Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
Section1 MAC frame formats
更新于2008-07-03 21:09:03

The format of the MAC frames is specified in this clause. A STA shall be able properly to construct a subset of the frames specified in this clause for transmission and to decode a (potentially different) subset of the frames specified in this clause upon validation following reception. The particular subset of these frames that a STA constructs and decodes is determined by the functions supported by that particular STA, as specified in 7.5. All STAs shall be able to validate every received frame using the frame check sequence (FCS) and to interpret certain fields from the MAC headers of all frames.

7.1 MAC frame formats

Each frame consists of the following basic components:

a) A MAC header, which comprises frame control, duration, address, and sequence control
information, and, for QoS data frames, QoS control information;

b) A variable length frame body, which contains information specific to the frame type and subtype;

c) A FCS, which contains an IEEE 32-bit CRC.

7.1.1 Conventions

The MPDUs or frames in the MAC sublayer are described as a sequence of fields in specific order. Each figure in Clause 7 depicts the fields/subfields as they appear in the MAC frame and in the order in which they are passed to the physical layer convergence procedure (PLCP), from left to right.

In figures, all bits within fields are numbered, from 0 to k, where the length of the field is k + 1 bits. The octet boundaries within a field can be obtained by taking the bit numbers of the field modulo 8. Octets within numeric fields that are longer than a single octet are depicted in increasing order of significance, from lowest numbered bit to highest numbered bit. The octets in fields longer than a single octet are sent to the PLCP in order from the octet containing the lowest numbered bits to the octet containing the highest numbered bits.

Any field containing a CRC is an exception to this convention and is transmitted commencing with the coefficient of the highest-order term.

MAC addresses are assigned as ordered sequences of bits. The Individual/Group bit is always transferred first and is bit 0 of the first octet.

Values specified in decimal are coded in natural binary unless otherwise stated. The values in Table 7-1 are in binary, with the bit assignments shown in the table. Values in other tables are shown in decimal notation.

Reception, in references to frames or fields within frames (e.g., received Beacon frames or a received
Duration/ID field), applies to MPDUs or MAC management protocol data units (MMPDUs) indicated from the PHY layer without error and validated by FCS within the MAC sublayer. Without further qualification, reception by the MAC sublayer implies that the frame contents are valid, and that the protocol version is supported (see 7.1.3.1.1), with no implication regarding frame addressing or regarding whether the frame type or other fields in the MAC header are meaningful to the MAC entity that has received the frame.

Parentheses enclosing portions of names or acronyms are used to designate a set of related names that vary based on the inclusion of the parenthesized portion. For example,

— QoS +CF-Poll frame refers to the three QoS data subtypes that include “+CF-Poll”: the QoS
Data+CF-Poll frame, subtype 1010; QoS Data+CF-Ack+CF-Poll frame, subtype 1011; and QoS CFAck+CF-Poll frame, subtype 1111.

— QoS CF-Poll frame refers specifically to the QoS CF-Poll frame, subtype 1110.

— QoS (+)CF-Poll frame refers to all four QoS data subtypes with CF-Poll: the QoS CF-Poll frame,
subtype 1110; the QoS CF-Ack+CF-Poll frame, subtype 1111; the QoS Data+CF-Poll frame,
subtype 1010; and the QoS Data+CF-Ack+CF-Poll frame, subtype 1011.

— QoS (+)Null frame refers to all three QoS data subtypes with “no data”: the QoS Null (no data)
frame, subtype 1100; the QoS CF-Poll (no data) frame, subtype 1110; and the QoS CF-Ack+CF-Poll
frame, subtype 1111.

— QoS +CF-Ack frame refers to the three QoS data subtypes that include “+CF-Ack”: the QoS
Data+CF-Ack frame, subtype 1001; QoS Data+CF-Ack+CF-Poll frame, subtype 1011; and QoS CFAck+
CF-Poll frame, subtype 1111.

— Whereas (QoS) CF-Poll frame refers to the QoS CF-Poll frame, subtype 1110, and the CF-Poll
frame, subtype 0110.

Reserved fields and subfields are set to 0 upon transmission and are ignored upon reception. 

