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Introduction to Computer Networks
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: [email protected]).
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Wireless LAN - 2
Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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IEEE 802.11
IEEE 802.11 is designed for a limited geographical area (homes, offices, campuses, stations)
The signals propagating through space
Also known as Wi-Fi
IEEEE 802.11 supports additional features
Power management and
Security mechanisms
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IEEE 802.11 Physical layer
Original 802.11 standard defined two radio-based physical layer standard
One using the frequency hopping
Over 791-MHz-wide frequency bandwidths
Second using direct sequence
Using 11-bit chipping sequence
Both standards run in the 2.4-GHz and provide up to 2 Mbps
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IEEE 802.11 Standards
Then physical layer standard 802.11b was added
Using a variant of direct sequence 802.11b provides up to 11 Mbps
Uses license-exempt 2.4-GHz band
Then came 802.11a which delivers up to 54 Mbps using OFDM
802.11a runs on license-exempt 5-GHz band
Then came 802.11g which is backward compatible with 802.11b
Uses 2.4 GHz band, OFDM and delivers up to 54 Mbps
Most recent standard is 802.11n which delivers up to 108Mbps, with multiple wireless signals and antennas, called MIMO technology.
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IEEE 802.11 – Hidden node problem
Assume each of four nodes is able to send and receive signals that reach just the nodes to its immediate left and right
For example, B can exchange frames with A and C, but it cannot reach D
C can reach B and D but not A
Example of a wireless network
B DA C
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IEEE 802.11 – Hidden node problem
Suppose both A and C want to communicate with B and so they each send it a frame.
A and C are unaware of each other since their signals do not carry that far
These two frames collide with each other at B
But unlike an Ethernet, neither A nor C is aware of this collision
A and C are said to hidden nodes with respect to each other
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IEEE 802.11 – Hidden node problem
“Hidden Node” ProblemAlthough A and C are hidden from each
other, their signals can collide at B. 雙方雖然聽不到對方的訊號, 但同時傳送給相同的對象會造成衝撞
B DA C
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IEEE 802.11 – Exposed node problem
Exposed node problem
Suppose B is sending to A. Node C is aware of this communication because it hears B’s transmission.
It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission.
Suppose C wants to transmit to node D.
This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B.
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IEEE 802.11 – Exposed node problem
“Exposed Node” ProblemAlthough B and C are exposed to each other’s
signals, there is no interference if B transmits to A while C transmits to D.雙方雖然聽得到對方的訊號, 但同時可傳送給不同的對象
B DA C
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IEEE 802.11 – CSMA/CA
802.11 addresses these two problems with an algorithm called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
Key Idea
Sender and receiver exchange control frames with each other before the sender actually transmits any data.
This exchange informs all nearby nodes that a transmission is about to begin
Sender transmits a Request to Send (RTS) frame to the receiver.
The RTS frame includes a field that indicates how long the sender wants to hold the medium
- Length of the data frame to be transmitted
Receiver replies with a Clear to Send (CTS) frame
This frame echoes this length field back to the sender
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IEEE 802.11 – RTS/CTS Frames
802.11 using RTS and CTS frames to reserve the wireless channel for a time period
(duration field in the RTS and CTS frames)
B CRTS
CTSRTS CTS
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IEEE 802.11 – RTS and CTS frames
Any node that sees the CTS frame
it is close to the receiver, therefore
cannot transmit for the period of time specified in the CTS frame
Any node that sees the RTS frame but not the CTS frame
is not close enough to the receiver to interfere with it, so is free to transmit
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IEEE 802.11 – RTS and CTS frames
If two or more nodes detect an idle link and try to transmit an RTS frame at the same time
Their RTS frame will collide with each other
So the senders realize the collision has happened when they do not receive the CTS frame after a period of time
Each sender waits a random amount of time before trying again.
The amount of time is defined by the same exponential backoff algorithm used on the Ethernet.
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IEEE 802.11 – Not Support Collision Detection
802.11 does not support collision detection
How to know the sent frame was received successfully or ?
