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The data link layer within 802.11 consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). 802.11 uses the same 802.2 LLC and 48-bit addressing as other 802 LANs, allowing for very simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLANs.
Remember that 802.11 MAC was developed to work seamlessly with standard Ethernet to ensure that wireless and wired nodes on an enterprise LAN are logistically indistinguishable. The 802.11 MAC is necessarily different from the wired Ethernet MAC, but any such differences are masked by an AP that connects a WLAN channel to a LAN backbone.
For starters, 802.11 defines both a frame format and MAC scheme that differs from standard Ethernet. In fact, its robust frame format enables a number of compelling features such as fast acknowledge, handling hidden stations, power management, and data security. The WLAN standard uses a carrier sense multiple access with collision avoidance (CSMA/CA) MAC scheme, whereas standard Ethernet uses a carrier sense multiple access with collision detection (CSMA/CD) scheme. CSMA/CD protocol regulates how Ethernet stations establish access to the wire and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN. In an 802.11 WLAN, collision detection is not possible due to what is known as the “near/far” problem: to detect a collision, a station must be able to transmit and listen at the same time, but in radio systems the transmission drowns out the ability of the station to “hear” a collision
CSMA/CA attempts to avoid collisions by using explicit packet acknowledgment (ACK), which means an ACK packet is sent by the receiving station to confirm that the data packet arrived intact.CSMA/CA works as follows. A station wishing to transmit senses the air, and, if no activity is detected, the station waits an additional, randomly selected period of time and then transmits if the medium is still free. If the packet is received intact, the receiving station issues an ACK frame that, once successfully received by the sender, completes the process. If the ACK frame is not detected by the sending station, either because the original data packet was not received intact or the ACK was not received intact, a collision is assumed to have occurred and the data packet is transmitted again after waiting another random amount of time.
Frame receipt acknowledgment provides one good example of differences between the 802.11 MAC and the wired Ethernet MAC, along with the advantage the difference affords in a wireless system. Most LANs rely on a receiving node to send an acknowledge message to verify that it received an incoming data frame. In Ethernet and most wired LANs, however, the acknowledge message is handled above the MAC layer.
The 802.11 standard specifies that the MAC layer handle acknowledgment and resend lost frames resulting in more efficient usage of the available bandwidth and quicker acknowledgment. The 802.11 frame format relies on an interframe spacing of 50 ms. The standard requires that the receiving station send an acknowledgment 10 ms after the end of each frame, providing the CRC check is correct. The 10-ms limit ensures that the receiving station can take immediate control of the airwaves rather than competing with other nodes for medium access, as would be required if it waited past the 50-ms interframe spacing. LANs that handle acknowledgment in layers above the MAC canít meet the strict timing requirements and, therefore, essentially compete for medium access and send a standard frame to convey each acknowledgment. The MAC-layer implementation eliminates the latencies of medium access and allows the acknowledgment to use some of the interframe spacing time period in which no other activity would occur in any case.
When a wireless server is used, the bandwidth efficiency becomes doubly important in a BSS or ESS configuration because many frames must be sent twice ñ first from node A to an AP, and then from the AP to node B. In such a case, the AP must send an acknowledgment to node A, and node B sends an acknowledgment to the AP.