This document provides an overview of the IEEE 802.11 WiFi standard in 3 parts. Part 1 discusses advantages and disadvantages of WiFi networks. Part 2 describes the physical layer specifications including spectrum, modulation techniques, and OFDM. Part 3 covers the media access control layer and protocols like CSMA/CA, RTS/CTS, and acknowledgments that provide reliability. The document is intended for teaching purposes and draws from various academic sources.
1. WiFi – IEEE 802.11
Damien Magoni – University of Bordeaux
& Liam Murphy et al. – University College Dublin
2015/09/01
Version 3
2. Attribution
• The material contained inside is intended for teaching.
• This document is licensed under the CC BY-NC-SA license.
• All figures and text borrowed from external sources retain the rights
of their respective owners.
2
3. Acknowledgments
• These slides are originating from Prof. Liam Murphy’s lectures on WiFi
at the school of CS at UCD.
• They were subsequently extended by J. Fitzpatrick, S. Djahel, and N.
Cranley at UCD.
• I have completed the existing parts and have added part 2 on the
physical layer and part 6 on 802.11e.
• Part 2 material comes from the presentation of Imad Aad at IN’Tech
2002 on 802.11 and from the lecture notes of Herbert Haas on
modulation techniques.
• Part 6 material comes from the IEEE standard.
3
4. Table of Contents
1. Overview
2. PHY layer
3. MAC layer
4. Improvements
5. Applications
6. QoS and 802.11e
4
6. Advantages
• It’s wireless… mobility
• Plug and play
• Requires less infrastructure
• Quick, easy to install and maintain, simplified wiring
• Cost-efficient
• Flexible topologies – Infrastructure, Mesh, Ad-hoc networking
• Easily expandable, scalable
• Handover capabilities and “roaming”
• Backup for cellular network
6
7. Disadvantages
• Bandwidth and speed – Slower than wired networks
• Security – Much harder to secure than wired
• Reliability – Radio suffers from interference
• Range – Typical ranges under normal indoor conditions are 20m-50m
• Struggles with large number of users .. CSMA/CA
• Non-deterministic
• Lots of WLAN standards and constantly evolving
7
8. WiFi
• Most successful local wireless networking technology
• Using Unlicensed ISM Band
• Wi-Fi Alliance
• Industry organisation
• Promotes interoperability
• Compatibility
• Standard compliance
• Certification
8
9. IEEE 802.11
• IEEE: Not-for-profit Professional association
• 802 is the LAN/MAN standards committee
• WLAN technology is standardised by the IEEE
• Part .11 is for WLAN
• Also have ISO/IEC equivalents
• Following standards ensures compatibility and
interoperability
9
12. 802.11 in the OSI Protocol Stack
Application
Presentation
Session
Transport
Network
Data Link
Physical
• Open System Interconnect
(“OSI model”)
• 7 Layers of abstract
functional blocks for data
communication
• Each provides service to
block above/below
• Allows changes to each
without modifying entire
system
HTTP, FTP, SSH
TCP, UDP, SCTP
IPv4, IPv6
Ethernet, 802.11, ISDN, ATM
Wired, Wireless, Optical802.11
12
14. WLAN Network Types
Independent BSS / Ad-hoc Infrastructure BSS
• BSS (Basic Service Set) – A group of stations that can communicate
• Defined by propagation characteristics of the wireless medium
• SSID (Service Set Identifier) – Advertised Network Name
• STA (Station) – User terminal connected to other STA or AP
• AP (Access Point) – Station with access to external network
14
15. Extended Service Sets (ESS)
• BSS only covers limited area
• ESS allows creation of larger
network
• Multiple APs connected to the
same DS (Distribution System)
• Backbone network used to forward
packets between APs
• APs have the same SSID
• STA’s can roam from BSS1 to BSS2
15
16. Access Point (AP)
• Basic Parameters
• Service Set Identifier (SSID) to identify your network
• Protocol 802.11a/b/g/n …
• Channel assignment
• Power levels
• The AP sends out beacons announcing the SSID, data rate, …
• Security
• Protocols: WEP (don’t use it), WPA, WPA2 (IEEE 802.11i), WPS (don’t use it)
• Encryption protocols: TKIP (RC4 based), CCMP (AES based)
• Authentication protocols (EAP): EAP-TLS, EAP-TTLS, PEAP-TLS, EAP-FAST…
16
17. Client (STA) Association
• Client scans all channels
• Client listens to beacons
• Client associates with the AP
• Obtains the MAC and security settings
• AP/Radius server authenticates the client
• Client keeps scanning – just in case the network is poor
17
18. Beacons and Scanning
• Beacon Frames are used to announce the existence of the network
• Periodically transmitted by APs (e.g. every 100ms)
• Lowest data rate, maximum power
• Provides synchronisation, BSS and address information
• The user station’s behaviour
