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Wireless LAN - 1/59
IEEE 802.11 Wireless LAN
All rights reserved. No part of this publication and file may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of Professor Nen-Fu Huang (E-mail: nfhuang@cs.nthu.edu.tw).
Wireless LAN - 2/59
Wireless LAN Architecture
Four major differences between Wireless LAN and Wired LANs:
Destination Address Does not Equal Destination Location. In wired LANs an address is equivalent to a physical address. In 802.11 the addressable unit is a station (STA). The STA is a message destination, but not a fixed location.
Wireless LAN - 3/59
The Media Impacts the DesignThe PHY layers used in 802.11are different from wired media. 802.11 PHYs:
Have limited physical point to point connection ranges.
Use a shared medium. Are unprotected from outside signals. Are significantly less reliable than wired PHYs. Have dynamic topologies.
Wireless LAN Architecture
Wireless LAN - 4/59
Wireless LAN Architecture
Impact of Handling Mobile StationsA portable station is one that is moved from location to location, but is only used while at a fixed location.
Mobile stations actually access the LAN while in motion.
Propagation effects blur the distinction between portable and mobile stations.
Wireless LAN - 5/59
802.11Wireless LAN Characteristics 1-108 Mbps
IEEE 802.11b is for 11 Mbps IEEE 802.11g is for 54 Mbps IEEE 802.11n is for 108 Mbps
IEEE 802.11 CSMA/CA Frame Transmission Medium: Radio CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance) ProtocolProvides priority scheme
Provides delay guaranteed transmission service. (PCF reservation-based)
Bandwidth Fairness is not guaranteed. By employing the CSMA/CA protocol, the bandwidth employed by each station may be different.
Wireless LAN - 6/59
Frame TypesClass 1 frames
Control Frames(1) RTS(2) CTS(3) ACK(4) Poll
Management Frames(1) Probe Request/Response(2) Beacon(3) Authentication
RTS
CTS
Data
ACK
Source Destination
Optional
Wireless LAN - 7/59
Class 2 FramesData Frames
Asynchronous data. Direct data frames only (FC control bits “To DS and from DS” both false)
Management Frames(1) Privacy Request/Response(2) ATIM (Ad-Hoc Traffic Indication Map,
ATIM)(3) Association Request/Response
Frame Types
Wireless LAN - 8/59
Frame TypesClass 3 Frames
Data Frames Asynchronous data. Indirect data frames allowed (FC
control bits "To DS and from DS" may be set to utilize DS Services)
Management Frames(1) Reassociation Request/Response(2) Disassociation
CF Data Frames (Coordination Function)(1) CF DATA(2) CF DATA + ACK
CF Control Frames(1) CF END
Wireless LAN - 9/59
Frame and MPDU Formats
Each frame should consist of three basic components:A MAC Header, which includes control information, addressing, sequencing fragmentation identification and duration.
A variable length Frame BodyAn IEEE 32-bit CRC frame check sequence
Wireless LAN - 10/59
Frame and MPDU Formats
Frame
Control
2 2 6 6 6 2 6 0-2304 4 bytes
Duration/ Conn ID
Addr 1 Addr 2 Addr 3 Sequence Control
Addr 4 FCSdata
Protocol Version
2 2 4 1 1 1 1 2 1 1 bit
SubtypeLastFlag
EP TypeTo DS
From DS
RetryPowerMang.
Rsvd
MAC Header
Wireless LAN - 11/59
Frame FieldsFrame Control Field :
Protocol Version, Type, Subtype, ToDS, From DS, Last Fragment, Retry, Power Management and Element Present.
Retry : Indicates that the frame is a retransmission of an earlier frame. A station may use this indication to eliminate duplicate frames.
Power Management : Indicates power management state and buffered traffic state of the station
00 = Active Mode (CAM or TAM), with more buffered frames 01 = PSP - Power Save, Polling 10 = PSNP - Power Save, No Polling 11 = Active Mode (CAM or TAM), without more buffered
frames
Wireless LAN - 12/59
Duration or Connection ID :Used to distribute a value (us) that shall update the Network Allocation Vector (NAV) in stations receiving the frame.
During the contention free period, this field may be replaced with a connection ID field.
Only contention free time-bounded data used a connection ID; contention based data and contention free asynchronous data do not use connection IDs.
Frame Fields
Wireless LAN - 13/59
Frame FieldsAddress Fields : Indicate the BSSID, SA, DA,
TA (Transmitter address), RA (Receiver address), each of 48-bit address.
