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This tutorial explains the 802.11n concepts in a simple and easy to understand manner. It talks about different types of MIMO improvements (diversity, MRC, spatial multiplexing, space time coding). It also provides a summary of 802.11n packet formats and various capacity/throughput related optimizations proposed in 802.11n (PLCP improvements, BlockAcks, A-MPDU & A-MSDU Aggregation).
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www.airtightnetworks.net
A Brief Tutorial on IEEE 802.11n
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AirTight Networks
Outline
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AirTight Networks
IEEE 802.11
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802.11 protocol suite
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802.11 MAC and PHY enhancements
PhysicalPMD
PLCP
MACData link802.11e802.11i
802.11n
802.11n
802.11w
Security QoS
Capacity & Coverage
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Two-slide primer on 802.11 MAC (1)
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) �&��� & ��� � ���1* # � 8* � 3
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Two-slide primer on 802.11 MAC (2)
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Example of DCF CSMA/CA (1)
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Example of DCF CSMA/CA (2)
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Example of DCF CSMA/CA (3)
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Example of DCF CSMA/CA (4)
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Example of DCF CSMA/CA (5)
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Example of DCF CSMA/CA (6)
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Example of DCF CSMA/CA (7)
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Motivation for multicarrier modulation
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Multicarrier modulation
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Orthogonal frequency division multiplexing (OFDM)
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OFDM in 802.11
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802.11n PHY Enhancements
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What is MIMO?
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RxTx
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Spatial diversity
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Receiver diversity
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Receiver diversity: Selection combining
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Receiver diversity: Maximum Ratio Combining (MRC)
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Radio
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Tx
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DSP
Rx
Radio Radio
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Receiver diversity: Maximum ratio combining
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Transmitter diversity: Channel-aware
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Transmitter diversity: Channel-unaware
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)s(t)h (h0.5 21 +
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Transmitter diversity: Alamouti scheme
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y = = Hs
9 ��z = HHy = HHHs = (|h12| + |h2
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z1 = h1*z1 + h2z2 = (|h12| + |h2
2|)s1
z2 = h2*z1 – h1z2 = (|h12| + |h2
2|)s2
h1 h2
h2* -h1
*
s1
s2
AirTight Networks
Transmitter diversity: Alamouti scheme
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Practical significance: array gain and diversity gain
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Array gain
Diversity gain
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Practical significance: array gain and diversity gain
Pe
SNR
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Spatial multiplexing
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Spatial multiplexing
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b1 b3 b5
b2 b4 b6
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b2 b4 b6
b1b2b3b4b5b6
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Spatial multiplexing gain vs. diversity gain trade-off
Div
ersi
ty g
ain
Spatial multiplexing gain
0, MN
1, (M-1)(N-1)
2, (M-2)(N-2)
k, (M-k)(N-k)
Min(M, N), 0
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802.11n channels
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802.11n Modes of OperationPLCP Enhancements
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802.11n: Modes of Operation
For use of legacy devices also
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Signalling(See next slide)
> �# � � ���� � �:5 � %�# �? � %�� � �(� &� �1� ������ ���� � ��" ��� �� ��9 * ��� �� ��3
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Detection of PPDU,timing & coarse freq acquisition
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L-SIG (MM) & HT-SIG (MM & GF)Encoded value indicatingDuration of rest of the packet
Always 6 Mbps
L-SIG of Mixed Mode
Refer to next slides
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Set to a non-zero number, to indicate the difference between the number of space time streams (NSTS ) and the number of spatial streams (NSS) indicated by the MCS.Set to 00 to indicate no STBC (NSTS = NSS)
STBC
Set to 1 to indicate that the PPDU in the data portion of the packet contains an AMPDU otherwise, set to 0.Aggregation
Set to 1Reserved
Set to 0 indicates that PPDU is a Sounding PPDUSet to 1 indicates that the PPDU is not a sounding PPDU
Not Sounding
Set to 1 indicates that channel estimate smoothing is allowedSet to 0 indicates that only per-carrier independent (unsmoothed) channel estimate is recommended
Smoothing
The number of octets of data in the PSDU in the range 0-65535Length
Set to 0 for 20 MHz or 40 MHz upper/lower Set to 1 for 40 MHzCBW 20/40
Index into the MCS table.Modulation and Coding Scheme
Explanation and codingField Name
HT-SIG
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CRC of bits 0-23 in HT-SIG1 and bits 0-9 in HT-SIG2CRC
Indicates the Number of extension spatial streams (NESS).Set to 0 for no extension spatial streamSet to 1 for 1 extension spatial streamSet to 2 for 2 extension spatial streamsSet to 3 for 3 extension spatial streams
Number of extension spatial streams
Set to 1 to indicate that the short GI is used after the HT training.Set to 0 otherwiseShort GI
Set to 1 for LDPCSet to 0 for BCC
LDPC coding
Explanation and codingField Name
HT-SIG
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Modulation & Coding Scheme (MCS)
# * ������� � ) � �� ��� � � � ������ ��1��� ? 3���� �� �����
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� ����� � �'
• MCS 0 to 15 at 20 Mhz (at AP)• MCS 0 to 7 at 20 Mhz (at client STA)
� 9 ������� �����
• MCS 16 to 76 are optional• All MCS at 40 Mhz
� �' @ @ ���@ � � ��� % ���� ��� ����
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Rate Dependent Parameters (20 MHz and Mandatory MCS)
NSS = 1
NSS = 2
What is the formula?
