The High-Throughput (HT) Immediate Block Ack was also introduced in 802.11n along-with aggregation of data packets. The combination of Immediate Block Ack and aggregation was a significant improvement on the Contention Free burst and block ack request/block ack mechanism. The earlier legacy block ack mechanism required the WLAN device to send a block ack request […]

Similar to delayed block Ack, an HT-Delayed Block Ack session provides the WLAN station to receive/transmit a Block Ack frame at a later time from the time when an A-MPDU was transmitted. HT-Delayed Block Ack is an optional feature. The HT-Delayed Block Ack support is advertised in the HT capabilities information field. Fig Courtesy – […]

The Block Ack session is negotiated per Traffic Identifier (TID). Hence, in essence if a Block Ack session is negotiated for each TID – then The Block Ack bitmap for each of the sessions need to be saved. Also, the packets that are awaiting an Ack via the Block Ack frame need to be saved […]

The 20/40 BSS Coexistence element is used by STAs to exchange information that affects 20/40 BSS coexistence. The 20/40 co-existence element is shown below. Fig Courtesy: 802.11 Standard As per the standard, the fields are defined below Information request field The Information Request field is used to indicate that a transmitting STA is requesting the recipient […]

In the legacy Block Ack Mechanism, the bitmap size was 128 bytes. This was due to the fact that fragmentation was allowed in 802.11a/b/g. A block Ack frame could ack 64 complete MPDUs and each MPDU could in theory be broken into 16 fragments. Hence, a packet could comprise of 2 octets (bytes) in the […]

An 802.11n HT station supports three modes of transmissions. Legacy 802.11 transmission Mode HT-Mixed Mode Transmission Mode HT-Green Field Mode Transmission Mode Legacy 802.11 transmission Mode The Frame Transmission is understandable by all 802.11 stations including legacy 802.11a/b/g stations. L-STF – Legacy Short Training Field L-LTF – Legacy Long Training Field L-SIG – Legacy Signal […]

With the advent of channel bonding in the 802.11n standard, a mechanism had to be developed that would allow a station operating in a 20 MHz channel to co-exist with a station operating in a 40 MHz channel. The station could be a legacy pre-802.11n device or an 802.11n device that was operating in 20 […]

An HT Station can perform a pure HT transmission (termed green-field mode) which is not understandable to legacy 802.11 stations (802.11b/g).  An HT station that intends to perform HT transmissions which will not be decipherable by a non-HT station needs to perform protection of such transmissions. The HT Operation element describes the HT protection field […]

PLCP Stands for PHY Layer Convergence Protocol. It is the lower most encapsulation of the 802.11 packet. It contains the Frame Preamble that is required by 802.11 receivers to detect whether an incoming transmission is an 802.11 Transmission or not. The PLCP also contains other information, such as length of packet, data rate and HT […]

802.11b specified two PLCP formats – long and short PLCP. The PLCP formats are provided below Fig Courtesy – 802.11 standards As can be seen from the pictorial representation for long and short 802.11b plcp – the format is different in the number of sync preamble bits that are sent out. The SFD/SIGNAL/SERVICE/LENGTH and CRC […]