wireless lan - wlan
DESCRIPTION
Wireless LAN - WLAN. Mohamed Mokdad Ecole d’Ingénieurs de Bienne. Agenda. WLAN Basics WLAN Configurations Customer considerations Components Frequencies Frames. First approach. (Wired) LAN is a data network Wireless LAN is an RF extension to LAN Data over the air interface No wires - PowerPoint PPT PresentationTRANSCRIPT
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Wireless LAN - WLAN
Mohamed Mokdad
Ecole d’Ingénieurs de Bienne
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Agenda
• WLAN Basics
• WLAN Configurations
• Customer considerations
• Components
• Frequencies
• Frames
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First approach
• (Wired) LAN is a data network
• Wireless LAN is an RF extension to LAN
• Data over the air interface
• No wires– i.e. Connectivity & Mobility
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Why WLAN?
• Mobility for data– As for voice, i.e. GSM (PWLAN)
• Installation's speed and simplicity– No wiring needed
• Installation flexibility– Easy reconfiguration
• Scalability– WLAN can expand easily (Roaming)
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The air interface technologies
• Narrowband– Classical RF transmission
• Spread spectrum (DSSS)– Sounds like noise if not tuned– Reliability, Integrity & Security
• Infrared– Very high frequencies just below visible
spectrum (Not very used in WLAN)
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Spread Spectrum Technology
• Direct-Sequence - DSSS– Robust to noise & interference– Resistant to multipath effects– More secure, harder to detect– Higher throughput
• Frequency-Hopping - FHSS– Simple implementation – Less power consumption– Can add overlapping capacity with orthogonal
hopping sequences
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Typical configuration
• An Access Point - AP - connects– The LAN to– Wireless LAN
• An AP can support a small group of users in a range of 100 to 300 m.
• The antenna is attached to the AP and defines the coverage area.– Isotropic or directional antennas
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Peer to peer network
- This configuration needs no AP and no network in order to connect the 2 PCs
- The existing PCMCIA Cards generally allow this configuration
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Clients and Access Points
LAN
Typical configuration
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Multiple access points & roaming
Roaming or Handover?This means mobiles can move between different coverage areas
LAN
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Use of an extension point
LAN
The extension point extends the coverage area of the AP without addingany complexity to the AP architecture. E.g. Hot Spot in Airports
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The use of directional antennas
LAN
LAN
Does this make sense?
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Customer considerations• Range and coverage
– Distance over which equipment can communicate
– Radiated power in the coverage area - CA
• Throughput– The mean bit rate (bps) a user can expect– N users need less bandwidth than N x times
• Integrity and reliability– Protect your WLAN from taping - Small CAs– Protect your AP - Encrypted access & data
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Customer considerations 2
• Compatibility with the existing network– Many different backbones supported– WLAN should be multivendor compatible
• Interoperability of wireless devices– Air transmission technology (FHSS, DSSS)– Channel allocation (not all channels available)
• Interference and Coexistence– Spectrum overlap– Different vendors can interfere
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Customer considerations 3
• WLAN Licensing issues– Free spectrum or licensed - Hyperlan? ISM?