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Rate Dependent Parameters (40 Mhz & Mandatory MCS)
NSS = 1
NSS = 2
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Other Optional MCSs
# * � � ����D>
� �' �0 A �?
� �$ ��( �' �? +
• 216.7 Mbps (20 Mhz)• 450 Mbps (40 Mhz)
# * � � ����D4
� �' �3 A ? �
� �$ ��( �' �? +
• 288.9 Mbps (20 Mhz)• 600 Mbps (40 Mhz)
$ �� � �# * �
� 1 ) � �� �����
• MCS 32• Useful under very high noise• Lowest rate of 40 Mhz (bpsk)• 6.7 Mbps max rate
� �' � - ������ ������������
• Use with – Tx beamforming – STBC
• MCS 33 – 38 (4 SS)– Max rate 495 Mbps
• MCS 39 – 52 (4 SS)– Max rate 495 Mbps
• MCS 53 – 76 (4 SS)– Max rate 495 Mbps
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MAC Enhancements
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/ ��) �� � � � � ���� �
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MPDU
Motivation
� ) � � ��' ��9 * �%�# � * �� ! �� �� ��
* ���� ��) � & ) �� � ����� ! ��&�� �" ��1���� ��� ��� � �� ? �3
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MPDU1PLCPDCF PLCP ACK MPDU2PLCPDCF PLCP ACK
PLCPDCF PLCP ACK
SIFS
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Physical Level Aggregation (A-MPDU)
�����������% � �� � � B � ��� ������������ ����% � � 6��������& ���1 ) ' 6C � #�� �����D!, , �, ����E ��� ����% ��� ��7 �
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Physical Level Aggregation (A-MPDU)
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� �� � � B �� �� ����,
• 0 indicates no restriction• Else, ranges from 1/4 to 16 usecs• Realized by using Delimiters with MPDU length 0
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F $ ��� ���� ��������!5 � � B ���������� = �' ���� ����
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Max Rx Factor(x): 0 to 3 [2^13+x]Min spacing: 0.25 to 16 usecs
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A-MSDU
!5 ' � B �������������� ����& � ����
!�� ' � B � � ���������& � ����% ��& ������� ����% �
F ��� ' � B & ����, �������) 6� (= �' �����+
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A-MSDU
' �� � � �������� ���� ����% � - �������� ���������, ��(��������!5 � � B += �' � �� � � B ���� *� ���!�7 � ������ " ���7 !�7 �, �������� �����- ���� ��!5 ' � B ���& ��� �����= �'
���� ����� � ����" !�������
!5 ' � B ��������������� !I ����5��������������� ' � B �� !�!5 ' � B ���& �) $ �����E��5 ' � B ��������$ � � ��� �� ����% �����
����% �
� 6�� ������������� ��� ' � B � ���& �) $ �% � ��� ��� ����% �������������
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Block ACK (BA)
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Block Ack Packet Exchange
� 6 6 E � �7 ; � ����� � ��� �������� �E � �� ���� �
� 6 6 E � �7 �� � �� �� � �(�� ) �8� G � ����� �� ���� ��
/ ��) ��� (���� ���� ��� � �� $ � �� �� ���� � �� � ���! &"� ) ������& ���$ ��- ����� � ������
������
� ) ������& ���� ���% ��- ��� ��7 ��� ����� ������
� ) ������& ����������� ������) I 4 � �
E &� � �� � �7 ; �� � ��� ��� &�� ������E &� � �� * �� �� � �� �(��)
6 < 9 E � ��� � ��� �� � ) ���� ���E � �� ���� �
AirTight Networks
Block ACK Sessions (ADDBA)
* �� � � � " ����E &� � ��� * ����� �� � ��� ������ 6 6 �� ; � ��8� �� � �� 8� & �
6 ��&� � ��� � ������� ) ����� �� (����6 �(� � �� ; 8� �� � ��
����) � � �� ��1� (�� � ����� ? ���&�� 3
� �) � ������ �� �� ���� �� ������ ��1 � � �� �3�(� � �� ��� ����� ���� ������� ��!
