802.11 – WIFI IFs

Inter frame spacing is some of the magic in WiFI. Its also one of the more confusing aspects of studying and understanding how WMM, and processes like Point coordination function work. Inter frame spacing is used to help avoid collisions on the wireless spectrum.

When a station wants to send a frame there are two methods that have to be OK before it can do this. 1 – Physical Carrier sense. Physical Carrier Sense is the process of checking to see if the wireless medium is busy. It does this my checking the wireless medium for RF energy, if it is above a certain threshold then it will wait to send.

The second check is Virtual Carrier sense. This method looks at passing wireless frames and sets a local MAC layer timer called the Network allocation vector or NAV to the amount of time found in the passing frame’s Duration field. When this reaches 0 it attempt a Physical Carrier sense. If the NAV is 0 and Physical Carrier sense shows RF energy below its threshold – the station will then wait the length of a Interframe space, then wait through the random backoff algorithm time (decided by the contention window), run through the carrier sense process again – then will actually send the frame.

The importance of the IFS is that the station has to wait that given amount of time. In the section below all the different IFS are listed with the its length of time. When studying for the CWAP exam I had a hard time remembering them, so hence the blog entry.

Once the station has the okay to send (Carrier sense is good) then it will wait the given IFS length. This is a big deal because not only does the STA wait the length of time before attempting to send the frame, but it can be used to give priority to certain frames over others. We do this by having that station wait less time then others based on the type of frame they are sending. By Priority I mean it asks the client to wait a smaller amount of time before sending the packet with a higher priority – This is called Probabilistic priority.

The following is a quick list of interframe spacing methods and are ordered by the duration time – shortest to longest time.

RIFS – Reduced IFS – Shorter than SIFS. Came out in 802.11n , to help minimize overhead. RIFS are used when one transmitter is sending packets when no SIFS reponses are expected. This IFS is not used in 802.11ac.

SIFS – Short Interframe Spacing – Shortest of all IFs . Used for ACKs, CTS in response to RTS, and the first data frame following the CTS.

PIFS – Point Coordination interframe space. – Used by the Point Coordination function to gain access to the medium. Used when switching from Distributed Coordination to Point Coordination

DIFS – DCF (Distribution coordination function) Interframe spacing– used with normal data frames. After the completion of sending a data frame the duration of a DIFS must be waited before attempting to send another.

AIFS – Arbitration IFS – Used in a kind of QoS manner. When a QoS station send data frames it waits an AIFS. Also used on a few control frames such as PS-Poll, CTS when not responding to a RTS (Thats a SIFS), BlockACKs, and RTS frames. A neat thing abou the AIFS is that the time is configurable on the AP, and then transmitted to clients by the Beacon frame.

EIFS – Extended IFS – Extended for a reason – When a frame is received with errors or corrupted the receiver will respond with  EIFS which tells the sender to wait for a longer period so it can see if the packet was delivered correctly. Once this happens, it will start go back to using a normal DIFS or AIFS .

Inter frame spacing gives us another mechanism to help avoid collisions in 802.11. It does this by having the Station wait a given amount of time (IFS) according to the type frame being sent.  We do this by utilizing Carrier sense, random back off timer, IFS and contention windows.

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