05 Novembre 2003 1

Wireless LAN at CERN

Leena Chandran-WadiaIT Division, CERN

TNC, 7 June 2005

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What is CERN ?

European Organization for Nuclear Research(European Laboratory for Particle Physics)

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 Tunnel is 27 km in circumference

Large Electron/Positron Ring (used till 2000) Large Hadron Collider (LHC) as of 2007

Detectors as ‘big as cathedrals’ Four LHC detectors

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 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 backbones90 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

• 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• 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)

– 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

902 MHz

928 MHz

26 MHz 83.5 MHz 200 MHz

2.4 GHz2.4835 GHz

5.15 GHz5.35 GHz

33cm 12cm 5cm

5.75 GHz

100 MHz

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IEEE 802.11 standards family

MAC

MIB

DSSS FH IR

PHY

WEP

LLC

MAC Mgmt

802.11b5,11 Mbps

802.11g20+ Mbps

802.11a6,9,12,18,24

36,48,54 Mbps

OFDM

802.11isecurity

802.11fInter Access Point Protocol

802.11eQoS Enhancements

802.11h: DFS and TPCEnhancements to 802.11a

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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

30 million transmitted frames, 52 million frames with Frame Check Sequence (FCS) errors!

Signal to Noise Ratio (SNR)Building 28, main floor.

Single b/g access point in corridor

Tool: AirMagnet Surveyor

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Office environments

– Sources of noise, absorption and multi-path interference

• 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

Building 28: 802.11b/g base station running in the b/g compatible mode

Relatively low levels of usage is common!

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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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Casino KursaalInterlakenCHEP’04

525 attendees300+ Laptops200+ connectedat a time..

Conference usage

Tool: AirMagnet Surveyor

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No automatic load balancing

Users mustexplicitlyselect the ‘a’ channel!

Of 220 online,150+ servedby 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 coverageGood S/N levels

Small Cells

Plan of AP placements4flr 1 6 11

16 11 1

611 1 6

111 6 11

1R 6 11 1

6S 11 1

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 Cells Adjust to Load Balance

Failed AP 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!