• Simplicity/Ease of Use– A configured Wireless LAN can be moved with
little or no modification at all
• Security– WLANs are less secure than LANs
• Cost– Main factors: Coverage area # of APs
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Customer considerations 4
• Scalability– Extension of coverage area– Support of additional APs at no costs
• Battery Life for Mobile Platforms– This is rather a hand held computer issue– End user wireless products with less power
• Safety– Much less than hand held cellular phones– Safety deals with human safety
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ISM
• Industrial/Scientific/Medical bands• 902-928 MHz
• Crowded: Cordless phone, wireless speaker, garage door (telemetry)
• 2400-2483.5 MHz• Medium use, Microwave oven
• 5725-5850 MHz• US & Canada only• Light use, some radar • Expensive
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OSI Layers
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Configurations
• Wireless LAN– Specified in IEEE 802.11– Specify Physical and MAC layers
• Stations– Portable: fixed when used– Mobile: mobile during usage
• Emitted Power– 100 mW (1 W in US and 10 mW in Japan)– Impacts directly coverage area (bandwidth)
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BSS & BSA
BSS: Basic Service SetThis figures out a coverage areaSTA1: Station 1
BSA: Basic Service Area BSS
Dynamic association between BSS and STAx
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DS & ESS
DS: Distribution SystemAP: Access Point
2 BSSs can overlap – Co-located coverage areas
LAN
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11b Operating Channels
Overlapping - close coverage areas should not overlap
Non overlapping
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11b Channels sets for Europe
SetNumber of channels
HR/DSSS Channel number
1 3 1,7,13
2 6 1,3,5,7,9,11
HR&DSSS: High Rate / Direct Sequence Spread Spectrum
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11b Regulation agencies
Code point Regulatory agency Region
X'10' FCC United States X'20' IC Canada X'30' ETSI Most of Europe X'31' Spain Spain X'32' France France X'40' MKK Japan
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Channels allocationRegulatory domains
X'10' X'20' X'30' X'31' X'32' X'40' CHNL_ID Frequency FCC IC ETSI Spain France MKK1 2412 MHz X X X — — —2 2417 MHz X X X — — —3 2422 MHz X X X — — —4 2427 MHz X X X — — —5 2432 MHz X X X — — —6 2437 MHz X X X — — —7 2442 MHz X X X — — —8 2447 MHz X X X — — —9 2452 MHz X X X — — —10 2457 MHz X X X X X —11 2462 MHz X X X X X —12 2467 MHz — — X — X —13 2472 MHz — — X — X —14 2484 MHz — — — — — X
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2 state variables & 3 logical states
State 1Unauthenticated
Unassociated
State 3Authenticated
Associated
State 2AuthenticatedUnassociated
Class 1Frames
Class 1,2Frames
Class 1,2,3Frames
SuccessfulAuthentication
SuccessfulAssociation orReassociation
DeauthenticationNotification
DeassociationNotification
DeauthenticationNotification
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Frame Classes
• Control• Management• Data• Class 1
– Control, Management & Data
• Class 2– Management
• Class 3– Control, Management & Data
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Control Frames
• Acknowledgment (ACK)
• CF-End + CF-Ack
• Clear To Send (CTS)
• Contention-Free (CF)-End
• Power Save (PS)-Poll
• Request To Send (RTS)
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Management Frames• Announcement traffic indication message (ATIM)• Association request• Association response• Authentication• Beacon• Deauthentication• Disassociation• Probe request• Probe response• Reassociation request• Reassociation response
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Data Frames
• CF-Ack (no data)
• CF-Ack + CF-Poll (no data)
• CF-Poll (no data)
• Data + CF-Ack
• Data + CF-Ack + CF-Poll
• Data + CF-Poll
• Null function (no data)
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MAC frame format - General
FrameControl
DurationID
Address1
FrameBody
FCSAddress
2Address
3Sequence
ControlAddress
3
MAC Header
ProtocolVersion
TypeSubType
ToDS
FromDS
MoreFragment
RetryPowerMgmt
MoreData
WEP Order
Power Management = From Station to say 1: power save or 0: Active mode Retry for corrupted framesMore fragments for segmented framesDuration ID = either AID (Station Association Identity) or Frame durationSequence Control Field = Frames (12 bits) and Fragments (4bits) numbering
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MAC frame format - Control
FrameControl
FCSDuration
MAC Header
ProtocolVersion
TypeSubType
ToDS
FromDS
MoreFragment
RetryPowerMgmt
MoreData
WEP Order
RA TA
Example of RTS control frame
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Address Fields
DA Destination Address of the MSDURA Receiver Address - Address of STA in the APSA Source Address of the MSDUTA Transmitter Address - Address of STA in the APBSSID BSS ID
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Address Fields bis
• To DS = 0 & From DS = 0– A data frame direct from one STA to another STA
within the same IBSS, as well as all management and control type frames.
• To DS = 1 & From DS = 0– Data frame destined for the DS.
• To DS = 0 & From DS = 1– Data frame exiting the DS.
• To DS = 1 & From DS = 1– Wireless distribution system (WDS) frame being
distributed from one AP to another AP.