7.1.2 General frame format

The MAC frame format comprises a set of fields that occur in a fixed order in all frames. Figure 7-1 depicts the general MAC frame format. The first three fields (Frame Control, Duration/ID, and Address 1) and the last field (FCS) in Figure 7-1 constitute the minimal frame format and are present in all frames, including reserved types and subtypes. The fields Address 2, Address 3, Sequence Control, Address 4, QoS Control, and Frame Body are present only in certain frame types and subtypes. Each field is defined in 7.1.3. The format of each of the individual subtypes of each frame type is defined in 7.2. The components of management frame bodies are defined in 7.3. The formats of management frames of subtype Action are defined in 7.4

The Frame Body field is of variable size. The maximum frame body size is determined by the maximum MSDU size (2304 octets) plus any overhead from security encapsulation.




Figure 7-1—MAC frame format


7.1.3 Frame fields

7.1.3.1 Frame Control field

The Frame Control field consists of the following subfields: Protocol Version, Type, Subtype, To DS,
From DS, More Fragments, Retry, Power Management, More Data, Protected Frame, and Order. The format of the Frame Control field is illustrated in Figure 7-2.


7.1.3.1.1 Protocol Version field

The Protocol Version field is 2 bits in length and is invariant in size and placement across all revisions of this standard. For this standard, the value of the protocol version is 0. All other values are reserved. The revision level will be incremented only when a fundamental incompatibility exists between a new revision and the prior edition of the standard. A MAC entity that receives a frame with a higher revision level than it supports shall discard the frame without indication to the sending STA or to LLC.

7.1.3.1.2 Type and Subtype fields

The Type field is 2 bits in length, and the Subtype field is 4 bits in length. The Type and Subtype fields together identify the function of the frame. There are three frame types: control, data, and management.
Each of the frame types has several defined subtypes. In data frames, the most significant bit (MSB) of the Subtype field, b7, is defined as the QoS subfield. Table 7-1 defines the valid combinations of type and subtype. (The numeric values in Table 7-1 are shown in binary.)


Each Subtype field bit position is used to indicate a specific modification of the basic data frame (subtype 0). Frame Control bit 4 is set to 1 in data subtypes that include +CF-Ack, bit 5 is set to 1 in data subtypes that include +CF-Poll, bit 6 is set to 1 in data subtypes that contain no Frame Body field, and bit 7 is set to 1 in the QoS data subtypes, which have QoS Control fields in their MAC headers.

7.1.3.1.3 To DS and From DS fields

The meaning of the combinations of values for the To DS and From DS fields are shown in Table 7-2.


7.1.3.1.4 More Fragments field

The More Fragments field is 1 bit in length and is set to 1 in all data or management type frames that have another fragment of the current MSDU or current MMPDU to follow. It is set to 0 in all other frames.
7.1.3.1.5 Retry field

The Retry field is 1 bit in length and is set to 1 in any data or management type frame that is a retransmission of an earlier frame. It is set to 0 in all other frames. A receiving STA uses this indication to aid in the process of eliminating duplicate frames.

7.1.3.1.6 Power Management field

The Power Management field is 1 bit in length and is used to indicate the power management mode of a STA. The value of this field remains constant in each frame from a particular STA within a frame exchange sequence defined in 9.12. The value indicates the mode in which the STA will be after the successful completion of the frame exchange sequence.

A value of 1 indicates that the STA will be in PS mode. A value of 0 indicates that the STA will be in active mode. This field is always set to 0 in frames transmitted by an AP.

7.1.3.1.7 More Data field

The More Data field is 1 bit in length and is used to indicate to a STA in PS mode that more MSDUs or MMPDUs are buffered for that STA at the AP. The More Data field is valid in directed data or management type frames transmitted by an AP to a STA in PS mode. A value of 1 indicates that at least one additional buffered MSDU or MMPDU is present for the same STA.

The More Data field may be set to 1 in directed data type frames transmitted by a CF-Pollable STA to the PC in response to a CF-Poll to indicate that the STA has at least one additional buffered MSDU available for transmission in response to a subsequent CF-Poll.

The More Data field is set to 0 in all other directed frames.

The More Data field is set to 1 in broadcast/multicast frames transmitted by the AP when additional
broadcast/multicast MSDUs or MMPDUs remain to be transmitted by the AP during this beacon interval. The More Data field is set to 0 in broadcast/multicast frames transmitted by the AP when no more broadcast/ multicast MSDUs or MMPDUs remain to be transmitted by the AP during this beacon interval and in all broadcast/multicast frames transmitted by non-AP STAs.

For a non-AP STA that has the More Data Ack subfield set in its QoS Capability information element and also has APSD enabled, an AP may set the More Data field to 1 in ACK frames to this non-AP STA to indicate that the AP has a pending transmission for the non-AP STA.

