2 the access point responds with a response control

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2 . The Access Point responds with a response control packet called CTS , which includes the same duration information. Receipt of the CTS packet indicates to the transmitter that no collision occurred, and permission is granted to start the data transmission . If the transmitter does not receive a CTS packet, then it repeats part 1 until it either receives ACK or times out after a given number of re-transmissions.
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64 3 The CTS frame is received by all the stations in the cell, notifying them that another unit will transmit during the following X microseconds. These stations record this information so they will know when the medium will again be available. Some stations may not have received the RTS packet because the original transmitting unit is out of range. 4 . The transmitting station sends its data frame to the access point. After the data frame is transmitted, the access point checks the CRC of the packet and, if correct, returns an ACK packet to confirm successful transmission. 5. If the final destination is another station on the WLAN, the access point then reserves the medium with a CTS packet. It proceeds to retransmit the data frame. The destination station checks the CRC of the packet and, if correct, returns an ACK to the access point
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65 The duration information in the CTS packet protects the data transmission from collisions . Because the RTS and CTS are very short packets , this mechanism also reduces the overhead of collisions. If the data packet is very short , the RTS packet may include all the data to be transmitted . If the RTS contains data, the CTS packet contains a duration of zero , and simply acts as an ACK to the transmitter that the RTS packet with data was received. Typical WLAN protocols use packets several hundred bytes long (up to 1518 bytes ). These packets << than Ethernet packets. It is preferable to use smaller packets in a wireless environment for several reasons: Due to the higher BER of a radio link, the probability of a packet getting corrupted increases with the packet size. In the case of packet corruption (due to collision or noise), a smaller the packet requires less overhead if it is necessary to re-transmit.
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66 Voice support in IEEE 802.11 DCF mode, with CSMA voice has priority over data (short IFS) voice users transmit staggered "black bursts", of length proportional to waiting time (i.e., speech bytes in buffer) voice user who waited longest wins (longest black burst) positive ACK guarantees success (no hidden term.) voice connections tend to evenly spread out in time frame Possible Improvement: instead of pos ACK, neg ACK receiver "invites" the sender with neg ACK if did not receive packet after time out.
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67 The 802.11 Protocol Stack LLC MAC DLL LLC sublayer, hides the differences between the different 802 variants and make them undistinguishable for the network .
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