Basic Access Method

In IEEE 802.11, priorities for accessing to the wireless medium are controlled by the use of inter frame space (IFS) intervals, i.e. time intervals between the transmissions of consecutive frames. The standard defines four different IFS intervals:

short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS), and extended IFS (EIFS). The SIFS is the smallest followed by PIFS, DIFS, and EIFS. After an SIFS, only acknowledgement (ACK), CTS and data frames may be sent. The use of PIFS and DIFS is to separate the PCF and DCF modes.

Under the basic access method, a station, when starting a new frame transmission, shall sense the wireless medium to determine if another station is transmitting. If the station detects that the wireless medium has been idle for more than a time interval called DIFS, the station can transmit the data frame immediately. If the medium is sensed as busy, the station waits until the channel becomes idle, then defers for an

extra DIFS interval. If the medium remains idle, the MAC starts the backoff procedure by selecting a random backoff count since collisions are most likely to happen just after the medium becomes free. The backoff counter (how to select the random backoff count is detailed below) is decremented as long as the channel is sensed idle, frozen when a transmission is detected on the channel, and reactivated when the channel is sensed idle again for more than a DIFS. The station transmits when the counter reaches zeros.

To select the random backoff count, each station maintains a contention window (CW) value. The backoff count, in the unit of Slot_Time, is determined as a random integer drawn from a uniform distribution over the interval [0, CW]. The value of CW

is initially assigned a CWmin, and increases exponentially when a transmission fails.

After any unsuccessful transmission attempt, another backoff is performed with a new CW value determined as follows:

(2.1)

2 ( 1) 1

CW ⇐ ⋅ CW + −

The set of CW values are sequentially ascending integer powers of 2 minus 1. Once the CW value reaches the value of CWmax, it remains at the value of CWmax until it is reset. The CW value is reset to CWmin after a successful transmission or after reaching the maximum retry limit. If the maximum retry limit (ShortRetryLimit or LongRetryLimit) is reached, the retransmission attempts shall cease, and the frame shall be discarded.

Every station maintains a station short retry count (SSRC) as well as a station long retry count (SLRC), both of which have an initial value of zero. SSRC indicates the retransmission number of the RTS frames or the data frames transmitted by the basic access method. SLRC indicates the retransmission number of data frames when

the RTS/CTS technique is used. The specified limits of SSRC and SLRC are ShortRetryLimit and LongRetryLimit (typically 7 and 4), respectively. The SSRC is reset to 0 whenever a CTS is received in response to an RTS or whenever an ACK is received in response to a data frame. The SLRC is also reset to 0 whenever an ACK is received in response to a data frame when the RTS/CTS technique is used.

One important characteristic of the IEEE 802.11 MAC is that an ACK frame shall be transmitted by the receiver after a successful data frame reception. Only after receiving the ACK frame correctly, the transmitter assumes that the data frame was delivered successfully. SIFS, which is smaller than DIFS, is the time interval between the data frame reception and ACK frame transmission. Using this smallest IFS interval between transmissions within the frame exchange sequence prevents other stations, which are required to wait for the medium to be idle for a longer time interval, e.g. at least a DIFS time, from attempting to use the medium, thus giving priority to completion of the frame exchange sequence in progress. If the ACK frame is not received within an ACK_Timeout period after the data transmission, the data frame is retransmitted after another random backoff. When the data frame is correctly transmitted and the corresponding ACK is received, the station performs a DIFS deference and another random backoff process, which is often referred to as

“post-backoff” to avoid channel capture.

When a data frame is transmitted, all the other stations hearing the data frame adjust their network allocation vector (NAV), which is used for virtual carrier sensing at the MAC layer indicating the period of time in which the channel will remain busy, based on the duration field value in the data frame, which includes an SIFS interval and the transmission time of the ACK frame following the data frame.

Based on the above discussion, we notice that a transmission cycle under the

basic access method consists of the following phases: the DIFS deferral phase, backoff phase if necessary, data transmission phase, SIFS deferral phase, and ACK transmission phase. The timing of successful frame transmission is illustrated in Figure 2.1(a). In Figure 2.1(b), if no ACK frame is received after an SIFS interval, due possibly to collision or an erroneous reception, i.e. received with an incorrect frame check sequence (FCS), the transmitter will contend again for the medium to retransmit the data frame after an ACK_Timeout. However, if an ACK frame is received in error, the transmitter will recontend for the medium to retransmit the data frame after an EIFS interval, as shown in Figure 2.1(c).