Block acknowledgement (BA) was initially defined in
IEEE 802.11e
IEEE 802.11e-2005 or 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of quality of service (QoS) enhancements for wireless Local area network, LAN applications through modifications to the media access control (MA ...
as an optional scheme to improve the MAC efficiency.
802.11n amendment ratified in 2009 enhances this BA mechanism then made it as mandatory to support by all 802.11n-capable devices (formally known as HT - High Throughput devices).
Instead of transmitting an individual
ACK for every
MPDU (i.e.,
frame
A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent.
Frame and FRAME may also refer to:
Physical objects
In building construction
*Framing (con ...
), multiple MPDUs can be acknowledged together using a single BA frame. Block Ack (BA) contains bitmap size of 64*16 bits. These 16 bits accounts the fragment number of the MPDUs to be acknowledged. Each bit of this bitmap represent the status (success/failure) of a MPDU.
Block acknowledgement consist of a setup and tear-down of the session phases. In the setup phase, capability information such as buffer size and BA policy are negotiated with the receiver. Once the setup phase completed, the transmitter can send frames without waiting for ACK frame. Finally the BA session is torn down with a so-called DELBA frame.
Block Ack policies
1. HT Immediate BlockAck
2. HT Delayed BlockAck
The frames used in these agreements are:
*BAR (Block Acknowledgement Request)
To request an acknowledgement to the recipient station, confirming that has received a block of frames.
*BA (Block Acknowledgement) response
BA Variants
* Normal (Basic) Block Ack (obsolete)
* Compressed Block Ack
** It is an enhanced version of BA defined in 802.11n. In compressed BA, Fragmented MSDUs cannot be transmitted and hence the bitmap size is reduced from 1024 (64*16) bits to 64 (64*1) bits.
* Multi TID Block Ack
** This scheme is applicable only with
QoS. Previously described BA variants are capable of acknowledging a single TID (traffic identifier).
The confirmation from the recipient station, stating which frames have been received, this is explicitly mentioned in a matrix (part of the BAR) call the "bit map".
See also
*
802.11 frame types
References
{{reflist
External links
Status of the project 802.11nIEEE Task Group TGn
B