tracked down the source of the interference and knocked, antennas in hand, on the door of the poor unsuspecting lady whose box was at fault. She clearly thought she had been tracked down for television licence evasion, though why she would think this if she had a licence is rather confusing. Obviously the signals were strong enough to merit intervention from the authorities, who took the action of tracking and closing down the problem rather than allowing it to continue.
Two things are surprising about this story. Firstly, the fact that the transmissions from the set-top box were strong enough to raise the alert with the satellite network. EPIRBs operating on 121.5 MHz typically use a power of 50mW or more. The malfunction of the set-top box must have caused some device within the box to oscillate and these oscillations were then radiated back through the TV antenna or the down-lead. To get a 50mW signal in such circumstances, however, is pretty good going. Even a well designed VHF oscillator would struggle to provide a stable 50mW of power on a fixed frequency without drifting as the oscillator got warmer or cooler. And given the poor performance of the antenna at these frequencies, it is likely that the power generated by the set-top box would need to be significantly more than this. It is no surprise that a ground-based search was also able to track down the signal! It is worrying in many ways that this is the case. Had the transmission been on a different frequency (for example 121.9 MHz) it would have interfered with air-traffic control (in this case at Heathrow airport). Such events do regularly occur, however there are no satellites that can pin-point the location of the transmission and many go unchecked.
The second thing which is surprising is that the box could produce such radiations in the first place. All electronic equipment has to conform to a set of standards known (in Europe) as the EMC directive. This requires all manufacturers to certify that their equipment does not suffer when in the presence of nearby radio transmissions AND that it does not cause unintentional radiation. The set-top box would have had to conform to this standard and thus should have been checked for emissions. Clearly, in this case, the set-top box was malfunctioning, which means that the tests conducted by manufacturers are not extensive enough to capture the results of fault conditions. Does that mean that I could produce a toaster which, if held upside down radiated a couple of Watts in the FM band and claim it conformed when used normally, to toast bread?
What's most worrying is that there are probably hundreds more cases of such radiation which go on unnoticed. In one respect the fact that they are unnoticed means that they are not causing anyone any problems. On the other, it does question whether opening up the use of the radio spectrum in a less controlled manner would really cause the mass devastation that many people seem to think it would.
Thursday 2 February, 2006, 21:28 - Radio RandomnessNo doubt a lot of people received cordless phones in their Christmas stocking, not least because with cellular phones all the fashion, it seems rather restrictive to have to use a fixed phone at home. Cordless phones come in many flavours, but in particular come in analogue and digital varieties. It's not always that simple to identify, however, which phone is which. In the radio sense, an analogue phone is one which uses analogue modulation (usually FM) to carry the audio to and from the phone. Conversely, a digital phone is one which uses digital modulation. However some unscrupulous retailers who believe that the 'digital' tag is something which sells, often call phones with digital answerphones (i.e. that record messages digitally) as 'digital cordless phones' - they aren't!
Why does all this matter? Well, only true digital phones offer any level of security over eavesdropping on your calls. Analogue phones are as very easy to tune into (more of which later...) I know of people who have bought 'digital cordless phones' in the belief that their calls were secure. A brief demonstration using a portable receiver as to how easy it is to overhear their calls sent them rushing back to the retailer from who they purchased the phone. Would the retailer swap it for a proper digital phone - not without them paying an additional fee (It's true to say that true digital phones still command a slight margin over analogue phones).
So how do you know whether the cordless phone you have is digital or not? Well, firstly if it is clearly marked as a 'DECT' phone (usually shown as the letters 'DECT', standing for Digital Enhanced Cordless Telephony, inside an oval shaped blob) then you're fine. As well as being digital, DECT, a European standard, normally operates at frequencies between 1880 and 1900 MHz which means that signals don't tend to travel far outside the house. There is an equivalent American DECT standard known as DCT-U and alternative American digital standards, all operating around 900 MHz.
For a practical test, try holding your phone next to an electrically noisy device such as a food blender, electric drill or lift/elevator motor. If you can hear crackles or whooshes over the top of your call, chances are you are using an analogue phone.
In the UK, there are three bands in which analogue cordless telephones, termed CT0 - cordless telephony 0 - to represent the '0th' generation of technology - can legally operate (phones imported from other European countries or outside Europe may operate in different bands).
The first of these bands, specified in UK standard MPT 1322 has a base transmitter on frequencies between 1.642 and 1.782 MHz, just off the end of the medium-wave broadcast band, paired with handset transmitters on frequencies between 47.44375 and 47.54375 MHz with a maximum radiated power of 10mW (erp). They can be identified as the base unit has a wire, up to 3 metres long, which trails out of the back. Using such a low frequency, the base transmitters can radiate over quite some distance. These phones are being phased out in preference to the band identified below, but a tune over the lower frequency band will usually yield a number of units still in operation. The handsets, being low power and with small antennas, tend not to radiate over such a wide area.
In 1996 the Radiocommunications Agency, then responsible for managing the radio spectrum in the UK, realised the need for additional channels for analogue cordless telephony to relieve the congestion that was occuring in the existing bands. A further 8 channels were released (specification MPT 1384) with base units transmitting in the range 31.0375 to 31.2125 MHz, and handsets in the range 39.9375 to 40.1125 MHz again at a power of 10mW (erp). Ranges for the base transmitters vary but can extend to over a km.
One final range of frequencies is used for long range cordless phones (complying with UK standard MPT 1371) with only two channels available and a maximum radiated power of 100mW (erp). Base units transmit on either 47.43125 or 47.41875 MHz and handsets on 77.5125 or 77.5500 MHz.
If you have a receiver that tunes across the range of frequencies used by these analogue phones, particularly those used by the base units, have a listen (though obviously I haven't told you to do this as doing so would force you to break the law and I can't possibly condone that kind of thing). It's amazing what range 10mW can give at some of these frequencies. Far be it for me to suggest that you could make a game out of guessing which of your neighbours regularly visits a 'medium', or which one's son has just come out as or which has insurmountable debts or ...! And remember, if you buy a cordless phone for yourself, make sure it's digital!