High Density Wireless - USENIX · High Density Wireless LISA 2012 13 Fixes Lots of access points...
Transcript of High Density Wireless - USENIX · High Density Wireless LISA 2012 13 Fixes Lots of access points...
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High Density WirelessWhy doesn't the conference network work?
David LangIntuitAG6AH
[email protected] materials available at
http://talks.lang.hm/talks/topics/Wireless
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High Density Wireless LISA 2012 2
How I became the Wireless Manager for SCaLE
March 2008, Me to SCaLE networking folks:
Wireless at ScaLE didn't work, what did you do? <cringe> That's complely wrong, I can help.
Jan 2010, SCaLE networking folks to me:
You said you know something about wifi, our vendor just backed out and we will have around 1200 people arriving in a month, can you help us?
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High Density Wireless LISA 2012 3
Why does wireless always suck?● Wifi works great when setup and tested. Then
lots of people try to use it, then it doesn't degrade gracefully, it just collapses.
● Is this a fundamental limitation of Wifi?– Yes and No. It's primarily a Radio side issue
● Yes, there is limited radio airtime and when you run out collapse happens
● No, there are many things you can do to delay the collapse
● Do 'Enterprise Class' AP's solve the problem?– No, many of them make it worse
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High Density Wireless LISA 2012 4
Defining the problem
● Conferences have lots of people in small areas
● Tech Conferences have lots of devices– Scale 10x 1965 people 1935 devices
● 875 connected at once
● Radios only allow one thing to talk on a given channel at any point in time
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High Density Wireless LISA 2012 5
Radio Problems
● Not enough Channels (especially on 2.4GHz)– Should not use the same channel in nearby
locations. Ideally at least 3x service range between re-use
● Hidden Transmitters● A small amount of interference will trash the
entire packet
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High Density Wireless LISA 2012 6
Protocol Problems
● Wifi Speed fallback● “Housekeeping” traffic
– Just staying connected to an AP generates traffic
– When you get too many devices connected to one AP you have no airtime left to actually pass user data
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High Density Wireless LISA 2012 7
Digital Network Issue
● Bufferbloat– In an attempt to prevent packet loss (and with
cheap memory) network buffers have become gigantic
– If there is congestion, the packets can sit in the buffers long enough that the sender retransmits them before they arrive, so data gets sent multiple times, adding to the congestion
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High Density Wireless LISA 2012 8
How the collapse happens
● The combination of– Retries
– Hidden Transmitters
– Fallback to slow speeds
– Wasted packets due to bufferbloat
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High Density Wireless LISA 2012 9
Power Levels and Antennas
● Turning up power seldom helps● In fact it usually hurts● Better antennas are almost always better than
amplifiers
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High Density Wireless LISA 2012 10
A quick note on Enterprise APs
● Many radios in one spot● Some with directional antennas
– Hidden transmitter problem caused by antenna patterns
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High Density Wireless LISA 2012 11
This is bad, now what● So now that we've seen how the networks
collapse, what can be done to make things better?
● First off, find out what you are up against– Site survey
● MySpy spectrum analyzer● Kismet● Wifi analyzer● Bring an AP● Find the network jacks
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High Density Wireless LISA 2012 12
Fixes
Encourage 5GHz
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High Density Wireless LISA 2012 13
Fixes
● Lots of access points● Turn down power on 2.4Ghz
– How far down? Waaaay down● Scale 10x had the AP's set to 4mw
– Remember that it doesn't help to use more power than the devices you are talking to.
● Take advantage of things that block the signal– Walls, Bodies, etc. Don't create more hidden
transmitters
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High Density Wireless LISA 2012 14
Fixes
● Use advanced antennas carefully– To direct the signal away from areas more than
towards them
– To reach into areas where you don't have wires
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High Density Wireless LISA 2012 15
Digital Issues
● SSIDs– One allows roaming
– Many allow users to specify best AP
● Use one per band
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High Density Wireless LISA 2012 16
Config Items● Enable Wireless Isolation● Beacon Interval
– Advertise the SSID less frequently, delays connections but saves airtime
– Prime value to reduce sync issues
● Disable slow speeds● Kernel buffers/FQ_Codel queuing● Disable conntrak (netfilter)● Short inactivity timers
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High Density Wireless LISA 2012 17
Scale 8x LAX Westin 2010
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High Density Wireless LISA 2012 18
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 10x
2011
LAX Hilton
AP locations
1st Floor
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High Density Wireless LISA 2012 19
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 9x
2011
LAX Hilton
AP locations
2nd Floor
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High Density Wireless LISA 2012 20
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 9x
2012
LAX Hilton
AP locations
2nd Floor
(1st Floor same as 2011)
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High Density Wireless LISA 2012 21
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 10x
2012
LAX Hilton
AP locations
2nd Floor
(1st Floor same as 2011)
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High Density Wireless LISA 2012 22
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 11x
2013
LAX Hilton
AP locations
1st Floor
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High Density Wireless LISA 2012 23
http://www.hilton.com/en/hi/hotels/meeting_space.jhtml?ctyhocn=LAXAHHH
ScaLE 11x
2013
LAX Hilton
AP locations
2nd Floor
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High Density Wireless LISA 2012 24
Equipment
● 2010– Fry's FR300 2.4GHz only Stock firmware
– Netgear 5GHz only Stock firmware
● 2011– Fry's FR300 2.4GHz only DD-Wrt firmware
– Netgear 5GHz only Stock firmware
● 2012-2013– Netgear WNDR3800 dual band OpenWrt firmware