Thursday, 10 July 2014

Cambridge public Wi-Fi project

A brief diversion from the technical routing and switching bits and a bit of climbing about on roofs (although not as good as this!)...

We recently engaged on a project to Wi-Fi chunks of the centre of Cambridge, focussing on the route of stage 3 of the Tour de France, which started in Cambridge and finished in London.  The presented a number of problems - getting Wi-Fi to places we didn't have cabling to, coping with interference on the RF bands and also handling the large number of clients.

Mesh link to the Guildhall

We've done Wi-Fi bridges before but we had to get one particularly tricky one going: from the New Museums Site over to the city council-owned Guildhall so we could cover the Market Square.

The uplink end was fixed on top of the Austin Building on the New Museums Site: the picture below shows the view over to the Guildhall.  The Guildhall end of the link is pointed to by the red arrow — if you zoom the image to full size, you'll see the other end: it's not the bigger white blob at the tip of the arrow (that's an unfortunately-placed satellite dish) but the tiny two-pixel dot slightly up and to the right.  That location was restricted by the space the city council would give us on the roof (hence the satellite dish almost in the way!)

An Aruba AP-175P was fixed to the railings on the roof and a pair of antennae were attached: an ANT-2X2-5614, a directional 5GHz link antennae pointing over to the Guildhall to run the link, and an ANT-2X2-D805, a ~180-degree 2.4GHz antenna to cover the car park on the New Museums Site (to avoid the 2.4GHz radio going to waste).  You can see those both here:

... the AP is in the sun shield on the left hand side, the D805 is the panel pointing down and to the right and the 5614 you can just see end-on, just to the left of the D805.

The AP itself you can see here, with it's lightning arresters / band pass filters (BPFs) sticking out of the antenna terminals:

The Guildhall end was stuck on a pole and had an AP-175AC mounted on the wall, just below.  Here you can see the AP removed and Alexander Cox (our wireless surveyor and installer in the high-vis jacket) and Giles Scott (from Aruba, who assisted with an RF issue, below) poking about:

The 5614 here points back at the New Museums Site end and the D805 covers the Market Place:

We've since then used the PoE ethernet port on the AP-175AC as a bridge of the management network so, in addition to the link running the AP itself, it extends to another AP-175P on the Guildhall, enhancing coverage of the Market Place with a pair of D805s on (one for 2.4GHz and another for 5GHz).  The D805 on the link AP now points down Petty Cury.

RF interference

So did it work?  Well - not at first, we had problems with the 2.4GHz antenna giving very poor coverage: client devices could see the AP with a good signal but couldn't connect — the AP reported no attempts to connect.  It was like the client could receive but the signal wasn't making it back to the AP.

After some more investigation, this turned out to be exactly what it was — we were getting a lot of interference from mobile phone signals in the city centre area.  In particular, the top of the Guildhall also has some other antennae on it:

These are Vodafone 3G masts, transmitting at 2.1GHz (a frequency used by UMTS in Europe) causing lots of interference.  The Aruba AP-175 series don't have Band Pass Filters (BPFs) and so are susceptible to interference around the Wi-Fi frequencies.

A colleague had spoken to a third party supplier regarding lightning arresters and if the Aruba ones were suitable.  The third party supplier suggested some alternative model and initially these were used, in place of the Aruba ones.

However, the Aruba ones also contain BPFs: the LAR-1 filters everything outside 2-5GHz and the LAR-24 outside 2.3-2.5GHz.  The LAR-1 should be used on the 5GHz antennae and the LAR-24 on the 2.4GHz antennae.

Once the lightning arresters with the BPFs were installed, everything sprung into life.  As I write this, the link is showing up at 300Mbit/s in both directions and has been stable for over 7 days.  Considering there's that annoying satellite dish in the way, that's pretty good!

There were several other locations where this occurred, although some were also fine.  However, all of them have since been swapped.

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