Chapter 15

Wireless LANs

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Chapter6: Outline

15.1 15.1 INTRODUCTIONINTRODUCTION

15.2 15.2 IEEE 802.11 PROJECTIEEE 802.11 PROJECT

15.3 15.3 BLUETOOTHBLUETOOTH

15.3

15-1 INTRODUCTION15-1 INTRODUCTION

Wireless communication is one of the fastest-growing technologies.

The demand for connecting devices without the use of cables is increasing everywhere.

15.4

Figure 15.1: Isolated LANs: wired versus wireless

15.5

Figure 15.2: Connection of a wired LAN and a wireless LAN to other networks

15.6

15.15.3 Access Control15.15.3 Access Control

An important issue concerning a wireless LAN is access control—

Answering the question of how the wireless host can get access to the shared medium.

The shared medium is free space for a wireless LAN.

15.7

15.15.3 Access Control15.15.3 Access Control

The CSMA/CD algorithm does not work in wireless LANs for many reasons including these:

1.Wireless signal amplitude degrades proportional to the inverse square of the distance. This makes relative amplitudes somewhat meaningless with wireless communication.

1.The hidden station problem prevents collision detection.

15.8

Figure 15.3: Hidden station problem

15.9

15-2 IEEE 802.11 PROJECT15-2 IEEE 802.11 PROJECT

IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, whichcovers the physical and data-link layers. It is sometimes called wireless Ethernet.

15.10

15.2.1 Architecture15.2.1 Architecture

The standard defines two kinds of services:

• BSS, the basic service set, and • ESS, the extended service set (ESS).

15.11

Figure 15.4: Basic service sets (BSSs)

15.12

Figure 15.6: Extended service set (ESS), (two or more BSSs)

15.13

15.2.2 MAC Sublayer15.2.2 MAC Sublayer

Like wired Ethernet, IEEE 802.11 defines two sublayers within the data-link layer:

LLC (logical link control) handles framing, error control, flow control.

MAC (media access control) physical addressing

15.14

15.2.2 MAC Sublayer15.2.2 MAC Sublayer

IEEE 802.11 defines two sublayers withing the MAC sublayer:

DCF the distributed coordination function, andPCF and point coordination function.

15.15

15.2.2 MAC Sublayer15.2.2 MAC Sublayer

DCF the distributed coordination function

•Uses CSMA/CA as the access method•Implements a persistence strategy with exp-backoff•Implements a DIFS after the media is idle.

15.16

15.2.2 MAC Sublayer15.2.2 MAC Sublayer

DCF the distributed coordination function, andPCF and point coordination function.

15.17

Figure 15.6: MAC layers in IEEE 802.11 standard

DIFS & SIFS

DIFS <= 50 microseconds SIFS<= 10 microseconds

15.19

Figure 15.7: Distributed Coordination Funcion: DCF

NAV

PCF

Point Coordination Function, optional layer to allow for transmissions needing higher priority (quality of service)

15.21

Figure 15.8: Example of repetition interval for PCF

15.22

Figure 15.9: The frame format has 9 fields

Frame Format

FC = Frame control (type of frame: control, data, etc)

D = duration SC = sequence control used for

“fragmentation” Frame body up to 2312 bytes. FCS = CRC error detection

Table 15.1: Subfields in FC field

15.24

15.25

Figure 15.10: Control frames

Table 15.2: Values of subfields in control frames

15.26

15.27

15.2.3 Addressing Mechanism15.2.3 Addressing Mechanism

The IEEE 802.11 addressing mechanism specifies four cases, defined by the value of the two flags in the FC field.

Each flag can be either 0 or 1, resulting in four different situations.

Table 15.3: Addresses

15.28

15.29

Figure 15.11: Addressing mechanisms

15.30

Figure 15.12: Exposed station problem

15.31

802.11-Physical Layer802.11-Physical Layer

The unlicensed frequency bands in these three ranges

902–928 MHz, 2.400–4.835 GHz, and 5.725–5.850 GHz.

Are known as the ISM bands, Industrial, Scientific and Medical.

Table 15.4: Specifications (DSSS = Direct Sequence...)

15.32

OFDM

Orthogonal Frequency Division Multiplexing

OFDM uses QAM for modulation.

15.34

15-3 BLUETOOTH15-3 BLUETOOTH

Bluetooth is yet another wireless LAN technology requiring short distances between stations.

15.35

15-3 BLUETOOTH15-3 BLUETOOTH

A Bluetooth LAN is an ad hoc network.

The devices, (aka gadgets), find each other and make a network called a piconet.

15.36

15.3.1 Architecture15.3.1 Architecture

Bluetooth defines two types of networks:

● piconet and ● scatternet.

Bluetooth Piconet

Up to 8* stations One station is designated the primary The other stations are secondary

Bluetooth Piconet

The secondary stations synchronize their clocks with the primary station.

The secondary stations receive their hopping sequence from the primary station.

Bluetooth Piconet

Communication is one-to-one or One-to-many.

Bluetooth Piconet

While limited to seven “active” secondary stations. It is possible to put a station in a “parked state” to allow another station on the piconet.

15.41

Figure 6.17: Piconet

15.42

Figure 15.18: Scatternet

Bluetooth V1

1 Mbps bandwidth 2.4-2.483GHz (overlaps with 802.11b

& g) FHSS – 1600 hops per second 79 frequency channels of 1MHz each FSK modulation

Bluetooth V2

3 Mbps bandwidth

Bluetooth V3

24 Mbps bandwidth Bluetooth establishes the link and uses

802.11 to achieve the 24Mbps data rate.

Bluetooth Range

Power Class 1 20db ~100 m

Power Class 2 4 db ~10 m Power Class 3 ~1 m

15.47

15.3.2 Bluetooth Layers15.3.2 Bluetooth Layers

Bluetooth uses several layers that do not exactly match those of the Internet model defined in the text book. Figure 15.19 shows these layers.

15.48

Figure 15.19: Bluetooth layers

Bluetooth Access Method

Polling-select The primary poles the secondary

stations on even clock cycles. If a secondary is polled, it transmits on

the next odd clock cycle.

15.50

Figure 6.21: Single-secondary communication (diagram error!) microseconds, not milliseconds

Slot Bandwidth

• A slot is 625 microsecs• 259 microsecs are used for overhead• 366 microsecs for data 1 Hz per bit,

366 bits per slot

15.52

Figure 6.22: Multiple-secondary communication

15.53

Figure 6.23: Frame format types

Bluetooth V1 Throughput

1-slot frames 1600slots/sec * (366-(72+54)) bits/slot =

384Kbps

5-slot frames 5*625-259 = 2866 bits / 5 slots 2866 – (72+54) = 2740 bits 1600slots/sec * 2740/5 = 876.8Kbps