• Passive Scanning: Scans each available channel looking for beacon frames
• Active Scanning: Does not scan; rather, transmits a probe request – AP will respond
with a probe response containing beacon information
• Active scanning is faster but more intrusive. Active scanning is now more common
(allows faster re-connection)
• Association/Authentication: having found an AP to attach to, the user
station must register with the AP
18
19. WLAN Reliability
• Wireless is challenging
• Interference, obstacles etc…
• Wireless networks have unclear boundaries
• Received signal = transmitted signal + interference + noise
• Signal strength decays with distance
• STA hears signal only if
• Received Power > Carrier Sense Threshold (CST)
• WLAN has built-in reliability
19
32. Complementary Code Keying (CCK)
• Modulation scheme adopted to supplement the Barker code
• Due to the shorter chipping sequence in CCK (8 bits versus 11 bits in Barker
code)
• Less spreading higher data rate more susceptible to narrowband
interference shorter radio transmission range
• CCK also has more chipping sequences to encode more bits
• 8 chipping sequences at 5.5 Mbit/s and 64 chipping sequences at 11 Mbit/s
• Barker code only has a single chipping sequence
• CCK uses approximately the same bandwidth and utilise the same
preamble and header as pre-existing 1 and 2 Mbit/s 802.11
• 802.11g networks employ CCK when operating at 802.11b speeds
32
40. MAC Functions
• Frame delimiting and recognition
• Addressing of destination stations
• both as individual stations and as groups of stations
• Access control to the physical transmission medium
• DCF, PCF
• Protection against errors
• Frame Check Sequences
• ACKs
• RTS/CTS
40
42. MAC Access Modes detailed
• DCF (Distributed Coordination Function)
• Contention based medium access
• Uses CSMA/CA: listen-before-transmit
• Random backoff
• Most prevalent MAC access mechanism
• PCF (Point Coordination Function)
• Contention Free (CF)
• Point coordinator (PC) (in the AP) controls medium access
• Not widely implemented
• Both can operate simultaneously
42
43. Distributed Coordination Function
• Enables automatic medium sharing
• DCF = CSMA/CA + ACKs for unicast traffic
• CSMA/CA
• CS = Carrier Sensing, i.e. listening to signal = wait
• MA = Multiple Access, i.e. fair shared medium, best effort
• CA = Collision Avoidance, i.e. more waiting if there is congestion, only
transmit if the medium is idle
• Even if there is only one STA in the BSS
• STA is contending with the AP to transmit data
• Try uploading and downloading high bandwidth video simultaneously
43
44. Carrier Sense (CS)
• Physical and virtual CS functions are used to determine the state of
the medium (busy/idle)
• Physical CS mechanism provided by the PHY layer
• Virtual CS mechanism provided by the MAC sublayer
• Using a Network Allocation Vector (NAV)
• Enables to predict future traffic
• CS combines the NAV state, the STA transmitter status and the
physical CS to determine the state of the medium (busy/idle)
44
46. Inter-Frame Spaces (IFS)
• Access and transmission on the medium is controlled by a number of
precise time intervals or IFS, listed from shortest to longest:
• SIFS: short interframe space
• PIFS: PCF interframe space
• DIFS: DCF interframe space
• EIFS: extended interframe space
• SIFS < PIFS < DIFS < EIFS
• These are MANDATORY gaps preceding Control and Data frames
46
47. Short IFS (SIFS)
• Minimum Inter Frame Space, determined per PHY
• 28 µs for FH, 10 µs for DS, 16 µs for OFDM
• Used to separate control transmissions belonging to a single dialog
• Data ACK and CTS frames
• the second or subsequent MPDU of a fragment burst
• by a STA responding to any polling by the PCF
• by a PC for any types of frames during the CFP
47
Sender
Node A
Receiver
Node B
RTS
CTS
S
I
F
S
Sender
Node A
Receiver
Node B
DATA
ACK
S
I
F
S
48. Point Coordination Function IFS (PIFS)
• Only used in PCF mode
• Used by STAs to gain priority access to the medium
• A STA using the PCF shall be allowed to transmit CF traffic after its CS
mechanism determines that the medium is idle at the TxPIFS slot
boundary
• PIFS = aSIFSTime +aSlotTime (determined by PHY)
• At the beginning of the CFP, all STAs except the AP are required to
wait for DIFS before they can transmit, the AP can access the medium
before the other STAs as PIFS < DIFS
48
49. Distributed Coordination Function IFS (DIFS)
• Used in DCF mode
• A DIFS must follow after a busy period
• Then a STA can start a new transmission if
• Backoff time has expired
• Medium is idle
• DIFS = aSIFSTime + 2 x aSlotTime
49
Sender
Node A
Receiver
Node B
DATA
ACK
S
I
F
S
D
I
F
S
T
S
DATA
ACKS
I
F
S
T
S
T
S
D
I
F
