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IEEE 802.11 defines a data frame exchange process that enables the stations and APs, to negotiate the timing of the exchange of data between devices over the various shared channels in the 2.4 and 5 GHz bands. In WLAN systems using the IEEE 802.11 standards, frames exchanged between stations (including APs) are classified into management frames, control frames, and data frames. The management frame is a frame used for exchanging management information that is not forwarded to higher layers of a communication protocol stack. The control frame is a frame used for controlling access to the transmission medium. The data frame is a frame used for transmitting data that will be forwarded to higher layers of the communication protocol stack [2]. Each frame's type and subtype are identified using a type field and a subtype field included in a control field of the frame, as described in the applicable standard.

Traditionally, at the frame level, Wi‐Fi WNs use Request to Send/Clear to Send (RTS/CTS) to take advantage of the shared medium. The AP only issues a CTS packet to one WN at a time. When the WN receives the CTS, it sends its entire frame to the AP; the WN then waits for an acknowledgment (ACK) frame from the AP indicating that it received the packet correctly; if the WN does not get the ACK in a specified amount of time, it postulates that the packet in fact collided with some other WN transmission – at that point the WN transitions into a period of binary exponential backoff; it will try to access the medium and retransmit its packet after the backoff time expires.

Clearly, data are transmitted using MAC framing and channel management mechanisms along with PHY resources. As alluded to earlier, at the MAC layer, the following frames are utilized:

 A data frame is used for the transmission of data forwarded to a higher protocol layer. The WLAN device transmits the data frame after performing backoff if a Distributed Coordination Function (DCF) Inter‐Frame Space (IFS) (known as DIFS) interval has elapsed, during which such DIFS interval, the medium has been idle.

 A management frame is used for exchanging management information that is not forwarded to a higher protocol layer. Subtype frames of the management frame include a beacon frame, an association request/response frame, a probe request/response frame, and an authentication request/response frame.

 A control frame is used for controlling access to the medium. Subtype frames of the control frame include a RTS frame, a CTS frame, and an ACK frame.

IFSs are waiting periods between transmission of frames operating in the MAC sublayer. These waiting periods are used to prevent collisions as defined in IEEE 802.11‐based WLAN standards; they represent the time period between completion of the transmission of the last frame and starting transmission of the next frame, apart from the variable backoff period. These are techniques used to prevent collisions as defined in IEEE 802.11‐based WLAN standard. Specifically, IFS is the time period between the completion of the transmission of the last frame and the start of transmission of the next frame, apart from the variable backoff period [16]. The list that follows enumerates the different types of IFSs starting from the shortest duration (highest priority) to the longest duration (lowest priority):

 Reduced Inter‐frame Space (RIFS)

 Short Inter‐frame Space (SIFS)

 Point Coordination Function (PCF) Inter‐frame Space (PIFS)

 Distributed Coordination Function (DCF) Inter‐frame Space (DIFS)

 Arbitration Inter‐frame Space (AIFS)

 Extended Inter‐frame Space (EIFS)

The DCF is a required technique utilized to prevent collisions in 802.11‐based WLANs. When using the DCF, a station is required to sense the status of the wireless channel before it can place its request to transmit a frame. DIFS is the time interval that a station must wait before it sends its request frame. SIFS is the time interval required by a WLAN device between receiving a frame and responding to the frame. See Table 2.3.

Figure 2.5 illustrates IFS relationships; the figure illustrates a SIFS, a PIFS, a DIFS, and an AIFS corresponding to an Access Category (AC) “i” (AIFS[i]) [2].

Before making a transmission, a WLAN device assesses the availability of the wireless medium using a CCA procedure. If the medium is occupied, CCA establishes that it is busy, while if the medium is available, CCA determines that it is idle. A WLAN device performs a backoff procedure when the WLAN device that is ready to transfer a frame finds the medium busy. In addition, a WLAN device operating according to the IEEE 802.11n and 802.11ac standards performs the backoff procedure when the WLAN device infers that a transmission of a frame by the WLAN device has failed. The backoff procedure includes determining a random backoff time composed of N backoff slots, each backoff slot having a duration equal to a slot time and N being an integer number greater than or equal to zero. The backoff time may be determined according to a length of a Contention Window (CW). All backoff slots occur following a DIFS or EIFS period during which the medium is determined to be idle for the duration of the period. When the WLAN device detects no medium activity for the duration of a particular backoff slot, the backoff procedure decrements the backoff time by the slot time. When the WLAN determines that the medium is busy during a backoff slot, the backoff procedure is suspended until the medium is again determined to be idle for the duration of a DIFS or EIFS period. The WLAN device may perform transmission or retransmission of the frame when the backoff timer reaches zero. The backoff procedure operates so that when multiple WLAN devices are deferring and execute the backoff procedure, each WLAN device may select a backoff time using a random function, and the WLAN device selecting the shortest backoff time may win the contention, reducing the probability of a collision [2]. When the control frame is not a response frame of another frame, the WLAN device transmits the control frame after performing backoff if a DIFS has elapsed, during which DIFS, the medium has been idle. When the control frame is the response frame of another frame, the WLAN device transmits the control frame after a SIFS has elapsed without performing backoff or checking whether the medium is idle.

