Data Transfer
The exchange of data in an 802.11 network is bidirectional between the radio card and access point. As mentioned earlier, data frames in an infrastructure wireless LAN do not travel directly between wireless users. Instead, the access point relays the data.
A radio card or access point (802.11 station) having the destination MAC address of the data frame replies with an acknowledgement (ACK) frame. This adds significant overhead to a wireless LAN as compared to an Ethernet network that does not require ACKs for every data frame. The ACKs are necessary with 802.11 due to the nature of the shared radio medium; data loss is much more likely with the wireless medium, so wireless LANs perform error detection and error correction at Layer 2.
If an 802.11 station sending a data frame does not receive an ACK after a specific period of time, the station retransmits the frame. These retransmissions occur up to a particular limit, which is generally three to seven times. After that, higher-layer protocols, such as Transmission Control Protocol (TCP), must provide error recovery.
To allow for extended range, 802.11 includes automatic data rate shifting. For example, an 802.11 station generally lowers its transmission data rate if a retransmission is necessary. Access points support multiple data rates to facilitate this kind of operation, where different remote stations might transmit data upstream at different rates.
A radio card or access point (802.11 station) having the destination MAC address of the data frame replies with an acknowledgement (ACK) frame. This adds significant overhead to a wireless LAN as compared to an Ethernet network that does not require ACKs for every data frame. The ACKs are necessary with 802.11 due to the nature of the shared radio medium; data loss is much more likely with the wireless medium, so wireless LANs perform error detection and error correction at Layer 2.
If an 802.11 station sending a data frame does not receive an ACK after a specific period of time, the station retransmits the frame. These retransmissions occur up to a particular limit, which is generally three to seven times. After that, higher-layer protocols, such as Transmission Control Protocol (TCP), must provide error recovery.
To allow for extended range, 802.11 includes automatic data rate shifting. For example, an 802.11 station generally lowers its transmission data rate if a retransmission is necessary. Access points support multiple data rates to facilitate this kind of operation, where different remote stations might transmit data upstream at different rates.