Sequence ControlDialog Token (12-bit) : An incrementing value. The same value shall be used for all fragments of the same MSDU.
Fragment Number (4-bit) : Indicates the number of each individual fragment.
Frame Body: 0 - 2304 bytes.CRC (4 octets)
Wireless LAN - 14/59
Frame Fields
MACHDR CRC
Frame Body
MACHDR CRC
Frame Body
MACHDR
CRC
Frame Body
MACHDR CRC
Frame Body
MSDU
Fragment 1 Fragment 2 Fragment 3 Fragment 4
Wireless LAN - 15/59
Format of Individual Frame Types
RTS Frame FormatIn an infrastructure LAN, the DA shall be the
address of the AP with which the station is associated.
In an ad hoc LAN, the DA shall be the destination of the subsequent data or management frame.
CTS Frame FormatThe DA shall be taken from the source
address field of the RTS frame to which the CTS is a response.
Wireless LAN - 16/59
ACK Frame FormatThe DA shall be the address contained in the
Address 2 field of the immediately previous Data or Management frame.
Poll Frame FormatThe BSS ID shall be the address of the AP. The
SID shall be the value assigned by the AP in the Associate Response frame.
Format of Individual Frame Types
Wireless LAN - 17/59
Format of Individual Frame Types
Frame Control Duration DA SA FCS
MAC Header
Frame Control
Duration DA FCS
MAC Header
Frame Control
Duration BSS ID SA FCS
MAC Header
Frame Control
Duration DA FCS
MAC Header
RTS Frame
CTS Frame
ACK Frame
Poll Frame
Wireless LAN - 18/59
Format of Individual Frame Types
Data FramesThe contents of the Address fields shall be
dependent upon the values of the To DS and From DS bits.
A station shall use the contents of Address 1 to perform address matching for receive decisions.
The DA shall be the destination of the frame (MSDU).
The RA shall be the address of the AP in the wireless DS that is the next immediate intended recipient of the frame.
The TA shall be the address of the AP in the wireless DS that is transmitting the frame.
Wireless LAN - 19/59
802.11 Architecture Components
STA 4
Distribution System (DS)
APSTA 2
STA 1
AP STA 7
BSS 1
BSS 2
AP: Access Point
AP STA 3
STA 5 STA 6
BSS 3
Wireless LAN - 20/59
The BSSID The AP address, if the station is an AP or
associated with an AP. The BSS ID of the ad hoc LAN, if the station is a
member of an ad hoc LAN.
Data Subtype During the contention period: 0000 During the contention free period
– 0000, 0011, 0110, and 0111 shall only be sent by a PCF.– 0000, 0001, 0100, and 0101 may be sent by any CF-aware station.
Format of Individual Frame Types
Wireless LAN - 21/59
Data Frames
Frame Control
Duration/
Conn ID
Addr 1 Addr 2 Sequence Number
Fragment
Number
Addr 4 Data
MAC Header
Addr 3
To DS From DS Addr 1 Addr 2 Addr 3 Addr 4
0 0 DA SA BSSID N/A
0 1 DA BSSID SA N/A
1 0 BSSID SA DA N/A
1 1 RA TA DA SA
FCS
Wireless LAN - 22/59
MAC Architecture
MACExten
t
Contention FreeService
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
Contention BasedService
Wireless LAN - 23/59
MAC Architecture
Distributed Coordination Function (DCF)The fundamental access method for the 802.11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
Shall be implemented in all stations and APs.
Used within both ad hoc and infrastructure configurations.
Wireless LAN - 24/59
Point Coordination Function (PCF)An alternative access methodShall be implemented on top of the DCFA point coordinator (polling master) is used to determine which station currently has the right to transmit.
Shall be built up from the DCF through the use of an access priority mechanism.
Different accesses of traffic can be defined through the use of different values of IFS.
MAC Architecture
Wireless LAN - 25/59
MAC Architecture
Shall use a Point IFS (PIFS) < Distributed IFS (DIFS)
Point coordinated traffic shall have higher priority to access the medium, which may be used to provide a contention-free access method.
The priority access of the PIFS allows the point coordinator to seize control of the medium away from the other stations.
Wireless LAN - 26/59
MAC Architecture
Coexistence of DCF and PCFBoth the DCF and PCF shall coexist without interference.
They are integrated in a superframe in which a contention-free burst occurs at the beginning, followed by a contention period.