7.1.3.1.8 Protected Frame field

The Protected Frame field is 1 bit in length. The Protected Frame field is set to 1 if the Frame Body field contains information that has been processed by a cryptographic encapsulation algorithm. The Protected Frame field is set to 1 only within data frames and within management frames of subtype Authentication. The Protected Frame field is set to 0 in all other frames. When the Protected Frame field is set to 1, the Frame Body field is protected utilizing the cryptographic encapsulation algorithm and expanded as defined in Clause 8. The Protected Frame field is set to 0 in Data frames of subtype Null Function, CF-ACK (no data), CF-Poll (no data), and CF-ACK+CF-Poll (no data) (see 8.3.2.2 and 8.3.3.1, which show that the frame body must be 1 octet or longer to apply the encapsulation).

7.1.3.1.9 Order field

The Order field is 1 bit in length and is set to 1 in any non-QoS data frame that contains an MSDU, or fragment thereof, which is being transferred using the StrictlyOrdered service class. This field is set to 0 in all other frames. All QoS STAs set this subfield to 0.

7.1.3.2 Duration/ID field

The Duration/ID field is 16 bits in length. The contents of this field vary with frame type and subtype, with whether the frame is transmitted during the CFP, and with the QoS capabilities of the sending STA. The contents of the field are defined as follows:

a) In control frames of subtype PS-Poll, the Duration/ID field carries the association identifier (AID) of
the STA that transmitted the frame in the 14 least significant bits (LSB), and the 2 most significant
bits (MSB) both set to 1. The value of the AID is in the range 1–2007.

b) In frames transmitted by the PC and non-QoS STAs, during the CFP, the Duration/ID field is set to a fixed value of 32 768.

c) In all other frames sent by non-QoS STAs and control frames sent by QoS STAs, the Duration/ID
field contains a duration value as defined for each frame type in 7.2.

d) In data and management frames sent by QoS STAs, the Duration/ID field contains a duration value as defined for each frame type in 7.1.4.

When the contents of a received Duration/ID field, treated as an unsigned integer and without regard for address values, type, and subtype (even when type or subtype contain reserved values), are less than 32 768, the duration value is used to update the network allocation vector (NAV) according to the procedures defined in 9.2.5.4 or 9.9.2.2.1, as appropriate.

When the contents of a received Duration/ID field, treated as an unsigned integer, are greater than 32 768, the contents are interpreted as appropriate for the frame type and subtype or ignored if the receiving MAC entity does not have a defined interpretation for that type and subtype.

The encoding of the Duration/ID field is given in Table 7-3.


7.1.3.3 Address fields

There are four address fields in the MAC frame format. These fields are used to indicate the basic service set identification (BSSID), source address (SA), destination address (DA), transmitting STA address (TA), and receiving STA address (RA). Certain frames may not contain some of the address fields.

Certain address field usage is specified by the relative position of the address field (1–4) within the MAC header, independent of the type of address present in that field. For example, receiver address matching is always performed on the contents of the Address 1 field in received frames, and the receiver address of CTS and ACK frames is always obtained from the Address 2 field in the corresponding RTS frame, or from the frame being acknowledged.

7.1.3.3.1 Address representation

Each Address field contains a 48-bit address as defined in 5.2 of IEEE Std 802-1990.

7.1.3.3.2 Address designation

A MAC sublayer address is one of the following two types:

a) Individual address. The address assigned to a particular STA on the network.

b) Group address. A multidestination address, which may be in use by one or more STAs on a given
network. The two kinds of group addresses are as follows:

1) Multicast-group address. An address associated by higher level convention with a group of
logically related STAs.

2) Broadcast address. A distinguished, predefined multicast address that always denotes the set of
all STAs on a given LAN. All ones are interpreted to be the broadcast address. This group is
predefined for each communication medium to consist of all STAs actively connected to that
medium; it is used to broadcast to all the active STAs on that medium.

The address space is also partitioned into locally administered and universal (globally administered)
addresses. The nature of a body and the procedures by which it administers these universal (globally administered) addresses is beyond the scope of this standard. See IEEE Std 802-1990 for more information.

7.1.3.3.3 BSSID field

The BSSID field is a 48-bit field of the same format as an IEEE 802 MAC address. This field uniquely
identifies each BSS. The value of this field, in an infrastructure BSS, is the MAC address currently in use by the STA in the AP of the BSS.

The value of this field in an IBSS is a locally administered IEEE MAC address formed from a 46-bit random number generated according to the procedure defined in 11.1.3. The individual/group bit of the address is set to 0. The universal/local bit of the address is set to 1. This mechanism is used to provide a high probability of selecting a unique BSSID.

