High Speed Wireless LANs

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High Speed Wireless LANs

• Principle of Network Design
• University of Tehran
• Dept. of Electrical and Computer Engineering
• By Dr. Nasser Yazdani
• Lecturer Peyman Teymoori

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Topics

• IEEE 802.11
• Network
• MAC Format
• IEEE 802.11e
• Paper Review
• Performance Analysis and Enhancement for the
  Current and Future IEEE 802.11 MAC Protocols
• Aggregation with Fragment Retransmission for Very
  High-Speed WLANs
• IEEE 802.11n

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IEEE 802.11 Topology

• Independent basic service set (IBSS) networks
  (Ad-hoc)
• Basic service set (BSS), associated node with an
  AP
• Extended service set (ESS) BSS networks
• Distribution system (DS) as an element that
  interconnects BSSs within the ESS via APs.

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IEEE 802.11 Topology
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Medium Access in WLANs

• IEEE 802.11
• MAC frame format
• CSMA/CA
• RTS/CTS
• IEEE 802.11e

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IEEE 802.11 Reference Model
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MAC Frame Format
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Frame Control Field (1)

• Protocol Version (2 bits) current version of the
  standard
• Type (2 bits) differentiates among a management
  frame (00), control frame (01), or data frame
  (10)
• Subtype (4 bits) further defines the type of
  frame
• Type 00, subtype 0000 association request
• Type 00, subtype 0001 association response
• Type 01, subtype 1011 RTS
• Type 01, subtype 1100 CTS
• Type 01, subtype 1101 ACK
• Type 10, subtype 0000 data
• Many others

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Frame Control Field (2)

• To/from DS (1 bit each) flags set when the frame
  is sent to/from the distribution system
• More Fragment (1 bit) flag set when more
  fragments belonging to the same frame are to
  follow
• Retry (1 bit) indicates that this frame is a
  retransmission
• Power Management (1 bit) indicates power
  management mode (active, power saving)
• More data (1 bit) more frames buffered by
  station for the same destination
• WEP (1 bit) payload encrypted with WEP
• Order (1 bit) strictly-ordered service

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Other Fields

• Duration ID (2 bytes) for data frames, it
  contains the duration of the frame
• Sequence control (2 bytes) sequence
• Frame body (0 to 2312 bytes)
• FCS (4 bytes) Frame Check Sequence (32 bit CRC)
• Address fields (6 bytes each) may contain BSSID,
  source/destination address, transmitting/receiving
  station address
• Interpretation depends on values of
  ToDS/FromDSbits

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Address Fields
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Indirection by Distribution System
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PHY

• MAC Protocol Data Unit (MPDU) is encapsulated by
  PLCP
• Format of PLCP PDU different for IEEE 802.11
  (DSSS, FHSS, IR), IEEE 802.11b (long
  preamble/short preamble), IEEE 802.11a
• PLCP PDU for IEEE 802.11b with long preamble
  compatible with PLCP PDU for IEEE 802.11 DHSS
• In this lecture, we will focus on IEEE 802.11b
  PLCP PDU

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802.11b Long Preamble PLCP PDU

• Compatible with legacy IEEE 802.11 systems
• Preamble (SYNC Start of Frame Delimiter) allows
  receiver to acquire the signal and synchronize
  itself with the transmitter
• Signal identifies the modulation scheme,
  transmission rate
• Length specifies the length of the MPDU
  (expressed in time to transmit it)

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802.11b Short Preamble PLCP PDU

• Not compatible with legacy IEEE 802.11 systems

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802.11 Medium Access

• Distributed Coordination Function (DCF)
• Stations contend for the medium and transmit when
  the medium becomes idle
• Mandatory in 802.11 standard
• Point Coordination Function (PCF)
• Works in conjunction with DCF
• Optional
• Access point polls stations during contention
  free periods and grants access to individual
  station

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Why not use CSMA/CD?

• In IEEE 802.3 (Ethernet), nodes sense the medium,
  transmit if the medium is idle, and listen for
  collisions
• If a collision is detected, after a back-off
  period, the node retransmits the frame
• Collision detection is not feasible in WLANs
• Node cannot know whether the signal was corrupted
  due to channel impairments in the vicinity of the
  receiving node
• IEEE 802.11 uses Carrier Sense Multiple Access
  (CSMA), but adopts collision avoidance, rather
  than collision detection

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CSMA

• Station waits a random amount of time before
  transmitting, while still monitoring the medium
• Avoids collisions due to multiple stations
  transmitting immediately after they sense the
  medium as idle
• Loss of throughput due to the waiting period is
  compensated by fewer retransmissions
• No explicit collision detection
• Retransmissions are triggered if ACK is not
  received
• Exponential backoff similar to IEEE 802.3
• Optionally, transmitting and receiving nodes can
  exchange control frames to reserve the channel

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Network Allocation Vector (NAV)

• Counter maintained by each station with amount of
  time that must elapse until the medium will
  become free again
• Contains the time that the station that currently
  has the medium will require to transmit its frame
• Station cannot transmit until NAV is zero
• Each station calculates how long it will take to
  transmit its frame (based on data rate and frame
  length) this information is included in the
  Duration field of the frame header
• This information is used by all other stations to
  set their NAV

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Timeline
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Timeline Discussed

• DCF Distributed Coordinated Function
• Basic access method for 802.11 (uses CSMA/CA)
• DIFS DCF Inter Frame Space
• Stations must listen to an idle medium for at
  least that amount of time before transmitting
• SIFS Short Inter Frame Space
• Period between reception of the data frame and
  transmission of the ACK
• SIFS lt DIFS
• What happens if another station starts listening
  to the medium exactly during the idle period
  between data transmission and acknowledgment?

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SIFS/DIFS

• SIFS makes transmission atomic
• Example Slot Time 1, CW 5, DIFS3, PIFS2,
  SIFS1,

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Hidden Node Problem

• Node A is not aware that node B is currently busy
  receiving from node C, and therefore may start
  its own transmission, causing a collision

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Exposed Node Problem

• Node B wants to transmit to node C but mistakenly
  thinks that this will interfere with As
  transmission to D, so B refrains from
  transmitting (loss in efficiency)

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RTS/CTS

1. Sender transmits a Request to Send (RTS)
   indicating how long it wants to hold the medium
2. Receiver replies with Clear to Send (CTS) echoing
   expected duration of transmission
3. Any node that hears the CTS knows it is near the
   receiver and should refrain from transmitting for
   that amount of time
4. Nodes that hear the RTS but not the CTS are free
   to transmit
5. Receiver sends ACK to sender after successfully
   receiving a frame. All nodes must wait for the
   receiver to ACK before attempting to transmit

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Timeline with RTS/CTS
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Special Frames ACK, RTS, CTS

• Acknowledgement
• Request To Send
• Clear To Send

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AP vs. Ad-hoc
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IEEE 802.11e

• MAC enhancements to support quality of service
  (QoS) in IEEE 802.11a/b/g
• Defines different categories of traffic
• Each QoS-enabled station marks its traffic
  according to its performance requirements
• Stations still contend for the medium, but
  different traffic types are associated with
  different inter frame spacing and contention
  window
• Qualitative, comparative QoS(no guarantees)

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802.11 STA vs. 802.11e STA
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Service Differentiation
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EDCA Review

• TXOP (Transmission Opportunity)
• An interval of time when a particular STA has the
  right to access the wireless medium.
• TID (Traffic identifier)
• TID value is specified in the QoS Control field
  of the 802.11e QoS datas frame MAC header.
• There are 16 possible TID values , where the
  value from 0-7 specify the user priority value
  of a frame, and the value from 8-15 specify the
  traffic stream which the frame belongs to.
• Block Ack (BA)
• During a TXOP, a STA (or AP) can transmit a
  number of frames without receiving any Ack. After
  frame transmissions completed, transmitter sends
  a control frame (Block Ack request, BAR) . Then
  the receiver respond with BA.

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802.11e TXOP and block ACK
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Wireless networking protocols

• The 802.11 family of radio protocols are commonly
  referred to as WiFi

• 802.11a supports up to 54 Mbps using the 5 GHz
  ISM and UNII bands.
• 802.11b supports up to 11 Mbps using the 2.4 GHz
  ISM band.
• 802.11g supports up to 54 Mbps using the 2.4 GHz
  ISM band.

• 802.11n supports up to 300 Mbps using the 2.4
  GHz and 5 GHz ISM and UNII bands.
• 802.16 (WiMAX) is not 802.11 WiFi! It is a much
  more complex technology that uses a variety of
  licensed and unlicensed frequencies.

