3 LANS. THIS IS ACTUALLY INCORRECT, ASIDE FROM THE FACT THAT THEY...

802.3 LANs. This is actually incorrect, aside from the fact that they use MAC addresses.Wireless LANs use the 802.11 frame structure, and you can encounter multiple types offrames. To get a better understanding, you can begin by learning the three types of wire-less frames. Once you are familiar with the three types of wireless frames, you can fur-ther your knowledge by taking a deeper look at interframe spacing (IFS) and why it isnecessary.Wireless Frame TypesWireless LANs come in three frame types:

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Management frames:Used for joining and leaving a wireless cell. Managementframe types include association request, association response, and reassociation re-quest, just to name a few. (See Table 7-2 for a complete list.)

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Control frames:Used to acknowledge when data frames are received.

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Data frames:Frames that contain data.Now that you have an idea of what frames are used, it is helpful to see how these framesare sent. For this, you need to understand a few more terms that might be new to you. Be-cause all the terms meld together to some degree, they are explained in context through-out the next section.Sending a FrameRecall that wireless networks are half-duplex networks. If more than one device were tosend at the same time, a collision would result. If a collision occurs, the data from bothsenders would be unreadable and would need to be resent. This is a waste of time and re-sources. To overcome this issue, wireless networks use multiple steps to access the net-work. Wireless LANs use carrier sense multiple access collision avoidance (CSMA/CA),which is similar to the way 802.3 LANs work. The carrier sensepart means that a stationhas to determine if anyone else is sending. This is done with clear channel assessment(CCA), and what it means is that you listen. You can, however, run into an issue where twodevices cannot hear each other. This is called the hidden node problem. This issue is over-come using virtual carrier sense (VCS). The medium is not considered available until boththe physical and virtual carrier report that it is clear.Each station must also observe IFS. IFS is a period that a station has to wait before it cansend. Not only does IFS ensure that the medium is clear, but it ensures that frames are notsent so close together that they are misinterpreted. The types of IFS periods are as follows:

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Short interframe space (SIFS):For higher priority and used for ACKs, amongother things

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Point-coordination interframe space (PIFS):Used when an AP is going to con-trol the network

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Distributed-coordination interframe space (DIFS):Used for data frames and isthe normal spacing between framesEach of these has a specific purpose as defined by the IEEE.SIFS is used when you must send a frame quickly. For example, when a data frame is sentand must be acknowledged (ACK), the ACK should be sent before another station sendsother data. Data frames use DIFS. The time value of DIFS is longer than SIFS, so the SIFSwould preempt DIFS because it has a higher priority.Figure 7-1 illustrates the transmission of a frame. In the figure, Station A wants to send aframe. As the process goes, both the physical and virtual carrier need to be free. Thismeans the client has to listen. To listen, the client chooses a random number and begins acountdown process, called a backoff timer. The speed at which the countdown occurs iscalled a slottimeand is different for 802.11a, b, and g. Select a random timer (29), 28, 27, 26....1 Key TopicListen during countdown.2 4 I was at 18; add 45 to that and continue (63, 62, 61...).Station A To Distribution Frame Duration3 Send for 45 slots.Station B Figure 7-1 Sending a Frame: Part 1It works like this: