The CSMA/CA distributed algorithm is mandated that a gap of a minimum specified duration exists between the contiguous frame sequences. A transmitting STA shall ensure that the medium is idle for this required duration before attempting to transmit the next frame. If the medium is determined to be busy by either physical or virtual carrier-sense mechanism, the STA shall defer a period and listen to the channel until the end of the current transmission. After the deferral, or prior to attempting to transmit packet again immediately after a successful transmission, the STA shall select a random backoff time interval and shall decrease the backoff time counter as long as the medium goes to idle state.
The time interval between the continuous two frames is called the Inter-Frame Space (IFS). Four different IFSs are defined to provide different priorities for accessing to the wireless medium in the IEEE 802.11 [1]. The four IFSs are listed in order here, from the shortest one to the longest one, that are Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), and Extended IFS (EIFS). These different IFSs shall be independent of the STA bit rate. Each IFS timing shall be defined as time gaps on the
Figure5. RTS/CTS/Data/ACK and NAV setting
DIFS
Defer Access back-off After Defer
Source
Destination
Others
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medium, and shall be fixed for each PHY (even in multi-rate-capable PHYs).
The basic time slot is the aSlotTime, and every counting step is based on the time slot. The aSlotTime for the DSSS PHY shall be the sum of the Rx-to-Tx turnaround time (5µs), measured at the MAC/PHY interface, and the energy detecting time (15µs). The propagation delay shall be regarded as being included in the energy detecting time.
The SIFS shall be used for an ACK frame, a CTS frame, the second or the subsequent MPDU frames of a fragment burst, and by a STA responding to any polling from the PCF mode. The SIFS is the time interval from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface. The IEEE 802.11 defines the SIFS time to be:
And its implementation shall not allow the tolerance between two contiguous frames that are defined to be separated by a SIFS time, as measured on the medium, to vary from the nominal SIFS value by more than ?10% of the aSlotTime for the PHY in use.
The PIFS shall be used only by the STAs operating under the PCF mode to gain the priority access to the medium at the start of the CFP in a superframe. According to the definition, the PIFS duration is:
aSlotTime aSIFSTime
aPIFSTime ? ?
? ?
2.2A STA using the PCF shall be allowed to transmit the contention-free traffics after its carrier-sense mechanism determines that the medium is idle for at least aPIFSTime.
The DIFS shall be used by the STAs operating under the DCF mode to transmit the data frames (MPDUs) and the management frames (MMPDUs). The aDIFSTime is defined as:
aSlotTime aSIFSTime
aDIFSTime ? ? 2?
? ?
2.3A STA using the DCF shall not transmit within an EIFS period after it determines that the medium is idle following the reception of a frame for which PHYRXEND.indication primitive contained an error or a frame for which the MAC FCS value was not correct. A STA may transmit after subsequent reception of an error-free frame, re-synchronizing the STA. This allows the STA to transmit using the DIFS following that frame.
The EIFS shall be used by the DCF whenever the PHY has indicated by the MAC that a frame transmission was begun that did not result in the correct reception of a complete MAC frame with a correct FCS value. The duration of an EIFS is:
rLength
where, ACKSize is the length, in bytes, of an ACK frame; and
( 8xACKSize + aPreambleLength + aPLCPHeaderLength ) is expressed in microseconds required to transmit at the PHY’s lowest mandatory rate.
The EIFS is defined to provide enough time for another STA to acknowledge what was, to this STA, an incorrectly received frame before this STA commences transmission. Reception of an error-free frame during the EIFS resynchronizes the STA to the actual busy/idle state of the medium, so the EIFS is terminated and the normal medium access (using DIFS and, if necessary, backoff algorithm) continues following reception of that frame.
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The basic access mechanism is illustrated in the Figure 6. A STA with a new packet to transmit shall monitor the channel activity through both physical carrier-sense mechanism CCA and virtual carrier-sense mechanism NAV. If the channel is determined to be idle for a period of time equal to the DIFS, the STA transmits the packet immediately. Otherwise, if the channel is sensed as busy (either immediately or during the DIFS), the STA persists to monitor the channel until it is measured to be idle for a DIFS period. At this point, the STA generates a random backoff time interval before transmitting to minimize the probability of collision with packets being transmitted by other stations. In addition, to avoid channel capture, a STA must wait a random backoff time between two consecutive new packet transmissions, even if the medium is sensed idle after a DIFS time.