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WLAN vs. Other Solutions
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Paper Review

• Performance Analysis and Enhancement for the
  Current and Future IEEE 802.11 MAC Protocols
• Yang Xiao, Jon Rosdahl

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High Data Rates

• The industry is seeking Higher Data Rates (HDR's)
  over 100Mbps (in 2002)
• More data rate intensive applications exist such
  as
• Multimedia conferencing,
• MPEG video streaming,
• Consumer applications,
• Network storage, and
• File transfer
• Finally, there is a great demand for higher
  capacity WLAN networks in the market such as
• Hotspots,
• Service providers,
• Wireless back haul, and
• An increasing number of users per access point

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High Data Rates

• We explore the overhead of HDR's to find out
  whether the MAC is good enough
• We prove that a theoretical throughput upper
  limit and a theoretical delay lower limit exist
  for IEEE 802.11 protocols
• In order to reduce overhead, we propose a burst
  transmission and acknowledgement ( BTA )
  mechanism

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PPDU Frame Format of IEEE 802.11a
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IEEE 802.11a

• Data rates for IEEE 802.11a
• 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
• Some IEEE 802.11a parameters
• Tslot 9µs (Slot time),
• Tsifs 16µs (SIFS time),
• Tp 16µs (Physical layer's preamble),
• CW0 CWmin 16,
• Tsim 4µs (Symbol time),
• Tdifs 34µs (DIFS time),
• Tphy 4µs (PHY header time), and
• t 1µs (Propagation delay).

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IEEE 802.11a Best-Case Performance

• Ldata length of the payload
• Tdata and Tack transmission times of a data
  frame and an ACK, respectively.
• MT Maximum throughput
• MD Minimum delay

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IEEE 802.11a Best-Case Performance

• BE bandwidth efficiency
• TUL theoretical throughput upper limit
• DLL theoretical delay lower limit

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IEEE 802.11a Best-Case Performance
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Burst Transmission and Acknowledgement

• A BTA sequence
• MAC frame format (FC Frame Control DU
  Duration A Address QoS QoS Control FB Frame
  Body) (Size is in bytes)

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Burst Transmission and Acknowledgement

• BurstAckReq frame format (FC Frame Control DU
  Duration RA Receiver Address TA Transmitter
  Address BAR BAR Control R Reserved) (Size is
  in bytes)
• BurstAck frame format (FC Frame Control DU
  Duration RA Receiver Address TA Transmitter
  Address R Reserved W Wait SC Sequence
  Control BM Ack Bitmap) (Size is in bytes)

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Burst Transmission and Acknowledgement

• Tr time required to transmit the burst
  acknowledgement request frame,
• Ta time required to transmit the burst
  acknowledgement frame
• Tpo time required to transmit the CF-Poll
  frame
• Nb number of burst

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Burst Transmission and Acknowledgement
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Burst Transmission and Acknowledgement
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Burst Transmission and Acknowledgement
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Paper Review

• Aggregation with Fragment Retransmission for Very
  High-Speed WLANs
• Tianji Li, Qiang Ni, David Malone, Douglas Leith,
  Yang Xiao, Thierry Turletti,

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Outline

• Goal
• To design a new MAC with high efficiency for very
  high-speed next-generation WLAN (e.g. 802.11n)
• Difficulty
• Overhead at MAC and PHY
• Solution
• aggregation at MAC

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Goal

• Now
• 802.11b PHY rate 11Mbps, MAC throughput
  7011 7 Mbps
• 802.11a PHY rate 54Mbps, MAC throughput
  5054 27 Mbps
• Future
• PHY rate gt 216 Mbps (up to 648 Mbps),
• MAC throughput ???

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DCF The Current MAC

• Overhead DIFS, backoff, SIFS, PHY headers, and
  ACKs.

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What if using DCF in Very High-Speed ?
MAC throughput lt 50 Mbps for ever !
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Why DCF so Slow?

• Tframe frame size / R, it scales with 1/R.
• Tack ack size / R, it scales with 1/R.
• But, other items in denominator are constant,
  which leads to
• Solution We need to make all in denominator
  scale also with 1/R.

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Prior Work (1/2)

• Burst ACK proposed in early versions of 802.11e
• Tdifs and Tbackoff scale with 1/R.
• Block ACK in the current 802.11e
• Tdifs , Tbackoff and TACK scale with 1/R.

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Prior Work (2/2)

• Aggregation from Ji et. al.
• Tdifs , Tbackoff , Tack and Tsifs scale with 1/R.
• Aggregation from Kim et. al.
• All in denominator scale with 1/R, then why I am
  here

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What are still missing?

• How to have very large frames?
• Wait or not if no enough information?
• How much time to wait for?
• Is there a limit for the frame size? What is the
  best size?
• What is the best size for retransmission?
• What the delay will look like?

