new ccna objectives

new ccna objectives.The duration 45 in the header of the frame sent by Site B is called the Network Allocation Vector (NAV, NetworkAllocationVector) and reserves a medium for its sending of the frame that contains the total time required to send the frame, wait for the SEFS, and receive the ACK from the AP. 5. Site A adds 45 to 18 and continues the countdown. All the time that Site A waits before sending is called the contention window(attentionwindow) o 6. After Site A's timer reaches 0, it is ready to send its own frame, as shown in Figure 7.2. At this point, the mediumIt should be idle. Figure 7.2 Sending frames: Part 2 Countdown is over, I can now sendIf Site A fails to transmit, the evasion timer is reset to a new random value and the countdown begins again. When these frame transmissions fail, the value of the evasion timer becomes larger. For example, the value of the timer may initially be any number between 0.31. After the first failed transmission, its value will become an arbitrary number between 0.127. After the next failure, the timer's value will be doubled, and after another failure, it will be doubled again.The whole process is called DistributedCoordinationFunction ( DCF,DistributedCoordinationFunction) o This simply means that each site is responsible for coordinating the sending of its data. An alternative to DCF for media access is the PointCoordinationFunction (PCF), which means that the AP is responsible for coordinating the data transmission.If the frame is sent successfully, an ACK must be sent to acknowledge it. The ACK uses the SIFS counter value to ensure that the frame is sent quickly. It is normal to have some silence interval between frames. SIFS is the shortest silence interval and the NAV can predetermine this value; the normal silence interval is DIFS. ACK uses SIFS again, because you want it to be sent immediately. The station sending the ACK waits for SIFS until the duration of the ACK becomes 0. This indicates the end of the transmission.7.2 Basic Topics 85When a client connects to a wireless cell, either the client or the AP can leave the connection by sending an unauthentication message. The unauthentication message has information in its body message as to why it is leaving. Alternatively, a client can send a disassociate message that separates that client from the cell, but still maintains that client's authentication. The next time the client returns to the wireless cell, it sends only a re-association message, and the AP sends a re-association response - omitting the need to re-connect to the cell for authentication.IEEE 802.11Bata. flags: . Type/Subtype: Data (0x26) p Frame Control: OxOAOB (Normal) Version: 0 Type: Dataframe (2) Subtype: D pFlags: QxA DSstatus: Frame frow DStoa STAvia AP( ToDS:0 FromDS:1)(0x02) =MoreFragments: This is the lastfragment . Retry: Frame is being retransmitted -PWR HOT:STAwill stayup =MoreData:No databuffered .Protected flag: Datais not protected worker Orderflag: Notstrictly ordered Duration: 44 Destinationaddress: Apple^ab:14:26 (00: le:c2:ab: 14:26)BSS Td:asco-Li^Sd: 21:3d (00:12:17:0d:21:3d) Sourceaddress: asco-Li_0d:21:3b (00:12:17:0d:21:3b) F ragnent number: 0 Sequence number: 1419Figure 7.3 Wireless frame capture diagram 1