h igh s peed w ireless lan s principle of network design university of tehran dept. of electrical...

HIGH SPEED WIRELESS LANS

Principle of Network Design

University of TehranDept. 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 9

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) 14

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 < 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, DIFS=3, PIFS=2, SIFS=1,

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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 A’s transmission to D, so B refrains from transmitting (loss in efficiency)24

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 25

TIMELINE WITH RTS/CTS

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SPECIAL FRAMES: ACK, RTS, CTS

Acknowledgement

Request To Send

Clear To Send

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FrameControl

DurationReceiverAddress

CRC

2 2 6 4bytes

ACK

FrameControl

DurationReceiverAddress

TransmitterAddress

CRC

2 2 6 6 4bytes

RTS

FrameControl

DurationReceiverAddress

CRC

2 2 6 4bytes

CTS

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 data’s 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

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• 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.

WLAN VS. OTHER SOLUTIONS

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Wired LAN

High performance WLAN

80

2.1

1a/g

80

2.1

1b

WLAN

Mbps (Tx Rate)1 10 1000.1

Out

door

Fixed

Walk

Vehicle

Indo

or

Fixed/Desktop

Walk

Mobility

UMTS

Wideband Cellular

WAN

GSM

& IS

-95

Bluetooth

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2.1

1n

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 τ = 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 = 70%*11 = 7 Mbps802.11a: PHY rate = 54Mbps, MAC throughput = 50%*54 = 27 Mbps

Future:PHY rate >= 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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DIFS frame ACK

backoff

SIFSPHYhdr PHYhdr

the real thing

WHAT IF USING DCF IN VERY HIGH-SPEED ?

54MAC throughput < 50 Mbps for ever !

WHY DCF SO SLOW?

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acksifshdrphybackoffdifs

frame

TTTTT

T

,*2

R while0

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.

PRIOR WORK (1/2)

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DIFSBurstACK SIFSFramePHYhdr ACKSIFSFramePHYhdr ACK SIFSSIFS

SIFSFramePHYhdrBlockACK FramePHYhdr

backoff

DIFS

backoff

BlockAck

SIFSBlockAckRequest

SIFS

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.

PRIOR WORK (2/2)

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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…

FramePHYhdrMobicom2004

FrameSub

PHYhdrDIFS

backoff

ACKSIFSFrameSub

PHYhdr

PHYhdrWoWMoM2005 (802.11n)

DIFS

backoff

ACKSIFSFrameMD FrameMD FrameMD

WHAT ARE STILL MISSING?

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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?

OUR SAMPLE SCHEME: AFR

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The Aggregation with Fragment Retransmission (AFR)

MACheader

MAC Layer

LLC Layer

PHY layer

frame

Packet 1 Packet 2 Packet nTCP/IP Layer

FCS

Fragment 1.0Fragment

1.1

fragmentheaders

Fragment n.0 Fragment n.1

Fragment n.0 Fragment n.1

FCSFragment 1.0

Fragment1.1

Fragment2.0

FCS

FCS

FCS

FCS

FCS

ZERO-WAITING

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Question:how much time should we wait for enough information to aggregate?

Answer:Zero-waiting: transmit immediately

Why:In heavily loaded networks, aggregation happens automaticallyIn slightly loaded networks, AFR degenerates to the legacy DCFZero-waiting is proven to be stable where feasible

MAXIMUM FRAME SIZE

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Constant throughput is possible with increasing frame sizes

Maximum frame size: 65536 bytes

FRAGMENT SIZES (1/2)

62Fragmentation is necessary with large frame in bad channels

FRAGMENT SIZES (2/2)

63A single fragment size can be found for near-optimal efficiency

MAC DELAY

64CSMA/CA delay for a ‘frame’ is worse than in DCF

MAC + QUEUE DELAY

65Total delay is much better due to ‘pipeline-like’ ability

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

71A structure containing multiple MSDUs, transported within a single (unfragmented) data MPDU

A-MPDU AGGREGATION FRAME STRUCTURE

72A structure containing multiple MPDUs, transported as a single PSDU by the PHY

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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TX

TX

RX

RX

MIMO

Processor

Input Output

MIMO Channel

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

HT Control field

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Data Frame

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 attentionAny question?

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