Ch 14. Wireless LANs

14.1 IEEE 802.11

• Specification for a wireless LAN– Cover physical and data link layers

• Basic service sets (BSS) and extended service set (ESS)

BSS ESS

MAC Sublayer Architecture

• MAC in IEEE 802.11 standard

Distributed Coordination Function (DCF)

• CSMA/CA – why not CSMA/CD?– A wireless station may not send data and receive collision

signals at the same time– Collision may not be detected due to the hidden station

problem (will discuss later)– Signal fading prevents a station of one end from hearing a

collision at the other end • Control frames– Request-To-Send (RTS): sent by the sender when it wants send

a frame– Clear-To-Send (CTS): sent by the receiver when it receives an

RTS and can receive the data

RTS/CTS Exchanges of DCF

• Network Allocation Vector (NAV)– RTS/CTS includes the duration of data transmission time– Stations affected by the transmission create a NAV timer to wait the end of the

transmission

PCF

• Point coordination function (PCF)– Centralized, contention-free “polling” access – Use a shorter inter-frame space (PIFS)– Useful for time-sensitive service

Frame Format

(RTS, CTS, ACK)

Transmission type for addressing (See Fig. 14.9)

Duration Sequence number of the frame

Transmission Type for Addressing

Control Frames

• RTS/CTS and ACK

Hidden Station Problem

• Station C cannot hear the transmission of station B (to A), and vice versa

Hidden Station Problem

• Solved by RTS/CTS exchange

Exposed Station Problem

• Station C can send to station D, but it is exposed to the transmission of station A (to B)

Exposed Station Problem

• Cannot solved by RTS/CTS exchange

Normally, C does not transmit an RTS here.

The figure shows that even if C transmits an RTS, it does not solve the problem

Physical Layers

14.2 Bluetooth

• Bluetooth: a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on– Ad hoc network

• Piconet and scatternet

Bluetooth Layers (1)

• Radio layer– 10m range– 1 Mbps data rate– 2.4GHz ISM band, FHSS, GFSK modulation

Bluetooth Layers (2)

• Baseband layer (MAC)– TDD-TDMA (Time Division Duplex TDMA)– Single-Secondary

Communication

– Multiple-Secondary Communication

Time for frequency hopping control

Homework

• Exercise in Chapter 14– 11– 12