802.11ah-2017

  • 900 MHz
  • Good for IOT!
    • Lower power consumption

Physical Layer

Like 802.11ac but downclocked by 10X

  • 1/2/4/8/16 MHz channels
    • 2/4/8/16 MHz channels in place of 20/40/80/160 MHz in ac
  • New 1 MHz channel!
    • 1 MHz also allows a new MCS 10 which is MCS0 with 2x repetition
    • Allows 9 times longer reach than 2.4GHz
  • Both the 20 MHz 11ac and 2 MHz 11ah have 64 FFT size and 48 data subcarriers + 4 pilots, but 1/10th inter-carrier spacing
  • Symbols are 10x longer to allow for 10x delay spread
    • Can handle longer multi-path
  • All times (SIFS, ACKS) are 10x longer
  • Adjacent channel bonding
  • Up to 4 spatial streams (compared to 8 in ac)
  • Beamforming possible, to create sectors

802.11ah MAC

  • Large number of devices per Access Point (AP)
    • Hierarchical Association Identifier (AID)
  • Relays are used to allow connectivity outside the coverage area. Limited to 2-hops.
  • Power Savings Enhancements:
  • Allows stations to sleep and save energy.
  • AP negotiates a Target Wake Time (TWT) for individual stations
  • Speed frame exchange allows stations to exchange a sequence of frames for a TXOP.

Protocol Version

  • PV0 - same as b/g/a/n/ac
  • PV1 - optimised for IOT
    • Short header
    • Null data packets
    • Speed frame exchange
    • Improved channel access

Short MAC Header

  • Shortened by 12 to 26 bytes
  • From the original 802.11 PV0
    • Removed throughput control
    • Removed QoS
    • Removed Duration field
    • 2 byte AID instead of 6 byte AID
  • Optional 3rd address
  • Frame Control indicates what protocol version is being used
  • Sequence field indicates if 3rd/4th addresses are present

Null Data Packet (NDP)

802.11ah removes the MAC header from the RTS/CTS/ACK packets.

They are instead identified by their MCS scheme at the PHY layer.
(ACK, Block ACK, CTS, etc... all use different MCS)

Speed Frame Exchange

aka Bi-Directional Transmit (BDT)

  • The initiator can send a frame with the response indicator set to "long response"
    • The receiver can reply with data instead of an ACK within a SIFS
    • Frames are sent until there re no more frames - block ACK at the end

Types of Stations

  • High traffic - Listen to the Traffic Indication Map (TIM) in beacons, and transmit accordingly within a restricted access window (TIM stations)
    • Always awake, to monitor all beacons
  • Periodic low traffic - Negotiate transmission time, does not listen to beacons (Non-TIM stations)
  • Very low traffic - Send a poll to AP and get a transmission opportunity in response (Unscheduled stations)

Page Segmentation

  • Only wake up once every few segments

  • Every Delivery TIM (DTIM) interval - the AP announces which segments have pending data and downlink/uplink periods

  • Each station knows what segment(s) they belong to

  • They wake up every DTIM interval and find out which beacon they should listen to

  • If the map data in the beacon indicates pending data for the station, they will send a PS-Poll to get the packet

Response Indication Deferral (RID)

  • Carrier sense mechanism that is used (due to the duration field being removed from the MAC PV1)
  • RID is set after reception of the PHY header
  • Set depending on the 2-bit response indication field
    • Normal: RID = SIFS + ACK or Block ACK time
    • NDP Response: RID = SIFS + NDP Frame Time
    • No Response (broadcasts): RID = 0
    • Long Response: RID = SIFS + Longest transmission time (Speed Frame Exchange)

Association Identifier

802.11b/g/n/ac use 11-bit identifiers, 802.11ah uses 16-bit identifiers.

There are 8 pages of ~2^11 stations each.
Currently only page 0 is used

First two bits are set to 11 to distinguish the AID from the duration and others

Restricted Access Window (RAW)

Allows a set of slots to be restricted to a group of stations - to reduce contention

  • A TIM station can be allocated slots in the RAW to transmit/receive
  • A RAW schedule is transmitted at the beginning of the RAW interval
  • A station can tell the AP that it has a frame to transmit using an Uplink Data Indication (UDI) bit
    • Helps to AP to work out which stations need access in the next round

There also exists other types of RAWs

  • Periodic RAW (PRAW) - Period and duration of PRAWs announced by A for periodic stations
  • Non-TIM RAW - Protects the transmission of non-TIM stations
    • Prevents TIM stations from hogging the channel
  • Sounding RAW - Sector sounding
  • AP Power Management RAW - Used by AP to announce the time when it will be asleep itself
  • Triggering Frame RAW - Used to allow stations to send PS-Poll frames indicated their need to transmit

Target Wake Time (TWT)

Stations can notify the AP for how long they will be asleep for, so that station does not need to waste time/energy including information about that station for that duration of time.

  • Target Wake Time
  • Minimum Wake Duration
  • Wake Interval Manitssa

The AP sends an NDP to a station at its target wakeup time containing its buffer status. The station can then send a PS-Poll to get its frames.

Sectorisation

The AP can divide the space in sectors.
Each station is told which sector it belongs to.

Beacons announce which sectors can trasmit in this sector interval.
Some sector intervals may be for omni-directional transmissions.
Allows spatial reuse, and increase throughput