S
T
S
T
S
DATA
BO=3 BO=2
50. Extended IFS (EIFS)
• Used by the DCF when a frame transmission was begun that did not result
in the correct reception of a complete MAC frame with a correct FCS value
• EIFS shall begin following indication by the PHY that the medium is idle
after detection of the erroneous frame, without regard to the virtual CS
mechanism
• EIFS is defined to provide enough time for another STA to acknowledge
what was, to this STA, an incorrectly received frame before this STA
commences transmission
• EIFS = aSIFSTime + DIFS + ACKTxTime
• Reception of an error-free frame during the EIFS resynchronizes the STA to
the actual busy/idle state of the medium, so the EIFS is terminated and
normal medium access (using DIFS and, if necessary, backoff) continues
following reception of that frame
50
51. Exponential Backoff and Contention Window
• STA wants to transmit a frame
• When the medium is idle, STA will wait = DIFS (or EIFS if error in last frame
received) + Contention Window
• Backoff Time = Random() × aSlotTime (= 20 µs for DSSS)
• Random() = Pseudo-random int drawn from a uniform distribution over [0, CW]
• CW = 2p
- 1
• aCWmin = 31 (for DSSS, p = 5) ≤ CW ≤ aCWmax = 1023 (p = 10)
• Collisions only occur if 2+ STAs select the same random value
• If collision occur, p is increased by 1 after every consecutive collision
• Reduces the probability of the nodes selecting the same random value again
• Once a frame is successfully transmitted or the retry limit is reached, CW is reset to CWmin
51
52. Collision Avoidance (CA)
• STA detects that the medium has been idle for DIFS
• It sets its BackOff (BO) counter to the randomly selected value drawn
from [0, CW]
• It decrements the BO for each aSlotTime that the medium is idle
• If the BO counter reaches 0, it transmits its data frame
• If another STA begins transmitting before the BO reaches 0, it pauses
decrementing the value until it detects that the medium is idle again
52
54. Reliability and Retransmissions
• For each data frame transmitted, an ACK is transmitted by the receiver
• If the ACK is not received, the sender will try to retransmit the frame again
• Retry Counters: the number of times the sender will try to retransmit the
frame
• Short Retry Counter: number of times to retransmit a short frame
• Long Retry Counter
• When the Retry Counter reaches 0, the frame is discarded
• Reliability mechanism is designed to overcome 2 issues which can occur:
• Hidden Nodes
• Exposed Nodes
54
55. Hidden Nodes
• B can “hear” A and C but A cannot “hear” C.. They are hidden from each other
• A is transmitting to B
• But C can’t “hear” this transmission
• C transmits to B
• … COLLISION … both transmissions may be lost
A
B
C
Nodes A and C
Are hidden from each other
55
56. Solution to Hidden Nodes
• 802.11 MAC uses 2 control frames
• Request to Send (RTS)
• Clear to Send (CTS)
• These are very short frames
• Aim to reduce the collision period
• Better to collide on a shorter frame than a longer data frame
56
57. RTS/CTS Operation
• Node A wants to transmit to Node B
• Case 1:
• A sends an RTS and waits for a CTS from B
• B sends a CTS to A, BUT C also “hears” the CTS
• Node A receives the CTS .. Its ok to transmit
• Node A transmits the DATA
• Node B transmits the ACK
• Node C “hears” the ACK … and knows that it can try again to transmsit
• Case 2:
• A and C BOTH send an RTS to B
• Collision occurs
• But since RTS are very short .. Its only a short collision
57
58. 58
A B C
RTS
CTS
DATA
Node C can “hear” the
CTS transmitted by Node B
RTS RTS
ACK
Node C can “hear” the
ACK transmitted by Node B
Case 1
Case 2
Short collision
59. Impact of RTS/CTS
• Node A sends an RTS
• All nodes in range of A must WAIT until transmission is completed
• Node B sends the CTS
• All nodes in range of B must WAIT until they “hear” the ACK from Node B
59
A
B
C
Inefficient!!
Not needed if ALL
STA are in range
WAITING!! WAITING!!
60. Exposed Nodes Reduced Throughput
60
• Node A is transmitting to Node D
• Node B wants to transmit to Node C
• But Node B “thinks” that if it transmits, it will interfere with Node A
• … So Node B WAITS …. Inefficient and reduced throughput
A B C
D
Exposed
Node
61. dot11RTSThreshold Parameter
• The MAC layer parameter dot11RTSThreshold indicates the minimum
required length of a frame for the frame to be preceded by RTS and
CTS frames
• Short frames are not preceded by RTS & CTS frames
• If Length > dot11RTSThreshold use RTS/CTS
• Can be set on a per-STA basis
• The default value of dot11RTSThreshold is 2347 Bytes (i.e., no
RTS/CTS)
61
62. Network Allocation Vector (NAV)
• How does a STA know how long to wait while a transmission is taking
place?
