bit to ONE) or not (bit to ZERO); - TID_INFO: this four-bit coded subfield provides the value of the TID (Traffic Identifier) field, for which a block acknowledgment is required. The BAR or BA Information field contains the value of the sequence number of the first transmitted MAC frame. If the block acknowledgment applies to several priority levels, the sequence number is indicated for each priority level. The BlockAck control frame is transmitted by the recipient of the MAC frames for block acknowledgment. Each bit in the bitmap of the Information field acknowledges (bit to ONE) or not (bit to ZERO) the frame that has this offset from the initial sequence number. 4.2. PLCP sub-layer The PLCP (Physical Layer Convergence Procedure) sub-layer supports the following three modes: - NON_HT mode: the PLCP header is identical to that defined for the 802.11a/g interfaces (Figure 4.6). NON_HT mode support is required; - HT_MF (Mixed Format) mode: the PLCP header contains a preamble compatible with that defined for the 802.11a/g interfaces SO that it can be processed by mobiles that do not handle the 802.11n interface (Figure 4.6). HT_MF mode support is required; 802.11n Interface - HT_GF (Greenfield) mode: the PLCP header does not contain fields compatible with the 802.11a/g interfaces (Figure 4.6). Support for HT_GF format is optional. A mobile that does not support the HT_GF mode must be able to detect that a transmission of an HT_GF frame is in progress. SERVICE NON_HT L-STF L-LTF L-SIG HT_MF L-STF L-LTF L-SIG HT-SIG HT-STF HT-LTF HT-LTF HT-LTF HT-LTF HT LTF DATA HT LTF Extension HT_GF HT-GF-STF HT-LTF1 HT-SIG HT-LTF HT-LTF HT-LTF HT-LTF HT LTF DATA HT LTF Extension Figure 4.6. PLCP frame structure L-STF (Non-HT Short Training Field): this field is identical to the short training sequence of the 802.11a/g interfaces. L-LTF (Non-HT Long Training Field): this field is identical to the long training sequence of the 802.11a/gi interfaces. L-SIG (Non-HT Signal): this field is identical to the SIGNAL field of the 802.11a/g 