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S3 Ep41: Crashing iPhones, PrintNightmares, and Code Red memories [Podcast]

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[01’32”] We explain how a format string bug could lock your iPhone out of your own network.  [08’53”] We revisit the PrintNightmare saga, which is sort-of fixed but not really.  [12’50”] We look back at the 20-year-old Code Red virus.  [18’30”] We look at what cybercriminals spend money on (hint: more cybercrime).  [29’10”] And in this week’s “Oh! No!”, we learn how farm animals can disrupt your network. (Seriously!)

With Doug Aamoth and Paul Ducklin.

Intro and outro music by Edith Mudge.


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Interested by the brief discussion of Fresnel Zones – and the need to keep them completely clear – a view backed by a quick search of other sources.
I went to “other sources” (OK Wikipedia) because the need to be completely clear contrasted with my experience with UK digital terrestrial TV reception.
In my area all the websites say we have to pickup our signal from a relay transmitter of which we have line of sight. Trouble is this only relays 3 of the 8 multiplexed signals in which UK Freeview is packaged for transmission. For full 8 mux reception you need to pickup the main transmitter.
There is a hill in the way of the main transmitter (with a wood on top of it) – just to get line of sight (never mind any fresnel zone) you would have to shave about 20 to 30 metres off it (Google Earth and elevation diagrams are quite helpful for such calculations). Yet point your aerial at it (with a little bit of extra upward elevation) and you get a signal good enough to receive the full 8mux with very rare visible drop out. I notice it (reception problems) once every couple of months when I might have to switch from the HD channel to the SD channel for a few minutes – but from the same transmitter.
My aerial installer says this is due to “diffraction” over the hill and that we are “lucky with the woods”.
This seems to be in contrast to the Oh No fresnel zone issue.
Is this because digital TV has so much redundancy in the signal that you can get away with a lot of signal degradation – unlike “data networks” ?


Not sure. TV transmitters are usually pretty high (on masts so no interference at the sending end) and powerful. Wi-Fi, on the other hand, is designed to run at very low power and across lots of frequency bands (thanks, Hollywood star Hedy Lamar!) in both directions.

IIRC the Fresnel zone doesn’t *have* to be 100% free of physical obstruction but it is a good start.

As you say, you don’t get a lot of interference but you do get some, every couple of months…


The science of radio waves and aerial technology has always slightly perplexed me and my late father would not teach me even though he worked his entire life in radar (for the UK government and subject to the Official Secrets Act), so if anyone knows a good textbook that can pick up from A level physics (or struggling undergraduate) level …
The height of the transmitter is surely just to maximise the area/population that has line of sight.
My local main transmitter transmits at either 100kW or 50kW (Full Freeview on the Pontop Pike (County Durham, England) transmitter)
But received power down the reception aerial is very very much lower (that 100kW is being radiated across a huge area and a TV aerial will only intercept a minute portion of it).

Digital TV receiving technology works at much lower powers than analogue (part of the benefit of going digital – more can be “packed in”). For example:
Digital TV tuners … will generally process signals from about -50 dBm (strong) to about -80 dBm (weak).(US Source)Getting Technical with Over-the-Air TV Reception (AKA: Why Can’t I Watch This Channel?)
OK it’s mixing units but wifi strength is usually stated in dBm (reference to 1 mW) as well:
The scale runs from -30 to -90. If you see -30, you have a “perfect connection,” and likely, are standing next to the Wi-Fi router. However, if you spot a Wi-Fi signal listed at -90, the service is so weak you probably can’t connect to that network. An excellent connection is -50 dBm, while -60 dBm is likely good enough to stream, handle voice calls, and anything else.How to Check Your Wi-Fi Signal Strength
(I’m receiving 55dBm on my laptop with the router about 4m away in a different room – stone walls)

So signal strengths and wavelengths are similar (at least to orders of magnitude), so I guess that for WiFi it’s that every bit matters (requiring resends for correction etc), whilst for Digital TV we won’t notice the odd pixel in the wrong colour or tiny blips on the sound? With different tolerances of errors, WiFi gets to the state of dropping out sooner? So the farmer’s bull could disrupt his data but not his TV?


There is, of course, the not entirely unlikely possibility that the whole thing is a cock-and-bull story, or at least that the role of the bull in the story was coincidental.

After all, even if the part of the story about raising the transmitters and subsequently improving reliability is true, we have no way of knowing whether it was the *raising* that did the trick, or merely that they may have been better aligned after the adjustment, or that the mounting poles were better secured, or that some other sloppiness in the original installation and connection of the equipment, including any antennas involved, was resolved during the maintenance work.

After all, there is nothing in the story to suggest that the presence of the bull was relevant, or even that the connection ever failed exactly when the bull was present, only that the bull “had a favourite spot” and happened to be loafing in it when the technicians went to investigate.

The story doesn’t say what technology was in use, which part of the spectrum it used, whether it was at or beyond its expected operating parameters, how much the transmitters were raised, or indeed which other sorts of connection problem this “troubleshooting” fixed at the same time – details which might make the cause-and-effect part of the story easier to determine.


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