TABLE 2.3 Inter‐frame Space Types

Inter‐frame Space Type	Description
Inter‐frame Space (IFS)	The time period between completion of the transmission of the last frame and starting transmission of the next frame. Various types of waiting times Distributed Coordination Function (DCF) scheme are listed below (these values typically depend on the standard, e.g. 802.11n, 802.11 ac).
Reduced Inter‐frame Space (RIFS)	A very short‐duration IFS spacing that has been used to send a burst of high‐priority frames. When a station needs to send multiple frames, RIFS is introduced between the individual frames to ensure that no other station finds an opportunity to occupy the channel within the frame burst. The use of RIFS is obsoleted from 802.11ac amendment onward (for compatibility, it is now set to zero).
Short Inter‐frame Space (SIFS)	The time interval required by a station that runs between receiving a frame and responding to the frame. It is used in the DCF scheme, which is the baseline technique used to prevent collisions. It is the IFS spacing maintained before and after the transmission of an acknowledgment frame and Clear To Send (CTS) frame. For example, in 802.11ac the SIFS is 16 μs.
Point Coordination Function (PCF) Inter‐frame Space (PIFS)	An IFS spacing used in the DCF. In coordinating the communications centrally, the AP waits for PIFS duration to acquire the channel. Because PIFS is less than the DIFS duration, the AP always has the priority to access the channel over the other stations. For example, in 802.11ac, the slot time is 9 μs and the PIFS is 25 μs.
Distributed Coordination Function (DCF) Inter‐frame Space (DIFS)	The time interval that a station should wait before it sends its request frame: with DCF, a station needs to sense the status of the wireless channel before it can place its request to transmit a frame. The following relationship holds: DIFS = SIFS +2 x Slot Time. For example, in 802.11ac, the slot time is 9 μs and the DIFS is 34 μs; as noted above, SIFS was 16 μs, that the resulting DIFS value of 34 μs.
Arbitration Inter‐frame Space (AIFS)	Timing to support stations that need to operate in a prioritized manner based upon the Access Category (e.g. for video or voice traffic). Here the waiting period (that is, the AISF) of a station is shortened or expanded before the station can transmit its frame: higher priority stations are assigned shorter AISF (a higher priority station has to wait for a shorter time interval before it can transmit its frame).
Extended Inter‐frame Space (EIFS)	An additional waiting period used in case of corrupted frames: If a previously received frame contains an error, then a station must defer EIFS duration instead of DIFS before transmitting a new frame.

FIGURE 2.5 Inter‐frame Space relationships [2].

A WLAN device that supports a Quality of Service (QoS) functionality may transmit the frame after performing backoff if an AIFS for an associated Access Category (AC) (AIFS[AC]), has elapsed. When transmitted by the QoS station, any of the data frame, the management frame, and the control frame, which is not the response frame, may use the AIFS[AC] of the AC of the transmitted frame.

Figure 2.6 illustrates a CSMA/CA‐based frame transmission procedure for avoiding collision between frames in a channel. The figure shows a first station, STA1 transmitting data, a second station, STA2 receiving the data, and a third station, STA3 that may be located in an area where a frame transmitted from the STA1, a frame transmitted from the second station STA2, or both can be received. STA1 may determine whether the channel is busy by carrier sensing. The STA1 may determine the channel occupation based on an energy level in the channel or autocorrelation of signals in the channel or may determine the channel occupation by using a Network Allocation Vector (NAV) timer. After determining that the channel is not used by other devices (that is, that the channel is IDLE) during a DIFS (and performing backoff if required), STA1 may transmit an RTS frame to STA2. Upon receiving the RTS frame, after a SIFS, STA2 may transmit a CTS frame as a response of the RTS frame. If Dual‐CTS is enabled and the second station STA2 is an AP, the AP may send two CTS frames in response to the RTS frame: a first CTS frame in the legacy non‐HT format and a second CTS frame in the HT format.

FIGURE 2.6 Carrier Sense Multiple Access/Collision Avoidance‐based frame transmission procedure.

When the third station STA3 receives the RTS frame, it may set a Network Allocation Vector (NAV) timer of the third station, STA3 for a transmission duration of subsequently transmitted frames (for example, a duration of SIFS+CTS frame duration+SIFS+data frame duration+SIFS+ACK frame duration) using duration information included in the RTS frame. When the third station STA3 receives the CTS frame, it may set the NAV timer of the third station STA3 for a transmission duration of subsequently transmitted frames using duration information included in the CTS frame. Upon receiving a new frame before the NAV timer expires, the third station STA3 may update the NAV timer of the third station STA3 by using duration information included in the new frame. The third station STA3 does not attempt to access the channel until the NAV timer expires. When STA1 receives the CTS frame from the second station STA2, it may transmit a data frame to the second station STA2 after SIFS elapses from a time when the CTS frame has been completely received. Upon successfully receiving the data frame, the second station STA2 may transmit an ACK frame as a response of the data frame after SIFS elapses. When the NAV timer expires, the third station STA3 may determine whether the channel is busy using carrier sensing. Upon determining that the channel is not used by other devices during a DIFS after the NAV timer has expired, the third station STA3 may attempt to access the channel after a CW according to a backoff process elapses [2].

When Dual‐CTS is enabled, a station that has obtained a transmission opportunity (TXOP) and that has no data to transmit may transmit a CF‐End frame to cut short the TXOP. An AP receiving a CF‐End frame having a BSSID of the AP as a destination address may respond by transmitting two more CF‐End frames: a first CF‐End frame using STBC and a second CF‐End frame using non‐STBC. A station receiving a CF‐End frame resets its NAV timer to 0 at the end of the PPDU containing the CF‐End frame.

Figure 2.6 also shows the second station STA2 transmitting an ACK frame to acknowledge the successful reception of a frame by the recipient.