Mobile Intelligence

3.3 BACKGROUND

The IEEE 802.11 MAC layer covers three functional areas: reliable data delivery access, control, and security. We will discuss mainly about first two functions, as they are more closely related to our proposal in this chapter.

3.3.1 Reliable Data Delivery

Like any wireless network, a wireless LAN using IEEE 802.11 physical and MAC layer is not reliable. Noise, interference, and other propagation have adverse effect and cause significant number of frame loses. The reliability mechanism can be handled at the higher level as TCP. But the times used at higher levels are usually in the order of seconds. It is, therefore, more efficient to deal with errors at the MAC layer level. To solve this problem, IEEE 802.11 includes a frame-exchange protocol, called RTS/CTS handshaking. When a station receives a data frame from another station, it returns an acknowledgment (ACK) frame to the source station. This exchange should not be interrupted by a transmission from any other station. If the source station does not receive an ACK within a certain period of time, it will retransmit the frame. To enhance further reliability to this scheme, a four-way-handshaking scheme has been introduced. In this scheme, the source issues a RTS frame to the destination. The destination then responds with a CTS. After receiving the CTS, the source transmits the data frame and the destination responds with a ACK. Both RTS and CTS alert all the neighboring nodes that are within transmission range ...