1 Wireless & Personal Communication Systems – CSE5807 Lecture: 07 Wireless Personal Communications Systems – CSE5807 Lecture: 07 Stephen Giles and Satha K. Sathananthan School of Computer Science and Software Engineering Monash University Australia These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .
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1Wireless & Personal Communication Systems – CSE5807Lecture: 07
Wireless Personal Communications Systems – CSE5807
Lecture: 07
Stephen Giles and Satha K. Sathananthan
School of Computer Science and Software Engineering
Monash University
Australia
These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .
2Wireless & Personal Communication Systems – CSE5807Lecture: 07
Locating a WLAN • Service Set Identifier (SSID):
– Unique, case sensitive, alphanumeric value from 2-32 characters long.
– Used as a network name.
– Sent in beacons, probe requests, probe responses and other types of frames.
• Beacons:– To organize and synchronize wireless communications.
• From AP to station in infrastructure mode.
• From station to station in ad hoc mode. – Provide functions including
• Time synchronization
• FH or DS parameters
• SSID information
• Traffic Indication Map
• Supported rates
3Wireless & Personal Communication Systems – CSE5807Lecture: 07
Locating a WLAN • Scanning:
– Passive Scanning• Process of listening for beacons on each channel for a specific
period of time.
• Continuing process even after association.
– Active Scanning• Sending of probe request frame by a wireless station when it seeks
a network to join.
• The probe request frame contains either a particular network’s SSID or broadcast SSID.
4Wireless & Personal Communication Systems – CSE5807Lecture: 07
Authentication • Wireless client’s identity is verified by the network/access
point.
• Access Point => Accept/Deny
• Authentication Methods:– Open System Authentication
• Based on SSID only.
• Option of using WEP for only encrypting data.
– Shared Key Authentication
• Use WEP.
APLAN
Authentication Request Frame
Authentication Response Frame
5Wireless & Personal Communication Systems – CSE5807Lecture: 07
Association • Allowed to pass data through access point => “Associated”.
• Authentication => Association
• Wireless client can authenticate more than one access point at a time but can associate only one access point.
APLAN
Association Request Frame
Association Response Frame
6Wireless & Personal Communication Systems – CSE5807Lecture: 07
Roaming
• Wireless client determines based on the signal strength.
• IEEE802.11 does not define how should be performed.
– But some basic building blocks for this process.
• Active and passive scanning, reassociation process.
– New IEEE802.11f standard for roaming.
– Inter Access Point Protocol (IAPP).
APDisassociation Frame
Reassociation Frame
AP
7Wireless & Personal Communication Systems – CSE5807Lecture: 07
Load Balancing
• Multi-cell structure with co-located access points creating a common coverage area.
– Wireless clients automatically associate with the access point that is less loaded and provides the best signal quality.
A
B
BA
B
AB
8Wireless & Personal Communication Systems – CSE5807Lecture: 07
Adaptive Rate Selection (ARS)
• Speed adjustment with varying distance and interference.
– Switched between specified data rates.
• Important in planning:
– Network throughput
– Cell sizes
– Power outputs of access points and wireless clients
– Security
A
A
11 Mbps
2 Mbps
9Wireless & Personal Communication Systems – CSE5807Lecture: 07
Power Management
• Continuous Aware Mode:
– Uses full power and no sleep mode.
– Wireless client determines based on the signal strength.
• Power Save Polling (PSP):
– Wireless client powers down for a very short amount of time.
– In BSS, traffic indication map (TIM) is used to notify buffered traffic.
– In ad hoc, “Ad hoc traffic indication messages” are used to notify buffered traffic.
10Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Physical Layer
• Physical Layer Convergence Protocol (PLCP):– Responsible for carrier sensing assessment and forming packets for
different physical layers.
• Physical Medium Dependent (PMD) protocol:– Defines modulation and coding technique for signaling.
• Physical Layer Management:– Decides on channel tuning to different options for each physical layer.
Data Link Layer
Physical Layer
MAC
PLCP
MAC Management
PHY
LLC
PMD Management
11Wireless & Personal Communication Systems – CSE5807Lecture: 07
IEEE802.11 Standards
IEEE802.11 IEEE802.11b IEEE802.11a IEEE802.11g
Frequency 2.4GHz 2.4GHz 5GHz 2.4GHz
Max. Rate 2Mbps 11Mbps 54Mbps 54Mbps
Modulation FHSS
DSSS
DSSS OFDM OFDM
12Wireless & Personal Communication Systems – CSE5807Lecture: 07