������� � � ���� �� �� ��� � �� � ��� �� � �! � ��� � � ����� �� ; � ���� � ��� �
� 6��������� ������� � ��������" ���7 !�<
Request Response
AirTight Networks
Block Ack Parameter Set Field used in ADDBA Action Management Frames
E &� � ��� � � ����) � � �� ��(� &�� ! ' � B ���� �����& �����- ������ � �����" !�������
� " ���7 !�7 � ���������� ��������!�< �� �� �������
• Delayed is sent at a slightly later time after receiving a Block AckReq (See next slide)
� ) 6� ��������������) �����6������� �����(�������6� �����, ��� ���� ���������������� = �' �����+
� " ���� ��2 ���������& ���� �
• Recipient controls the buffers that can be supported
802.11n
802.11e
AirTight Networks
Immediate BlockAck
Delayed BlockAck
AirTight Networks
Block ACK Sessions (DELBA)
6 < 9 E � ��� � ��� �� �� �� � � ��� ���� ��� ? � &�� ��&"
�������� � ���� �� �� ��� � �� � ��� �� �� � �� � �� � �! � �� (�+ � �� ��������
������� � �6 < 9 E �
DELBA Parameter set
AirTight Networks
BlockAckReq (BAR)
802.11n
802.11e
See Next Slide
AirTight Networks
Fields of BlockAckReq Frame
E � 7 �* � ��� � &� " !9 !�< � �����(1 ) 5
�����������+
• Normal ACK• No ACK
� ���5) 6�
• Does BAR consist of req for different QoS streams?
� ���� �����
• Support for ACK for fragments in BA?
� ) 6� J 6*� 4
• Info about each TID
��� � ����� ��� � �� ��E � �� � &�� "
� ������������������; ��������" ��
��E &� � ��� � &�� " �(� &� (!� � " !+
� 6��������������������1 ) ��������
; 1 ) �������� ���������E � 7 �� * ��
�� &�� "
• Negotiated between HT stations as a part of HT capabilities
• Extensions for using BA with 802.11n features such as frame aggregation (A-MPDU)
AirTight Networks
BlockAckReqEncoding
-MT BAR-TID_info contains number of TIDs-BAR info contains seq number for that many TIDs
Per TID INFO
-Basic BAR, Compressed BAR-TID info contains TID for which the . req has been made
BAR Info Field
AirTight Networks
BlockAck frame
E &� � �� � � � �� � � ��� * �� ������) �� �
< ? �� ��(� � ) ���� � �� ��� ���� � �� � � ��� �1� �� ? ���&�� 3
AirTight Networks
BA Information for each BA encoding
Basic BA128 byte bitmap
Compressed BA Mandatory8 bit bitmapNo support for fragments
MTBA (repeatedFor each TID)
AirTight Networks
HT Protection Mechanisms
AirTight Networks
Protection Requirements
-Protection may be required if Non-HT stations are present or Non-greenfield stations are present
-Types of protection that an HT station provides-RTS/CTS using a legacy rate-CTS to self using a legacy rate- Transmit 1st frame in a backward compatible mode
-1st frame Tx using a Non-HT preamble and then switch to HT mode-1st frame Tx using a MM preamble and then switch to Greenfield operation
-Setting of L-SIG values in preamble to protect the current transmission-L-SIG TxOP (See next slide)
AirTight Networks
L-SIG TxOP Protection
������������& �- ����1 ) ' ) !� ����� � � ������� �(�������������1 ) ��� �& ������6F ��� ��+
� �����, ����� ��� ' � B � (��, ��� !) !K !�< �9 ) ' .�) ' +����, ��� , � �� �������� �% ��� ��#5' 6C� #5' 6C � � ����- ���& ��� �% ��� ���� !������ E��� ����% ����
*��51 ) ' ) !� D���7 E ������ ������������% ��% ��, ���, ���� , �� ���L
!� � ����& ���1 ) 5 �$ ������) $ ����
AirTight Networks
HT Parameter NegotiationInformation Elements
AirTight Networks
Advertising HT Capabilities using MAC Frames
HT Capability Information Element (E.g., Beacon, Probe Response)
Refer to next slides
AirTight Networks
Set to 0 if not supportedSet to 1 if supported
Indicates support for the transmission of PPDUs using STBCTx STBC