• 802.11 frames carry a Duration field
• Specifies the transmission time required for the frame, during which the medium will
be busy
• Duration information announced in RTS/CTS frames
• Duration information available in the MAC headers of all frames sent during the CP
• Each STA sets and updates its NAV using these Duration fields
• NAV prevents other stations from accessing the medium until the
transmission is complete
• NAV counting down to zero at a uniform rate (timer)
• NAV > 0 -> medium is busy, NAV = 0 -> medium is idle
• Medium considered busy when the STA is transmitting
62
63. NAV Timeline
63
Sender
Node A
Receiver
Node B
NAV
Node C
RTS
CTS
Frame
ACK
NAV RTS
NAV CTS
SIFS
SIFS
SIFS
DIFS
t
t
t
RTS Duration Field = CTS + Data/Mgmt + ACK + 3xSIFS
CTS Duration Field = Data + ACK + 2xSIFS
65. NAV for Power Save
• Wireless stations are often battery powered, so in order to conserve
power the stations may enter a power-saving mode.
• Constantly sensing the medium consumes power
• When the NAV = 0, an STA will wakeup to sense the medium again.
65
66. Point Coordination Function
• Optional, not widely implemented
• Wireless channel is divided into superframes
• Superframe = Contention Free Period (CFP) + Contention Period (CP)
• At the beginning of CFP, the Point Coordinator (PC) (in the AP) contends for
access of the wireless channel
• If an AP acquires the channel, a polling/granting policy is applied for
transmission
• Used in 802.15.4 Zigbee for WSN
66
67. B B
CFP repetition interval
Superframe
CFP
Busy
medium B
CP (DCF)CP (DCF)
Superframe
• Superframe = CFP + CP
• Designed for delay-sensitive/real-time applications
• B = Beacon frame (sent by AP to indicate start of CFP)
• CPF = Contention-Free Period (reserved for real-time traffic)
• CP = Contention Period (normal DCF operation)
CFP
Reduced CFP due to the busy medium (it is not
possible to cut off active DCF transmissions)
67
68. Protecting CFP from CP
CFP operation is guaranteed in two ways
1. The NAV value in the beacon signal = length of CFP
2. Usage of PIFS (instead of DIFS) within the CFP
• PIFS < DIFS
B B
CFP
Busy
medium B
CP (DCF)CP (DCF)
CFP
NAV NAV
68
69. Polling during the PCF of the CFP
B
SIFS
Set by beacon frame
S1 DATA
SIFS SIFS
PC (AP)
STAs
NAV
SIFS PIFS SIFS
S3
CPCFP
Poll S1 Poll S2 Poll S3 + data CFP end
Begin of CFP
AP sends a Beacon containing the NAV so the duration of the CFP is known by all
AP polls STA1, STA1 sends its data
AP polls STA2, STA2 does NOT have any data to send, wait for PIFS
AP polls STA3 and also sends some data to STA3, STA3 has some data to send
End of CFP
69
70. Shortcomings of the PCF
• PCF designed for RT applications but does not offer extensive QoS
• No differentiation possible between traffic types/classes
• CPF length cannot be dynamically changed according to traffic needs
• No mechanism for STAs to communicate QoS requirements to the AP
• Different maximum packet lengths cannot be enforced
• Increased delays – waiting to be polled in a round robin
• Wasteful – polling STAs that have nothing to send
• AP needs to contend for the channel at the beginning using DCF
• Effective period of CFP may vary
70
71. General Frame Format
• Generic MAC frame
• The 802.11 MAC uses a different frame structure than Ethernet
• 4 Address fields
• Not used all the time
• Overhead of 34 bytes
• Max payload 2312 bytes
71
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4Bytes
MAC Header
72. Frame Control Field
72
2 2 6 6 6 2 6 0-2312 4Bytes
Protocol
Type=
data
Sub type
To
DS
2 2 4 1
From
DS
1
More
Frag
1
Retry
1
Pwr
Mgmt
1
More
Data
1
Prot
Frame
1
Order
bit
1Bits
• Protocol Version: version of 802.11 MAC (currently 0)
• Type: Frame Type (e.g. Management, Control or Data)
• Sub Type: Specific Frame Subtype
– e.g. 1000 = beacon, 1011 = RTS
• To DS / From DS: Specify if frame is destined for the DS or is coming
from the DS
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
73. Frame Control Field cont’
• More Frag: Specifies if frame has been fragmented
• Retry: Is a retransmission
• Power Management: Indicates that sending station will power
