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.

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