This presentation provides some technical details on the IEEE 802.11ah (2016), a specification for WLAN communications in the sub 1GHz frequency band. This new technology is the basis of the Wi-Fi HaLow (the low power, long range Wi-Fi approach for the Internet of Things - aka IoT - ).
Good Stuff Happens in 1:1 Meetings: Why you need them and how to do them well
IEEE 802.11ah: what lies beneath Wi-Fi HaLow
1. IEEE 802.11ah
Eduard Garcia-Villegas, Elena López-Aguilera
Dept. of Network Engineering
eduardg@entel.upc.edu
elopez@entel.upc.edu
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
by wilgengebroed
Download this presentation (PDF) from:
http://ocw.upc.edu/download.php?file=15016145/802.11ah_wi-fi_iot-5709.pdf
2. Contents
IEEE 802.11ah: sub 1GHz WLAN for IoT
o Purpose, scope and use cases
o PHY
o MAC
o Power saving
o Other remarkable features
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by wilgengebroed
3. Purpose, scope and use cases
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
4. IEEE 802.11ah: purpose
Defines operation of license-exempt (ISM)
IEEE 802.11 wireless networks in frequency
bands below 1 GHz
o excluding the TV White Space bands (802.11af).
4
by Atmel Corp.
IEEE 802.11 WLAN
user experience for
fixed, outdoor,
point to multi point
applications
IEEE 802.11ah: sub 1GHz WLAN for I0T
5. IEEE 802.11ah: scope
Defines an OFDM PHY operating in the license-exempt
bands below 1 GHz
o and enhancements to the IEEE 802.11 MAC to support
this PHY, and to provide mechanisms that enable
coexistence with other systems in the bands (e.g. IEEE
802.15.4 P802.15.4g)
The PHY is meant to optimize the rate vs. range
performance of the specific channelization in a given band.
o transmission range up to 1 km
o data rates > 100 kbit/s
The MAC is designed to support thousands of connected
devices
5IEEE 802.11ah: sub 1GHz WLAN for I0T
6. IEEE 802.11ah: use cases
Use Case 1 : Sensors and meters
o Smart Grid - meter to pole
o Environmental monitoring
o Industrial process sensors
o Healthcare
o Home/Building automation
o Smart city
Use Case 2 : Backhaul sensor and meter data
o Backhaul aggregation of sensor networks
o Long point-to-point wireless links
Use Case 3 : Extended range Wi-Fi
o Outdoor extended range hotspot
o Outdoor Wi-Fi for cellular traffic offloading
6IEEE 802.11ah: sub 1GHz WLAN for I0T
8. IEEE 802.11ah: PHY (1)
Advantages of transmitting in sub 1 GHz:
o Spectrum characteristics
• good propagation and penetration
• large coverage area and one-hop reach
• license-exempt, light licensing
o Reliability:
• less congested frequency band
• high sensitivity and link margin
• available diversity – (frequency, time, space)
o Battery operation
• long battery life
• short data transmissions
8IEEE 802.11ah: sub 1GHz WLAN for I0T
9. IEEE 802.11ah: PHY (2)
Channelization:
o Each regulatory domain
defines a different band and
different tx power limits
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o Configurable bandwidth
(channel bonding) of: 1,
2, 4, 8 and 16MHz
• Example of bandwidth
options in the US
1MHz
2MHz
4MHz
8MHz
16MHz
902MHz 928MHz
750 MHz 1 GHz
China
Europe
Japan
Korea
Singapore
USA
(755-787 MHz)
(863-868 MHz)
(916.5-927.5 MHz)
(917.5-923.5 MHz)
(866-869 MHz) (920-925)
(902-928 MHz)
IEEE 802.11ah: sub 1GHz WLAN for I0T
10. IEEE 802.11ah: PHY (3)
Inherited from IEEE 802.11ac (adapted to S1G):
o OFDM
• 10 times down-clocking .11ac
– symbol duration x 10 40µs
• Same number of OFDM subcarriers: bandwidth /10