down after transmission
• More Data: Informs a sleeping station that data is available
• Protected Frame: Security is being used on Frame
• Order Bit: Strict ordering must be used
Protocol
Type=
data
Sub type
To
DS
2 2 4 1
From
DS
1
More
Frag
1
Retry
1
Pwr
Mgmt
1
More
Data
1
Prot
Frame
1
Order
Bit
1Bits
Bytes
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4
73
74. Duration/ID Field
• Duration: Three Purposes
• Setting the NAV: Defines the amount of time in msec the
medium will be busy for
• CFP Frames: Essentially sets the NAV to the maximum
value during contention free periods
• PS-Poll Frames: Used by stations to inform the AP that they
have woken from power saving state
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4Bytes
74
75. Address Fields
• All addresses are 48 bit IEEE MACs
(01:23:45:67:89:ab)
• Destination: Identifies the final recipient
• Source: Identifies the original source
• Receiver: Identifies the receiver which should process the
frame (Intermediate node)
• Transmitter: Identifies the transmitter of the frame
(Intermediate node)
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4Bytes
75
76. Frame Body
• Also called the “Data Field”
• Contains the higher layer payload
• 802.11 supports up to 2312 bytes of data
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4Bytes
76
77. Seq-Ctl & FCS
• Sequence Control – Used for detecting and discarding
of duplicate frames
• Frame Check Sequence – Often called the CRC, used
to check frame integrity
• G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7
+ x5 + x4 + x2 + x + 1
Frame
Control
Duration
/ID
Address 1 Address 2 Address 3 Seq-ctl Address 4
Frame
Body
FCS
2 2 6 6 6 2 6 0-2312 4Bytes
77
78. Control Frames Format: RTS
• RA: identifies the immediate receiver
• TA: identifies the transmitter of the RTS
• Duration = CTS + Data/Mgmt + ACK + 3xSIFS
• measured in micro-seconds
• i.e., CTS->SIFS->DATA->SIFS->ACK->SIFS
78
Frame
Control
Duration
RA TA FCS
2 2 6 6Bytes
4
MAC Header
79. Control Frames Format: CTS
79
• RA: identifies the immediate receiver, i.e. the sender
of the previous RTS
• Duration = Data + ACK + 2xSIFS
Frame
Control
Duration
RA FCS
2 2 6Bytes
4
MAC Header
80. Control Frames Format: ACK
80
• RA: identifies the immediate receiver, i.e. the sender
of the previous DATA frame
• Duration = 0 if More Frag = 0
…otherwise:
• Duration = DATA + ACK + 2xSIFS
Frame
Control
Duration
RA FCS
2 2 6Bytes
4
MAC Header
82. The 802.11 Alphabet Soup
• Only one standard… but a large number of 802.11 Amendments
• Naming scheme is done per letter, as the working group is established
• Not when it was published!
• Some define both MAC/PHY, others just apply to the MAC
• Some operate in conjunction with others, e.g. 802.11g/802.11e
82
83. Common 802.11 Amendments
• 802.11e – QoS enhancements to MAC layer
• 802.11f – Inter access point protocol
• 802.11s – Mesh networking
• 802.11k – Radio resource management
• 802.11n – MIMO
• 802.11ac – MU-MIMO
83
84. Lastest IEEE 802.11 standards/amendments
• 802.11af-2013: Television White Spaces (TVWS) Operation
• 802.11ac-2013: Enhancements for Very High Throughput for Operation in
Bands below 6 GHz
• 802.11ad-2012: Enhancements for Very High Throughput in the 60 GHz
Band
• 802.11ae-2012: Prioritization of Management Frames
• 802.11aa-2012: MAC Enhancements for Robust Audio Video Streaming
• 802.11-2012: Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications (latest global version, 2793 pages!)
• 802.11u-2010: Interworking with External Networks
84
87. 802.11n
• Features
• Multiple Input (tx antennas) Multiple Output (rx antennas) (MIMO) exploits
multipath, each rx antenna gets a slightly different version of the signal
allowing for successful decoding
• 40 MHz channel bandwidth
• Frame aggregation using Block ACKs from .11e
• Optional features
• 2 to 4 spatial streams
• Transmit Beamforming (TxBF)
• 400 ns short guard interval (SGI)
• 64-QAM, 40 MHz channel, 400 ns GI, 4 spatial streams -> 600 Mbps
87
88. Multipath
88
Multipath with 11a/b/g :
Reflections can interfere and distort
the signal.
