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© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 2
Wireless LANs (WLAN)
�Based on IEEE 802.11 protocol
�Otherwise known as WiFi
�Uses unlicensed frequencies
�Half-duplex medium
CSMA/CA
RTS – CTS plus ACK
�Clients make association decision
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 3
Wireless Agencies and Standards Bodies
� Institute of Electrical and Electronics Engineers (IEEE)
� Federal Communications Commission (FCC)
� European Telecommunications Standards Institute (ETSi)
� Wi-Fi Alliance
� Creates and maintains operational standards
� Regulates the use of wireless devices in the US
� Chartered to produce common standards in Europe
� Promotes and tests for WLAN interoperability
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 4
Unlicensed Frequencies (US)
� 3 bands released by FCC
� 900Mhz and 2.4Ghz are aka Industrial, Scientific, and Medical (ISM) bands
� 5Ghz band aka Unlicensed National Information Infrastructure (UNII) band
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 5
802.11
� First standard released in 1997
� Worked at 1Mbps and 2Mbps
� Runs in 2.4Ghz frequency
� Very few products released at the time
� 3 Non overlapping channels in the US
22Mhz wide channels
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 6
802.11b
� Most widely deployed
� Operates at 2.4Ghz unlicensed spectrum
� Max data rate of 11Mbps
� Allows for rate shifting
Occurs on a transmission by transmission basis
� Uses modulation technique call Direct Sequence Spread Spectrum (DSSS)
� Furthest range (350 feet)
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 7
802.11g
� Backwards compatible with 802.11b
� Also operates in 2.4Ghz
� Supports speeds up to 54Mbps
� Uses modulation technique of Orthogonal Frequency Division Muliplexing (OFDM)
� Medium range (300 feet)
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 8
802.11a
� Operates in the 5Ghz band
� US there are 23 Non-overlapping channels
12 channels if not using DFS/TPC
20Mhz wide channels
� Speeds up to 54Mbps
� NOT backwards compatible with 802.11b or g
� DFS (Dynamic Frequency Selection) needed on some of the channels where the AP detects radar interference and unoccupies the channel [802.11h]
� TPC (Transmit Power Control) also needed for some of the channels. [802.11h]
� Short Range (200 feet)
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 9
Typical WLAN Network Topology
Basic Service Set (BSS)
WiFi Clients
Access PointAccess Point
Basic Service Set (BSS)
Extended Service Set (ESS)
Ethernet Distribution System (DS)
Access Points manage all 802.11 traffic and details for the clients attached to them. Clients cannot talk to each other, must go through AP. Otherwise known as infrastructure mode.
Radiating Service Set Identifier (SSID)
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 10
Independent Basic Service Set (IBSS)
SSID
- Otherwise known as AdHoc
- Generally used for small networks
- NO Access point
- Connects two or more clients in a peer-to-peer wireless network
- All clients are equal
- Area covered is called Independent Basic Service Set (IBSS)
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 11
Security
� Open
Typically seen at hotspots (cafes, airports, hotels)
� WEP (IEEE 802.11)
Uses shared-key authentication
AP sends challenge text packet
Either 40 bits or 128 bits
Can be easily hacked
� MAC Filters
Place client MAC addresses on AP to allow/prevent access
Administrative nightmare!
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 12
Security (con’t)
� WPA (WiFi Alliance)
Stands for WiFi Protected Access
Uses TKIP encryption as mandatory, AES is optional
Can use either Pre-share Key (PSK) or 802.1x (WPA Enterprise)
� WPA2 (IEEE 802.11i)
AES is mandatory
Also uses either PSK or 802.1x
Most secure methodology
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 13
MIMO40Mhz
ChannelsPacket
AggregationBackward
Compatibility
Technical Elements of 802.11n
MIMO 40Mhz ChannelsPacket
AggregationBackward
Compatibility
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 14
Performance
Aspects of 802.11n
Beam Forming Spatial MultiplexingMaximal Ratio Combining
MIMO (Multiple Input, Multiple Output)
MIMO 40Mhz ChannelsPacket
AggregationBackward
Compatibility
Performed by Transmitter (Talk Better)
Ensures Signal Received in Phase
Increases Receive Sensitivity
Works with non-MIMO and MIMO Clients
MIMO AP
HALLWAY
Without Beam Forming Transmissions Arrive out of Phase
With Beam Forming Transmissions Arrive in Phase, Increasing Signal Strength
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 15
Aspects of 802.11n
Beam Forming Spatial MultiplexingMaximal Ratio Combining
MIMO (Multiple Input, Multiple Output)
40Mhz ChannelsPacket
AggregationBackward
Compatibility
Performed by Receiver (Hear Better)
Combines Multiple Received Signals
Increases Receive Sensitivity
Works with non-MIMO and MIMO Clients
Performance
Multiple Signals Sent; One Signal Chosen
Without MRC
MIMO AP
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 16
Aspects of 802.11n
Beam Forming Spatial MultiplexingMaximal Ratio Combining
MIMO (Multiple Input, Multiple Output)
40Mhz ChannelsPacket
AggregationBackward
Compatibility
Performed by Receiver (Hear Better)
Combines Multiple Received Signals
Increases Receive Sensitivity
Works with non-MIMO and MIMO Clients
Performance
Multiple Signals Sent and Combined at the Receiver Increasing Fidelity
With MRC
MIMO AP
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 17
Aspects of 802.11n
Beam Forming Spatial MultiplexingMaximal Ratio Combining
MIMO (Multiple Input, Multiple Output)
40Mhz ChannelsPacket
AggregationBackward
Compatibility