The value of all 1s is used to indicate the wildcard BSSID. A wildcard BSSID shall not be used in the
BSSID field except for management frames of subtype probe request.

7.1.3.3.4 DA field

The DA field contains an IEEE MAC individual or group address that identifies the MAC entity or entities  intended as the final recipient(s) of the MSDU (or fragment thereof) contained in the frame body field.

7.1.3.3.5 SA field

The SA field contains an IEEE MAC individual address that identifies the MAC entity from which the
transfer of the MSDU (or fragment thereof) contained in the frame body field was initiated. The individual/ group bit is always transmitted as a zero in the source address.

7.1.3.3.6 RA field

The RA field contains an IEEE MAC individual or group address that identifies the intended immediate
recipient STA(s), on the WM, for the information contained in the frame body field.

7.1.3.3.7 TA field

The TA field contains an IEEE MAC individual address that identifies the STA that has transmitted, onto the WM, the MPDU contained in the frame body field. The Individual/Group bit is always transmitted as a zero in the transmitter address.

7.1.3.4 Sequence Control field

The Sequence Control field is 16 bits in length and consists of two subfields, the Sequence Number and the Fragment Number. The format of the Sequence Control field is illustrated in Figure 7-3. Sequence Control field is not present in control frames.


7.1.3.4.1 Sequence Number field

The Sequence Number field is a 12-bit field indicating the sequence number of an MSDU or MMPDU. Each MSDU or MMPDU transmitted by a STA is assigned a sequence number. Sequence numbers are not assigned to control frames, as the Sequence Control field is not present.

Non-QoS STAs, as well as QoS STAs operating as non-QoS STAs because they are in a non-QoS BSS or non-QoS IBSS, assign sequence numbers, to management frames and data frames (QoS subfield of the Subtype field is set to 0), from a single modulo-4096 counter, starting at 0 and incrementing by 1 for each MSDU or MMPDU.

QoS STAs associated in a QoS BSS maintain one modulo-4096 counter, per TID, per unique receiver
(specified by the Address 1 field of the MAC header). Sequence numbers for QoS data frames are assigned using the counter identified by the TID subfield of the QoS Control field of the frame, and that counter is incremented by 1 for each MSDU belonging to that TID. Sequence numbers for management frames, QoS data frames with a broadcast/multicast address in the Address 1 field, and all non-QoS data frames sent by QoS STAs are assigned using an additional single modulo-4096 counter, starting at 0 and incrementing by 1 for each MSDU or MMPDU. Sequence numbers for QoS (+)Null frames may be set to any value.

Each fragment of an MSDU or MMPDU contains a copy of the sequence number assigned to that MSDU or MMPDU. The sequence number remains constant in all retransmissions of an MSDU, MMPDU, or fragment thereof.

7.1.3.4.2 Fragment Number field

The Fragment Number field is a 4-bit field indicating the number of each fragment of an MSDU or
MMPDU. The fragment number is set to 0 in the first or only fragment of an MSDU or MMPDU and is
incremented by one for each successive fragment of that MSDU or MMPDU. The fragment number remains constant in all retransmissions of the fragment.

7.1.3.5 QoS Control field

The QoS Control field is a 16-bit field that identifies the TC or TS to which the frame belongs and various other QoS-related information about the frame that varies by frame type and subtype. The QoS Control field is present in all data frames in which the QoS subfield of the Subtype field is set to 1 (see 7.1.3.1.2). Each QoS Control field comprises five subfields, as defined for the particular sender (HC or non-AP STA) and frame type and subtype. The usage of these subfields and the various possible layouts of the QoS Control field are described 7.1.3.5.1 through 7.1.3.5.7 and illustrated in Table 7-4.


7.1.3.5.1 TID subfield

The TID subfield identifies the TC or TS to which the corresponding MSDU, or fragment there of, in the Frame Body field belongs. The TID subfield also identifies the TC or TS of traffic for which a TXOP is being requested, through the setting of TXOP duration requested or queue size. The encoding of the TID subfield depends on the access policy (see 7.3.2.30) and is shown in Table 7-5. Additional information on the interpretation of the contents of this field appears in 6.1.1.2 .


For QoS Data+CF-Poll, the TID subfield in the QoS Control field indicates the TID of the data. For all QoS (+)CF-Poll frames of subtype Null, the TID subfield in the QoS Control field indicates the TID for which the poll is intended. The requirement to respond to that TID is nonbinding, and a STA may respond with any frame.

