Transcript Chapter 5 Lectures Notes

CWNA Guide to Wireless
LANs, Second Edition
Chapter Five
IEEE 802.11 Media Access Control and
Network Layer Standards
1
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)
Basic Service Set (BSS)
4
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)
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)
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
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
Control frame
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MAC Frame Formats (continued)
• Data frame: Carries information to be transmitted to
destination device
Data frame
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802.11 MAC
Addressing
Host A to Host B
General 802.11 Frame
X
xxx
Y
Distribution System (DS)
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Access Point 1
Access Point 2
A
B
aaa
bbb
C
D
• Address 1 – Receiver address
• Address 2 – Transmitter address
• Address 3 – Ethernet SA, Ethernet DA, or BSSID
• Transmitter: Sends a frame on to the wireless medium, but
doesn’t necessarily create the frame.
• Receiver: Receives a frame on the wireless medium, but may
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not be the destination, i.e. may be the access point.
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)
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)
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)
Open system authentication
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Joining the WLAN: Authentication
(continued)
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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Medium Access – CSMA/CA
All stations detect the
collision
ACK
CSMA/CD
CSMA/CA
• Both CSMA/CD and CSMA/CA are half-duplex architectures
• Ethernet uses CSMA/CD – Collision Detection
– Ethernet devices detect a collision as when the data is transmitted
• 802.11 uses CSMA/CA – Collision Avoidance
– 802.11 devices only detect a collision when the transmitter has not
received an Acknowledgement (coming).
– Stations also use CS/CCA – coming
– Stations also use a virtual carrier-sense function, NAV (coming)28
Medium Access – CSMA/CA
All stations detect the
collision
ACK
CSMA/CD
CSMA/CA
• The 802.11 standard makes it mandatory that all stations implement
the DCF (Distributed Coordination Function), a form of carrier sense
multiple access with collision avoidance (CSMA/CA). Coming!
• CSMA is a contention-based protocol making sure that all stations first
sense the medium before transmitting (physically and virtually).
Coming!
• The main goal of CSMA/CA is to avoid having stations transmit at the
same time, which will then result in collisions and eventual
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retransmissions. Coming!
DCF and PCF
• IEEE mandated access mechanism for 802.11 is DCF
(Distributed Coordination Function)
– Basis for CSMA/CA
– Discussed in detail next
• There is also the PCF (Point Coordination Function)
– Point Coordinators (PC), ie.Access Points, provide point
coordination for contention-free services.
– Restricted to Infrastructure BSSs
– Stations can only transmit when allowed to do so by PC (AP).
– PCF is not widely implemented and will not be discussed
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DCF Operation
• In DCF operation, a station wanting to transmit :
– Checks to see if radio link is clear, CS/CCA – Carrier Sense,
Clear Channel Assessment (Later in PHY presentation)
– Checks its Network Allocation Vector (NAV) timer to see if
someone else is using the medium.
– If medium is available DCF uses a random backoff timer to avoid
collisions and sends the frame.
• Transmitting station only knows the 802.11 frame got there if it
receives an ACK.
• May also use RTS/CTS to reduce collisions (coming)
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Duration Field
General 802.11 Frame (more on this later)
• Duration/ID field – The number of microseconds (millionth of a
second) that the medium is expected to remain busy for transmission
currently in progress.
– Transmitting device sets the Duration time in microseconds.
– Includes time to:
• Transmit this frame to the AP (or to the client if from an AP)
• The returning ACK
• The time in-between frames, IFS (Interframe Spacing)
• All stations monitor this field!
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• All stations update their NAV (Network Allocation Vector) timer.
NAV Timer
General 802.11 Frame (more on this later)
•
•
•
•
•
All stations have a NAV (Network Allocation Vector) timer.
Virtual carrier-sensing function
Protects the sequence of frames from interruption.
Martha sends a frame to George.
Since wireless medium is a “broadcast-based” (not broadcast frame)
shared medium, all stations including Vivian receive the frame.
• Vivian updates her NAV timer with the duration value.
• Vivian will not attempt to transmit until her NAV is decremented to 0.
• Stations will only update their NAV when the duration field value 33
received is greater than their current NAV.
Interframe Spacing (IFS)
• 802.11 uses four different interframe spaces used to determine
medium access (note: microsecond = millionth of a second):
– DIFS – DCF Interface Space (50 microseconds in DSSS)
• Minimum amount of medium idle time until contention-based services
begin.
– PIFS – PCF Interframe Space (30 microseconds in DSSS)
• Used by PCF
– SIFS – Short Interframe Space (10 microseconds in DSSS)
• Used for highest priority transmission, ACKs, RTS, CTS
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Wanting to transmit (1/3)
Random backoff slots
• Station wanting to transmit.
• Carrier Sensing:
– Physical: Physically senses medium is idle (CS/CCA – coming).
– Virtual: NAV timer is 0
• Waits DIFS (DCF Interface Space) period of 50 microseconds
– Minimum amount of medium idle time until contention-based services
begin.
– Once DCF is over, stations can contend for access.
• Contention window begins.
– Uses random backoff algorithm to determine when it can attempt to35
access the medium. (next)
Wanting to transmit (2/3)
Contention Window Begins
• (Detail of random backoff algorthim has been left out, but this will be
sufficient.)
• The random backoff algorithm randomly selects a value from 0 to
255 (maximum value varies by vendor and stored in the NIC).
• The random value is the number of 802.11 slot times the station must
wait after the DIFS, during the contention window before it may
transmit.
• Stations pick a random slot and wait for that slot before attempting to
access the medium.
• With several stations attempting to transmit, the station that picks the
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lowest slot, lowest random number, wins.