We can use CSMA protocol, but the performance is not good enough
802.11 using ACK frame in CSMA/CA
Receiver sends an ACK frame to the sender after successfully receiving a frame
All nodes must wait for this ACK frame before trying to transmit
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802.11 Frame Types
Class 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 Station Destination Station
Optional
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Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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IEEE 802.11 – Distribution System
Nodes are free to move around
Directly reachable nodes may change dynamically
To deal with mobility and partial connectivity,
802.11 defines additional structures on a set of nodes
some nodes are allowed to roam
some are connected to a wired network infrastructure
Access Points (AP) and
connected to each other by a so-called distribution system (DS)
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A distribution system that connects many APs
The distribution network runs at layer 2 of the ISO architecture
IEEE 802.11 – Distribution System
APs connected to a distribution network
B
D
A
C E
G
F
H
AP1
AP2AP3
Distribution System (DS)
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IEEE 802.11 – Distribution System
The idea behind this configuration is
Each nodes associates itself with one AP
A C: A AP1 C
A E: A AP1 DS AP3 E
B
D
A
C EG
F
H
AP1
AP2AP3
Distribution System (DS)
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IEEE 802.11 – Distribution System
How do the nodes select their APs ?
How does it work when nodes move from one cell to another ?
The technique for selecting an AP is called scanning
The node sends a Probe frame
All APs within reach reply with a Probe Response frame
The node selects one of the APs and sends that AP an Association Request frame
The AP replies with an Association Response frame
A node runs this protocol whenever
it joins the network, and
when it wants to change another AP (better signal)
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IEEE 802.11 – Distribution System
Scanning to select an associated AP(Probe + Response)
B
A
E
G
F
AP1
AP2AP3
Distribution System (DS)
response
probe
response
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Active Scanning
When node C moves from the cell serviced by AP-1 to the cell serviced by AP-2.
As it moves, it sends Probe frames, which eventually result in Probe Responses from AP-2.
At some point, C prefers AP-2 over AP-1 , and so it associates with AP-2.
This is called active scanning since the node is actively searching for an AP
IEEE 802.11 – Distribution System
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IEEE 802.11 – Distribution System
Node Mobility with active scanning(Probe + Response)
B
DE
G
F
AP3
Distribution System (DS)
CC
C CA
H
AP1AP2
C
C
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IEEE 802.11 – Distribution System
Passive Scanning
APs also periodically send a Beacon frame that advertises the capabilities of the AP; these include the transmission rate supported by the AP
This is called passive scanning
A node can change to this AP based on the Beaconframe simply by sending it an Association Request frame back to the AP.
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IEEE 802.11 – Distribution System
Node Mobility with passive scanning(Beacon + Association Request)
DE
G
F
AP3
Distribution System (DS)
A H
AP1AP2
C
B
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Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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IEEE 802.11 – Frame Format
Frame Control
2 6 6 6 6 2 6 0-2312 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 bits
Subtype LastFlag
EPTypeTo DS
From DS
Retry PowerMang.
Rsvd
MAC Header
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Source and Destination addresses: each 48 bits
Data: up to 2312 bytes
CRC: 32 bits
Control field: 16 bits
Contains three subfields (of interest)
6 bit Type field: indicates whether the frame is an RTS or CTS frame or being used by the scanning algorithm
A pair of 1 bit fields : called ToDS and FromDS
IEEE 802.11 – Frame Format
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IEEE 802.11 – Frame Format
Frame contains four addresses
How these addresses are interpreted depends on the settings of the ToDS and FromDS bits in the frame’s Control field
This is to account for the possibility that the frame had to be forwarded across the Distribution System
Simplest case
When one node is sending directly to another, both the DS bits are 0, Addr1 identifies the target node, and Addr2 identifies the source node
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IEEE 802.11 – Frame Format
Most complex case
Both DS bits are set to 1
Indicates that the message went from a wireless node distribution system another wireless node
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 DA TA RA SA
BSSID: MAC address of an AP SA: Original Source address DA: Final Destination address TA: Transmitter address RA: Receiver address
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IEEE 802.11 – Frame Format
E
F
AP3
Distribution System (DS)
A
AP1AP2
C
Frame Control
Duration/Conn ID
Sequence ControlAddr 1 Addr 2 Add 3 Addr 4 FCSDataAP1 A E NAAP3 AP1 E AE AP3 A NA
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Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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IEEE 802.11 MAC Architecture
MACExtent
免競爭式服務(具時限傳輸)
Distributed Coordination Function (DCF)
競爭式服務(非同步傳輸)
Point Coordination Function (PCF)
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IEEE 802.11 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 configurations:
Ad hoc
Infrastructured
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Point Coordination Function (PCF)
An alternative access method
Shall be implemented on top of the DCF
A 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 (Inter-Frame Space).
IEEE 802.11 MAC Architecture
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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.
