cisco ccna r&s exam topics

cisco ccna r&s exam topics.In a wireless network environment, the device operates in a completely different way, and there are two main methods of listening to carriers.Physical carrier listening: When a workstation stops sending data, it senses the presence of other frames in the channel. This is sometimes called ClearChannelAssessment (CCA).O Virtual carrier listening: When a workstation sends a frame, it fills in the frame header with a time value indicating the estimated time to send the frame. Other workstations can use this value to estimate how long the channel will be busy.For similar reasons, it is not possible to perform conflict detection. Therefore, wireless workstations need to avoid conflicts, and so use a mechanism called congestion avoidance. This mechanism relies on a backofftimer for its operation. o In addition to the backoff period, each wireless workstation needs to wait for an additional period of time, called interframe space time, before sending frames. The reason for waiting this extra time is to reduce the probability of conflict and to allow an additional buffer time between frames.The 802.11 standard defines several interframe interval timers. The standard interframe interval timer is called the Distributed InterframeThe standard interframe interval timer is called the Distributed Interframe Spacing (DIFS) timer. The process of sending frames from a wireless workstation basically consists of the following 3 steps. Step 1. detects if the channel is free.Step 2. selects the delay based on the fallback timer. During the delay, if the channel is busy, the evasion timer is turned off.When the channel is clear again, restart the evasion timer. Step 3. Wait for an additional DIFS period. Figure 1.34 shows the process of a workstation sending frames within the WLAN. As can be seen from the figure, Workstation A is ready to send frames. Itfirst detects the medium, selects a period of evasion time, and then sends the frame. The duration (send) time of the frame will be included in the field in the frame header. Workstations B and C will wait until Workstation A has finished sending frames, then wait for the DIFS duration, and then will start the evasion timer. Since Workstation C's evasion timer expires before Workstation B, Workstation C will send frames before Workstation B. Workstation B detects that the channel is busy and will turn off the evasion timer. Workstation B waits for the new send time, DIFS period, and remaining evasion timer, and then sends the frame.Client A|KB-___