onsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random "backoff factor" that determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is 13.4 IEEE 802.11 SYSTEM Server DISTRIBUTION SYSTEM Station BSS-B Station Station Station BSS-A Figure 13.1 An ESS can provide campus-wide coverage. [From IEEE 802.11 tutorial (Ref. 5).] clear, the transmitting node decrements its backoff counter. (When the channel is busy, it does not decrement its backoff counter.) When the backoff counter reaches zero, the node transmits the packet. Since the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet, cannot be used for the radio-frequency transmissions of IEEE 802.11. The reason for this is that when a node is transmitting, it cannot hear any other node in the system that may be transmitting, since its own signal interferes with any others arriving at the node. Whenever a packet is to be transmitted, the transmitting node first sends out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received success- fully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is necessary to avoid the "hidden node" problem. This may be described by considering three nodes of a network: A, B, and C. As is often the case in these situations, node A can communica