Transcript Document

Wireless LAN at CERN
Leena Chandran-Wadia
IT Division, CERN
05 Novembre 2003
TNC, 7 June 2005
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What is CERN ?
European Organization for Nuclear Research
(European Laboratory for Particle Physics)
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An international non-profit research organization located across the
Swiss/French border near Geneva
Frontier of Human Scientific Knowledge, endeavouring to create
‘Big bang’-like conditions
Accelerators with latest super-conducting technologies
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Detectors as ‘big as cathedrals’
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Tunnel is 27 km in circumference
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Large Electron/Positron Ring (used till 2000)
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Large Hadron Collider (LHC) as of 2007
Four LHC detectors
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ALICE, ATLAS, CMS, LHCb
Inventor of the World-Wide Web to:
“Tie all the physicists together – no matter
where they are” (Tim Berners-Lee)
World-wide participation
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Europe (20 member states) plus USA, Canada,
Brazil, Japan, China, Russia, Israel, etc.
Slide by W. von Rüden
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CERN’s Campus Network
Two distinct multi-Gigabit backbones
90 Gigabit Ethernet Routers – 1200 subnets
800 Switches – roughly 40,000 ports
600 Ethernet Hubs – roughly 15,000 ports
20,000+ Active connections & 40,000 sockets
2,000 Km of UTP cable & 2,500 Km fibers
250+ Star-Points with 20 to 1,000 outlets
All equipment is Multi-Manufacturer, standards compliant
Slide by Danny Davids
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Features of the wired network
– Extremely dynamic
• 1,500+ requests for Moves-Adds-Changes per month
– Extremely diverse
• contains everything, from PLCs, to PCs and PDAs
– Users expect 100% availability
– Entire network run by less than 30 people!
– Very high level of automation (CERN specific)
• configuration of network devices
– The database is the center of network operations and
management
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Wireless requirements at CERN
– 430 buildings, roaming within buildings
– Require 3,000 to 6,000 APs for full coverage
– Only about 200 installed so far
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Meeting rooms, cafeterias, conference rooms
LHC tunnel – 100m below ground
Atlas experimental pit
Equipment assembly halls with sensitive magnets etc.
Old, heavy concrete buildings
– Need to integrate configuration, monitoring and
management with wired network
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Contents
– Experience sharing
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Is 802.11a necessary?
For 802.11b/g, should we use large cells or small ones?
What kind of hardware? Access Points (APs) and Clients
Performance and Management concerns
– Unusual installations
• Wireless on VDSL in the LHC tunnel
• Wireless distribution systems – Atlas Cavern
• Leaky Feeder Cables as Antennas
– Security
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IEEE 802.11 WLANs
– Wireless LAN standard defined in the unlicensed spectrum
(2.4 GHz and 5 GHz U-NII bands)
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33cm
26 MHz
902 MHz
12cm
83.5 MHz
2.4 GHz
928 MHz
5cm
200 MHz
5.15 GHz
2.4835 GHz
100 MHz
5.75 GHz
5.35 GHz
– 2.4 GHz band also used by Cordless Phones, Bluetooth,
and Microwave Ovens
– 5 GHz band by Defense! (only indoors usage allowed)
• Earth Exploration Satellite Systems, Space Research
Systems, Radars
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IEEE 802.11 standards family
LLC
802.11i
security
WEP
802.11f
Inter Access Point Protocol
MAC
802.11e
MIB
PHY
QoS Enhancements
DSSS
802.11h: DFS and TPC
Enhancements to 802.11a
MAC
Mgmt
FH
802.11b
IR
OFDM
5,11 Mbps
802.11a
802.11g
20+ Mbps
6,9,12,18,24
36,48,54 Mbps
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IEEE 802.11b/g
– 802.11b - very successful technology
– 802.11g hampered by requirement of b/g
compatibility
• Reduces available bandwidth greatly (14 Mbps shared)
– Only 3 non-overlapping channels (20 MHz each)
• Interference between neighboring APs
• Adjusting cell size can help to partly overcome problem
• Price: lower bandwidth
– Noisy (2.4 GHz band crowded)
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Signal measurements
Signal to
Noise
Ratio (SNR)
Building 28,
main floor.
Single b/g
access point
in corridor
30 million transmitted frames, 52 million frames
with Frame Check Sequence (FCS) errors!
Tool: AirMagnet Surveyor
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Office environments
– Sources of noise, absorption and multi-path
interference
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Wet walls, floors
Fish tanks
People
Foliage
Tinted Glass
Metal, Concrete
Elevator shafts
– Signals spill out of glass windows on to other floors
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Small cells vs. large cells
– Use many base stations
in a controlled way
(small cells)
• To benefit from higher
transmission speeds
• For load balancing
– 55 simultaneous users
and over 30%
retransmissions (GNEW)
– Large cells make sense
• For few users
• Difficult coverage
situations
source: Proxim
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Some statistics
Relatively
low levels
of usage is
common!