When using MIMO and its
multiple receiving antennas,
however, the effects of multipath
become additive -
multiple messages can be received
by multiple antennas, and combined.
http://www.radio-electronics.com/info/antennas/mimo/formats-siso-simo-miso-mimo.php
89. 802.11ac
• Operates in the 5GHz frequency band
• Multiple User-MIMO (MU-MIMO) enables multiple users to simultaneously
access the same channel by leveraging users’ interferences (spatial
signature)
• New mandatory features
• 80 MHz channel bandwidth
• New optional features
• 5 to 8 spatial streams
• 160 MHz channel bandwidth (contiguous 80 + 80)
• 80 + 80 MHz channel bonding (discontiguous 80 + 80)
• MCS 8/9 (256-QAM), 10/11 (1024-QAM, non standard)
• 256-QAM, 160 MHz channel, 400 ns GI, 8 spatial streams
-> 8 x 866.7 Mbps = ~6934 Mbps
89
90. 802.11ad - WiGig
• 4 bands around 60GHz
• 2.16GHz channel bandwidth
• Can not penetrate walls but can propagate by reflection
• Beamforming for ranges up to 10 m
• Backward compatibility for tri-band (2.4, 5 and 60GHz) devices
• Low power single carrier data rate up to 2.5Gbps
• 64-QAM, -47dBm sensitivity, -26dB Error Vector Magnitude (EVM) in
constellation -> ~6757 Mbps
90
91. 802.11af - WhiteFi, Super WiFi
• WLAN operation in TV white space spectrum in the VHF and UHF bands
between 54 and 790 MHz
• Frequency channels 6 to 8 MHz wide, depending on the regulatory domain
• OFDM as in 802.11ac
• Range up to 1 km, increased compared to 802.11 a/b/g/n/ac
• Up to four channels may be bonded in either one or two contiguous blocks
• MIMO operation possible with up to 4 streams used for either space–time
block code (STBC) or multi-user (MU-MIMO) operation
• 256-QAM, 8MHz channel, 2.25µs GI, 4 spatial streams, 4 bonded channels
-> ~569 Mbps
91
93. WiFi is slow
• Application types and hungry apps
• Lack of bandwidth
• Poor configuration
• Real throughput
93
94. Application Types
• Voice
• Video
• Data
• E-mail
• Browsing/Surfing
• Business applications
• File transfer/sharing
• Gaming
94
95. Voice
• Not bursty, not bandwidth demanding
• Small packets sent frequently
• Eg 10ms voice per pkt = 100pkts per sec
• Very strict latency and loss rate requirements
• People are particularly sensitive to latency
• VoIP quality is measured using the E-model
• Ro = SNR
• Is = combination of all impairments which occur more or less simultaneously with the voice signal
• Id = impairments caused by delay
• Ie-eff = effective equipment impairment factor represents impairments caused by low bit-rate codecs and
impairments due to randomly distributed packet losses
• A = compensation of impairment factors
• WiFi can not guarantee or even bound latency or loss
95
96. Video
• Bandwidth demanding
• Somewhat loss resilient but
error propagation issues
• Encoded as a series of frames
• Example: 20fps = 1 frame every 50 ms
• Different frame types: I, P, B
• Group of Pictures (GoP) = periodic sequence of P+B frames
between I-frames
• Example: 20fps might have GoP sequence (I-PbbPbbPbb)x2
• I-frames are large and generate a burst of packets
• Encode video at a CBR to avoid bursts
• No as CBR is the average smoothed bit rate, cannot avoid packet
bursts generated by I-frames
Frame size
96
97. Gaming
• Fast interactivity -> Low latency, low loss rate
• Asynchronous and bursty
• Often adds VoIP on top for team cooperation
• May be high bandwidth for Web gaming
97
98. Upgrade WiFi to WiGig
• Throw bandwidth at the problem, that’ll make it all go away …
• Upgrade to a higher rate PHY (theoretical limits)
• 802.11n: 600Mbps
• 802.11ac/ad: 6Gbps
• Temporary quick fix
• Contention issues remain
• Hungrier apps, e.g. HDTV
• Backward compatibility and device chipsets
98
99. Typical WiFi Config Parameters
Parameter If Increased… If Decreased…
Beacon interval Better throughput and longer battery
life
Faster mobility and handoff
RTS/CTS
On/Off
Off: Better throughput On: Hidden node access
RTS Threshold Frames > Threshold use
RTS/CTS
Increase throughput if no hidden
terminals
Higher throughput with many
hidden terminals
Fragmentation
Threshold
Frames > Threshold are
fragmented
Increased throughput in error free
channel
Increased throughput in error
prone channel + increased
latency
Long/short retry
limits
Max number of
retransmission attempts
Lower pkt loss rate, increased backoff
and reduced throughput
Higher pkt loss rate, smaller
buffer required
Auto-rate
adaptation
AP can be in auto mode, dynamically switch PHY rates to limit interference
Power Increase antenna power to increase coverage and range.