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Our Sample Scheme AFR
The Aggregation with Fragment Retransmission
(AFR)
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Zero-waiting

• Question
• how much time should we wait for enough
  information to aggregate?
• Answer
• Zero-waiting transmit immediately
• Why
• In heavily loaded networks, aggregation happens
  automatically
• In slightly loaded networks, AFR degenerates to
  the legacy DCF
• Zero-waiting is proven to be stable where
  feasible

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Maximum Frame Size
Constant throughput is possible with increasing
frame sizes Maximum frame size 65536 bytes
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Fragment sizes (1/2)
Fragmentation is necessary with large frame in
bad channels
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Fragment sizes (2/2)
A single fragment size can be found for
near-optimal efficiency
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MAC Delay
CSMA/CA delay for a frame is worse than in DCF
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MAC Queue Delay
Total delay is much better due to pipeline-like
ability
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AFR vs DCF
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HDTV (simulation)
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802.11n

• The latest approach toward High-Speed WLANs
• What we review
• Some New MAC Concepts
• 802.11n Features
• Performance Evaluation

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MAC Definitions

• MPDU stands for MAC Protocol data unit. MPDUs are
  messages (Protocol data units) exchanged between
  MAC entities in a communication system based on
  the layered OSI model.
• In systems where the MPDU may be larger than the
  MSDUs, then the MPDU may include multiple MSDUs
  as a result of Packet aggregation.
• In systems where the MPDU is smaller than the
  MSDU, then one MSDU may generate multiple MPDUs
  as a result of Packet segmentation.

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MAC Definitions

• Packet aggregation is the process of joining
  multiple packets together into a single
  transmission unit, in order to reduce the
  overhead associated with each transmission
• A-MPDU
• A-MSDU

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A-MSDU Aggregation Frame Structure
A structure containing multiple MSDUs,
transported within a single (unfragmented) data
MPDU
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A-MPDU Aggregation Frame Structure
A structure containing multiple MPDUs,
transported as a single PSDU by the PHY
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IEEE 802.11n Features

• MIMO-OFDM physical layer
• Aggregation
• Block ACK
• Reverse direction

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MIMO-OFDM

• The most commonly used method is to increase the
  raw data rate in the PHY layer
• MIMO can effectively enhance spectral efficiency
  with simultaneously multiple data stream
  transmissions
• Orthogonal frequency division multiplexing (OFDM)
  transmission scheme has been used to increase PHY
  layer transmission rate
• With this enhancement in the PHY layer, the peak
  PHY rate can be boosted up to 600 Mbps

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Aggregation

• The key feature to improve the 802.11 MAC
  transmission efficiency
• designed as two-level aggregation scheme
• A-MSDU
• A-MPDU
• The maximum length of an A-MSDU, 3839 or 7935
• These MSDUs must be in the same traffic flow
  (same TID) with the same destination and source
• The TID of each MPDU in the same AMPDU might be
  different.
• The maximum size limit of A-MPDU is 65535 bytes

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Two-level aggregation in IEEE 802.11n
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Block ACK

• Problem frame error rate is higher as the size
  of the frame increases!
• Large frames in high bit-error-rate (BER)
  wireless environment have a higher error
  probability and may need more retransmission
• To overcome this drawback in aggregation, the
  block ACK mechanism in 802.11n is modified to
  support multiple MPDUs in an A-MPDU. When an
  A-MPDU from one station is received and errors
  are found in some of the aggregated MPDUs, the
  receiving node sends a block ACK only
  acknowledging those correct MPDUs. The sender
  only needs to retransmit those non-acknowledged
  MPDUs.
• Note, block ACK mechanism only applies to AMPDU,
  but not A-MSDU!
• The maximum number of MPDUs in an A-MPDU is
  limited to 64 as one block ACK bitmap can only
  acknowledge at most 64

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Block ACK with aggregation
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Reverse Direction

• Reverse direction mechanism allows the holder of
  TXOP to allocate the unused TXOP time to its
  receivers to enhance the channel utilization and
  performance of reverse direction traffic flows
• The major enhancement in reverse direction
  mechanism is the delay time reduction in reverse
  link traffic
• This feature can benefit a delay-sensitive
  service like VoIP

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Reverse Direction
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802.11n MAC Frame Format

• Data Frame

• HT Control field

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802.11n MAC Frame Format

• BlockAckReq frame
• BA Information field (BlockAck)

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802.11n MAC Frame Format

• A-MSDU structure
• A-MSDU subframe structure

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802.11n MAC Frame Format

• A-MPDU format
• A-MPDU subframe format

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Block ACK performance
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• Thanks for you attention
• Any question?