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Short Block Ack

Short Block Ack

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Short Block Ack

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  1. Short Block Ack Date: 2012-07-16 Authors:

  2. Introduction • 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

  3. 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

  4. 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

  5. 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 Basic BA frame Short BA frame STF LTF1 SIG Starting Sequence Control Block Ack Bitmap SIG Block Ack ID MCS, CRC, Tail

  6. 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)

  7. Conclusions • 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)

  8. References [1] 11-12-0324-02-00ah-short-ack [2] 11-12-0643-00-00ah-short-cts

  9. Straw Poll 1 • Do you support to include in the specification framework the definition of a new short Block Ack frame with • Block ACK ID (length TBD) • Starting Sequence Control (12 bits) • Block Bitmap with length (TBD) • Other fields (TBD) STF LTF1 SIG