Wireless Local Area NetworksWireless Local Area Networks

By Edmund Gean August 2, 2000

Why Wireless?Why Wireless?

• extension of wired LAN

• alternative for a wired LAN

• ease of installation

• mobile users

Wireless TerminologyWireless Terminology

• WLAN• 802.11• basic service set (BSS, cell)• DSSS vs FSSS• Hardware vs Software Access Points• channel• SSID• peer-to-peer mode• infrastructure mode

802.11802.11

• 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 BandsISM Frequency Bands

Physical LayerPhysical 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 FHSSDSSS vs FHSS

Media Access Control LayerMedia 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 EquipmentCompaq Wireless Equipment

ConfigurationConfiguration

•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

ConclusionConclusion

• interoperability of WLAN networking interoperability of WLAN networking equipmentequipment

• use of WLAN at Cal State LAuse of WLAN at Cal State LA