Typ. max power value of 20 dBm or 100 mW which covers an area of 100 meters
But in reality these are not always available …. 99
100. WiFi Auto-Rate Adaptation
• Clients can shift data rates on a
transmission-by-transmission basis
• AP can support multiple clients at
multiple speeds depending upon the
location of each client
• Higher rates require stronger signals at
the receiver
• Lower rates have a greater range
• STAs always try to communicate at
highest rate
• STAs reduce the rate only if transmission
errors and transmission retries occur
100
101. Interaction with Higher Layers
STA-A
STA-B
AP
Wired
Ethernet
1. UDP with RTS/CTS
2. UDP without RTS/CTS
3. TCP
101
102. 1. UDP with RTS/CTS
Starts sensing the
medium (idle)
DataAP
STA-A
STA-B
MAC
ACK
Defers access
timeRTS
CTS
RTS
DIFS
SIFS
BO: RandInt[0,CWmin]
102
103. 2. UDP without RTS/CTS
Starts sensing the
medium (idle)
DataAP
STA-A
STA-B
MAC
ACK
Defers access
time
Data
What happens if the
MAC-ACK is not received?
Could be the Data packet
was lost or the MAC-ACK
was lost.
Retransmission: Receiver of the frame should
transmit an ACK immediately. If no ACK is
received then sender retransmits the frame
after a random backoff.
But how useful is retx for real-time VoIP or
video?
DIFS
SIFS
BO: RandInt[0,CWmin]
103
104. 3. TCP … it gets worse...
Starts sensing the
medium (idle)
TCP Data
AP
STA-A
STA-B
MAC
ACK
Defers access
time
TCP ACK
MAC
ACK
Data
What happens if the TCP-Data, TCP-MAC-ACK, TCP-Ack, TCP-MAC-ACK_ACK gets
lost? Not even lost – but maybe just delayed…. Not only are there
retransmissions at the MAC layer, but then the TCP congestion control kicks in
invoking the TCP-retransmission and the Congestion control.
Very lucky here.. But what if STA-B won the right to
transmit instead.
DIFS
SIFS
BO: RandInt[0,CWmin]
104
108. IEEE 802.11e-2005
• Amendment incorporated into the published 802.11-2007 standard
• Hybrid Coordination Function (HCF) with 2 modes
• EDCA
• HCCA
• Can cooperate with non-QoS devices
• nQSTA are regular stations: only DCF (optional PCF)
• QSTA are QoS enabled stations: DCF and HCF are present
108
109. Enhancements of 802.11e
• Contention Free Bursts (CFB) allow stations to send several frames in
a row without contention, if the allocated Transmission Opportunity
(TXOP) permits
• Block Ack (optional)
• No Ack needed for applications where retransmission cannot be used
due to strict delay requirements
• Direct Link Setup (DLS) enables direct communication between STAs
without involving the AP
109
111. Hybrid Coordination Function (HCF)
• Hybrid Coordinator (HC) in the QoS enabled AP (QAP)
• HCF has 2 access modes
• Enhanced Distributed Channel Access (EDCA) similar to DCF but with different
priority levels for different services (such as DiffServ)
• HCF Controlled Channel Access (HCCA) is a CSMA/CA-compatible polling-
based access method similar to PCF (optional, not widely implemented)
111
112. Enhanced Distributed Channel Access (EDCA)
• EDCA divides the traffic into 8 different User Priorities (UP, same as in
802.1D) which are mapped onto 4 Access Categories (AC)
• All MSDUs having the same UP belong to a Traffic Category (TC)
• Each AC’s access is controlled by using 4 differentiating parameters
• Transmission Opportunity (TXOP) specifies the starting time and maximum
duration during which a station can transmit 1+ frames
• Minimum idle duration time is not the DIFS but the Arbitration Interframe
Space (AIFS) which is a variable DIFS
• Minimum contention window size (aCWmin)
• Maximum contention window size (aCWmax)
112
114. Transmission Opportunity (TXOP)
• Under HCF, the basic unit of allocation of the right to transmit onto
the medium is the TXOP
• Each TXOP is defined by a starting time and a defined maximum
length/duration
• TXOP may be obtained in 2 ways
• by a QSTA winning an instance of EDCA contention (EDCA TXOP, length
defined in beacon frame) during the CP
• by a QSTA receiving a QoS(+)CF-Poll frame (HCCA TXOP or polled TXOP,
includes its length) from HC during the CFP or the CP
114
115. Arbitration Inter-Frame Space (AIFS)
• AIFS are used by QSTAs to transmit all data and management frames and
the following control frames: PS-Poll, RTS, BlockAckReq
• A QSTA using EDCA shall obtain a TXOP for an AC if the CS mechanism
determines that the medium is idle at the AIFS[AC] slot boundary, after a
correctly received frame, and the backoff time for that AC has expired
• AIFS Number (AIFSN)
• nb of slots after a SIFS a non-AP QSTA should defer before either invoking a backoff
or starting a transmission (minimum value = 2 for non-AP, 1 for QAP)
• AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime
• A QSTA using EDCA shall not transmit within an EIFS-DIFS+AIFS[AC] plus any
backoff time after that QSTA determines that the medium is idle following
reception of an incorrect frame
115
118. Scheduling the AC queues
• Consider an AP has some VoIP,
video, and background data to
send to STA
• Internal collision resolution: High
priority AC wins the right to
transmit, but low priority AC back
off as if it experiences a collision
• Highest priority traffic “statistically”