Wireless LAN - 27/59
MAC Architecture
Contention Free
Period
Super Frame
Contention
Period
Wireless LAN - 28/59
Distributed Coordination Function
Allows for automatic medium sharing between PHYs through the use of CSMA/CA and a random backoff time following a busy medium condition.
All directed traffic uses immediate positive ack (ACK frame) where retransmission is scheduled by the sender if no ACK is received.
Carrier Sense shall be performed both through physical and virtual mechanisms.
Wireless LAN - 29/59
The virtual Carrier Sense mechanism is achieved by distributing medium busy reservation information through an exchange of special small RTS and CTS frames (contain a during field) prior to the actual data frame. Unicast only, not used in multicast/broadcast.
The use of RTS/CTS is under control of RTS_Threshold (payload length, under which without any RTS/CTS prefix).
All stations are required to be able to receive any frame transmitted on a given set of rates, and must be able to transmit at (at least) one of these rates. This assures that the Virtual Carrier Sense mechanism still works on multiple rates environments.
Distributed Coordination Function
Wireless LAN - 30/59
Physical Carrier Sense MechanismA physical carrier sense mechanism shall be provided by the PHY.
Virtual Carrier Sense MechanismProvided by the MAC, named Net Allocation Vector (NAV), which maintains a prediction of future traffic based on duration information announced in RTS/CTS frames.
Distributed Coordination Function
Wireless LAN - 31/59
MAC-Level Acknowledgments (Positive Acknowledgment)To allow detection of a lost or errored frame an ACK
frame shall be returned immediately following a successfully received frame. The gap between the received frame and ACK frame shall be SIFS.
The frame types should be acknowledged with an ACK frame:
Data Poll Request Response
The lack of an ACK frame means that an error has occurred.
Distributed Coordination Function
Wireless LAN - 32/59
Distributed Coordination Function --Inter-Frame Space (IFS)
A station shall determine that the medium is free through the use of carrier sense function for the interval specified.
Three different IFS's are defined to provide priority levels.
Short-IFS (SIFS)Shall be used for an ACK frame, a CTS frame, by a
station responding to any polling, and between frames in the sequences described in Page 41.
Any STA intending to send only these frame types shall be allowed to transmit after the SIFS time has elapsed following a busy medium.
Wireless LAN - 33/59
PCF-IFS (PIFS)Shall be used only by the PCF to send any of the
Contention Free Period frames.The PCF shall be allowed to transmit after it
detects the medium free for the period PIFS, at the start of and during a CF-Burst.
DCF-IFS (DIFS)Shall be used by the DCF to transmit
asynchronous MPDUs. A STA using the DCF is allowed to transmit after
it detects the medium free for the period DIFS, as long as it is not in a backoff period.
Distributed Coordination Function --Inter-Frame Space (IFS)
Wireless LAN - 34/59
Distributed Coordination Function --Random Backoff Time
Before transmitting asynchronous MPDUs, a STA shall use the carrier sense function to determine the medium state.
If busy, the STA shall defer until after a DIFS gap is detected, and then generate a random backoff period for an additional deferral time (resolve contention).
Backoff time = INT(CW * Random()) * Slot time
Wireless LAN - 35/59
Where CW = An integer between CWmin and CWmax
Random() =
Slot Time = Transmitter turn-on delay +
medium propagation delay +
medium busy detect response time
Distributed Coordination Function --Random Backoff Time
CWmax
CWmin 7 15 31
63
2nd Retransmission 1st Retransmission
Initial
127 255 255
3rd Retransmission
Wireless LAN - 36/59
Distributed Coordination Function --DCF Access Procedure
CSMA/CA ProtocolUsed when there is no PCF detected and when in
the Contention Period of a Superframe when using a PCF.
Basic AccessA STA with a pending MPDU may transmit when it
detects a free medium for greater than or equal to a DIFS time.
If the medium is busy when a STA desires to initiate a Data, Poll, Request, or Response MPDU transfer, and only a DCF is being used (or a Contention Period portion of a Superframe is active), the Random Backoff Time algorithm shall be followed.
Wireless LAN - 37/59
Distributed Coordination Function --DCF Access Procedure
DIFS
Busy Medium
PIFS
SIFSDIFS
Contention Window
Back off- Window
Next Frame
Defer Access
Slot time
Immediate access when medium is free >= DIFS
Wireless LAN - 38/59
Distributed Coordination Function --DCF Access Procedure
Backoff ProcedureA backoff time is selected first. The Backoff
Timer shall be frozen while the medium is sensed busy and shall decrement only when the medium is free (resume whenever free period > DIFS).
