WLAN (Wireless LAN) Architecture

• Ad hoc WLAN

– Wireless nodes transmit directly to each other– Use wireless NICs

• No intervening connectivity device

– Poor performance

• Many spread out users, obstacles block signals

• Access point (AP)

– Accepts wireless signals from multiple nodes

• Retransmits signals to network

– Base stations, wireless routers, wireless gateways

• Infrastructure WLAN

– Stations communicate with access point

• Not directly with each other

– Access point requires sufficient power, strategic placement

• WLAN may include several access points

– Dependent upon number of stations– Maximum number varies: 10-100

• Mobile networking allows roaming wireless nodes

– Range dependent upon wireless access method, equipment manufacturer, office environment

• Access point range: 300 feet maximum

• Can connect two separate LANs

– Fixed link, directional antennas between two access points

• Allows access points 1000 feet apart

• Support for same protocols, operating systems as wired LANs

– Ensures compatibility

802.11 WLANs

• Wireless technology standard

– Describes unique functions

• Physical and Data Link layers

– Differences

• Specified signaling methods, geographic ranges, frequency usages

– Developed by IEEE’s 802.11 committee

• Wi-Fi (wireless fidelity) standards

– 802.11b, 802.11a, 802.11g, 802.11n (draft)– Share characteristics

• Half-duplexing, access method, frame format

Access Method

• 802.11 MAC services

– Append 48-bit (6-byte) physical addresses to frame

• Identifies source, destination

• Same physical addressing scheme as 802.3

– Allows easy combination

• Wireless devices

– Not designed for simultaneous transmit, receive– Cannot quickly detect collisions– Use different access method

• CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)

– Minimizes collision potential– Uses ACK packets to verify every transmission

• Requires more overhead than 802.3

• Real throughput less than theoretical maximum

• RTS/CTS (Request to Send/Clear to Send) protocol

– Optional– Ensure packets not inhibited by other transmissions– Efficient for large transmission packets– Further decreases overall 802.11 efficiency

802.11b

• DSSS (direct-sequence spread spectrum) signaling

• 2.4-GHz band

– Separated into 22-MHz channels

• Throughput

– 11-Mbps theoretical– 5-Mbps actual

• 100 meters node limit

• Oldest, least expensive

• Being replaced by 802.11g

802.11a

• Released after 802.11b

• 5-GHz band

– Not congested like 2.4-GHz band

• Lower interference, requires more transmit power

• Throughput

– 54 Mbps theoretical– 11 and 18 Mbps effective

• Attributable to higher frequencies, unique modulating data method, more available bandwidth

• 20 meter node limit

• More expensive, least popular

802.11g

• Affordable as 802.11b

• Throughput

– 54 Mbps theoretical– 20 to 25 Mbps effective

• 100 meter node range

• 2.4-GHz frequency band

– Compatible with 802.11b networks

802.11n

• Ratification in late 2009

• Manufacturers

– Selling 802.11n-compatible transceivers

• Primary goal

– Wireless standard providing much higher effective throughput

• Maximum throughput: 600 Mbps

– Threat to Fast Ethernet

• Backward compatible with 802.11a, b, g standards

• 2.4-GHz or 5-GHz frequency range

• Compared with 802.11a, 802.11g

– Same data modulation techniques

• Compared with three 802.11 standards

– Manages frames, channels, encoding differently

• Allows high throughput

• Channel bonding

– Two adjacent 20-MHz channels bonded to make 40-MHz channel

• Doubles the bandwidth available in single 20-MHz channel

• Bandwidth reserved as buffers assigned to carry data

• Higher modulation rates

– Single channel subdivided into multiple, smaller channels

• More efficient use of smaller channels

• Different encoding methods

• Maximum throughput dependencies

– Number, type of strategies used– 2.4-GHz or 5-GHz band– Actual throughput: 65 to 600 Mbps

• Backward compatible

– Not all 802.11n features work

• Recommendation

– Use 802.11n-compatible devices