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CWNA guide to wireless LANs 2nd ch05

CWNA Guide to Wireless
LANs, Second Edition
Chapter Five
IEEE 802.11 Media Access Control and
Network Layer Standards


Objectives
• List and define the three types of WLAN
configurations
• Tell the function of the MAC frame formats
• Explain the MAC procedures for joining,
transmitting, and remaining connected to a WLAN
• Describe the functions of mobile IP

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IEEE Wireless LAN Configurations:

Basic Service Set
• Basic Service Set (BSS): Group of wireless
devices served by single AP
– infrastructure mode

• BSS must be assigned unique identifier
– Service Set Identifier (SSID)
• Serves as “network name” for BSS

• Basic Service Area (BSA): Geographical area of a
BSS
– Max BSA for a WLAN depends on many factors

• Dynamic rate shifting: As mobile devices move
away from AP, transmission speed decreases
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IEEE Wireless LAN Configurations:
Basic Service Set (continued)

Figure 5-1: Basic Service Set (BSS)

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IEEE Wireless LAN Configurations:
Extended Service Set
• Extended Service Set (ESS): Comprised of two or
more BSS networks connected via a common
distribution system
• APs can be positioned so that cells overlap to
facilitate roaming
– Wireless devices choose AP based on signal
strength
– Handoff



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IEEE Wireless LAN Configurations:
Extended Service Set (continued)

Figure 5-2: Extended Service Set (ESS)

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IEEE Wireless LAN Configurations:
Independent Basic Service Set
• Independent Basic Service Set (IBSS): Wireless
network that does not use an AP
– Wireless devices communicate between themselves
– Peer-to-peer or ad hoc mode

• BSS more flexible than IBSS in being able to
connect to other wired or wireless networks
• IBSS useful for quickly and easily setting up
wireless network
– When no connection to Internet or external network
needed
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IEEE Wireless LAN Configurations:
Independent Basic Service Set
(continued)

Figure 5-3: Independent Basic Service Set (IBSS)

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IEEE 802.11 Media Access Control
(MAC) Layer Standards
• Media Access Control (MAC) layer performs
several vital functions in a WLAN





Discovering WLAN signal
Joining WLAN
Transmitting on WLAN
Remaining connected to WLAN

• Mechanics of how functions performed center
around frames sent and received in WLANs

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MAC Frame Formats
• Packet: Smaller segments of a digital data
transmission
– Strictly speaking, other terms used to describe these
smaller segments

• Frames: Packet at MAC layer
– Or Data Link layer in OSI model
– IEEE MAC frames different from 802.3 Ethernet
frames in format and function
– Used by wireless NICs and APs for communications
and managing/controlling wireless network
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MAC Frame Formats (continued)
• Frame control field identifies:
– Specific 802.11 protocol version
– Frame type
– Indicators that show WLAN configuration

• All frames contain
– MAC address of the source and destination device
– Frame sequence number
– Frame check sequence for error detection

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MAC Frame Formats (continued)
• Management Frames: Initialize communications
between device and AP (infrastructure mode) or
between devices (ad hoc mode)
– Maintain connection

Figure 5-4: Structure of a management frame

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MAC Frame Formats (continued)
• Types of management frames:











Authentication frame
Association request frame
Association response frame
Beacon frame
Deauthentication frame
Disassociation frame
Probe request frame
Probe response frame
Reassociation request frame
Reassociation response frame

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MAC Frame Formats (continued)
• Control frames: Provide assistance in delivering
frames that contain data

Figure 5-5: Control frame

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MAC Frame Formats (continued)
• Data frame: Carries information to be transmitted to
destination device

Figure 5-6: Data frame

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Discovering the WLAN: Beaconing
• At regular intervals, AP (infrastructure network) or
wireless device (ad hoc network) sends beacon
frame
– Announce presence
– Provide info for other devices to join network

• Beacon frame format follows standard structure of
a management frame
– Destination address always set to all ones

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Discovering the WLAN: Beaconing
(continued)

Figure 5-7: Beaconing

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Discovering the WLAN: Beaconing
(continued)
• Beacon frame body contains following fields:







Beacon interval
Timestamp
Service Set Identifier (SSID)
Supported rates
Parameter sets
Capability information

• In ad hoc networks, each wireless device assumes
responsibility for beaconing
• In infrastructure networks beacon interval normally
100 ms, but can be modified
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Discovering the WLAN: Scanning
• Receiving wireless device must be looking for
beacon frames
• Passive scanning: Wireless device simply listens
for beacon frame
– Typically, on each available channel for set period

• Active scanning: Wireless device first sends out a
management probe request frame on each
available channel
– Then waits for probe response frame from all
available APs
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Discovering the WLAN: Scanning
(continued)

Figure 5-8: Active scanning

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Joining the WLAN: Authentication
• Unlike standard wired LANS, authentication
performed before user connected to network
– Authentication of the wireless device, not the user

• IEEE 802.11 authentication: Process in which AP
accepts or rejects a wireless device
• Open system authentication: Most basic, and
default, authentication method
• Shared key authentication: Optional
authentication method
– Utilizes challenge text
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Joining the WLAN: Authentication
(continued)

Figure 5-9: Open system authentication

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Joining the WLAN: Authentication
(continued)

Figure 5-10: Shared key authentication

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Joining the WLAN: Authentication
(continued)
• Open system and Shared key authentication
techniques are weak
– Open System: Only need SSID to connect
– Shared Key: Key installed manually on devices
• Can be discovered by examining the devices

• Digital certificates: Digital documents that
associate an individual with key value
– Digitally “signed” by trusted third party
– Cannot change any part of digital certificate without
being detected
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Joining the WLAN: Association
• Association: Accepting a wireless device into a
wireless network
– Final step to join WLAN

• After authentication, AP responds with association
response frame
– Contains acceptance or rejection notice

• If AP accepts wireless device, reserves memory
space in AP and establishes association ID
• Association response frame includes association
ID and supported data rates
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