Wireless LANPertemuan 16

Matakuliah : H0484/Jaringan KomputerTahun : 2007

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Learning Outcomes

Pada akhir pertemuan ini, diharapkan mahasiswa

akan mampu :

• Menggunakan LAN -Nirkabel

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Outline Materi

• Ad-Hoc• Infra structure• Standards

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MAC layers in IEEE 802.11 standard

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Wireless LAN Specifications

802.11b+ 802.11b 802.11g 802.11a

Data Transfer Rate Up to 22Mbps Up to 11Mbps Up to 54Mbps Up to 54Mbps

Frequency 2.4GHz 2.4GHz 2.4GHz 5GHz

Compatibiliy 802.11b 802.11g 802.11b 802.11a only

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802.11b Wireless LAN

• Utilise Direct Sequence Spread Spectrum (DSSS) technology

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

• Frequency range of DSSS is between 2.4 to 2.4835 GHz

• No. of channels at different part of the world varies (S.E.A. : 13 channels)

• Each channel bandwidth is approximately 20MHz

2400

2410

2420

2430

2440

2450

2460

2470

2480

2405

2415

2425

2435

2445

2455

2465

2475

Ch11Ch6Ch1

2400 2483.5

20MHz 20MHz 20MHz

2400

2410

2420

2430

2440

2450

2460

2470

2480

2405

2415

2425

2435

2445

2455

2465

2475

Ch11Ch6Ch1

2400 2483.5

20MHz 20MHz 20MHz

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Access Components

•Client (PCMCIA card)•External Antena•Access Points

•Enterprise Access Point•Residential Gateways•Access Servers (Public

Access)•Outdoor Routers (Outdoor)

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FHSS

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DSSS

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Two Modes in 802.11b

Ad-Hoc Mode (also known as “peer-to-peer” mode)

Screen Monitor II

PC equipped with wireless LAN

adapter

Notebooks equipped with wireless LAN

adapters

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Infrastructure Mode

Screen Monitor II

Screen Monitor II

Wired File Server

Wired Ethernet Network

Gateway to Internet

Internet

Wired Computer

with printer sharing

Hardware Access Point

Wireless Network

PC equipped with wireless LAN adapter

Notebooks equipped with wireless LAN

adapters

Two Modes in 802.11b

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Security Issue

•Access Control – By MAC Address, only the authorised network card (with

correct MAC address) can access the network•Data Privacy (Wired Equivalent Privacy -WEP)

– Protect the data while it is transferred from one station to another

– Encryption between wireless devices to prevent eavesdropping

– Rely on a secret key that is shared between the client machines and the access points

– The secret key (64/128-bit) is used to encrypt packets before transmitted

– 802.11b+ supports secret key with 64/128/256-bit

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Understanding the Antennas

• Type of Antenna– Omni, Patch, Panel, Yagi, etc

• Indoor or Outdoor• Gain (dBi)

– The higher the gain, the further the coverage distance

• Half Power Beam Width (HPBW)– Horizontal & Vertical

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Antennas Characteristics

Antenna Gain •Accomplished by compressing the radiation

pattern of the antenna into a narrower beamwidth and focusing the energy toward one direction.

•Defined as the ratio between radiation intensity of the main lobe and the radiation intensity of an isotropic radiator (having the same input power).

•Antenna Gain is expressed in dBi.•Gain = Focus.

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Position of Antenna

Omni Antenna

Best Coverage

Side View

Best Coverage

Top View

antenna

HPBW/ Horizontal = 3600

HPBW/Vertical

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Line of Sight

•Optical line of sight: –An imaginary straight line connecting the antennas on both sides of the link.

•Clear Line of Sight:–When no physical objects obstruct viewing one antenna from the location of the other.

•Clearance: –The distance between the optical Line of Sight and the nearest obstacle.

•Fresnel Zone: –The area of a circle around the line of sight.