CWAP MAC Sublayer and Functions - Beacon Frames

    The biggest part of the CWAP exam is "MAC Sublayer and Functions" which encompasses 25% of the exam. Because of this, you need a deep understanding of frame formats and what their purpose is in the environment. I started with management frames:


Management frames have type 00 in the Frame Control Field


    The Frame Control Field is in all 802.11 frames and it contains information like the frame subtype along with tons of other information that will be discussed later.


Management Frame Subtypes:

    As you may have noticed in the image above, the frame control field contains a Subtype. There are 16 management frame subtypes:



Each of the frame subtypes are important to managing the stations that are on the WLAN.


Beacon

    The beacon frame is something we see visualized without knowing it. This is visible to us when you go into the wireless settings on your device and see the SSID's of WiFi networks around you. The information on at that screen is gathered from beacon frames.

    Beacon frames are sent out regularly based on the Target Beacon Transmit Time (TBTT). The beacon is used to not only advertise the network, but to help clients manage power save or to tell clients when they have buffered frames. Each SSID on your network will broadcast a beacon around every 100 TU(Time Units) or roughly ten times a second:


    Beacons also contain information about the BSS, advertising the capabilities of the BSS. Listening to beacons is a way clients can passively scan for networks. Below is a screenshot of just some of the information listed in the beacon frame:



    Looking through this information can be incredibly helpful for understanding how the BSS operates. You can learn things like the mandatory data rates, the beacon interval, the HT/VHT/HE capabilities, and even vendor specific information about the BSS just by digging into the beacon frame.

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