Set to 0 if not supportedSet to 1 if supported
Indicates Short GI support for the reception of 40 MHz packetsShort GI for 40 MHz
Set to 0 if not supportedSet to 1 if supported
Indicates Short GI support for the reception of 20 MHz packetsShort GI for 20 MHz
Set to 0 if not supportedSet to 1 if supported
Indicates support for the reception of PPDUswith HT Greenfield format.Greenfield
Set to 0 for Static SM Power Save modeSet to 1 for Dynamic SM Power Save modeSet to 3 for SM enabled
The value 2 is reservedIndicates the Spatial Multiplexing (SM) Power Save mode.SM Power Save
Set to 0 if only 20 MHz operation is supportedSet to 1 if both 20 MHz and 40 MHz operation is supported
Indicates which channel widths the STA supportsSupported channel width set
Set to 0 if not supportedSet to 1 if supported
Indicates support for receiving LDPC coded packetsLDPC coding capability
EncodingDefinitionSubfield
HT Capabilities Info
AirTight Networks
Set to 0 for no supportSet to 1 for support of one spatial streamSet to 2 for support of one and two spatial streamsSet to 3 for support of one, two and three spatial streams
Indicates support for the reception of PPDUsusing STBCRx STBC
In Beacon, Measurement Pilot and Probe Response frames:Set to 0 if the BSS does not allow use of DSSS/CCK in 40 MHzSet to 1 if the BSS does allow use of DSSS/CCK in 40 MHzOtherwise:Set to 0 if the STA does not use DSSS/CCK in 40 MHzSet to 1 if the STA uses DSSS/CCK in 40 MHz
Indicates use of DSSS/CCK mode in a 40 MHz capable BSS operating in 20/40 MHz mode. DSSS/CCK Mode in 40 MHz
Set to 0 for 3839 octetsSet to 1 for 7935 octets
Indicates maximum AMSDU length. See 9.7b (A-MSDU operation).Maximum A-MSDU length
Set to 0 if not supportedSet to 1 if supported
Support indicates that the STA is able to accept an ADDBA request for HT-delayed Block Ack
Indicates support for HTdelayed BlockAckoperation.HT-delayed BlockAck
EncodingDefinitionSubfield
HT Capabilities Info
AirTight Networks
EncodingDefinitionSubfield
Set to 0 if not supportedSet to 1 if supported
Indicates support for the LSIG TXOP protection mechanism
L-SIG TXOP protection support
Set to 0 by an AP if the AP allows use of 40 MHz transmissions in neighboring BSSs.Set to 1 by an AP if the AP does not allow use of 40 MHz transmissions in neighboring BSSs.Set to 0 by a STA to indicate to its associated AP that the AP is not required to restrict the use of 40 MHz transmissions within its BSS.Set to 1 by a STA to indicate to its associated AP that the AP is required to restrict the use of 40 MHz transmissions within its BSS.
When sent by an AP, indicates whether other BSSs receiving this information are required to prohibit 40 MHz transmissions.When sent by a STA, indicates whether the AP associated with this STA is required to prohibit 40 MHz transmissions by all members of the BSS. Forty MHz Intolerant
In Beacon, Measurement Pilot and Probe Response frames transmitted by an AP.Set to 0 if the AP does not support PSMP operationSet to 1 if the AP supports PSMP operation
In Beacon frames transmitted by a non-AP STA:Set to 0
Indicates support for PSMP operation. SeePSMP support
HT Capabilities Info
AirTight Networks
Example Packet Trace Snippet of a DlinkAP
5 � �* �� ���&��" ���(� ����H � � � � � � � � � � �
���������������������������� �������������� ������������ ���
������������������������������������� !"� ���#��������������$%&� ��$�'���
������������������(�)���*'����������������� ����������� �
���+��������������'���������������������(���*,,-�.�����/�� !"
