Wireless Waffle - A whole spectrum of radio related rubbish

TP-Link WR702N Wireless Nano Routersignal strength
Monday 14 October, 2013, 18:42 - Radio Randomness, Equipment Reviews
Posted by Administrator
Whilst Wireless Waffle is branching out into equipment reviews the time seems right to give a plug to the snappily titled TP-Link WR702N wireless N nano router.

What is it? It's a small (hand-sized) wireless router that can be configured to do a number of jobs. In essence it's a wired network connection and a wireless network connection in a single box that can be configured in a number of ways. For example:
  1. It can add a WiFi connection to a normal wired network or internet connection.
  2. It can share an existing wired internet connection with multiple devices or users.
  3. It can extend the range of an existing WiFi network.
  4. It can act as a WiFi client, to give a wireless connection to things that don't have one (e.g. TV set-top-boxes).
And all this in a case no larger than a deck of playing cards and at an amazingly low price.

The device deserves a special mention because of both its flexibility in being able to do a number of different tasks, but most importantly because of its usefulness. The only downside is that it can be fiddly to set-up (see the CNET review of the device for more info on this).

hotel girl laptopImagine, if you can, being in an overseas hotel where there is a free wired internet connection but no WiFi and all you have to connect to the internet are your phone, a tablet and a laptop that's so slim and sexy that it doesn't have a wired network socket. You could turn on data roaming, but we all know how extortionate that is. You could get high, get moving, get naked and get clear to see if you can snag a signal from a rogue, unencrypted WiFi connection. You could wander down the street to the nearest coffee shop and hope they have WiFi. Or... you could have brought your TP-Link WR702N with you and plug it into the wired connection. Before you can say 'hey espresso', you have a ready made WiFi connection and both your phone and tablet are ready to update your Facebook profile!

Imagine moving into a new house to find that the WiFi connection covers every room except the kitchen. How will you read the morning e-newspaper over your coffee without WiFi in the kitchen? Simple, get your TP-Link WR702N and place it somewhere between the kitchen and your existing WiFi hub and before you can say 'special-k presto' you're back online.

tp link wr702nImagine discovering that you've missed the latest thrilling installement of 'Britain's Got Strictly Celebrity Antique Hunter On Ice'. Your satellite set-top-box has the option to run 'i-TV-Player-on-Demand' but it's only got a wired internet connection and you only have WiFi at home. Just break out the trusty TP-Link WR702N and before you can say 'tres fresco' you're streaming over-the-top video like a pro.

Finally, imagine trying to build a tower of plastic boxes that reaches the moon, to find you're a centimetre short. Just grab your TP-Link WR702N and put it on top and 'pay tesco' you've made it to the lunar surface.

Seriously though, for anyone who travels a lot and stays in hotels with no WiFi, who has patchy WiFi coverage at home, or wants to connect their blu-ray player to the internet to use Netflix, this little beauty does a great job and comes with the Wireless Waffle, 'Recommended Useful Broad Band Internet Signal Heighthener' seal of approvalness. Oops, that was supposed to be serious and look what happened. Maybe Wireless Waffle should just stick to being facetious.
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Topping TP20 Mark II - A Noisy Noise Annoys (Part 4)signal strength
Wednesday 25 September, 2013, 11:35 - Radio Randomness, Spectrum Management, Equipment Reviews
Posted by Administrator
In the past, Wireless Waffle has discussed various things that cause radio interference but which are not supposed to including, for example Power Line Telecommunications devices. This time around it's the turn of a Class T audio amplifier to come under the spotlight.

topping tp20 spotlight

Class T amplifiers are really Class D amplifiers but are supposedly more efficient. Any clearer? No, probably not. The idea behind these types of audio amplifiers (noting that the Class D principal is also used in some radio transmitters too) is that instead of amplifying an analogue signal in an analogue way, such that the output voltage is just an amplified version of the input voltage, they switch the output voltage on and off at a frequency higher than the audio signal, and then use a filter on the output to smooth the square wave that they produce back into a nice analogue signal. This method is known as pulse width modulation.


pulse width modulationThis switching technique is exactly the same one that is used in the majority of modern power supplies (SMPS) and has the prime advantage that as the transistors that do the switching are either turned on or off, they are never in some intermediate state where they would have to act as a resistor and in doing so dissipate power and heat. So they are highly efficient and it is possible to generate audio with over 90% efficiency meaning that more of the power is converted to sound and less is lost as heat, which is, after all, a very admirable quality.

