On Tue, 19 Feb 2013 20:09:07 +0000
"Clive D. W. Feather" wrote:
In message , d
wrote:
"The transponder sends a 2.4GHz data signal back to the reader."

2.4Ghz eh? And LU is rolling out wifi into stations. Hmm. What could
possibly
go wrong?

I'm not a techie - are you suggesting there is an interference risk?

Depends what frequencies around the 2.4Ghz band they use but if they use
ones on or near the wifi ones then yes. No doubt they used that band because
its license free but I wouldn't have thought you'd have needed a license if
its only ever used underground. I suspect the risk is small but why take it
at all?

You need a licence (or to be exempt) whether or not you're underground.

Bluetooth and WiFi in the 2.4 GHz band is limited to 10 mW and has other
restrictions on duty cycle and power density. Railway equipment in 2.446
to 2.454 is allowed 500 mW in a narrow channel ( 1.5 MHz). Given that
the application is a train passing straight over the transponder in the
four foot, I suspect that the WiFi signal won't even be noticed.

Thats all very well, but whats stopping someone shoving a bluetooth signal
through a linear amp to disrupt the comms? If you think thats a stupid thing
to do , well hackers tend to do stupid things.

Spud

In article ,
wrote:
On Tue, 19 Feb 2013 20:09:07 +0000
"Clive D. W. Feather" wrote:

Bluetooth and WiFi in the 2.4 GHz band is limited to 10 mW and has other
restrictions on duty cycle and power density. Railway equipment in 2.446
to 2.454 is allowed 500 mW in a narrow channel ( 1.5 MHz). Given that
the application is a train passing straight over the transponder in the
four foot, I suspect that the WiFi signal won't even be noticed.

Thats all very well, but whats stopping someone shoving a bluetooth signal
through a linear amp to disrupt the comms? If you think thats a stupid thing
to do , well hackers tend to do stupid things.

There was the case in China recently where repeated disruption to a
2.4GHz metro train control system was blamed on a high number of personal
wireless hubs built into passengers' equipment. This article is a bit
lacking in technical details but gives the outline:
http://english.caijing.com.cn/2012-11-20/112296950.html

Nick
--
"The Internet, a sort of ersatz counterfeit of real life"
-- Janet Street-Porter, BBC2, 19th March 1996

On Wed, 20 Feb 2013 10:18:00 +0000 (UTC)
Nick Leverton wrote:
In article ,
wrote:
On Tue, 19 Feb 2013 20:09:07 +0000
"Clive D. W. Feather" wrote:

Bluetooth and WiFi in the 2.4 GHz band is limited to 10 mW and has other
restrictions on duty cycle and power density. Railway equipment in 2.446
to 2.454 is allowed 500 mW in a narrow channel ( 1.5 MHz). Given that
the application is a train passing straight over the transponder in the
four foot, I suspect that the WiFi signal won't even be noticed.

Thats all very well, but whats stopping someone shoving a bluetooth signal
through a linear amp to disrupt the comms? If you think thats a stupid thing
to do , well hackers tend to do stupid things.

There was the case in China recently where repeated disruption to a
2.4GHz metro train control system was blamed on a high number of personal
wireless hubs built into passengers' equipment. This article is a bit
lacking in technical details but gives the outline:
http://english.caijing.com.cn/2012-11-20/112296950.html

Oh dear. Looks like LU may have saved a few pennies but potentially left
themselves with a serious problem especially if the new system on the Met
also uses 2.4 Ghz because thats mostly above ground. One hacker with a grudge
could probably bring the entire line to a halt. Or even worse, if they can
duplicate the comms protocol and override the real signal then they could
potentially cause a crash.

Spud

In message ,
d wrote:
Bluetooth and WiFi in the 2.4 GHz band is limited to 10 mW and has other
restrictions on duty cycle and power density. Railway equipment in 2.446
to 2.454 is allowed 500 mW in a narrow channel ( 1.5 MHz). Given that
the application is a train passing straight over the transponder in the
four foot, I suspect that the WiFi signal won't even be noticed.

Thats all very well, but whats stopping someone shoving a bluetooth signal
through a linear amp to disrupt the comms? If you think thats a stupid thing
to do , well hackers tend to do stupid things.

It would have to be a big linear amp. These transponders are used with
the train over them, so if the idiot is standing 1m from the platform
edge he needs about a factor of 800 amplification to reach the strength
of the transponder signal.

In any case, the only purpose of these transponders is to confirm the
train's exact position. It works something like this. The train starts
at a known position, call it zero. It count wheel rotations to work out
how far it's gone. Now suppose that these have an accuracy of 2%. Then
after it's gone what it thinks is 200 metres, its position is somewhere
in the range 196 to 204 metres from zero. After 500 metres it is
somewhere between 490 and 510, and so on. It therefore makes all its
safety decisions based on whichever limit is the more conservative.
Whenever it passes over one of these boxes, that resets its knowledge of
the position. They're supposed to be no more than 200 metres apart, so
that means it is at most 4 metres wrong in its calculations. If an idiot
disrupts one box, that simply means that the maximum error increases to
8 metres before resyncing. But the signalling will be designed to take
this into account and provide wider margins around a train that hasn't
synced recently. (Also recall that there's a 25m static margin anyway
for safety.)

--
Clive D.W. Feather | Home:
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On Thu, 21 Feb 2013 10:50:10 +0000
"Clive D. W. Feather" wrote:
It would have to be a big linear amp. These transponders are used with
the train over them, so if the idiot is standing 1m from the platform
edge he needs about a factor of 800 amplification to reach the strength
of the transponder signal.

A fair point. Though you often don't need much to cause enough interference
in a digital signal to make it unusable if there is poor or non existent
error correction.

In any case, the only purpose of these transponders is to confirm the
train's exact position. It works something like this. The train starts

Is that how the new Met system will work too?

Spud