– 20MHz 2MHz (52/64 data subcarriers)
o MIMO + MU-MIMO
• Up to 4 spatial streams (NSS > 2 are optional)
o PHY rates ranging from 150kbps to 347Mbps
• Min: MCS10 (BPSK 1/2 with repetition) x 1 stream x 1MHz x
Long Guard Interval (GI)
• MAX: MCS9 (256-QAM 5/6) x 4 streams x 16MHz x Short GI
10IEEE 802.11ah: sub 1GHz WLAN for I0T
11. IEEE 802.11ah: PHY (4)
Expected throughput vs. coverage
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1MHz
2MHz
4MHz
8MHz
16MHz
Mandatory
for STAs
(Globally
interoperable)Mandatory
for APs
Range
Rate
0.15 – 4.40Mbps x NSS
5.85 - 86.67Mbps x NSS
2.92 – 43.33Mbps x NSS
1.35 – 20.00Mbps x NSS
0.65 – 8.67Mbps x NSS
NSS = number of spatial streams
IEEE 802.11ah: sub 1GHz WLAN for I0T
12. IEEE 802.11ah: PHY (5)
Expected throughput vs. coverage (min and max)
12
Additional step
thanks to MCS10
(only available with
1MHz and NSS 1)
NSS = number of spatial streams
IEEE 802.11ah: sub 1GHz WLAN for I0T
14. IEEE 802.11ah: MAC (1)
Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Short MAC headers:
• Removed fields (Duration, QoS control, HT control,
optionally Sequence control)
• Option to use only two addresses (instead of three)
– Option to use 2B AID instead of 6B MAC address
• Example: send frame with 100 Bytes of data
– Legacy: 100B of data + 36B of header + FCS 26%
overhead!
– 11ah short MAC header: 100B of data + 14B of
header + FCS 12% overhead
14IEEE 802.11ah: sub 1GHz WLAN for I0T
AID = Association ID (unique value assigned to a STA during association)
15. IEEE 802.11ah: MAC (2)
Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o NULL Data Packets (NDP)
• Concentrate relevant information of control frames in
the PHY header (avoid MAC header + payload)
• Example:
– 11ah transmission of 100B frame at lowest rate (1MHz
x NSS 1 x MCS10) takes ~8ms
» Legacy ACK: ~1.5ms (20% of the data frame!)
» NDP ACK: ~0.5ms (6% of the data frame)
o Short Beacons
• Beacons are sent frequently at the lowest rate
short (more frequent) and full beacons (less frequent)
15IEEE 802.11ah: sub 1GHz WLAN for I0T
16. IEEE 802.11ah: MAC (3)
Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Implicit acknowledgement (no ACK needed)
• Bidirectional TXOP (BDT): extension of 802.11n’s
Reverse Direction protocol (RD)
– With RD: exchange of uplink and downlink frames
during a single TXOP
– With BDT: reception of next data frame implies that
previous data was successfully received (no ACK
needed).
• Reduces channel access attempts, number of frames
exchanged Increases channel efficiency, battery
lifetime
16IEEE 802.11ah: sub 1GHz WLAN for I0T
17. IEEE 802.11ah: MAC (4)
Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Implicit acknowledgement (no ACK needed)
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STA A
(BDT Init.)
STA B
(BDT Resp.)
DATA
More Data = 1
Long Resp.
ACK
DATA
More Data = 1
Long Resp.
DATA
More Data = 0
Long Resp.
DATA
More Data = 0
Normal Resp.
STA A
(legacy)
STA B
(legacy)
DATA
ACK
ACK
DATA
SIFS
SIFS
DIFS +
Backoff
DATA
ACK
ACK
DATA
IEEE 802.11ah: sub 1GHz WLAN for I0T
18. IEEE 802.11ah: MAC (5)
Need to support thousands of associated devices
(increases coverage increases reachable STAs)
o Legacy 802.11 limited to 2007 associated STAs
11ah increases to >8000
• Hierarchical Association ID (AID) assignment (uses
13bits): page/block/sub-block/STA
– Allows grouping STAs according to different criteria
» Device type, power constraints, application, location, etc.
