Thursday, 17 April 2014

Modern communications and disasters

The tragic sinking of The Sewol yesterday off the coast of South Korea has led to many sad stories, not the least of which are the text messages sent by people - many young - trapped in the ship as it tilted and sank.

Rumours continue that messages are being sent by people still alive within the ship. It is possible for people to be alive as it does not appear to be too deep; pictures show the tip of the bow protruding from the water. One report says that rescuers are pumping air into some parts of the hull to replenish any air pockets.

However, I doubt that many calls or text messages are being sent from within the ship at this time. Water attenuates RF signals rapidly, especially when at an angle to the receiver. It can be as much as a matter of a few inches or feet before the signal is fully attenuated, depending on the conditions. Although the effect is less than salt water, it increases as the signal's frequency increases.

So what might have happened? 
1) The people receiving the text messages may not have noticed them when they were sent, and picked up the phone later when they heard the news to see the message.

2) Text messages are not designed for reliability of data (*). They only display the time they were received at the network centre (called the short message centre). This includes a timezone which many phones do not compensate for, which may give the impression they were sent after the ship sank. In addition, many phones display the time the message was received by the phone, rather than the time in the message from the network centre. If the recipient's phone is switched off, the time displayed for the message would be the time the phone was switched on, not the time it was sent.

3) Finally, hope might make them think it was received after the ship sank even if evidence suggests otherwise.

4) Somehow, the people trapped inside the ship managed to send a signal through the water and to a distant mobile phone base station on the shore.

The first three of these effects will sadly give false hope to many families. But let's hope I'm wrong.

(*)From RFC5724:
In particular, SMS messages between different network operators sometimes take a long time to be delivered (hours or even days) or are not delivered at all, so applications SHOULD NOT make any assumptions about the reliability and performance of SMS message transmission.

Monday, 7 April 2014

Competitive walking

On a few occasions on this blog (and believe me, much more frequently in real life) I have mentioned various types of 'extreme' walks. One thing I don't like is the Olympics walking discipline, where people try to walk as fast as possible. To me, it appears a rather stupid type of movement. If you want to go fast, just run!

Gizmodo has an article about a form of extreme walking from Victorian times that I'd never heard of before: competitive walking. This is not the Olympic-style extreme walking, but instead involved walking vast distances with little rest. Naturally enough, this is much more my cup of tea.

People would walk along a short looper track for six days, excluding the Sabbath, and bunks were provided for them to catch a few hours of much-needed rest. They would often manage over 100 miles a day for all six days. The American races were front-page news, and massively popular spectator sports.

A detailed story can be found on the Planetultramarathon website, and there is also a book:
Pedestrianism: When Watching People Walk Was America's Favorite Spectator Sport

I could never hope to be the next Cliff Young, and I find the idea of walking even fifty miles in a day to be rather a tall order. But kudos to those who can, and especially those who did.

Friday, 4 April 2014

Network Rail and delays

It is expected that Network Rail will get a £70 million fine later this year for poor performance. Whilst the men and women on the ground have been doing a sterling job fixing the problems caused by the storms at Dawlish and elsewhere, the organisation as a whole has utterly failed to meet its performance targets.The Telegraph has a very balanced article on the problems.

In a way it is a victim of the railway's success: the network is busier than ever, nearly breaking peace-time records for the number of passengers carried. More trains on the same tracks means that any infrastructure-related problems effect more trains, and the infrastructure gets more wear and tear. In comparison, the performance of the latest generation of trains means that fewer delays are being caused by breakdowns.

Network Rail has to ensure that 92.5% of all trains arrive within five minutes of the scheduled time, or within ten minutes for long-distance trains. During the five year period to 2014, it managed a little under 90%, a large discrepancy.

According to Network Rail's own figures, 60% of all train delays are down to them. In their defence, a third of that 60% are down to factors outside their immediate control, such as trespass, cable theft, or the weather.

This is important politically: it looks likely that Labour are going to call for renationalisation of the railways in their next manifesto, by means of a failure to refranchise the train operators leases when they end. Leaving the wrongs and rights of such a move to one side for a moment, one of the factors fans of renationalisation have been saying is that Network Rail is already in public hands (and thanks to the government removing Brown's awful debt bodge, it is also now on the government's books). If it fails to perform well, it will set back the cause of railway renationalisation.