Studying 802.11

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Studying 802.11 1st lectureOverview of IEEE
802.11
Tadashi YASUMOTO
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In beginning

• To understand acronyms
• Overwhelming numbers
• To understand the difference with Ethernet
• To understand the 802.11 MAC layer

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What IEEE 802.11 is for.

• Allow WLAN to be identical to wired LAN
• With mobility
• With built-in power saving operations

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IEEE 802.11 defines

• Physical layers
• MAC management protocols and service
• MAC sublayer

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IEEE 802.11 defines

• Physical layers
• MAC management protocols and service
• MAC sublayer

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Physical layers

• PHYs Physical layers
• Some kinds of layers
• 802.11b
• HR/DSSS a high-rate DSSS
• 802.11a
• Based on OFDM using orthogonal frequency
  division multiplex
• 802.11g
• Using OFDM with backwards compatibility with 11b
• Common specification
• CSMA/CA
• Difference with CSMA/CD

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Physical layers

• CSMA/CA Carrier Sense Multiple Access with
  Collision Avoidance
• Dispersion Contorol (lt-gt Centralization Control)
• Difference with CSMA/CD
• Not always detect the collision
• Features
• If the other node are in use of the channel
• Wait for random delay time
• And have the right to send

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IEEE 802.11 defines

• Physical layers
• MAC management protocols and service
• MAC sublayer

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MAC layer (1/2)

• MAC sublayer
• LLC the logical link control
• A common link layer used by lower-layer LAN
  tecnology
• What is LLC -gt next slide
• MAC management
• 802.1D
• Bridging
• 802.1Q
• Virtual LAN(VLAN)

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MAC layer (2/2)

• LLC Logical Links Control

• One of the sublayer in Data Link Layer (L2 in
  OSI)
• In 802, all of lower-layers are under LLC

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802.11 Architecture

• Components
• Types of Networks
• Services
• Other

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802.11 Architecture

• Components
• Types of Networks
• Services
• Other

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Components
DS
Medium
APs
STAs
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Components

• STAs Stations
• Computing devices with wireless network
  interface(s)
• NICs familiar with in wired networks
• APs Access points
• Wireless-to-wired bridging
• Wireless medium
• PHYs
• DS Distribution system (later)
• Tracking the movements of mobile STAs

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Components

• Distribution Systems
• Provides services to STAs
• Tracking where a STA is
• Delivering frames

Ethernet is a part of DS
AP has at least two interfaces one is wired, the
other is wireless with bridging egngine
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802.11 Architecture

• Components
• Types of Networks
• Services
• Other

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Types of Networks(1/3)

• BSS the basic service set
• IBSS independent BSS
• A set of STAs creating a short-lived network
• Direct communication and No relay function
• Sometimes referred to AD-HOC networks
• Infrastructure BSS
• Using an AP
• Two hops communication with relay function
• More transmission capacity than IBSS
• Assist with STAs saving power
• STAs need to associate with an AP

later
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Types of Networks(2/3)

• ESS Extended service area
• An arbitrarily large size
• A set of BSSs
• With same SSID service set identifier
• Link with APs in layer 2

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Types of Networks(3/3)

• Virtual APs
• Some ESSs on same physical infrastructure
• To protect some different networks
• To create parallel networks with different
  security levels

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802.11 Architecture

• Components
• Types of Networks
• Services
• Other

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Services

• STA services
• Authentication
• Deauthentication
• Privacy
• Data delivery
• Distribution services
• Association
• Reassociation
• Disassociation
• Distribution
• Integration

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Services

• Distribution services

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Services

• Privacy (or Confidentiality)
• Provide a level of protection
• WEP protocol Wired Equivalent Privacy

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802.11 Architecture

• Components
• Types of Networks
• Services
• Other

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Other

• Interaction between services
• After
• Mobility support
• 3 types of transitions between AP s
• No transition
• BSS transition
• ESS transition

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Other

• Interaction between services

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Mobility support

• BSS transition
• No need to be aware of STA's location
• the cooperation of AP s

Same ESS
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Mobility Support

• ESS transition
• 802.11 does not support...

