Matakuliah : H0484/Jaringan Komputer
Tahun
: 2007
Wireless LAN
Pertemuan 16
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+
Data Transfer Rate
802.11b
802.11g
802.11a
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
20MHz
20MHz
20MHz
Ch1
•
•
•
•
•
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Ch6
2480
2475
2470
2465
2460
2455
2450
2445
2440
2435
2430
2425
2420
2415
2410
2405
2483.5
2400
2400
Ch11
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
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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Two Modes in 802.11b
Infrastructure Mode
Internet
Wired
Computer
with printer
sharing
Screen Monitor II
Wired File
Server
Gateway to
Internet
Wired Ethernet Network
Hardware
Access Point
Wireless Network
Screen Monitor II
PC equipped
with wireless
LAN adapter
Notebooks equipped
with wireless LAN
adapters
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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
Best Coverage
antenna
HPBW/Vertical
HPBW/ Horizontal = 3600
Side View
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Top View
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.
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