Overview of Wireless LANs

Use wireless transmission medium Issues of high prices, low data rates,

occupational safety concerns, & licensing requirements now addressed

key application areas: LAN extension cross-building interconnect ad hoc networking

Wireless LAN Requirements

Throughput - efficient use wireless medium No of nodes - hundreds of nodes across multiple cells Connection to backbone LAN - using control modules Service area - 100 to 300 m low power consumption - for long battery life on

mobiles Transmission robustness and security license-free operation handoff/roaming Dynamic configuration - addition, deletion, and

relocation of end systems without disruption to users

Wireless LAN at a glance

Wireless LAN

Medium

access

control

FDMA CS

MA

/CA

CDMA

Tran

smis

sion

med

ia

Infrared

Radio

Topologies

Ad hoc

Infrastructure

Applications

Sta

nd

ard

s

IEEE ETSI (Hipper LAN)

CDMA: Code Division Multiple Access ETSI: European Telecom. Standards InstituteCSMA/CD: CSMA with Collision Detection FDMA: Frequency Division Multiple AccessCSMA/CA: CSMA with Collision Avoidance TDMA: Time Division Multiple Access

CDMA: Code Division Multiple Access ETSI: European Telecom. Standards InstituteCSMA/CD: CSMA with Collision Detection FDMA: Frequency Division Multiple AccessCSMA/CA: CSMA with Collision Avoidance TDMA: Time Division Multiple Access

CSMA/CDT

DM

A

Pulse-position modulation

Direct modulation

Multi-subcarriermodulation

Single-carriermodulation

Transmission schemes

Carriermodulation

Spread spectrum

On-offkeying

Direct Sequence

Frequency hopping

AirportsWarehouses

Retail stores

Old buildings

Hospitals

Wireless LANs

IEEE 802.11 Basic service set (cell)

Set of stations using same MAC protocol Competing to access shared medium May be isolated May connect to backbone via access point

(bridge) Extended service set

Two or more BSS connected by distributed system

Appears as single logic LAN to LLC level

802.11 modes

Infrastructure mode All packets go through a base station Cards associate with a BSS (basic service set) Multiple BSSs can be linked into an Extended

Service Set (ESS) Handoff to new BSS in ESS is pretty quick Moving to new ESS is slower, may require

re-addressing Ad Hoc mode

Cards communicate directly. Perform some, but not all, of the AP functions

A BSS without an AP is called an ad hoc network;

a BSS with an AP is called an infrastructure network.

Note

Basic service sets (BSSs)

Wireless connections: Access Point as a Bridge

Extended service sets (ESSs)

Types of station

No transition Stationary or moves within direct

communication range of single BSS BSS transition

Moves between BSS within single ESS ESS transition

From a BSS in one ESS to a BSS in another ESS

MAC layers in IEEE 802.11 standard

Media Access Control

Distributed coordination function (DCF): CSMA/CA No collision detection After finding channel idle, the station waits for a time

period called distributed interframe space (DIFS). Then sends RTS (Request to send) & waits for time

period short interframe space (SIFS) Destination sends clear to send (CTS) to source station After time period of SIFS source sends data After successful transmission of data source waits for

acknowledgement Point coordination function (PCF): It can be implemented in Infrastructure network.

AP performs polling for all the stations.

CSMA/CA flowchart

CSMA/CA NAV (Network allocation vector)

Frame format

Duration of transmission

Sequence control

Subfields in FC field

Control frames

Addresses

Addressing mechanisms

Hidden station problem

The CTS frame in CSMA/CA handshake can prevent collision from

a hidden station.

Note

Use of handshaking to prevent hidden station problem

Exposed station problem

Use of handshaking in exposed station problem

WIRELESS LAN :Physical layers

Industrial, scientific, and medical (ISM) band

Physical layer of IEEE 802.11 FHSS

Physical layer of IEEE 802.11 DSSS

Physical layer of IEEE 802.11 infrared

BLUETOOTHBLUETOOTH

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

Bluetooth basics

Short-range, high-data-rate wireless link for personal devices Originally intended to replace cables in

a range of applications e.g., Phone headsets synchronization, remote controls

Operates in 2.4 GHz ISM band Same as 802.11 Frequency Hopping Spread Spectrum

across ~ 80 channels

Protocol Architecture

Piconet Architecture

One master and up to 7 slave devices in each Piconet:

Master controls transmission schedule of all devices in the Piconet

Time Division Multiple Access (TDMA): Only one device transmits at a time

Frequency hopping used to avoid collisions with other Piconets 79 physical channels of 1 MHz each, hop between channels 1600

times a sec

Scatternets

Combine multiple Piconets into a larger Scatternet Device may act as master in one Piconet and slave in another Each Piconet using different FH schedule to avoid interference

Can extend the range of Bluetooth, can route across Piconets

Frame format types

L2CAP data packet format