Feedback: Your letters

This weeks issue mainly sees GPS getting a pounding from you on all directions, fibre optic for all and pineapples skewing sea level readings?

Why UK needs a better network

Dan Stowell suggests peer-to-peer is a saviour and not the villain when it comes to network congestion (Letters, Vol 4, #2). Sadly, Dan, I doubt that most ISPs would share your view.

In an ideal world, we would all have a native (not buried in other transport mechanisms) Ethernet connection (via fibre or copper) to a street-level [Ethernet] switch; which itself is interconnected to all of the other town/city switches via high-speed (10Gb or faster) fibre. You and your neighbours would all be on a local-level ISP and then, when you wanted to peer-to-peer to your neighbour, the data would simply flow at local switch level, or maybe across a few switches in your local area.

This model is akin to the data networks you will find in business, where copper Cat 5/6 links to the desk are below 95m and fibre is cheaply and easily routed between 'comms rooms' to provide high-speed switch backbone interlinks. This is a highly controlled environment (no-one will stick a JCB through your cables) and is easy to manage and implement (although I am always wary of handling £1,000 fibre interface modules!).

In our current world, the [Internet] network has been designed around the client-server model where data is sent from a server to the end-user and no-where else. This is why, since the dark days of dial-up, the downlink speed has always been faster than the uplink: it is wrongly assumed you have little or nothing to upload. Peer-to-peer causes the ISPs a traffic nightmare as the uplinks are soon saturated. This, in turn, affects the downlinks for other customers, who complain.

The second issue is routing. If I send data to my friend in Devon who is on the same ISP, there may only be two or three routers between us and our traffic remains on the ISP's backbone. If I now want to send data to another friend who is on a different ISP, I could easily see 15 router hops and traffic passing through expensive high-speed links; maybe even rattling around Europe before coming back to the UK.

This generates a large amount of backbone traffic that the ISPs would rather not have to pay for; because we end up paying for it in higher charges and static technology: I have been stuck with ADSL for seven years and I am growing old and bored waiting for fibre to my home. I have data travelling between servers I manage, but they are all limited to 400kbps, which makes backing them up a slow and sometimes painful process; even with clever Unix programs like Rsync.

The UK needs a national fibre network (not just the cities) and it needs it now if peer-to-peer, IPTV and VoIP are to be effective. So please report with a spade and a pick: there is a lot of digging to do!

Gary Myers MIET, Biggleswade, Bedfordshire

GPS safety issues

In 'Where Would We Be Without It?' (Vol 4, #2), Mark Williamson mentions some issues that he suggests may be delaying the greater use of GPS in large airliners. He mentions GPS being an untried and untested system and the possibility of total or partial reduction in the service at the behest of the US government.

There is a third issue - that of system integrity and safety. The safety of modern airline operation is dependent on the continuous availability of accurate data for navigation and for the flight-control system, and this is usually achieved by means of multiple data sources and channels of computing with some kind of voting system to select a correct output.

In the application of GPS to aircraft, the aircraft has multiple receivers and there is a high level of redundancy in the satellite constellation - usually around seven satellites are visible and four are sufficient.

But there is only one propagation path through our atmosphere and, because the signal strength is so low, that element of the system is vulnerable to interference. This interference can be due to natural causes, accidental man-made sources or even deliberate interference. The interference could potentially have two forms - jamming of the GPS signals by noise, or attempting to transmit to aircraft pseudo-GPS signals that might appear to be credible.

Jamming by noise requires very few Watts of RF power to block GPS signals over a wide area around an airport. It is extremely unlikely that jamming of this type could cause an accident because it would be detected quite readily. Any aircraft using GPS in a fully-integrated way would simply revert to the existing ground-based systems.

It has been shown to be theoretically possible to mislead an aircraft by transmitting credible but incorrect signals (spoofing) but fortunately it is technically very difficult and, even if achieved, is unlikely to affect more than one aircraft in any one instance. Inevitably, the possibility of unintentional interference or intentional jamming, or even spoofing, will join with the commercial/political issues cited by Mark Williamson to delay the date at which commercial air transport can take full advantage of GPS.

John Campbell FIET, Sheffield, South Yorkshire

SatNav banana skins

I wish Mark Williamson had spoken to people other than vendors of GPS products. He might have discovered that GPS - in fact any GNSS - is inherently unsuitable for use in safety-related systems.

Very serious issues regarding the reliability and availability of GPS and the like occur in my area of interest: RFI, EMI and EMC. There are several examples of interference with GPS in 'The First 500 Banana Skins [new window]', a book of interference reports and anecdotes.

'Banana skin' No 222 shows how GPS over all of Monterey Harbour, California, which is often very foggy, and for 1km out to sea, was blocked by accidental jamming from a TV antenna booster on a yacht. The TV antenna booster concerned was just one of a 'bad batch' of that had been sold all over the US.

No 224 describes how a $40 jammer, which will fit in a shirt pocket, can intentionally block GPS for a radius of about 100 miles.

