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Chapter 14. Wireless LANs

1. IEEE 802.11

2. Bluetooth

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Basic Service Set (BSS)

• BSS

– The building block of a wireless LAN

• BSS with an AP

– Access Point (AP): central base station

• BSS without an AP

– Stand-alone network

– Cannot send data to other BSSs

– Ad hoc architecture

• BSS with an AP

– Called an infrastructure network

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BSS

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Extended Service Set (ESS)

• BSSs are connected through a distribution system: infrastructure network (usually wired LAN)

• Station Types: No-transition, BSS-transition, and ESS-transition mobility

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MAC Sublayer

• Two MAC sublayers: DCF and PCF

• DCF uses CSMA/CA as the access method

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CSMA/CA Flow Chart

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CSMA/CA and NAV

• Network allocation vector (NAV) shows how much time must pass before these stations are allowed to check the channel for idleness

• Collision During handshaking

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Point Coordination Function (PCF)• An optional access method that can be implemented in an AP• A centralized, contention-free polling access method• To give priority to PCF over DCF, another set of interframe spaces has

been defined: PIFS and SIFS• PIFS (PCF IFS) is shorter than the DIFS AP using PCF has priority• Repetition interval starts with a special control frame, called a beacon

frame

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MAC Layer Frame Format

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Frame Types

• Three categories of frames• Management frames for initial communication between stations and APs• Control frames for accessing the channel and acknowledging frames• Data frame for carrying data and control information

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Addressing Mechanism

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Hidden Station Problems

• The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

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Exposed Station Problems

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Physical Layer

• Industrial, scientific, and medical (ISM) band which defines three unlicensed bands in three ranges 902-928 MHz, 2.400-4.835 GHz, and 5.725-5.850 GHz

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Physical Layer• IEEE 802.11 FHSS

• IEEE 802.11 DSSS

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Physical Layer• IEEE 802.11 Infrared

• IEEE 802.11a OFDM– Common data rates are 18 Mbps (PSK) and 54 Mbps (QAM)

• IEEE 802.11b DSSS– High-rate direct sequence spread spectrum (HR-DSSS)

– Similar to DSSS method except for the encoding method called complementary code keying (CCK), Four data rates; 1, 2, .5, 11 Mbps

• IEEE 802.11g– Forward error correction and OFDM using 2.4 GHz ISM, 22- or 54-Mbps data rate

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Bluetooth: Piconet

• Bluetooth is a wireless LAN technology designed to connect devices oBluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameraf different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc nes, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneouslytwork, which means that the network is formed spontaneously

• Bluetooth network is called piconet, or a small net

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Bluetooth: Scatternet

• Piconet can be combined to form what is called a scatternet.Piconet can be combined to form what is called a scatternet.

• Bluetooth device has a built-in short range radio transmitter with 2.4 GHz bandwidth. A Bluetooth device has a built-in short range radio transmitter with 2.4 GHz bandwidth. A possibility of interference with IEEE 802.11bpossibility of interference with IEEE 802.11b

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Bluetooth Layers

• Radio layer

– 2.4 GHz ISM band divided into 79 channels of 1 MHz each

– FHSS: Bluetooth hops 1600 times per second, dwell time is 625 μsec (= 1/1600 sec)

– Modulation: GFSK (FSK with Gaussian bandwidth filtering)

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Bluetooth Layers

• Baseband layer

– TDD-TDMA (time division duplex TDMA): half-duplex communication

– Single-secondary communication

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Bluetooth Layers

• Baseband layer

– TDD-TDMA: multiple-secondary communication

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Bluetooth Layers• Physical links

– SCO (synchronous connection-oriented)

– ACL (asynchronous connectionless link)

• Frame format

• L2CAP (Logical Link Control and Adaptation Protocol)

– Multiplexing, segmentation and reassembly, QoS, group management