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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
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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:
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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:
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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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Transmitting on the WLAN: Distributed
Coordination Function (DCF)
• MAC layer responsible for controlling access to
wireless medium
• Channel access methods: Rules for cooperation
among wireless devices
– Contention: Computers compete to use medium
• If two devices send frames simultaneously, collision
results and frames become unintelligible
• Must take steps to avoid collisions
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
• Carrier Sense Multiple Access with Collision
Detection (CSMA/CD): Before networked device
sends a frame, listens to see if another device
currently transmitting
– If traffic exists, wait; otherwise send
– Devices continue listening while sending frame
• If collision occurs, stops and broadcasts a “jam” signal
• CSMA/CD cannot be used on wireless networks:
– Difficult to detect collisions
– Hidden node problem
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Figure 5-11: Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Figure 5-11 (continued): Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Figure 5-12: Hidden node problem
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
• Distributed Coordination Function (DCF):
Specifies modified version of CSMA/CD
– Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)
– Attempts to avoid collisions altogether
– Time when most collisions occur is immediately after
a station completes transmission
– All stations must wait random amount of time after
medium clear
• Slot time
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
• CSMA/CA also reduces collisions via explicit frame
acknowledgment
– Acknowledgment frame (ACK): Sent by receiving
device to sending device to confirm data frame
arrived intact
– If ACK not returned, transmission error assumed
• CSMA/CA does not eliminate collisions
– Does not solve hidden node problem
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Figure 5-13: CSMA/CA and ACK
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
• Request to Send/Clear to Send (RTS/CTS)
protocol: Option used to solve hidden node
problem
– Significant overhead upon the WLAN with
transmission of RTS and CTS frames
• Especially with short data packets
– RTS threshold: Only packets that longer than RTS
threshold transmitted using RTS/CTS
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Figure 5-14: Request to Send/Clear to Send (RTS/CTS)
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Transmitting on the WLAN: Interframe
Spacing
• Interframe spaces (IFS): Intervals between
transmissions of data frames
– Short IFS (SIFS): For immediate response actions
such as ACK
– Point Coordination Function IFS (PIFS): Time
used by a device to access medium after it has been
asked and then given approval to transmit
– Distributed Coordination Function IFS (DIFS):
Standard interval between transmission of data
frames
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Transmitting on the WLAN: Interframe
Spacing (continued)
Figure 5-15: CSMA/CA with one station transmitting
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Transmitting on the WLAN: Interframe
Spacing (continued)
Figure 5-16: CSMA/CA with two stations transmitting
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Transmitting on the WLAN:
Fragmentation
• Fragmentation: Divide data to be transmitted from
one large frame into several smaller ones
– Reduces probability of collisions
– Reduces amount of time medium is in use
• If data frame length exceeds specific value, MAC
layer fragments it
– Receiving station reassembles fragments
• Alternative to RTS/CTS
– High overhead
• ACKs and additional SIFS time gaps
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Transmitting on the WLAN: Point
Coordination Function (PCF)
• Polling: Channel access method in which each
device asked in sequence if it wants to transmit
– Effectively prevents collisions
• Point Coordination Function (PCF): AP serves
as polling device or “point coordinator”
• Point coordinator has to wait only through point
coordination function IFS (PIFS) time gap
– Shorter than DFIS time gap
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Transmitting on the WLAN: Point
Coordination Function (continued)
• If point coordinator hears no traffic after PIFS time
gap, sends out beacon frame
– Field to indicate length of time that PCF (polling) will
be used instead of DCF (contention)
• Receiving stations must stop transmission for that
amount of time
– Point coordinator then sends frame to specific
station, granting permission to transmit one frame
• 802.11 standard allows WLAN to alternate between
PCF (polling) and DCF (contention)
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Transmitting on the WLAN: Point
Coordination Function (continued)
Figure 5-18: DIFS and DCF frames
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Transmitting on the WLAN: Quality of
Service (QoS) and 802.11e
• DCF does not work well for real-time, timedependent traffic
• Quality of Service (QoS): Capability to prioritize
different types of frames
• Wi-Fi Multimedia (WMM): Modeled after wired
network QoS prioritization scheme
• 802.11e draft: defines superset of features
intended to provide QoS over WLANs
– Proposes two new mode of operation for 802.11
MAC Layer
