32
© 2007 Cisco Systems, Inc. All rights reserved. Cisco Confidential Presentation_ID 1 Basics of Wi-Fi

Basics of Wi-Fi - employees.mscon.comemployees.mscon.com/traning/Cisco101/Class 4 CCNA Wireless Intro.pdfBasics of Wi-Fi. Presentation_ID © 2007 Cisco Systems, Inc. All rightsreserved

Embed Size (px)

Citation preview

© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1

Basics of Wi-Fi

© 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

© 2007 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 32