IEEE 802.11 MAC Architecture
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Coexistence of DCF and PCF
Both the DCF and PCF shall coexist without interference.
Superframe : A contention-free burst occurs at the beginning, followed by a contention period.
IEEE 802.11 MAC Architecture
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免競爭訊框
超級訊框
Super Frame
需競爭訊框
IEEE 802.11 MAC Architecture
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Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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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 frame
Retransmission is scheduled by the sender if no ACK is received.
Carrier Sense shall be performed both through physical and virtual mechanisms.
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The Virtual Carrier Sense mechanism is achieved by distributing medium busy reservation information through an exchange of RTS and CTS frames (contain a duration 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).
Distributed Coordination Function
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Wireless LAN - 43
Distributed Coordination Function
Physical Carrier Sense Mechanism
A physical carrier sense mechanism shall be provided by the PHY.
Virtual Carrier Sense Mechanism
Provided by the MAC, named Net Allocation Vector (NAV), which maintains a prediction of future traffic based on duration information announced in RTS/CTS frames.
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Wireless LAN - 44
MAC-Level Ack (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
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Wireless LAN - 45
DCF -- Inter-Frame Space (IFS)
Priority levels: Three different IFS's are defined.
Short-IFS (SIFS)
Used for
an ACK frame,
a CTS frame,
by a station responding to any polling
Any STA intending to send only these frame types is allowed to transmit after the SIFS time has elapsed following a busy medium.
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Wireless LAN - 46
PCF-IFS (PIFS)
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.
DCF-IFS (DIFS)
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.
DCF -- Inter-Frame Space (IFS)
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Wireless LAN - 47
DCF -- Random Backoff Time
Before transmitting asynchronous MPDUs, a STA shall determine the medium state.
If busy, the STA shall defer until after a DIFS gap is detected, and then generate a random backoffperiod for an additional deferral time (resolve contention).
Backoff time = INT(CW * Random()) * Slot time
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Wireless LAN - 48
Where CW = An integer between CWmin and CWmax
Random() = a random number
Slot Time = Transmitter turn-on delay +
medium propagation delay +
medium busy detect response time
DCF -- Random Backoff Time
CWmax
CWmin7 15
3163
第二次重送第一次重送
第三次重送初始值
127
255 255
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Wireless LAN - 49
DCF Access Procedure
CSMA/CA Protocol
Used when there is no PCF detected and when in the Contention Period of a Superframe when using a PCF.
Basic Access
A 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.
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Wireless LAN - 50
DCF Access Procedure
DIFS
Busy Medium
PIFS
SIFS
DIFSContentionWindow
Back off-Window
Next Frame
Defer Access
Slot time
Immediate access when medium is free >= DIFS
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Wireless LAN - 51
DCF Access Procedure
Backoff Procedure
A 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 BackoffTimer 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.
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Wireless LAN - 52
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
DIFS DIFS DIFS
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Wireless LAN - 53
DCF Access Procedure
Setting the NAV Through Use of RTS/CTS Frames
RTS 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.
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Wireless LAN - 54
DCF Access Procedure
Data
DIFS
Defer Access
RTS
CTS ACK
NAV (RTS)
NAV (CTS)
DIFS
ContentionWindow
Back off-Window
Next Frame
SIFS
T1
SIFS
SIFS
T3
Back off after Defer
起始工作站
目的地工作站
其他工作站
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Wireless LAN - 55
DCF Access Procedure
Control of the Channel
The 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.
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Wireless LAN - 56
DCF Access Procedure
DIFS
PIFSFragment Burst
Back off-Window
Frag-ment 1
Ack1
SIFS SIFS
Frag-ment 2
Ack2
SIFS SIFS
Frag-ment 3
Ack3
SIFS SIFS
Src
Dst
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Wireless LAN - 57
DCF Access Procedure
Control of the Channel
If the source station does not receive an ackframe, it will attempt to retransmit the fragment at a later time (according to the backoffalgorithm).
When the time arrives to retransmit the fragment, the source station will contend for access in the contention window.
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Wireless LAN - 58
DCF Access Procedure
RTS/CTS Usage with Fragmentation
The 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.
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Wireless LAN - 59
RTS/CTS Usage with Fragmentation
DIFS
PIFS
Back off-Window
Frag-ment 1
Ack 1
SIFS SIFS
Frag-ment 2
Ack2
SIFS SIFS
Frag-ment 3
Ack3
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
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Wireless LAN - 60
DCF Access Procedure
RTS/CTS Usage with Fragmentation
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.