Transmission shall commence whenever the Backoff Timer reaches zero.
A STA that has just transmitted a frame and has another frame ready to transmit (queued), shall perform the backoff procedure (fairness concern).
Tends toward fair access on a FCFS basis.
Wireless LAN - 39/59
Distributed Coordination Function --DCF Access Procedure
A
B
C
D
E
DIFS
Backoff
Frame
Frame
Frame
Frame
Frame
CWindow = Contention Window = Backoff( 後退 )
= Remaining Backoff( 持續後退 )
19 us
15 us
10 us
9 us
5 us
7 us
4 us
2 us
2 us
Wireless LAN - 40/59
Distributed Coordination Function --DCF Access Procedure
RTS/CTS Recovery Procedure and Retransmit Limits
After an RTS is transmitted, if the CTS fails in any manner within a predetermined CTS_Timeout (T1), then a new RTS shall be generated (the CW shall be doubled).
This procedure shall continue until the RTS_Re-Transmit_Counter reaches an RTS_Re-Transmit_Limit.
The same backoff mechanism shall be used when no ACK is received within a predetermined ACK_Window(T3) after a directed DATA frame has been transmitted.
This procedure shall be continue until the ACK_Re-Transmit_Counter reaches an ACK_Re-Transmit_Limit.
Wireless LAN - 41/59
Distributed Coordination Function -- DCF Access Procedure
Setting the NAV Through Use of RTS/CTS FramesRTS and CTS frames contain a Duration field based on the medium occupancy time of the MPDU from the end of the RTS or CTS frame until the end of the ACK frame.
Wireless LAN - 42/59
Distributed Coordination Function --DCF Access Procedure
Data
DIFS
Defer Access
RTS
CTS ACK
NAV (RTS)
NAV (CTS)
DIFSContenti
on
Window
Back off-WindowNext Frame
SIFS
T1
SIFS
SIFS
T3
Back off after Defer
Source
Destination
Other Station
Wireless LAN - 43/59
Distributed Coordination Function --DCF Access Procedure
Control of the ChannelThe IFS is used to provide an efficient MSDU delivery
mechanism.Once a station has contended for the channel, it will
continue to send fragments until either all fragments of a MSDU have been sent, an ack is not received, or the station can not send any additional fragments due to a dwell time boundary.
If the source station does not receive an ack frame, it will attempt to retransmit the fragment at a later time (according to the backoff algorithm).
When the time arrives to retransmit the fragment, the source station will contend for access in the contention window.
Wireless LAN - 44/59
Distributed Coordination Function --DCF Access Procedure
DIFS
PIFSFragment
Burst
Back off-Window
Frag-ment 1
Ack 1
SIFS SIFS
Frag-ment 2
Ack 2
SIFS SIFS
Frag-ment 3
Ack 3
SIFS SIFS
Src
Dst
Wireless LAN - 45/59
Distributed Coordination Function --DCF Access Procedure
RTS/CTS Usage with FragmentationThe RTS/CTS frames defines the duration of the first frame and ack. The duration field in the data and ack frames specifies the total duration of the next fragment and ack.
The last Fragment and ACK will have the duration set to zero.
Each Fragment and ACK acts as a virtual RTS and CTS.
In the case where an ack is not received by the source station, the NAV will be marked busy for next frame exchange. This is the worst case situation.
Wireless LAN - 46/59
RTS/CTS Usage with FragmentationIf the ack is not sent by the destination, stations that can only hear the destination will not update their NAV and be free to access the channel.
All stations will be free to access the channel after the NAV from Frame 1 has expired.
The source must wait until the NAV (Fragment 1) expires before attempting to contend for the channel after not receiving the ack.
Distributed Coordination Function --DCF Access Procedure
Wireless LAN - 47/59
RTS/CTS Usage with Fragmentation
DIFS
PIFS
Back off-Window
Frag- ment 1
Ack 1
SIFS SIFS
Frag- ment 2
Ack 2
SIFS SIFS
Frag-ment 3
Ack 3
SIFS
SIFS
Src
Dst
RTS
CTS
SIFS SIFS
NAV(RTS) NAV(Frag 1) NAV(Frag 2)
NAV(CTS) NAV(ACK 1)
NAV(ACK 2)
Other
Wireless LAN - 48/59
RTS/CTS Usage with Fragmentation
DIFS
PIFS
Backoff-Window
Frag-ment 1
Ack 1
SIFS
SIFS
Src
Dst
RTS
CTS
SIFS SIFS
NAV(RTS) NAV(Fragment 1)
NAV(CTS) NAV(ACK 1)
Other
Wireless LAN - 49/59
Distributed Coordination Function --DCF Access Procedure
Directed MPDU Transfer Procedure Using RTS/CTS (see Page 52)STA shall use an RTS/CTS exchange for directed frames only when the length of the MPDU is greater than the RTS_Threshold (a managed object within the MAC MIB, 0... Max MPDU length).