• Increased TIM size (one bit per each associated STA)
– 1kB each Beacon frame!?! No, it can be compressed
18IEEE 802.11ah: sub 1GHz WLAN for I0T
19. IEEE 802.11ah: MAC (6)
Need to support thousands of associated devices
o Thousands of STAs huge collision probability!
o Restricted Access Window (RAW): regular RAW
• Divide STAs into groups (AID)
• Split channel access into time slots
• Assign slots to groups (AP indicates RAW allocation
and slot assignments in its Beacons)
– STAs are only allowed to transmit during its group’s slot
– Cross Slot Boundary option enables STAs to cross its
assigned RAW slot to complete the ongoing exchange.
– STAs can sleep during other groups’ slots
• Different backoff rules apply during RAW (due to
different contention conditions)
19IEEE 802.11ah: sub 1GHz WLAN for I0T
20. IEEE 802.11ah: MAC (7)
Need to support thousands of associated devices
o Thousands of STAs huge collision probability!
o Restricted Access Window (RAW): regular RAW
• Example:
– 2MHz
– MCS 5
– NSS 1
– Payload 1000B
– Saturation
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0.0
0.5
1.0
1.5
2.0
2.5
3.0
4 8 16 32 64 128 256
Throughut(Mbps)
Number of groups of equal size
256 STAs RAW 512 STAs RAW 1024 STAs RAW
256 STAs DCF 512 STAs DCF 1024 STAs DCF
IEEE 802.11ah: sub 1GHz WLAN for I0T
21. IEEE 802.11ah: MAC (8)
Need to support thousands of associated devices
o Thousands of STAs huge collision probability!
o Restricted Access Window (RAW): triggering frame
RAW and resource allocation (an example)
• RAW 1 reserved for triggering frames (e.g. PS-Poll for STAs with
pending UL or DL frames)
• AP’s scheduling algorithm distributes resources among STAs
• AP starts RAW 2 with Resource Allocation frame (contains
scheduling information)
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Beacon
Beacon Interval
RAW1
A P AP
* P: PS-Poll/Trigger frame, D: DATA, A: ACK, R: Resource Allocation
RAW2
AD AD
Slot duration = Ts1 Slot duration = Ts2 != Ts1
Beacon
Normal contention
R
IEEE 802.11ah: sub 1GHz WLAN for I0T
23. IEEE 802.11ah: power saving (1)
Need to reduce power consumption (battery
powered devices)
o PS mode allows STAs to remain inactive during
max idle period after which, the STA is
disassociated.
• Legacy max idle period: 16 bits (units of 1024ms)
1.024s ·(216 – 1) > 18h
– Some use cases require days/weeks of inactivity
waste of energy sending keep-alive messages.
• IEEE 802.11ah: two first bits used as scaling factor
(1, 10, 103 or 104) 104·(214-1) > 5 years sleeping!
23IEEE 802.11ah: sub 1GHz WLAN for I0T
24. IEEE 802.11ah: power saving (2)
Need to reduce power consumption (battery
powered devices)
o Beacons carry TIM bitmap (0 or 1 for each
associated STA depending on whether that STAs
has buffered frames) Beacons are too big!!
• TIM segmentation
– Some Beacons carry bitmap at page/block level
» Rest of the Beacons carry a partial bitmap at STA level
– A STA calculates the moment when the Beacon with its
corresponding TIM is going to be sent
» Sleep until then!
24IEEE 802.11ah: sub 1GHz WLAN for I0T
25. IEEE 802.11ah: power saving (3)
Need to reduce power consumption (battery
powered devices)
o Beacons carry TIM bitmap even receiving and
decoding Beacons consumes energy!!
• Target Wake Time (TWT): intended for STAs rarely
transmitting/receiving data (i.e. TWT STAs)
– TWT STA and AP negotiate when, for how long and how
frequently the TWT STA will be awake.
– AP STA frame exchanges occur only during those
TWT service periods.
• Recall that Beacons are used to distribute AP’s timer
reference for synchronization purposes
– Missing beacons other synchronization mechanisms
are needed for TWT STAs
25IEEE 802.11ah: sub 1GHz WLAN for I0T
27. IEEE 802.11ah: other features (1)
Multihop relay operation
o Extend (root) AP coverage
o STAs will require lower tx power
o STAs may use faster MCS (less tx time)
27
from IEEE P802.11ah™/D5.0
IEEE 802.11ah: sub 1GHz WLAN for I0T
28. IEEE 802.11ah: other features (2)
Fast association and authentication
o When AP (re)boots thousands of STAs
simultaneously requesting association/authentication
collapse channel access!!