ESS2
ESS1
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Workshop Presentation2nd

• Ryo Nakajima

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• Wireless and 802.11 introduction
• Time scheduling in 802.11
• Frame format in 802.11
• Particular situation
• Frame translation between wire and wireless

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802.11

• Wireless link
• Many interfere
• Microwave ovens, node itself
• Need power saving mode
• Mobile, battery operated
• Particular system acknowledgement, RTS/CTS,
  CSMA/CA

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Acknowledgement

• Each frame must be acknowledged
• Or the sender will re-transmit the same frame

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RTS/CTS

• The Hidden Node Problem
• Only node2 can know both node1 and node3

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RTS/CTS

• RTS/CTS keep other node silent

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CSMA/CA

• CA collision avoidance
• Like CSMA/CD
• Wait until the channel become idle
• Details in Scheduling

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Scheduling

• DCF (AD-HOC mode)
• Basic time scheduling in 802.11
• To avoid collisions, use empty time
• PCF (AP)
• Rare system
• To avoid collisions, AP(Access Point) gives
  priority to each node in order
• HCF
• Keep service quality, but now undertaking

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DCF

• Carrier sensing
• To find whether the channel is busy or not
• Time scheduling
• To avoid collisions by simultaneous transmission

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Carrier sensing

• Physical carrier sensing
• Expensive electronics
• Hidden nodes lurking everywhere
• Virtual carrier sensing
• Use Network Allocation Vector(NAV)
• NAV means time to be quiet
• NAV is included almost all frames
• So NAV can be heard every nodes

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Time scheduling

• NAV
• Interframe Spacing
• Waiting time after one frame transmitted

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Interframe Spacing

• SIFS
• Short range
• To higher priorityRTSCTS, DataAck, Fragment
• DIFS
• Long range, standard
• After DataAck, with Backoff time
• EIFS
• To detect transmit failure
• SIFSAckDIFS

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Time Schedule Example
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Backoff

• Time to wait after DIFS
• Random length
• Each node determine by itself
• To lower collisions
• Lower simultaneous beginning of transmission

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Error Recovery

• No ack?retry
• Short retry counter and long retry counter
• Incremented when frames are retransmitted
• Can hear ack?set to 0
• If reach to the limit, discard that frames
• Backoff time become long to reduce transmission
• If retry counter incremented, backoff time is
  doubled

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General Frame format
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Frame Control
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• Next Duration/ID

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Duration/ID

• NAV
• PS-PollAID(Associated ID)
• Nodes wake-up signal

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• Next Address field

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Address field

• Destination Address
• Source Address
• BSSID
• APs wireless NICs MAC address
• Receiver Address
• Transmitter Address

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• Next Sequence Control

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Sequence Control

• 4 bits Fragment number and 12 bits Sequence
  number
• All fragments have the same sequence number
• To detect duplicate frame

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• Next FCS

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FCS

• Frame Check Sequence
• Calculate the value with hash function

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Particular situation

• Group frames( broadcast multicast )
• Fragment
• RTS/CTS
• Power saving
• Multirate support

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Group frames

• No acknowledgement
• No fragment
• No NAV set

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Fragment

• SIFS between fragment

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RTS/CTS

• NAVRTSCTSfirst_datafirst_ack

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Power saving

• Node send PS-poll to AP after wake up
• AP send data to node which is buffered
• AP buffer data while node is sleeping
• APs response Immediate response or Deferred
  response
• Immediate use SIFS
• Deferred use DIFSBackoff

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Multirate Support

• How to choose the rate depends on the vendors
• Basic rate set every station in the system must
  support
• Control frame that start a frame exchange
• Such as RTS/CTS
• One of the rates in the basic rate set
• CTS no faster than RTS

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Wireless?Wire

• 1 integrity check
• 2 BSSID (APs MAC) check
• 3 decrypt if needed
• 4 Fragment check
• 5 copy address and type
• 6 FCS recalculate

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Wire?Wireless

• 1 Integrity check and destination node check
• 2 prepend SNAP header
• 3 power saving check
• 4 set sequence number, fragment number
• 5 encryption if needed
• 6 copy address
• 7 FCS recalculate