7.1.3.5.2 EOSP (end of service period) subfield

The EOSP subfield is 1 bit in length and is used by the HC to indicate the end of the current service period (SP). The HC sets the EOSP subfield to 1 in its transmission and retransmissions of the SP’s final frame to end a scheduled/unscheduled SP and sets it to 0 otherwise.

7.1.3.5.3 Ack Policy subfield

The Ack Policy subfield is 2 bits in length and identifies the acknowledgment policy that is followed upon the delivery of the MPDU. The interpretation of these 2 bits is given in Table 7-6.


Table 7-6—Ack Policy subfield in QoS Control field of QoS data frames




An MSDU is sent using an acknowledgment policy of Normal Ack or Block Ack if the service class
parameter in MA-UNITDATA.request primitive is set to QoSAck and of No Ack if the service class
parameter in MA-UNITDATA.request primitive is set to QoSNoAck.

7.1.3.5.4 TXOP Limit subfield

The TXOP Limit subfield is an 8-bit field that is present in QoS data frames of subtypes that include CF-Poll and specifies the time limit on a TXOP granted by a QoS (+)CF-Poll frame from an HC in a BSS. In QoS data frames with subtypes that include CF-Poll, the addressed STA is granted a TXOP that begins a SIFS

period after this frame and lasts no longer than the number of 32 μs periods specified by the TXOP limit value. The range of time values is 32 μs to 8160 μs. A TXOP limit value of 0 implies that one MPDU or one QoS Null frame is to be transmitted immediately following the QoS (+)CF-Poll frame. The TXOP limit is inclusive of the PHY and IFS overhead, and an AP should account for the overhead when granting TXOPs.

7.1.3.5.5 Queue Size subfield

The Queue Size subfield is an 8-bit field that indicates the amount of buffered traffic for a given TC or TS at the non-AP STA sending this frame. The Queue Size subfield is present in QoS data frames sent by STAs associated in a BSS with bit 4 of the QoS Control field set to 1. The AP may use information contained in the Queue Size subfield to determine the TXOP duration assigned to non-AP STA.

The queue size value is the total size, rounded up to the nearest multiple of 256 octets and expressed in units of 256 octets, of all MSDUs buffered at the STA (excluding the MSDU of the present QoS data frame) in the delivery queue used for MSDUs with TID values equal to the value in the TID subfield of this QoS Control field. A queue size value of 0 is used solely to indicate the absence of any buffered traffic in the queue used for the specified TID. A queue size value of 254 is used for all sizes greater than 64 768 octets. A queue size value of 255 is used to indicate an unspecified or unknown size. If a QoS data frame is fragmented, the queue size value may remain constant in all fragments even if the amount of queued traffic changes as successive fragments are transmitted.

7.1.3.5.6 TXOP Duration Requested subfield

The TXOP Duration Requested subfield is an 8-bit field that indicates the duration, in units of 32 μs, that the sending STA desires for its next TXOP for the specified TID. The range of time values is 32 μs to 8160 μs. If the calculated TXOP duration requested is not a factor of 32 μs, that value is rounded up to the next higher integer that is a factor of 32 μs. The TXOP Duration Requested subfield is present in QoS data frames sent by non-AP STAs associated in a BSS with bit 4 of the QoS Control field set to 0. The AP may choose to assign a TXOP duration shorter than that requested in the TXOP Duration Requested subfield. A value of 0 in the TXOP Duration Requested subfield indicates that no TXOP is requested for the MSDUs for the specified TID in the current SP.

TXOP Duration Requested subfield values are not cumulative. A TXOP duration requested for a particular TID supersedes any prior TXOP duration requested for that TID. A value of 0 in the TXOP Duration Requested subfield may be used to cancel a pending unsatisfied TXOP request when its MSDU is no longer queued for transmission. The TXOP duration requested is inclusive of the PHY and IFS overhead, and a STA should account for this when attempting to determine whether a given transmission fits within a specified TXOP duration.

7.1.3.5.7 AP PS Buffer State subfield

The AP PS Buffer State subfield, defined in Figure 7-4, is an 8-bit field that indicates the PS buffer state at the AP for a non-AP STA. The AP PS Buffer State subfield is further subdivided into three subfields: Buffer State Indicated, Highest-Priority Buffered AC, and AP Buffered Load.


Figure 7-4—QoS AP PS Buffer State subfield

The Buffered State Indicated subfield is 1 bit in length and is used to indicate whether the AP PS buffer state is specified. A value of 1 indicates that the AP PS buffer state is specified.