wins
118
120. Which AC Parameters?
• VoIP/Gaming – short AIFS, low CWmin, for quick access
• Packet every 10 or 20 ms
• Video – large TXOP to allow for bursts generated by an I-frame
• Packet bursts every 40 - 50 ms
• TCP – low AIFS and CW to minimise TCP-ACK RTT and hopefully avoid
TCP congestion control
• Best-effort and Background – values larger than the above
120
121. HCF Controlled Access (HCCA)
• Based on the HC using polling for controlling the traffic
• HC can poll stations during CFP and also during CP
• By using Controlled Access Phases (CAP)
• During the CP, all stations function in EDCA
• Traffic can be classified
• Use of Traffic Streams (TS)
• Traffic Specifications (TSPEC) in frames
• Traffic Classification (TCLAS) elements in devices
121
122. Traffic Streams (TS)
• QSTAs can send TSPECs describing the traffic characteristics and the QoS
requirements (rate, delay, …) of a TS
• Reserve resources within the HC and modify the HC’s scheduling behavior
• Allow other parameters to be specified that are associated with the TS (e.g., traffic
classifier and acknowledgment policy)
• A TS may have one or more TCLAS element associated with it
• The AP uses the parameters in the TCLAS elements to filter the MSDUs
belonging to this TS for delivery as part of the TS
• HC is thus not limited to per-station queuing and can provide per-session service
• Stations provide lengths of their queues for each TS and the HC can use this info
122
123. Controlled Access Phase (CAP)
• HCCA uses the HC’s higher priority to initiate frame exchanges and to
allocate TXOPs to itself and other STAs in order to provide limited-
duration Controlled Access Phase (CAP) for CF transfer of QoS data
• CAP is a time period when the HC maintains control of the medium,
after gaining access by sensing the channel to be idle for a PIFS
• CAP might span multiple consecutive TXOPs and can contain polled
TXOPs
• Delivery Traffic Indication Message (DTIM): interval between the
consecutive TxTimes of beacons containing a DTIM
123
125. TXOP Structure and Timing
• Any QoS data frame with CF-Poll contains a TXOP limit in its QoS Control
field
• The ensuing polled TXOP is protected by the NAV set by the Duration field of the
frame that contained the QoS (+)CF-Poll function
• Within a polled TXOP, a STA may initiate the transmission of 1+ frame exchanges
each separated by a SIFS
• The STA shall not initiate transmission of a frame unless the transmission and any
acknowledgment or other immediate response are able to complete prior to the end
of the remaining TXOP duration
• A TXOP or transmission within a TXOP shall not extend across
• TargetBeaconTxTime, dot11CFPMaxDuration (if during CFP), dot11MaxDwellTime (if
using an FH PHY), or dot11CAPLimit
• The HC shall verify that the full duration of any granted TXOP meets these
requirements so that STAs may use the time prior to the TXOP limit of a polled TXOP
without checking for these constraints
125
127. Block Ack
• Improves the channel efficiency
• Transmit a block of data frames
• each separated by a SIFS period
• Aggregating several acknowledgements into ONE frame
• Two types of Block Acks
• immediate for high-bandwidth, low latency traffic
• delayed for applications that can tolerate moderate latency
127
131. No Ack Policy
• Usage of No Ack is determined by the policy at the QSTA
• When No Ack policy is used, there is no MAC-level recovery
• Reduced reliability due to the increased probability of lost frames from
interference, collisions, or timevarying channel parameters
• A protective mechanism (e.g., HCCA, RTS/CTS) should be used to
reduce the probability of other STAs transmitting during the TXOP
131
132. Direct Link Setup (DLS)
• DLS is a protocol that enables a STA in a BSS to transmit frames directly to
another STA in the same BSS
• DLS enables to exchange rate set and data between the sender and the
receiver
• A recipient may be in PS mode, in which case it can be awakened only by
the AP
• DLS prohibits the STAs going into PS mode for the duration of the direct
stream as long as there is an active DLS between the two STAs
• Setup and teardown procedures go through the AP
• DLS does not apply in an IBSS, where frames are always sent directly from
one STA to another
132
133. Considerations
• QoS can be finely configured with HCCA
• QSTA can request specific transmission parameters (data rate, jitter, …)
• Requirements for implementing HCCA
• Stations only need to be able to respond to poll messages
• On the AP side, a scheduler and queuing mechanism is needed
• .11e not widely supported especially HCCA
• Difficulty to set optimal settings for the different traffic types and ACs
• PCF is not widely implemented in WLAN but is in WSN
• With the takeoff of WSNs due to IoT and M2M, PCF might make a comeback
133
