Interframe Spacing

As with traditional Ethernet, the interframe spacing plays a large role in coordinating access to the transmission medium. 802.11 uses four different interframe spaces. Three are used to determine medium access; the relationship between them is shown in Figure 3-6.

Figure 3-6. Interframe spacing relationships

Figure 3-6. Interframe spacing relationships

We've already seen that as part of the collision avoidance built into the 802.11 MAC, stations delay transmission until the medium becomes idle. Varying interframe spacings create different priority levels for different types of traffic. The logic behind this is simple: high-priority traffic doesn't have to wait as long after the medium has become idle. Therefore, if there is any high-priority traffic waiting, it grabs the network before low-priority frames have a chance to try. To assist with interoperability between different data rates, the interframe space is a fixed amount of time, independent of the transmission speed. (This is only one of the many problems caused by having different physical layers use the same radio resources, which are different modulation techniques.) Different physical layers, however, can specify different interframe space times.

Short interframe space (SIFS)

The SIFS is used for the highest-priority transmissions, such as RTS/CTS frames and positive acknowledgments. High-priority transmissions can begin once the SIFS has elapsed. Once these high-priority transmissions begin, the medium becomes busy, so frames transmitted after the SIFS has elapsed have priority over frames that can be transmitted only after longer intervals.

PCF interframe space (PIFS)

The PIFS, sometimes erroneously called the priority interframe space, is used by the PCF during contention-free operation. Stations with data to transmit in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic.

DCF interframe space (DIFS)

The DIFS is the minimum medium idle time for contention-based services. Stations may have immediate access to the medium if it has been free for a period longer than the DIFS.

Extended interframe space (EIFS)

The EIFS is not illustrated in Figure 3-6 because it is not a fixed interval. It is used only when there is an error in frame transmission.

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