As with switch mode power supplies a good filter is critical in ensuring that none of the original square waves find their way to the output. Square waves are very good at producing harmonics and therefore are equally good at generating radio signals and, of course, radio interference. There have been many cases of switch mode power supplies causing such radio interference and their use in, for example, LED lighting, means that the number of possible sources of interference is ever increasing.

The main problem is that, in many cases, the device will work without the filter fitted - if (and only if) the device that it is powering is not too fussy about all those square waves (e.g. an LED) or has a method of smoothing them out itself (e.g. a loudspeaker). A loudspeaker is basically a large inductor, which is what the filters in switching amplifiers also comprise. Feeding the nasty square waves on the output of the switcher directly into a loudspeaker will not result in a noticeable loss in fidelity (assuming the switching frequency is well above the audible frequency range), nor any particular loss of efficiency. So why fit the filter? To stop radio interference, that's why.

topping tp20 examination

So step up to the examination table, the Topping TP20-MK2 Class T Digital Mini Amplifier. One of these was recently purchased for the Wireless Waffle office, so that we could listen to the oidar through a bigger set of speakers. Being compact, and efficient, and coming in a shiny silver case, it ticked all the right boxes. But ouch, what noise from yonder shiny case breaks? As soon as the amplifier was turned on, reception of radio signals on just about any frequency was wiped out by noise. Even an FM tuner sat receiving a strong local transmission which was previously a perfectly quieting signal, was sent into oblivion by the amplifier. Obviously, the filter on the output of the Topping amplifier is completely inadequate for the purposes of curbing radio interference.

In cases such as these, there is little that can be done. Other than taking the device apart and replacing the filter components with better ones (an idea that is not as daft as it sounds), the solution is to junk the device and use a traditional linear amplifier instead. Which is what has been done. Bye bye trendy, offendy Topping, hello dusty, trusty Sony.
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Choosing the clearest channels for WiFi... continuedsignal strength
Tuesday 12 February, 2013, 14:24 - Radio Randomness, Spectrum Management
Posted by Administrator
wireless connection iconYou might think that Wireless Waffle has a thing about WiFi as it is a topic that keeps coming back time and time again (for example under the title 'Wire-More Lan' here, here, here, here and here). But it's also a topic that is becoming increasingly important as mobile operators seek to try and offload data from their network onto WiFi hotspots in cafes, malls and shops to try and free up their cellular networks for those truly on the move who need it most.

One issue with WiFi that crops up regularly is that of selecting which channel to use. This is a subject on which several previous articles have been written, however the discovery of a 2004 paper by Cisco entitled 'Channel Deployment Issues for 2.4-GHz 802.11 WLANs', looks at whether or not it is possible to seat WiFi hotspots on neighbouring channels and the effect that channel separation has on performance. Though the paper is now nearly 10 years old, it remains one of the only references on the issue.