Example
I’m
waiting
I’m
waiting
Scenario:
• Both Vivian and George want to transmit frames.
• Both stations have same NAV values and physically sense when
the medium is idle.
• Both are waiting for Martha’s transmission to end and the
medium to become available.
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• The medium now becomes available.
Example
Random backoff slots
• George and Vivian are both wanting to transmit.
• Both perform the following:
• Both sense that medium is available using Physical and
Virtual Carriers Sensing:
– Physical: Physically senses medium is idle (CS/CCA – coming).
– Virtual: NAV timer is 0
• Both waits DIFS (DCF Interface Space) period of 10
microseconds
• Contention window begins.
– Uses random backoff algorithm to determine when it can
attempt to access the medium. (next)
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Example
Vivian (7), George (31)
• Both Vivian and George calculate their random backoff
algorithm to randomly selects a value from 0 to 255.
• Vivian has a slot time of 7, George a slot time of 31.
• Vivian wins.
• The destination of her frame is George
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Martha and George
receive “broadcastbased” 802.11 frame.
Others
update NAV
Example
(((
)))
General 802.11 Frame (more on this later)
• Vivian transmits, setting the Duration ID to the time
needed to transmit, ACK and IFSs.
• George with a higher slot will see the 802.11 frame from
Vivian and wait to transmit.
• Assuming their was not a collision from another station,
Martha and George update their NAVs.
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Transmitting on the WLAN: Distributed
Coordination Function (continued)
Hidden node problem
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RTS/CTS Solution
• Vivian attempts to reserve the medium using
•
an RTS control frame to the AP.
The RTS frame indicates to the AP and all
stations within range, that Vivian wants to
reserve the medium for a certain duration
of time, message, ACK, and SIFS.
• The hidden node stations cannot see the RTS.
• The AP replies to Vivian with a CTS, which all nodes,
including the hidden node can see.
• Vivian transmits the frame.
• The AP returns an ACK to Vivian.
• The AP sends the message to George who returns an ACK
to the AP.
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RTS/CTS Solution
• RTS/CTS consumes a fair amount of
•
capacity and overhead, resulting in
additional latency.
Normally used in high capacity
environments.
• The RTS/CTS procedure can be enabled/controlled by
setting the RTS threshold on the 802.11 client NIC.
• RTS/CTS is also used during frame fragmentation (coming).
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Setting the RTS Threshold on a Cisco
Client
RTS
Threshold
•
Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS
flow control. A small value causes RTS packets to be sent more often, which consumes
more of the available bandwidth and reduces the throughput of other network packets.
However, small values help the system recover from interference or collisions, which can
occur in environments with obstructions or metallic surfaces that create complex
multipath signals.
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Improving WLAN Performance with
RTS/CTS by Jim Geier (wi-fiplanet.com)
• If you enable RTS/CTS on a particular station (just the
hidden node station), it will refrain from sending a data
frame until the station completes a RTS/CTS handshake
with another station, such as an access point.
• Keep in mind, though, that an increase in performance
using RTS/CTS is the net result of introducing overhead
(i.e., RTS/CTS frames) and reducing overhead (i.e., fewer
retransmissions). If you don't have any hidden nodes,
then the use of RTS/CTS will only increase the amount of
overhead, which reduces throughput. A slight hidden node
problem may also result in performance degradation if you
implement RTS/CTS. In this case, the additional RTS/CTS
frames cost more in terms of overhead than what you gain
by reducing retransmissions. Thus, be careful when
implementing RTS/CTS.
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Improving WLAN Performance with
RTS/CTS by Jim Geier (wi-fiplanet.com)
• One of the best ways to determine if you should activate
RTS/CTS is to monitor the wireless LAN for collisions. If
you find a large number of collisions and the users are
relatively far apart and likely out of range, then try
enabling RTS/CTS on the applicable user wireless NICs.
You can activate the function by clicking "enable RTS/CTS"
somewhere in the user setup screens. You don't need to
enable RTS/CTS at the access point in this case. After
receiving a RTS frame from a user's radio NIC, the access
point will always respond with a CTS frame.
• Of course, keep in mind that user mobility can change the
results. A highly mobile user may be hidden for a short
period of time, perhaps when you perform the testing,
then be closer to other stations most of the time. If
collisions are occurring between users within range of each
other, the problem may be the result of high network
utilization or possibly RF interference.
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Frame Fragmentation
• Since we have already discussed RTS/CTS, let’s also
discuss frame fragmentation.
• Later, we will see that RTS/CTS and fragmentation are
typically combined.
• Frame fragmentation is a MAC layer function that is
designed to increase the reliability of transmitting frames
across a wireless medium.
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Frame Fragmentation
• In a “hostile wireless medium” (interference, noise) larger
frames may have more of a problem reaching the receiver
without any errors.
• By decreasing the size of the frame, the probability of
interference during transmission can be reduced.
• Breaking up a large frame into smaller frames, allows a
larger percentage of frames to arrive undamaged (without
errors).
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Frame Fragmentation
• Frame fragmentation can increase the reliability of frame
transmissions but there is additional overhead:
– Each frame fragment includes the 802.11 MAC protocol header.
– Each frame fragment requires a corresponding acknowledgement.
• If a frame fragment encounters errors or a collision, only that
fragment needs to be retransmitted, not the entire frame.
• The frame control field includes information that this is a
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fragmented frame.
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)
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
–
–
–
–
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)
Power management in infrastructure mode
Traffic Indication Map (TIM)
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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)
Mobile IP components
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WLAN Network Layer Standards:
Mobile IP (continued)
Computer relocated in Mobile IP
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WLAN Network Layer Standards:
Mobile IP (continued)
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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