Building 28: 802.11b/g base station
running in the b/g compatible mode
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IEEE 802.11a
– Slow to come to Europe
• 802.11h compliance requirements and HiperLAN
– Not subject to noise (5 GHz not crowded)
– Smaller wavelength of 802.11a generally translates
into smaller range
• Quality of radio compensates greatly
• Speeds fall off more slowly with distance
– Has many non-overlapping channels
• 8 in CH presently, 4 in France, eventually 19 everywhere!
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Conference usage
Casino Kursaal
Interlaken
CHEP’04
525 attendees
300+ Laptops
200+ connected
at a time..
Tool: AirMagnet Surveyor
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No automatic load balancing
Users must
explicitly
select the
‘a’ channel!
Of 220 online,
150+ served
by 3 b/g APs!
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More on 802.11a
– At CERN we are installing dual-band APs
• Use 802.11a to provide the bandwidth
• 802.11b/g for coverage and connectivity
– Separate SSIDs for the 802.11a and b/g network
• Have users explicitly select 802.11a
– Difficulty (comes from 802.11h): cannot specify
channel in 802.11a band
– Result: when multiple APs boot together, several
adjacent ones can come up on the same channel!
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Reflections on hardware
– Useful to have feature rich access points
• SNMP manageability
– software upgrades, configuration and monitoring
• Power over Ethernet,
• Wireless Distribution System
• Rogue Access Point Detection support
– Pays to invest in good radios
• Output power of APs must be 20dBm
• Good receive sensitivity, better than -85dBm (b/g)
• Good client utility
– stability in the presence of multiple APs
– Security
• WPA2 and 802.11i
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Rogue Access Points
Tool: AirMagnet Laptop Analyser
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Some interesting installations
• Hostel 39
– First complete installation – small cells
• LHC Tunnel
– wireless over VDSL
– wet walls!
• Hostel 38, old building, lots of concrete
– Leaky Feeder cable as Antenna
• Atlas Cavern and Assembly hall
– Wireless Distribution System (WDS)
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Hostel 39
Complete coverage
Good S/N levels
Small Cells
Plan of AP placements
4flr
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Tool: AirMagnet Surveyor
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LHC Tunnel: Wireless over VDSL
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Leaky Feeder cable for GSM
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Leaky Feeder Cable – Hostel 38
– Idea borrowed from GSM installation in tunnel
– Tests in corridor of concrete building ~ 60m long
– Preliminary results not as promising
• Carries well only to about 25 meters
• High background noise
• Coverage in adjacent rooms falls off sharply
– Possible reasons
• Transmit power too low - amplifier
• Coupling to antenna on AP very resistive
• Installation requirements not respected
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LFC: Projected coverage
Source: Radio Frequency Systems
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WDS in Atlas Cavern
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Security
– Presently
• Open network
– broadcasts SSID
– doesn’t use encryption
• Precautions
– registration process
– ‘safe applications’ (HTTPS, SSH…)
– Future (short-term)
• RADIUS for authentication
– for wired as well as wireless network
– main challenge is the diversity of devices on the network
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Some perspective
– Wireless not even as good as shared Ethernet
• All nodes cannot hear each other (fundamental
assumption in Ethernet)
• Radios are Half-duplex - cannot do Collision Detect
• CSMA/CD of Ethernet replaced by CSMA/CA
– with ACKs for collision detection and
– RTS/CTS (Request To Send, Clear to Send) for performance
– Translates into low bandwidth
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Viewpoint
– Wireless still requires too much manual adjustment
• Dynamic performance tuning for sudden high loads
– GNEW 2004, over 30% retransmissions for 55 users
– Switching on RTS/CTS would have helped
• Transmit power control
– For load balancing (full coverage scenario)
– To compensate for failures
– users are extremely tolerant of poor performance!
• because wireless is convenient and fun
– usage levels are still relatively low and sporadic
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Wireless Switching: The Promise
Heavily Loaded Cell
Failed AP
Cells Adjust to
Load Balance
Other Cells Adjust to
Provide Coverage
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Wireless Switching
– Concept
• Place Antennas only on the walls
• Concentrate all intelligence in a single Layer 2/3 switch
• Centralized management and coordination of wireless coverage
(using 802.11f protocol)
– Auto load balancing
– Auto failover
– Rogue base station detection and jamming
– Difficulties
• Separation of functionality between boxes on wall and central switch
not yet subject to any standards
• Difficult to use in Multi-vendor environments
• Exciting new features are still on paper
• Some initial offerings are using regular APs – no cost advantage
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Thank you!
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