• Centralized approach
– STAs choose a number [0, 1023] at random
– AP sets an Authentication Control Threshold (announced
in Beacons)
– STAs with random number < threshold are allowed to
attempt authentication (otherwise, wait for next Beacon)
• Distributed approach
– STAs wait a random time (e.g. several Beacon intervals)
before attempting authentication
– Each unsuccessful attempt increases window
28IEEE 802.11ah: sub 1GHz WLAN for I0T
29. IEEE 802.11ah: other features (3)
Subchannel selective transmission (SST)
o STAs with limited capabilities (e.g. sensor nodes)
may support only 1 and 2MHz (mandatory)
• APs are likely to support wider bandwidth
o SST APs allow the use of subchannels within a wider
bandwidth
• AP announces in Beacons which subchannels are
temporarily available for SST
– Beacons are duplicated on a set of different subchannels
• STAs choose the best subchannel (e.g. less affected by
fading)
29IEEE 802.11ah: sub 1GHz WLAN for I0T
30. IEEE 802.11ah: summary
Lower frequency
band
Longer OFDM
symbols
Robust modulation
and coding
schemes
30
Support for >8000
nodes
Grouping
RAW access
Reduced frame
formats
Efficient frame
exchanges
Enhanced power
saving
mechanisms
LONG RANGE SCALABILITY EFFICIENCY
IEEE 802.11ah: sub 1GHz WLAN for I0T
31. Special Thanks to:
M.Shahwaiz Afaqui
Víctor H. Baños
EETAC - UPC
Master's degree in Applied Telecommunications
and Engineering Management
IoT & Ubiquitous IP
Course offered at:
Notes de l'éditeur
In-order to achieve a higher bandwidth, 802.11ah maintains the same channel bonding method as in 802.11n and 802.11ac i.e several adjacent narrow channels are bonded together to yield a wider channel. As a result , 2 Mhz channel is composed of two adjacent 1 MHz channels.
US: 26 1MHz channel, 13 2Mhz channels, 6 4MHz channels, 3 8MHz channels and 1 16MHz channel
No wastage of spectrum at the edges
EU: 5 1MHz channels, 2 2MHz channels (600Khz, 868-868.6 as guard interval)
Limited spectrum makes 1 MHz channels necessary
Japan: Channelization starts at 916.5 and ends at 927.5. Channelization starts with 0.5 Mhz off-set because the japanese spectrum regulation specify center frequencies instead of start/stop bands. 11 1 MHz channels
Max BW limit in Japan makes 1 MHz necessary Channelization much smaller than 1 MHz would encourage modes which are difficult to design as interoperable modes with higher BW modes
China: 24 1MHz channels(755MHz to 779Mhz) + 8 1MHz channels (779MHz to 787 MHz) , 4 2MHz channels (779MHz to 787 MHz), 2 4MHz channels (779MHz to 787 MHz), 1 8 MHz channels (779MHz to 787 MHz)
Two possible options (contingent on regulatory developments
Ability to use 779~787MHz (TV Bands) Expansion of channel width to1 or 2 MHz in the future in some of the other bands
South korea: Starts from 917.5 MHz and ends at 923.5MHz. 0.5MHz offset is to reduce the possible mutual interference with wireless legacy systems at lower frequencies. 6 1MHz channels, 3 2MHz channels, 1 4MHz channel
x10 downclocking and keeping number of subcarriers = 11ac_bandwidth/10 (20 2MHz)
Increases robustness in front of multipath propagation in long outdoor links
If frame between STAs: three addresses needed anyway
AID: unique value assigned to a STA during association handshake
Long Response: The addressed recipient may return a response frame which is not an individual control response frame.
AID: unique value assigned to a STA during association handshake
TIM Traffic Indication Map (bitmap used to announce which STAs have pending frames so as to wake them up)