Interframe spaces
(RIFS, SIFS, PIFS, DIFS, AIFS, EIFS)
. . . .
Interframe spaces
- Interframe space (IFS) is a period of time that exists between transmissions of wireless frames.
- The actual length of time of each of the interframe spaces varies depending on the transmission speed of the network.
- Interframe spaces are one line of defense used by CSMA/CA to ensure that only certain types of 802.11 frames are transmitted following certain interframe spaces.
For example, only ACK frames, block ACK frames, data frames, and clear-to-send (CTS) frames may follow a SIFS. - *The two most common interframe spaces used are the SIFS and the DIFS.
- Interframe spaces are all about what type of 802.11 traffic is allowed next.
- Interframe spacing also acts as a backup mechanism to virtual carrier sense
- There are six interframe spaces of different durations of time, and the order is
RIFS > SIFS > PIFS > DIFS > AIFS > EIFS.
There are six types of interframe spaces, which are listed here in order of shortest to longest:
- Reduced interframe space (RIFS), highest priority
The 802.11e QoS amendment introduced the capability for a transmitting radio to send a burst of frames during a transmit opportunity (TXOP).
During the frame burst, a short interframe space (SIFS) was used between each frame to ensure that no other radios transmitted during the frame burst.
The 802.11n amendment defines a new interframe space that is even shorter in time, called a reduced interframe space (RIFS).
An 802.11a/n SIFS interval is 16 μs and an 802.11b/g/n SIFS is 10 μs. The much shorter RIFS interval is only 2μs.
A RIFS interval can be used in place of a SIFS interval, resulting is less overhead during a frame burst.
It should be noted that RIFS intervals can be used only when a Greenfield HT network is in place.
RIFS can only be used between HT radios, and no legacy devices can belong to the basic service set. - *Short interframe space (SIFS), second highest priority
- PCF interframe space (PIFS), middle priority
- *DCF interframe space (DIFS), lowest priority
- Arbitration interframe space (AIFS), used by QoS stations
- Extended interframe space (EIFS), used after receipt of corrupted frames
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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