Transmitter and Receiver Participate
Concurrent Transmission on Same Channel
Increases Bandwidth
Requires MIMO Client
Performance
stream 1
stream 2
Information Is Split and Transmitted on Multiple Streams
MIMO AP
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 18
20-MHz
20-MHz
40-MHzGained Space
MIMO (Multiple Input, Multiple Output)40Mhz Channels
Aspects of 802.11n
MIMO 40Mhz ChannelsPacket
AggregationBackward
Compatibility
Moving from 2 to 4 Lanes
40-MHz = 2 aggregated 20-MHz channels—takes advantage of the reserved channel space through bonding to gain more than double the data rate of
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 19
20-MHz
20-MHz
40-MHzGained Space
MIMO (Multiple Input, Multiple Output)40Mhz Channels
Aspects of 802.11n
MIMO 40Mhz ChannelsPacket
AggregationBackward
Compatibility
Moving from 2 to 4 Lanes
40-MHz = 2 aggregated 20-MHz channels—takes advantage of the reserved channel space through bonding to gain more than double the data rate of
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 20
40Mhz Channels
Aspects of 802.11n
Packet Aggregation
40Mhz ChannelsPacket
AggregationMIMO
Backward Compatibility
Carpooling Is More Efficient Than Driving Alone
Without Packet Aggregation
Data Unit
Packet
802.11n Overhead
Data Unit
Packet
802.11n Overhead
Data Unit
Packet
802.11n Overhead
With Packet Aggregation
Data Unit
Packet
802.11n Overhead
PacketPacket
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 21
Packet AggregationBackward Compatibility
Aspects of 802.11n
Packet Aggregation
Backward Compatibility
MIMO 40Mhz Channels
2.4GHz 5GHz
802.11ABG Clients Interoperate with 11n AND Experience Performance Improvements
11n Operates in Both
Frequencies
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 22
Channel Throughput by Protocol
Protocol Throughput (Mbps)
802.11b 7.2
802.11b/g mix 13
802.11g 24
802.11a 25
802.11n 150
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 23
Channel Re-use� Co-channel interference from Client Radios will be the single biggest obstacle
Data Rate Required
SNR (10% PER)
Data Rate Required
SNR (10% PER)
1* 0 12 5.5
2* 3 18 7.5
5.5* 6 24 10.5
6 2 36 12.5
9* 5 48 17
11* 9 54 19
* = DSSS rate
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 24
What is CCA?
� 802.11 is CSMA/CA – collision avoidance
� CCA is Clear Channel Assessment – and is the listen before talk component of Collision Avoidance
� 2 CCA methods supported by the specification:ED – Energy Detect (widely supported)
Preamble (not so much, requires more power and cycles and is slower)
� CCA – ED is -65 dBm for 802.11b/g/a
� CCA for 802.11a drops by 20 dBm if ED is positive and requires preamble at that level to clear – so -85 dBm
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 25
Contention – 802.11
� Virtual carrier sense – NAV also a factor in throughput
� Detection levels for 802.11b, 802.11g, and 802.11a separate
� CCA, CW, and NAV work together to prevent mid air collisions of packets on the air interface
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 26
Duty Cycle – and Spectrum Capacity
� Duty Cycle is the on time of a given transmitter
� It is measured as percentage of total time available, this relates directly to channel utilization
� 802.11 can only do essentially two things to recover in a challenging RF environment
Retransmit a Frame – Turn the radio on again to send information that has already been sent once = Increased Duty Cycle
Rate shift to a slower speed that can be supported – If retries are excessive, then the link will be rate shifted to a slower speed in an attempt to gain reliability
� Both of these will increase Duty Cycle and make the problem worse if it is a dense network
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 27
802.11B/G/A/N and Duty Cycle
Beacon SizeDSSS
100 200 250 300 350
1 896 1696 2096 2496 2896
2 496 896 1096 1296 1496
5.5 241 387 460 532 605
11 169 241 278 314 351
OFDM
6 153 287 353 420 487
12 87 153 187 220 253
24 53 87 103 120 137
54 35 50 57 64 72
130 26 32 35 38 42
300 23 25 27 28 29
Time µS
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 28
Receiver SensitivityExample for 2.4GHz Direct Sequence
� Indication of the ability of the receiver to decode the desired signal
� The minimum receivedsignal level, in the absence of interference, at which the desired signal can be decoded with a particular Bit ErrorRate (BER)
� Typically expressed in dBm
� The more negative the value,the better
� Function of the data rate: the higher the data rate, the higher the receiver sensitivity required Receiver Noise Floor
(Will Vary for Each Environment)
-92 dBmReceiver Sensitivity @ 2 Mbps
Receiver Sensitivity @ 1 Mbps
-94 dBm
-98 dBm
-90 dBmReceiver Sensitivity @ 5.5 Mbps
-87 dBmReceiver Sensitivity @ 11 Mbps
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 29
Coverage and Capacity example
� AP placement, antenna type and radio power all determine the cell coverage area
� Each cell handles multiple clients
� More cells = more wireless bandwidth (Capacity)
� Cell overlap is ok for different channel numbers but bad for same channel numbers
Coverage and Limited Capacity
Coverage AND Higher Capacity
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 30
Antenna Radiation Patterns
� Antenna choice plays a critical part in design for proper coverage
© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 31
Data Rate and Performance Variance
� Data rates decrease with the increase of distance from the radio source
� Throughput (performance) varies with the number of users
� Performance degrades with radio interference from other sources
� Critical deployment design goal is to achieve high data rate at cell boundary
High signal AND low noise