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Transmitting on the WLAN: Quality of
Service and 802.11e (continued)
Table 5-1: Wi-Fi Multimedia (WMM)
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Transmitting on the WLAN: Quality of
Service and 802.11e (continued)
• 802.11e draft (continued):
– Enhanced Distributed Channel Access (EDCA):
Contention-based but supports different types of
traffic
• Four access categories (AC)
• Provides “relative” QoS but cannot guarantee service
– Hybrid Coordination Function Controlled
Channel Access (HCCA): New form of PCF based
upon polling
• Serves as a centralized scheduling mechanism
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Remaining Connected to the WLAN:
Reassociation
• Reassociation: Device drops connection with one
AP and establish connection with another
– Several reason why reassociation may occur:
• Roaming
• Weakened signal
– When device determines link to current AP is poor,
begins scanning to find another AP
• Can use information from previous scans
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Remaining Connected to the WLAN:
Power Management
• When laptop is part of a WLAN, must remain
“awake” in order to receive network transmissions
– Original IEEE 802 standard assumes stations
always ready to receive network messages
• Power management: Allows mobile devices to
conserve battery life without missing transmissions
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Transparent to all protocols
Differs based on WLAN configuration
AP records which stations awake and sleeping
Buffering: If sleeping, AP temporarily stores frames
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Remaining Connected to the WLAN:
Power Management (continued)
Figure 5-19: Power management in infrastructure mode
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Remaining Connected to the WLAN:
Power Management (continued)
• At set times AP send out beacon to all stations
– Contains traffic indication map (TIM)
– At same time, all sleeping stations switch into active
listening mode
• Power management in ad hoc mode:
– Ad hoc traffic indication message (ATIM)
window: Time at which all stations must be awake
• Wireless device sends beacon to all other devices
– Devices that previously attempted to send a frame
to a sleeping device will send ATIM frame
indicating that receiving device has data to receive
and must remain awake
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WLAN Network Layer Standards:
WLAN IP Addressing
• In standard networking, IP protocol responsible for
moving frames between computers
– Network layer protocol
• TCP/IP works on principle that each network host
has unique IP address
– Used to locate path to specific host
– Routers use IP address to forward packets
– Prohibits mobile users from switching to another
network and using same IP number
• Users who want to roam need new IP address on
every network
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WLAN Network Layer Standards:
Mobile IP
• Provides mechanism within TCP/IP protocol to
support mobile computing
– Computers given home address,
• Static IP number on home network
– Home agent: Forwarding mechanism that keeps
track of where mobile computer located
– When computer moves to foreign network, a
foreign agent provides routing services
• Assigns computer a care-of address
• Computer registers care-of address with home agent
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WLAN Network Layer Standards:
Mobile IP (continued)
Figure 5-20: Mobile IP components
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WLAN Network Layer Standards:
Mobile IP (continued)
Figure 5-21: Computer relocated in Mobile IP
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WLAN Network Layer Standards:
Mobile IP (continued)
Figure 5-22: Encapsulated Mobile IP frame
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Summary
• A Basic Service Set (BSS) is defined as a group of
wireless devices that is served by a single access
point (AP)
• An Extended Service Set (ESS) is comprised of
two or more BSS networks that are connected
through a common distribution system
• An Independent Basic Service Set (IBSS) is a
wireless network that does not use an access point
• Frames are used by both wireless NICs and
access points for communication and for managing
and controlling the wireless network
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Summary (continued)
• The MAC layer provides four major functions in
WLANs: discovering the WLAN signal, joining the
WLAN, transmitting on the WLAN, and remaining
connected to the WLAN
• Discovery is a twofold process: the AP or other
wireless devices must transmit an appropriate
frame (beaconing), and the wireless device must
be looking for those frames (scanning)
• Once a wireless device has discovered the WLAN,
it requests to join the network; This is a twofold
process known as authentication and association
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Summary (continued)
• The IEEE 802.11 standard specifies two
procedures for transmitting on the WLAN,
distributed coordination function (DCF) and an
optional point coordination function (PCF)
• The 802.11 standard provides for an optional
polling function known as Point Coordination
Function (PCF)
• The 802.11e draft defines a superset of features
that is intended to provide QoS over WLANs
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Summary (continued)
• Power management allows mobile devices to be off
as much as possible to conserve battery life but not
miss data transmissions
• Mobile IP provides a mechanism within the TCP/IP
protocol to support mobile computing
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