������������������ �0�#����� �(1���"���2324�&5���
������������������(������������� ��!�(���5���'���6��6��� �����
���������������������.0��',������ �
������������������ ���#���� ��������������� ��0�',
����������+��������%� ������� ���� !"������� ���
�������������������%� ������� �7�� !"����������� ���
������������������(�)������������&������.����)����(1����%��8�� ��#&��
�������������++����������� �������0��$�9��&���
���������������+��'��%�7� !"������� !"���� �������������� ���
�������������������(�,������$�����&����5����������� ���
AirTight Networks
HT Capabilities IE: Supported MCS Set
7 ? �# * �E ��) �����������D� ���� �� �� ����� � � � ��(� � ������# *
� ? # * � ��6 (�� � �D�� �) ������ ���� ? 87 ? �# * ��� � ; ��&
A � �� 4 �) �? ���� �) ��� ���� ���� � � � � �
� ? �� ; ��&�) � � �&���� ����� � � � ��1���� ��� ��� � � �� &� � 3�) �" �� � �) �" ��� ��� ���� � � � � �
AirTight Networks
HT Extended Capabilities
� �4 �' �� � � �� � ��������$ �������� � ����
� !�� ���& �- ����� ; � � �3 � �2 �� � ����
� !- ����G� ��������5�$ ������H
�' ����& ��7
� ' �������� �% ��� �' ����& ��7
9 � 9 ��� ���� ����������� � � �� 9 �% � ���� ������ �����
� 4 � ��������� �- �� ���������� ��������� ��� ����� ��7 �& � �� ��� ��� ����
, ���! ��� ��� ����" ��) � & ) ������ ���� (��� ���� ! ��� �(��
AirTight Networks
HT Information Element
-Prev IE was about capabilities-This IE is about actual state of BSS at any point of time (more dynamic)-See next slide for more
AirTight Networks
HT Information Element
������� ������ � ���� �
� � ��� ��������
� ' ������ �������������
� �������- ������' ) !(�� � 3 � +
� � ���" �����
• Does AP Tx beacon in secondary channel?
� E ���� �# * � �
• Mandatory MCS for all STAs in BSS
• Similar to Basic rates of .11a/b/g
9 6� '
� ' �� � ��� � ��7 �, �� �
� F �, ��������(���� � ��- ��
�0 ������� ' 6� ' +
) $ & � �����
� " � ���C � � 9 6� ' � ��7 ��
4 % � ��� � ��, " ' ' � ������
� ����) ' � ������
� ' �����) ' �� ' ) " �; ��, ���
' ) !� ��� � ����
� ���" ' ' ��� � � �� #5' 6C ) I 4 �
� ������
-Phased Coexistence (PCO Parameters)-PCO Active-PCO phase (20 or 40 Mhz switch)
AirTight Networks
HT Information elementOperating mode
Non-GF STAs present
-Set to 0 -All STAs in BSS are 20/40 Mhz HT-All STAs in a 20 MHz HT BSS are 20 MhzHT
-Set to 1 (non-member protection)-Some members on the channel (maybe outside BSS) are non-HT
-Set to 2-At least one 20 Mhz only STA in a HT BSS
- Set to 3-MM (at least one legacy STA is present in BSS)
-Set to 0-All associated STAs in BSS are GF stations
-Set to 1-Some non-GF STAs present in a BSS
Protection
-Required for Operating mode 1 & 3
-Protection mechanisms discussed earlier can be used
- Operating mode can also be updated dynamically based on BSS constitution
AirTight Networks
Channel Switch & Extended Channel Switch Elements
* ���� &�� ��� ��� 6���������������� �������� ����% ����� ��� ��������
• Useful for 40 Mhz transmission • 0 indicates no sec channel, 2 is reserved• 1 means secondary is above primary, 3 means below
� " �������� �& �9 ��� �����
� ��������- ��������������� ����(!��������, ����� ����+
< ? � �� � �* ���� &�� ��� �� ' - ���������- ��������� �2 � �� ��� ��������(3 � �2 +���� �, ���� ������
� " �������� �& �9 ��� �����
� ��������- ��������������� ����(!��������, ����� ����+
AirTight Networks
Quick Overview of advanced .11n features Optional and/or not yet available today
AirTight Networks
HTControl
Parameters used for channel measurement procedures and antenna selection
AirTight Networks
When the MAI field is set to the value ASELI, this field is interpreted as defined in Figure n4 (ASELC subfield) and Table n3 (The ASEL Command and ASEL Data parts of the ASELC subfield).Otherwise, this field contains recommended MCS feedback.A value of 127 indicates that no feedback is present.
MCS Feedback and Antenna Selection Command/Data
MFB/ASELC
Set to the received value of MSI contained in the frame to which the MFB information refers.Set to 7 for unsolicited MFB
MFB Sequence IdentifierMFSI
When set to 14, the MAI field contains an Antenna Selection Indication(ASELI).Otherwise the MAI field is interpreted
MCS request or Antenna Selection Indication
MAI
Set to 1 to request the responder to transmit a sounding PPDU.When set to 0, the responder is not requested to transmit a sounding PPDU.
Sounding RequestTRQ
DefinitionMeaningField
HT Control: Link adaptation
AirTight Networks
RDP Exchange
AirTight Networks
Thank you
I��������� ���� %� � � �������J K ��� ��� ��� �� � ���� �