Post on 18-Dec-2015
transcript
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– i.e. Connectivity & Mobility
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)
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)
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
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
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
Clients and Access Points
LAN
Typical configuration
Multiple access points & roaming
Roaming or Handover?This means mobiles can move between different coverage areas
LAN
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
The use of directional antennas
LAN
LAN
Does this make sense?
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
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
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
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
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
OSI Layers
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)
BSS & BSA
BSS: Basic Service SetThis figures out a coverage areaSTA1: Station 1
BSA: Basic Service Area BSS
Dynamic association between BSS and STAx
DS & ESS
DS: Distribution SystemAP: Access Point
2 BSSs can overlap – Co-located coverage areas
LAN
11b Operating Channels
Overlapping - close coverage areas should not overlap
Non overlapping
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
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
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
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
Frame Classes
• Control• Management• Data• Class 1
– Control, Management & Data
• Class 2– Management
• Class 3– Control, Management & Data
Control Frames
• Acknowledgment (ACK)
• CF-End + CF-Ack
• Clear To Send (CTS)
• Contention-Free (CF)-End
• Power Save (PS)-Poll
• Request To Send (RTS)
Management Frames• Announcement traffic indication message (ATIM)• Association request• Association response• Authentication• Beacon• Deauthentication• Disassociation• Probe request• Probe response• Reassociation request• Reassociation response
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)
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
MAC frame format - Control
FrameControl
FCSDuration
MAC Header
ProtocolVersion
TypeSubType
ToDS
FromDS
MoreFragment
RetryPowerMgmt
MoreData
WEP Order
RA TA
Example of RTS control frame
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
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.
Type Coding Management
Type Coding Control & Data
Architectural services
• Station services– Authentication & Deauthentication– Privacy - WEP– MSDU delivery - Data delivery
• Distribution system services– Association, Disassociation & Reassociation– Distribution– Integration (to the LAN)
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)
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
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
Antenna
Antenna
Medium Access
• Carrier Sense Multiple Access with Collision Avoidance
CSMA/CA
• Carrier Sense Multiple Access with Collision Detection
CSMA/CD
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
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
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
Back-Off
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
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
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
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
Contention Window
Exposed Node
A B C
- B is sending to A- C wants to send to D (C = Exposed Node)
D
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
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