How do they block wifi

Abz Hussain · 25 Aug 2014 at 08:45

So on London Underground you can now get free wifi, whoopee. It is available on the platforms. What i want to know is this, I work from a portacabin, maybe about 10-20M below and the same distance away from one of these wifi enabled platforms/stations but i cannot pick up the virgin wifi signal from the cabin. I can pick it up on my phone no problem at all when on the platform. How are they preventing it from being accessible over a larger range? Is there some dirty tricks going on?

dalepearson · 25 Aug 2014 at 08:47

Most likely using directional antennas to reduce the wifi bleed out of the desired zone.

KIA · 25 Aug 2014 at 12:18

It isn't free. They have no need to artificially limit the coverage. https://www.tfl.gov.uk/campaign/station-wifi

novaenough · 25 Aug 2014 at 12:34

I'd guess that you being 20+ meters away, underground and in a metal box will be reducing the chances of you picking up a signal. Also - as dalepearson suggested - if they are using directional antenna that will make it even harder to pick up as you would be way off their desired axis.

Mattus · 25 Aug 2014 at 13:03

The access points are Cisco gear - they look like this.

Lisapete · 25 Aug 2014 at 13:04

Installed using this http://www.solwise.co.uk/wireless-leaky-feeder-cable.htm

If your 10-20 M away then range will be the factor. If your below said platform again it's not going to go through clay, mud and rock and reinforced concrete. If your portacabin has a metal skin again this wont help.

Caged · 25 Aug 2014 at 14:01

It's an incredibly challenging RF environment, hence the external antennas and the sheer quantity of access points. You've said nothing about what is in between you and the access point.

Abz Hussain · 25 Aug 2014 at 15:10

Caged said:
It's an incredibly challenging RF environment, hence the external antennas and the sheer quantity of access points. You've said nothing about what is in between you and the access point.

Shrubs.
The portacabin lol, i would hardly call it metal, more like chocolate.
It must be to prevent staff having access to it me reckons - which i am.

Mattus · 25 Aug 2014 at 15:13

Abz Hussain said:
It must be to prevent staff having access to it me reckons - which i am.

You reckon they're going to block the signal just so you can't access it?

They really don't care whether you use the wi-fi or not.

spoffle · 25 Aug 2014 at 15:16

Abz Hussain said:
Shrubs.
The portacabin lol, i would hardly call it metal, more like chocolate.
It must be to prevent staff having access to it me reckons - which i am.

Yeah, no.

Caged · 25 Aug 2014 at 16:26

How are there underground shrubs?

Abz Hussain · 25 Aug 2014 at 16:44

Caged said:
How are there underground shrubs?

theres a lot of thin 'shrubbery' between me and the platforms. the platforms are above me at an angle of about 45degs and 20m ish distance. i can hear passenger chatter on the platforms from where i am. Im not sure where the antenna is though, i will have to observe this next time.

Caged · 25 Aug 2014 at 18:39

Well then the reason you can't get Wi-Fi is a simple range issue. The power on the APs will be turned down to prevent interference from other units, and considering they often use 3 or 4 APs to cover an 80-100m platform why are you surprised that you can't pick a signal up through 20m of shrubs and a portakabin wall?

ubern00b · 25 Aug 2014 at 18:52

Caged said:
Well then the reason you can't get Wi-Fi is a simple range issue. The power on the APs will be turned down to prevent interference from other units, and considering they often use 3 or 4 APs to cover an 80-100m platform why are you surprised that you can't pick a signal up through 20m of shrubs and a portakabin wall?

Because a conspiracy is easier to understand than the underlying technology.

Abz Hussain · 26 Aug 2014 at 07:05

Caged said:
why are you surprised that you can't pick a signal up through 20m of shrubs and a portakabin wall?

Because i can pick up other private LUL wifi hotspots and residential/business BT WIFI hotspots, albeit with 2-3 bar signals.

PistolPete · 27 Aug 2014 at 17:56

Abz Hussain said:
Because i can pick up other private LUL wifi hotspots and residential/business BT WIFI hotspots, albeit with 2-3 bar signals.

Which are regular omnidirectional antenna access points and not using leaky co-ax feeders. Different technology, different "rules".