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IEEE 802.11ah
Eduard Garcia-Villegas, Elena López-Aguilera
Dept. of Network Engineering
eduardg@entel.upc.edu
elopez@entel...
Contents
 IEEE 802.11ah: sub 1GHz WLAN for IoT
o Purpose, scope and use cases
o PHY
o MAC
o Power saving
o Other remarkab...
Purpose, scope and use cases
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
IEEE 802.11ah: purpose
 Defines operation of license-exempt (ISM)
IEEE 802.11 wireless networks in frequency
bands below ...
IEEE 802.11ah: scope
 Defines an OFDM PHY operating in the license-exempt
bands below 1 GHz
o and enhancements to the IEE...
IEEE 802.11ah: use cases
 Use Case 1 : Sensors and meters
o Smart Grid - meter to pole
o Environmental monitoring
o Indus...
The PHY
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
IEEE 802.11ah: PHY (1)
 Advantages of transmitting in sub 1 GHz:
o Spectrum characteristics
• good propagation and penetr...
IEEE 802.11ah: PHY (2)
 Channelization:
o Each regulatory domain
defines a different band and
different tx power limits
9...
IEEE 802.11ah: PHY (3)
 Inherited from IEEE 802.11ac (adapted to S1G):
o OFDM
• 10 times down-clocking .11ac
– symbol dur...
IEEE 802.11ah: PHY (4)
 Expected throughput vs. coverage
11
1MHz
2MHz
4MHz
8MHz
16MHz
Mandatory
for STAs
(Globally
intero...
IEEE 802.11ah: PHY (5)
 Expected throughput vs. coverage (min and max)
12
Additional step
thanks to MCS10
(only available...
The MAC
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
IEEE 802.11ah: MAC (1)
 Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Short MAC he...
IEEE 802.11ah: MAC (2)
 Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o NULL Data Pa...
IEEE 802.11ah: MAC (3)
 Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Implicit ack...
IEEE 802.11ah: MAC (4)
 Need to reduce overhead: low data rates + short
frames (typical in some use cases)
o Implicit ack...
IEEE 802.11ah: MAC (5)
 Need to support thousands of associated devices
(increases coverage  increases reachable STAs)
o...
IEEE 802.11ah: MAC (6)
 Need to support thousands of associated devices
o Thousands of STAs  huge collision probability!...
IEEE 802.11ah: MAC (7)
 Need to support thousands of associated devices
o Thousands of STAs  huge collision probability!...
IEEE 802.11ah: MAC (8)
 Need to support thousands of associated devices
o Thousands of STAs  huge collision probability!...
Power saving
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
IEEE 802.11ah: power saving (1)
 Need to reduce power consumption (battery
powered devices)
o PS mode allows STAs to rema...
IEEE 802.11ah: power saving (2)
 Need to reduce power consumption (battery
powered devices)
o Beacons carry TIM bitmap (0...
IEEE 802.11ah: power saving (3)
 Need to reduce power consumption (battery
powered devices)
o Beacons carry TIM bitmap  ...
Other remarkable features
IEEE 802.11ah
sub 1GHz WLAN for IoT
What lies beneath Wi-Fi HaLow
IEEE 802.11ah: other features (1)
 Multihop relay operation
o Extend (root) AP coverage
o STAs will require lower tx powe...
IEEE 802.11ah: other features (2)
 Fast association and authentication
o When AP (re)boots  thousands of STAs
simultaneo...
IEEE 802.11ah: other features (3)
 Subchannel selective transmission (SST)
o STAs with limited capabilities (e.g. sensor ...
IEEE 802.11ah: summary
Lower frequency
band
Longer OFDM
symbols
Robust modulation
and coding
schemes
30
Support for >8000
...
Special Thanks to:
M.Shahwaiz Afaqui
Víctor H. Baños
EETAC - UPC
Master's degree in Applied Telecommunications
and Enginee...
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IEEE 802.11ah: what lies beneath Wi-Fi HaLow

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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 - ).

Published in: Technology

IEEE 802.11ah: what lies beneath Wi-Fi HaLow

  1. 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. 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 2 by wilgengebroed
  3. 3. Purpose, scope and use cases IEEE 802.11ah sub 1GHz WLAN for IoT What lies beneath Wi-Fi HaLow
  4. 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. 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. 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
  7. 7. The PHY IEEE 802.11ah sub 1GHz WLAN for IoT What lies beneath Wi-Fi HaLow
  8. 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. 9. IEEE 802.11ah: PHY (2)  Channelization: o Each regulatory domain defines a different band and different tx power limits 9 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. 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. 11. IEEE 802.11ah: PHY (4)  Expected throughput vs. coverage 11 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. 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
  13. 13. The MAC IEEE 802.11ah sub 1GHz WLAN for IoT What lies beneath Wi-Fi HaLow
  14. 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. 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. 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. 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) 17 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. 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. 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. 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 20 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. 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) 21 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
  22. 22. Power saving IEEE 802.11ah sub 1GHz WLAN for IoT What lies beneath Wi-Fi HaLow
  23. 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. 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. 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
  26. 26. Other remarkable features IEEE 802.11ah sub 1GHz WLAN for IoT What lies beneath Wi-Fi HaLow
  27. 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. 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. 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. 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. 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:

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