The Highest-Priority Buffered AC subfield is 2 bits in length and is used to indicate the AC of the highest priority traffic remaining that is buffered at the AP, excluding the MSDU of the present frame.

The AP Buffered Load subfield is 4 bits in length and is used to indicate the total buffer size, rounded up to the nearest multiple of 4096 octets and expressed in units of 4096 octets, of all MSDUs buffered at the QoS AP (excluding the MSDU of the present QoS data frame). An AP Buffered Load field value of 15 indicates that the buffer size is greater than 57 344 octets. An AP Buffered Load subfield value of 0 is used solely to indicate the absence of any buffered traffic for the indicated highest priority buffered AC when the Buffer State Indicated bit is 1.

When the Buffered State Indicated subfield is set to 0, the Highest-Priority Buffered AC subfield and the AP Buffered Load subfield are reserved; and the values of these subfields are either unspecified or unknown.

7.1.3.6 Frame Body field

The Frame Body is a variable length field that contains information specific to individual frame types and subtypes. The minimum frame body is 0 octets. The maximum length frame body is defined by the maximum length (MSDU + ICV + IV), where integrity check value (ICV) and initialization vector (IV) are the WEP fields defined in 8.2.1.

7.1.3.7 FCS field

The FCS field is a 32-bit field containing a 32-bit CRC. The FCS is calculated over all the fields of the MAC header and the Frame Body field. These are referred to as the calculation fields.

The FCS is calculated using the following standard generator polynomial of degree 32:

G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

The FCS is the ones complement of the sum (modulo 2) of the following:

a) The remainder of xk × (x31 + x30 + x29 + …+ x2 + x + 1) divided (modulo 2) by G(x), where k is the number of bits in the calculation fields, and

b) The remainder after multiplication of the contents (treated as a polynomial) of the calculation fields by x32 and then division by G(x).

The FCS field is transmitted commencing with the coefficient of the highest-order term.

As a typical implementation, at the transmitter, the initial remainder of the division is preset to all ones and is then modified by division of the calculation fields by the generator polynomial G(x). The ones complement of this remainder is transmitted, with the highest-order bit first, as the FCS field.

At the receiver, the initial remainder is preset to all ones and the serial incoming bits of the calculation fields and FCS, when divided by G(x), results in the absence of transmission errors, in a unique nonzero remainder value. The unique remainder value is the polynomial:

x31 + x30 + x26 + x25 + x24 + x18 + x15 + x14 + x12 + x11 + x10 + x8 + x6 + x5 + x4 + x3 + x + 1

7.1.4 Duration/ID field in data and management frames

Within all data frames containing QoS CF-Poll, the Duration/ID field  value is set to one of the following:
— One SIFS duration plus the TXOP limit, if the TXOP limit is nonzero, or

— The time required for the transmission of one MPDU of nominal MSDU size and the associated
ACK frame plus two SIFS intervals, if the TXOP limit is zero.

Within all data or management frames sent in a CP by the QoS STAs outside of a controlled access phase (CAP), following a contention access of the channel, the Duration/ID field is set to one of the following values:

a) For management frames, frames with QoS Data subfield set to 0, and unicast data frames with Ack Policy subfield set to Normal Ack,

1) The time required for the transmission of one ACK frame (including appropriate IFS values), if the frame is the final fragment of the TXOP, or

2) The time required for the transmission of one ACK frame plus the time required for the transmission of the following MPDU and its response if required (including appropriate IFS
values).

b) For unicast data frames with the Ack Policy subfield set to No Ack or Block Ack and for multicast/
broadcast frames,

1) Zero, if the frame is the final fragment of the TXOP, or

2) The time required for the transmission of the following MPDU and its response frame, if required (including appropriate IFS values).

c) The minimum of

1) The time required for the transmission the pending MPDUs of the AC and the associated ACKs, if any, and applicable SIFS durations, and

2) The time limit imposed by the MIB attribute dot11EDCATableTXOPLimit (dot11EDCAQAP- TableTXOPLimit for the AP) for that AC minus the already used time within the TXOP.

Within all data or management frames sent under HCCA, to ensure NAV protection for the entire CAP, the Duration/ID field is set to one of the following values:

— The remaining duration of the TXOP, if the frame is a nonfinal frame in a TXOP with multiple
frame exchanges.
— The actual remaining time needed for this frame exchange sequence, if the frame is the sole or final frame in the TXOP.

 

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