Directed MPDU Transfer Procedure Without RTS/CTS
Wireless LAN - 50/59
Distributed Coordination Function --DCF Access Procedure
Data
DIFS
Defer Access
ACK
DIFS Contention Window
Backoff-Window
Next Frame
SIFS
Backoff after Defer
Source
Destination
Other Station
Wireless LAN - 51/59
Point Coordination Function(PCF)
The PCF provides contention free services.It is an option for a station to become the
Point Coordinator(PC), which generates the Superframe (SF).
Not all stations must be capable of becoming the PC and transmitting PCF data frames.
The SF consists of a Contention Free (CF) period and a Contention Period.
The length of a SF is a manageable parameter and that of the CF period may be variable on a per SF basis.
Wireless LAN - 52/59
Point Coordination Function(PCF)
Free contention Period
Super Frame
Contention Period
Wireless LAN - 53/59
Point Coordination Function-- PCF Access Procedure
The PCF protocol is based on a polling scheme controlled by one special STA per BSS called the Point Coordinator.
The PC gains control of the medium at the beginning of the SF and maintains control for the entire CF period by waiting a shorter time between transmissions.
CF-Down Frames and CF-UP Frames.
Wireless LAN - 54/59
At the beginning of the SF, the PCF shall sense the medium. If it is free the PCF shall wait a PIFS time and transmit a Data frame with the CF-Poll Subtype bit set, to the next station on the polling list, or
a CF-End frame, if a null CF period is desired.
Point Coordination Function-- PCF Access Procedure
Wireless LAN - 55/59
The PCF uses the PCF priority level of the CSMA/CA protocol. The shorter PIFS gap causes a burst traffic with inter-frame gaps that are shorter than the DIFS gap needed by stations using the Contention period.
Each station, except the station with the PCF, shall preset it's NAV to the maximum CF-Period length at the beginning of every SF. The PCF shall transmit a CF-End frame, at the end of the CF-Period, to reset the NAV of all stations in the BSS.
Point Coordination Function-- PCF Access Procedure
Wireless LAN - 56/59
Point Coordination Function-- PCF Transfer Procedure
PCF Transfers When the PCF Station is Transmitter or RecipientStations shall respond to the CF-Poll immediately when a frame is queued, by sending this frame after an SIFS gap. This results in a burst of Contention Free traffic (CF-Burst).
For services that require MAC level ack, the ack is preferably done through the CF-Ack bit in the Subtype field of the responding CF-Up frame.
Wireless LAN - 57/59
Point Coordination Function-- PCF Transfer Procedure
NAV
SIFS
SIFS
Busy CF-D1
CF-U1
SIFS
CF-D2
CF-U2
SIFS
PIFSSIFS
CF-D3 CF-D4
CF-U4
PIFS
SIFS
CF-End
Contention free period
Superframe
CP
Reset NAV
CF-boundary
Dx = Down Traffic
Ux = Up Traffic
Wireless LAN - 58/59
Point Coordination Function-- PCF Transfer Procedure
PCF Transfers When the PCF Station is Neither Transmitter nor RecipientA CF-aware station, when polled by the PCF,
may send a Data frame to any station in the BSS an SIFS period after receiving the CF-Poll.
If the recipient of this transmission is not the PCF station, the Data frame is received and acknowledged in the same manner as a contention-based Data frame.
The PCF resumes (CF-Down) transmissions an SIFS period after the ACK frame. If not acknowledged, a PIFS period is employed.
Wireless LAN - 59/59
Point Coordination Function-- PCF Transfer Procedure
NAV
SIFS
Busy CF-D1
S-To-S
SIFS
ACK
CF-D2
SIFS
PIFS SIFS
CF-U2
SIFS
CF-End
CFP
Superframe
CP
Reset NAV
CF-boundary
Dx = Down TrafficUx = Up Traffic
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