AT THIS POINT, THE MEDIUM SHOULD BE CLEAR.IF STATION A SENDS BUT FA...

2. At this point, the medium should be clear.If Station A sends but fails, it resets the backoff timer to a new random number and countsdown again. The backoff timer gets larger as the frames fail in transmission. For example,the initial timer can be any number between 0 and 31. After the first failure, it jumps toany number between 0 and 127. It doubles for the next failure, then again, then again.This entire process is known as the distributed coordination function(DCF). This simplymeans that each station is responsible for coordinating the sending of its data. The alterna-tive to DCF is point coordination function (PCF), which means the AP is responsible forcoordination of data transmission.5 Countdown is over. Now I can send.Key TopicFrame DurationStation A To Distribution Station B Figure 7-2 Sending a Frame: Part 2If the frame is successful, an ACK must be sent. The ACK uses the SIFS timer value tomake sure it is sent quickly. Some amount of silence between frames is natural. The SIFS isthe shortest period of silence. The NAV reserves this time. A normal silence time is theDIFS. Again, the ACK uses SIFS because you want it to be sent immediately. The stationthat sends the ACK waits for the SIFS and then ACKs with the duration of 0. This is howthe end of the transmission is indicated.

Wireless Frame Headers

Figure 7-3 shows a wireless frame. Each of the fields has been expanded so you can see itmore clearly. It is beneficial to understand these fields and how they play a part in thesending and receiving of wireless frames. Figure 7-3 Wireless Frame Capture 1As you can see from the capture, a preamble is present, denoted with the Type/Subtypelabel, followed by a Frame Control field. The preamble can be anywhere from 76 to 156bytes. The Frame Control field is 2 bytes. It tells what type of frame it is, represented with2 bytes. In this case, it is a data frame.The Flags field indicates that the frame is travelingfromthe DS, not toward the DS. This isrepresented with a single byte. In the figure, this is a frame that is coming back to the client.Following the Flags field is a Duration field. The Duration field indicates how long themedium is reserved while this frame is being sent and includes time for an ACK to be sentin reply. The idea behind this process is to prevent collisions.A wireless frame can have up to three MAC addresses following the Duration field. This isa total of 18 bytes. In the figure, you can see the following:

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Destination MAC address

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BSS ID, which is also a MAC address

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Source MAC addressThe source address (SA) is the station that sent the frame. The transmitter address (TA) isthe address of the station that is emitting the frame; in Figure 7-3, a TA is not shown. Insome scenarios, a TA might vary from an SA. For example, if a wireless frame is relayedthrough a repeater, the TA would be the radio of the repeater, and the SA would be thesending device. The destination address (DA) is the final destination of the frame; in thiscase, it is the wireless client.The Sequence Control field (2 bytes) indicates whether the frame is a fragment. Again, inFigure 7-3, the Sequence Control field is indicated with Fragment Numberand showsthat this is number 0, or the last fragment. This leads to an interesting topic—fragmenta-tion. When and why would you fragment on a wireless network? The answer is that awireless frame is, by default, 2346 bytes long. Considering that the frame is going to moveto or from an Ethernet distribution that has a maximum transmission unit (MTU) of 1500bytes and can see frames as big as 1518 bytes or slightly larger (depending on the trunk-ing used), the frames on the wireless side are too big and need to be chopped up.120 CCNA Wireless Official Exam Certification GuideOptionally, you can see a fourth MAC address, a receiving address (RA), which is the ad-dress of the directstation that this frame is sent to; however, this is not seen in the figure.The frame could be relayed through a wireless bridge or repeater. This additional addressadds six more bytes.Finally, the frame body follows (not seen in the figure). It can be up to 2306 bytes and ref-erences only two MAC addresses, just like any other L2 frame. The frame body is encap-sulated inside the last header shown in the figure.In addition, you might see a 4-byte frame check sequence (FCS) following the L2 frame.This is common but not required.

Frame Types

For the most part, all frames are going to have the same type of header. The difference isin the body of the frame. The body is more specific and indicates what the frame is allabout. Table 7-2 shows some frame types.Table 7-2 Frame Types TableManagement Control DataBeacon Request to Send (RTS) Simple dataProbe Request Clear to Send (CTS) Null functionProbe Response Acknowledgment Data+CF-ACKAssociation Request Power-Save-Poll (PS-Poll) Data+CF-PollAssociation Response Contention Free End (CF-End) Data+CF-AckAuthentication Request Contention Free End + Acknowl-ACK+CF-Polledgment (CF-End +ACK)Authentication Response CF-ACKDeauthentication CF-ACK+CF-PollReassociation requestReassociation responseAnnouncement traffic indicationmessage (ATIM)Each frame type merits its owndiscussion to follow.Management FramesManagement frames, as their name indicates, are used to manage the connection. In look-ing at a frame capture, the Type field indicates Management, and the subtype tells whatkind of management frame it is. As Table 7-2 listed, there are 11 Management frame types.There are some more-often seen frames that you should be familiar with. These frametypes are discussed in the following sections. Beacons and ProbesFigure 7-4 shows a management frame with a subtype of 8. This indicates that it is a bea-con frame, which is used to help clients find the network.Figure 7-4 Management Frame CaptureFigure 7-5 shows a sample network where the AP is sending a beacon frame.SSID: CARROLL Beacon I hear beaconsfrom an AP.Wireless Client Figure 7-5 Sample Network Using Beacon FramesWhen the client hears the beacon frame, it can learn a great deal of information about thecell. In Figure 7-6, you can see that the beacon frame includes a timestamp that gives a ref-erence time for the cell, the beacon interval, and a field called Capability Information,which provides specifics for this cell. The Capability Information field includes informa-tion regarding power save mode, authentication, and preamble information.A beacon frame also includes the SSIDs that the AP supports, the rates that are supported,and six fields called Parameter Set that indicate modulation methods and such.Another field you will find is Traffic Indication Map (TIM), which indicates whether theAP is buffering traffic for clients in power-save mode.When a client sees a beacon frame, it should be able to use that information to determineif it is able to connect to the wireless Cell. Chapter 16, “Wireless Clients,” covers the