What the paper says, quite rightly, is that the extent to which a WiFi hotspot will cause interference to a neighbouring one is dependent on the extent to which interference from one is received by the other. This is partially a factor of the distance between them but is also related to their frequency separation. What the paper shows is that, in the USA, where there are 11 2.4 GHz WiFi channels available, only the set of channels 1, 6 and 11 are usable in such a way that a hotspot on one will not interfere with hotspots on the others. The paper examines what would happen if the set of channels 1, 4, 8 and 11 were used for four neighbouring hotspots. Cisco even did some tests which showed the following results:
Channels Used Throughput per Client
1, 1, 6 and 11 601 KB/second
1, 4, 8 and 11 349 KB/second

What the results indicate is that even though, in the first test, two of the hotspots were on the same channel, the average throughput was increased over the four channel example because the overall level of interference was reduced. The paper also notes that WiFi hotspots on the same channel recognise each other and use various measures to try and avoid interfering whereas those on neighbouring channels just see interference on the channel. This is one reason why it is better to use a set group of channels in a given area (and to see what your neighbours are doing) rather than just pick a channel at random.

Having read this paper, the question that immediately arose was whether or not the fact that in Europe (and certain other parts of the world) there are 13 2.4 GHz WiFi channels available and not 11, offers up the possibility of a four channel arrangement that actually works - channels 1, 5, 9 and 13. The diagrams below show the emissions that are produced by the hotspots in the original three and proposed European four channel arrangements.

3 channel wifi arrangement
Three channel arrangement

4 channel wifi arrangement
Four channel arrangement

The amount of interference that is caused between devices is the overlapping area of the emissions of one transmitter with that of the receiver of a device on another channel (shown shaded on the first diagram). Using the standard emission mask to represent the emissions from one device (in reality they are usually lower than this) and using the same mask to represent the characteristics of the receiver of another (again a reasonable assumption) the extent to which a transmission on one channel is received by a receiver on another channel can be easily calculated.

The chart below shows how much of the emissions of a transmitter on a given channel are received by a receiver which is offset by a certain amount from that channel.

wifi channel offset interference

An offset of zero means that both are on the same channel and therefore the interference is at 100% (shown as 0 dB). When the transmitter and receiver are separated by one channel (eg one hotspot is on channel 6, the other on channel 7), the amount of interference that one causes the other reduces by 1.25 dB (representing around 75% of the original value). When this offset is increased to two channels (eg one hotspot on channel 5, the other on channel 7), the interference falls by 3.1 dB (or around 50%). At the extreme, when the offset reaches 8 channels or more, the interference has fallen by nearly 37 dB (or to around 0.02% of the original value). So...
  • In the case of the use of channels 1, 6 and 11 where the offset is 5 channels it can be seen that the level of mutual interference between hotspots would be -27 dB (or around 0.2%).
  • In the case of the use of channels 1, 5, 9 and 13 where the offset is 4 channels it can be seen that the level interference between hotspots would be -22 dB (or around 0.6%).
  • In the case of the use of channels 1, 4, 8 and 11 where the offset falls to 3 channels, the level interference between hotspots rises dramatically -6.4 dB (or around 23%).

It is therefore, perhaps, no great surprise that using the four channel arrangement posed by Cisco that interference levels increased to the point that the throughput dropped by nearly half. It does, however, look feasible that in Europe the use of a four channel arrangement that uses channels 1, 5, 9 and 13 might be feasible. A lot will depend on the characteristics of the receivers in use and whether they are as good as the emission mask.

wifi squeezeDon't worry if you haven't understood everything presented here, the salient points are:
  • In the USA and other places where there are only 11 WiFi channels available, throughput will be maximised if everyone in the same neighbo(u)rhood stuck to using only channels 1, 6 or 11.
  • In Europe and other places where there are 13 WiFi channels available, it seems quite feasible that if everyone sticks to using channels 1, 5, 9 or 13, the performance of everyone's WiFi hotspots would be maximised.
  • Using a WiFi hotspot on a channel which is offset from a nearby hotspot by 1, 2 or 3 channels is a recipe for both hotspots to suffer interference and have degraded performance.
  • Randomly picking a channel for your WiFi hotspot (eg. setting it to your lucky number) is a recipe for poor performance all round.

It is perhaps worth noting that in the 5 GHz band, no such restrictions apply as, sensibly, all of the available channels are mutually independent from all the others!

Unfortunately, unlike Cisco, the Wireless Waffle team do not have a shed full of WiFi hotspots that can be used to do tests on throughput. Perhaps someone might like to take up this challenge and let us know what the results are?
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Hop to it!signal strength
Friday 25 January, 2013, 13:45 - Radio Randomness, Spectrum Management
Posted by Administrator
Wireless Waffle received an e-mail from Des of Ireland. Des writes:
Since early May I have been noticing many many frequencies being occupied by very short bursts of digital 'noise' which are random in their frequency and time but very recognisable. So far pattern emerged is that they follow an 8 kHz spacing right across the HF bands (from 3.4 MHz to 28.5 MHz), but mainly in 6 to 9 MHz region. Even 6622kHz Shanwick being clobbered ... These noise bursts in the HF bands intrigue me, I wondered if it is a basic military comms set-up in case satellites/internet/microwave/fible-cable are clobbered.

Take a look a the picture below (click on it to open a much larger version). It is a snapshot of the radio spectrum between roughly 6550 and 6950 kHz taken using the University of Twente's on-line receiver in the Netherlands (which is a marvel in itself). The snapshot was taken at about 07:00 GMT. The horizontal axis shows the frequency, the vertical axis is time (in thie case about a minute). Straight vertical lines represent constant transmissions. Dotted ones (such as the broken line just above 6600 kHz) are morse code. Other squiggles that are roughly vertical are all manner of other signals that can be found on the HF bands.

hf frequency hopper

What is of interest here are the horizontal dashes of which there are three at the top left hand corner (just under 6550 kHz), four just below 6950 kHz and various others scattered across the chart, seemingly randomly (see around 6665 kHz and 6555 kHz for two bright ones). These are not bugs in the University's software, nor are they local interference in Twente. What they are are bursts of data from a frequency hopping transmitter. If you tune into one of the frequencies just at the time when the transmission is taking place on that frequency, you will hear a 'chuff' noise which is the quick burst of data that is being sent. If you happen across a frequency that has multiple 'hops' on it, the effect is not totally unlike there being a steam train on the frequency (listen to this actual recording).

At HF, this hopping transmission is almost certainly military in nature. Frequency hopping at HF is not at all uncommon. Even back in the 1980s, Racal's TRA 931XH would happily hop around the HF bands. In the case of the '931XH it did this by changing frequency roughly every second. Transmissions were just SSB (with an initial data burst to synchronise the receiver and transmitter - this is essential so that the two follow the same sequence of frequencies). The Wireless Waffle team had the fun of seeing a demo of the '931XH which was set to hop from frequencies between around 6950 and 7450 kHz, right across the 41m broadcast band. The effect of the hopping was to change the background noise every second or so - sometimes with a loud whistle caused by the carriers of the broadcast signals. The effect to anyone who happened to listen on a frequency that was being used would have been that they would have heard speech for a second which would then disappear.

hop to itThere's nothing unusual about the use of frequency hopping transmitters. Your bluetooth headset does this, and most GSM networks are set up to use frequency hopping too. The reason for using frequency hopping can be many and various, such as:
  • Hopping around makes the transmission much more difficult to detect. Unless you know the sequence of frequencies being used, it's almost impossible to follow the transmission from one frequency to the next.
  • Hopping can overcome some kinds of interference. If one frequency is blocked (from a broadcast transmission for example) the information sent on that frequency is lost, but if most are clear of interference, the error correction schemes can be arranged to deal with missing blocks and the overall communication is unaffected.
  • Hopping can help overcome fading and propagation problems. In a GSM network for example, Rayleigh fading will cause some channels to have deep fades and others not. Hopping around makes sure that these 'dead' channels do not cause a total lack of communication.
It's not surprising then that the military are using hopping on the HF bands (nor anywhere else for that matter). The question that remains unanswered is whether the military still need HF given all their other channels of communication. Patently they do!
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