Wireless Local Area Networks
By Edmund Gean
August 2, 2000
Why Wireless?
• extension of wired LAN
• alternative for a wired LAN
• ease of installation
• mobile users
Wireless Terminology
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WLAN
802.11
basic service set (BSS, cell)
DSSS vs FSSS
Hardware vs Software Access Points
channel
SSID
peer-to-peer mode
infrastructure mode
802.11
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IEEE 802.11 standard created in 1997 to facilitate
interoperability between differing brands of wireless LANS
• specifies 2 Mbps data rate with fallback to 1 Mbps
• IEEE 802.11 High Rate standard ratified in 1999 allows
11 Mbps (backward compatible to 802.11)
• allows choice of different physical layers. Vendors use
either frequency hopping spread spectrum (FHSP) or direct
sequence spread spectrum (DSSS)
• specifies medium access control (MAC) and physical
characteristics for wireless LANs
• radio frequencies in range of 2.4 GHz - 2.483 GHz
ISM Frequency Bands
Physical Layer
• Direct Sequence Spread Spectrum
- employs a high speed code sequence, along
with the basic information being sent, to
modulate a RF carrier.
- 20 MHz bandwidth per channel
- 11 overlapping Channels in US
- 3 non-overlapping channels
•Frequency Hopping Spread Spectrum
- uses a code sequence to "hop" from frequency to
frequency over a wide band.
DSSS vs FHSS
Media Access Control Layer
•Uses Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
- distributed inter frame space (DIFS)
- exponential random backoff time
- short inter frame space (SIFS)
- no collision detection (hidden node problem)
- collision avoidance via RTS/CTS frames
•similar to CSMA/CD in standard IEEE
802.3 Ethernet
Compaq Wireless Equipment
Configuration
•Peer-to-peer (ad-hoc) mode
•Infrastructure mode
Ad-Hoc Mode
Infrastructure Mode
Software Access Point
Infrastructure Mode with Hardware
Access Point
Infrastructure Mode with Multiple
Hardware Access Points
Configuring Wireless Network using
Compaq Wireless Hardware Access
Point
•install Access Point Manager
•install hardware access point
•configure hardware access point using AP
Manager (IP address, channel #, community
string)
•install wireless PC card into PCMCIA slot of
notebook
•install wireless drivers onto notebook
•set value of Service Set Identifier (SSID) same
as that of hardware access point
Demonstration
Conclusion
• interoperability of WLAN networking
equipment
• use of WLAN at Cal State LA