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Short Block Ack. Date: 2012-07-16. Authors:. Introduction. 11ah STAs may have limited capabilities L ow TX power, low transmission rates, limited buffers capacity Sleep most of the time and wake up to TX/RX limited bursts of MPDUs

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short block ack
Short Block Ack

Date: 2012-07-16


  • 11ah STAs may have limited capabilities
    • Low TX power, low transmission rates, limited buffers capacity
    • Sleep most of the time and wake up to TX/RX limited bursts of MPDUs
  • Power imbalance between heterogeneous TX/RX STAs implies issues with MCS selection in response frames
  • Optimized energy saving requires optimized frame formats
  • Short frames were introduced, which improve MAC efficiency and simplify control response MCS selection [1, 2]
    • Short CTS and short Ackdefined as a PHY preamble only
  • An important control response frame is missing among them: the Block Ack
block acknowledgement
Block Acknowledgement
  • BA is useful in 11ah
    • Aggregation is possible/likely at 1 and >=2MHz rates: BA enables it
      • (~8 MPDUs@4Mbps and ~16 MPDUs@8Mbps, 1KB MPDU, TXOP 20ms)
    • BA allows to reduce the number of transmissions
      • E.g. a sensor sending a single BA to acknowledge multiple PPDUs, instead of sending multiple ACKs
  • However, the benefits may be significantly reduced when used in 802.11ah
    • Increased TX times for sending BA frames
      • Uncompressed Block Ack (TX time of~8.68ms@1MHzRep2)
      • Compressed Block Ack (TX time of ~2.28ms@1MHzRep2)
  • The proposed short Block reduces the TX time and energy consumption
    • Block ACK information stored in SIG field of PHY Preamble
      • Same approach as for short Ack frames [1]
    • TX time of 560us @ 1MHz, 240us @ >=2Mhz
block ack mcs selection rules
Block Ack MCS selection rules
  • Block Ack is a control response frame
    • Must obey rate selection rules imposed to control response frames
      • Must be sent with the highest MCS in the Basic Rate Set which is lower or equal to the MCS of the soliciting frame
    • Likely power imbalance between AP and STA causes problems when this rules applies
      • BA frames may not be received by the AP due to high MCS selected by above rule and low transmit power
  • Using short control response frames eliminates the need for rate selection rules
    • NDP frames consist only of the PHY preamble
    • ACK[1], CTS[2] already accepted
  • Short BA completes the set of short control response frames, eliminating the issue with MCS selection rules
short block ack1
Short Block ACK
  • Reload bits in SIG Field in the PHY Header to store Block Ack info
    • Short Block Ack indicated using one reserved value of MCS field (e.g., 11)
    • 22 reloadable bits for 1 MHz PHY Header format
    • 34 reloadable bits for 2 MHz PHY Header format
  • Use available bits in the SIG field for
    • Block ACK ID
    • Starting Sequence Control
    • Block Ack Bitmap


BA frame


BA frame




Starting Sequence Control

Block Ack Bitmap


Block Ack ID

MCS, CRC, Tail

short block ack details
Short Block Ack Details
  • No modifications required to the Block Ack protocol
    • Buffer size is negotiated during Block Ack session initiation
      • Buffer size specified by the Responder in the ADDBA Response is binding
  • Short Block Ack to be used for immediate Block Ack
    • Delayed block Ack is generally not used anyways
  • Short Block ACK frame includes
    • Block Ack ID (TBD)
    • Starting Sequence Control (12 bits)
      • Include Starting Sequence Number only
    • Block Bitmap (8/16 bits)
      • 8 bits for 1 MHz and 16 bits for 2MHz PHY Preamble
    • Other fields (TBD)
  • We propose a new short frame for Block Ack, using NDP format
    • Use a reserved MCS value to identify this frame type
    • Includes 8 or 16-bit bitmap (for 1 or 2 MHz)
    • No change to current BA protocol, only frame format change
  • Benefits
    • Resolve issue with MCS selection in case of UL/DL link imbalance
    • Reduce BA time from ~2.8ms to 560us at 1MHz rep2
  • Further developments
    • Same frame format could be used also for fragments acknowledgment (TBD)

[1] 11-12-0324-02-00ah-short-ack

[2] 11-12-0643-00-00ah-short-cts

straw poll 1
Straw Poll 1
  • Do you support to include in the specification framework the definition of a new short Block Ack frame


    • Block ACK ID (length TBD)
    • Starting Sequence Control (12 bits)
    • Block Bitmap with length (TBD)
    • Other fields (TBD)