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Wireless LAN - 61
RTS/CTS Usage with Fragmentation
If 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 Fragment 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.
DCF Access Procedure
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Wireless LAN - 62
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)
Other
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Wireless LAN - 63
DCF Access Procedure
Directed MPDU Transfer Procedure Using RTS/CTS
STA shall use an RTS/CTS exchange for directed frames only when the length of the MPDU is greater than the RTS_Threshold.
Directed MPDU Transfer Procedure WithoutRTS/CTS
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Wireless LAN - 64
DCF Access Procedure
Data
DIFS
Defer Access
ACK
DIFS ContentionWindow
Backoff-Window Next Frame
SIFS
Backoff after Defer
起始工作站
目的地工作站
其他工作站
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Wireless LAN - 65
Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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Wireless LAN - 66
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 Superframe consists of a Contention Free (CF) period and a Contention Period.
The length of a Superframe is a manageable parameter and that of the CF period may be variable on a per SF basis.
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Wireless LAN - 67
Point Coordination Function(PCF)
免競爭訊框
超級訊框
Super Frame
需競爭訊框
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Wireless LAN - 68
PCF Access Procedure
The PCF protocol is based on a polling scheme controlled by one special STA called the Point Coordinator (PC).
The PC gains control of the medium at the beginning of the SuperFrame and maintains control for the entire CF period by waiting a shorter time between transmissions (PIFS).
CF-Down Frames and CF-UP Frames.
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Wireless LAN - 69
At the beginning of the SF, the PC shall sense the medium.
If it is free the PC 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.
PCF Access Procedure
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Wireless LAN - 70
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.
PCF Access Procedure
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Wireless LAN - 71
PCF Transfer Procedure
When the PCF Station is Transmitter or Recipient
Stations 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 ackis preferably done through the CF-Ack bit in the Subtype field of the responding CF-Up frame.
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Wireless LAN - 72
PCF Transfer Procedure
NAV
SIFS
SIFS
媒介忙碌中 CF-D1
CF-U1
SIFS
CF-D2
CF-U2
SIFS
PIFSSIFS
CF-D3 CF-D4
CF-U4
PIFS
SIFS
CF-End
免競爭週期
超級訊框
競爭週期
重設 NAV
CF-邊界
Dx = Down Traffic
Ux = Up Traffic
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Wireless LAN - 73
PCF Transfer Procedure
When the PCF Station is Neither Transmitter nor Recipient
A CF-aware station, when polled by the PCF, may send a Data frame to any station 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.
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Wireless LAN - 74
PCF Transfer Procedure
NAV
SIFS
媒介忙碌中 CF-D1
S-To-S
SIFS
ACK
CF-D2
SIFS
PIFS SIFS
CF-U2
SIFS
CF-End
免競爭週期
超級訊框
競爭週期
重設 NAV
CF-邊界
Dx = Down TrafficUx = Up Traffic
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Wireless LAN - 75
Outline
Introduction
Distributed System
IEEE 802.11 Frame format
IEEE 802.11 MAC Architecture
Distributed Coordination Function (DCF)
Point Coordination Function (PCF)
IEEE 802.11 Standards
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Wireless LAN - 76
IEEE 802.11 Standards
IEEE 802.11, 2Mbps
IEEE 802.11b, 11Mbps
IEEE 802.11a, 54 Mbps
IEEE 802.11g, 54Mbps
IEEE 802.11n, 108Mbps
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Wireless LAN - 77
Summary
Hidden node problem
雙方雖然聽不到對方的訊號, 但同時傳送給相同的對象會造成衝撞
Exposed node problem
雙方雖然聽得到對方的訊號, 但同時可傳送給不同的對象
IEEE 802.11 wireless communication no collision detection
Use RTS/CTS frames to reserve the channel for large frames
A duration field in the RTS/CTS frames
Use ACK frame to confirm the correct frame
Two ways to sense the carrier
Physical
Virtual (NAV) – duration field
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Wireless LAN - 78
Summary
CSMA/CA (Collision Avoidance), sense the carrier
Idle, wait a DIFS then transmit
Busy, wait channel to idle + wait a DIFS + wait random backoff time, then transmit
Three Priority levels
SIFS < PIFS < DIFS
Superframe : A contention-free burst occurs at the beginning, followed by a contention period.
The PCF protocol is based on a polling scheme controlled by the Point Coordinator.