Jac Spaans, president of the Netherlands Institute of Navigation, warns against relying on GPS in No 223. And despite efforts to control EMI, No 420 describes how GPS was 'knocked out' by solar storms over the entire sunlit side of the Earth by a solar storm in December 2006.

Despite the litany of reasons why GPS is unsuitable for use where safety is an issue, it seems that organisations all over the world are planning on using GPS even for safety-critical applications - presumably the fact that it is a cheap (sorry, 'low-cost') technology is just too appealing for them. I would like to see their risk analyses, and what value they put on a life.

Keith Armstrong, Brocton, Stafford

Trusting in GPS

'A New Contellation' (Vol 4, #2) quotes satellite industry analyst Max Engel as saying, with reference to using GPS to control commercial airliners during landing: "No matter how benign the US Department of Defense is, you simply cannot trust absolute life-and-death responsibilities to a single source."

GPS and Galileo rely on very low level L-Band signals from distant (23,222km) satellites. Experience in Monterey Bay has shown that accidental jamming by malfunctioning consumer equipment can disrupt the signals and that it can be very difficult to trace the source(s). If that jamming were to be deliberate, it could be applied to both GPS and Galileo very easily.

It therefore follows that using two satellite navigation systems should be treated as using a single source and that maintaining and using the existing system is a way of providing a second source.

ILS and, hopefully, its successors are radiated locally by log-periodic aerial arrays at the end of the runways which makes them much harder to jam.

Kristen Cadman CEng MIEE, Marlow, Buckinghamshire

Arctic evidence

It's good to see from 'Explorers Take to the Ice' (Vol 4,#2) that investigation on-site in the Arctic is still the way to find out facts about the world. Explorer Pen Hadow follows in the footsteps of George Hubert Wilkins (Sir Hubert) who first plumbed the Arctic Ocean and flew across it in his endeavour to set up worldwide meteorological stations to enable better weather forecasting for the benefit of farming. His remarkable life is set out in 'The Last Explorer: Hubert Wilkins Australia's Unknown Hero' by Simon Nasht. It is a gripping read.

I have no doubt that Hadow's investigations will show that the IPCC is being unduly alarmist, as with other 'facts' highlighted in the article. For example; an expert on sea-level changes, Dr Nils-Axel Mörner, has found that the average sea level at Tuvulu has not changed over the last 30 years. They are in trouble because of over-use of their aquifers for growing pineapples for Japanese entrepreneurs. Dr Mörner has also gathered evidence on-site that the sea level in the Maldives appears to be falling and not rising.

Henry Broadbent, Somers, Victoria, Australia

Half the story

The government's 'Digital Britain' report, promising to make broadband available to every UK home by 2012, is an important step forward in helping blind and partially-sighted people play their full part in the British economy. But that's only half the story.

Guide Dogs is a pioneer in using new technology to help people with disabilities break through the barriers they face every day. And our experience shows that much thought needs to go into how the broadband potential is to be realised.

For example, all of our visually impaired volunteers and employees have software 'readers' that allow them to access their computers in general and their email specifically, even when they are away from their homes or base offices.

Britain is a leader in software innovation. 'Digital Britain' offers a great deal of potential for new opportunities. So let's hit the return button and get on with the business of changing the world - for the better.

Bridget Warr, Chief executive, Guide Dogs, Berkshire


The letter in E&T Vol 4, #3 headed 'TV Aspect Ratio' should have been attributed to IET member John L Mack, not Alan J Aldous, whose letter 'Dyson's School' appears in the same issue.

Ernest Wotton has asked us to point out that a significant sentence was omitted from his letter 'Office Blues' in Vol 4, #2. Recalling how he noticed that the red stripes in his tie appeared brownish while the blue tended to 'shriek' in a drawing office where he had been told that 'cool white' lamps had been replaced with 'full spectrum' fluorescent lamps, his letter should have gone on to say: "This should not have occurred under full-spectrum lighting since the lamps had good colour-rendering properties."


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What they're talking about on the Internet

A contributor to the transport discussion forum on the IET website asked: "A railway communication system is not as essential as a signalling system or automatic train control system. The safety and reliability requirement is not as stringent, but there should be some standards governing performance and quality. How should that be?"

It depends on how safety-critical it is. Is it to tell the passengers what the next station is, or to tell the controller that the embankment has given way?

The communication system in the railway includes subsystems with different kinds of equipment. Each has different requirements for availability and survivability - a level of assurance that the system will have essential functions available when needed.

Failure of any of these systems should not result in a train crash because none of them will cause the signalling system to display green instead of red. The message may be safety critical, but the message transmission system should not be. 

The CENELEC standard is EN 50159, parts 1 and 2. If you are sending safety-related messages, the communications network must achieve a tolerable hazard rate compatible with the safety function.

Surely this can be achieved with appropriate handshaking and error-checking algorithms, meaning point-to-point single-links can be SIL0, and you only need to worry about redundancy and availability. I thought this was common in TCP/IP and similar interfaces, which are not deterministic and there's no guaranteed delivery of packets?

It is possible to achieve SIL4 integrity in the message, leaving the communication system open.

Join the debate, or start one of your own [new window].

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