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Type Coding Management
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Type Coding Control & Data
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Architectural services
• Station services– Authentication & Deauthentication– Privacy - WEP– MSDU delivery - Data delivery
• Distribution system services– Association, Disassociation & Reassociation– Distribution– Integration (to the LAN)
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Security
• Service set identifier (SSID)– SSID associated with an AP or a group
of APs. Client must know SSID
• Media Access Control (MAC) address filtering – No access to client’s MAC not in this list.
• Wired Equivalent Privacy (WEP) – Encryption (not strong)
• Others: IP range definition (e.g. WPA1/2)
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WEP: Wired Equivalent Privacy
• WEP provides equivalent functionality to the wired LAN– It is reasonably strong: from 40 to 128 bits– It is self-synchronizing: in each packet– It is efficient: it works– It may be exportable: US & Europe issue
• E.g Internet Explorer @ 128 kbps
• It is optional: It’s an IEEE option
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WEP
• 40 to 256 bits
• Devices can work with lower encryption level - 120 bits can work @ 64 and 40 bits
• Encryption level determined by key length
• Lower level with 10 Hex characters– i.e. 5 ASCII characters
• Highest level with 64 Hex characters– i.e. 32 ASCII characters
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Antenna
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Antenna
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Medium Access
• Carrier Sense Multiple Access with Collision Avoidance
CSMA/CA
• Carrier Sense Multiple Access with Collision Detection
CSMA/CD
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LAN - CSMA/CD
• Supports two or more nodes on common Supports two or more nodes on common busbus
• Node postpones transmission of data Node postpones transmission of data packets until network is clear of trafficpackets until network is clear of traffic
• Other nodes transmit in event of collisionOther nodes transmit in event of collision
• Back off period before retransmissionBack off period before retransmission
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LAN - CSMA/CD
A B
B will detect tranmission from AIt stops its transmission andSend the ethernet jam sequence, i.e. 32 bitsCollision detected with higher current values
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Back-Off• Detect collision N
– Start with N = 0
• After a collision– Increment N and Increment K (if K < 10)– Calculate number set and pick R (Random)– {0,1,2,3} for N = 2 (and K = 2)– {0,1,2,3,4,5,6,7} for N = 3 (and K = 3)– Items number = 2K (max 2K = 1024)– R defines the wait time, i.e. R*51.2 μs
• The collision probability gets lower
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Back-Off
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WLAN - CSMA/CA
• Cannot detect data packet collisionCannot detect data packet collision• Detects medium busyDetects medium busy • Node can transmit Request to Send (RTS) Node can transmit Request to Send (RTS)
to destinationto destination • Destination can transmit Clear to Send Destination can transmit Clear to Send
(CTS) to originating node(CTS) to originating node • Destination can transmit Destination can transmit
Acknowledgement when data packet has Acknowledgement when data packet has been receivedbeen received
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ACK
WLAN - CSMA/CA
Node A
Node B
Node C
D MPDU S A D CW MPDU
D = DCF Interframe SpaceDCF = Distributed Coordination Function MPDU = MAC Protocol Data UnitS = Short Interframe SpaceCW = Contention WindowA = Acknowledgment
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Hidden Node B
A B C
- A is sending to B- C is out of range of A’s transmission
C CSs and transmits Collision
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RTS
Use RTS & CTS
ACK
Node A
Node B
Node C
D MPDU S A D
CTS
C Not allowed for sending
RTS ) Ready To SendCTS ) Clear To Send
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Contention Window
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Exposed Node
A B C
- B is sending to A- C wants to send to D (C = Exposed Node)
D
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Solve
A CB
C waits since it hears B’s CTS
RTS
CTS CTS
A CB
C doesn’t wait since it doesn’t hear A’s CTS
CTS
RTS RTS
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MIBs and SNMP
• MIB (format)– ieeedot11– rfc1213– Proprietary, e.g. Cisco
• SNMP (+ SMNP Agent in device)– Access objects– 1.3.6.1.2.1.11.25.0– iso.org.dod.internet.mgmt.mib-2.snmpOutGetRequests.0
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