解釋：
The DCF mechanism is also based on the use of the optional RTS/CTS (Request- To-Send and Clear-To-Send) scheme. If a node has a packet to send, it firstly transmits an RTS packet to request the channel. If the receiver is ready to receive data, it replies with a CTS packet. After the sender receives the CTS packet successfully, it transmits the actual data packet. Figure 3, shows an overview of 802.11 module architecture in the ns-3 simulator. The WifiNetDevice class holds together WifiChannel, WifiPhy, WifiMac, and WifiRemoteStationManager. If transmission is initiated by an application, the WifiNetDevice interface sends the packet to WifiMac class which handles high MAC level functions. Each WifiNetDevice is aggregated to a StationManager which stores all transmission parameters (e.g. payload data rate, RTS/CTS, fragmentation, etc.) The main classes implementing the IEEE 802.11 DCF scheme are DcfManager and DcaTxop classes. DcaTxop can handle only one packet. If other transmissions have been initiated, WifiMac pushes the packets to WifiMacQueue. DcaTxop is used to handle the request access to the channel from DcfManager. When access is granted, DcaTxop pushes the packet to MacLow for transmission. MacLow initiates data transmission. WifiPhy class is designed to support all physical layer related issues, such as channel sensing, SINR computation, interferences, etc. Its design is sufficiently generic that it is able to support the implementation of different MAC designs. YansWifiPhy is the implementation of the IEEE 802.11 physical layer. It models an additive Gaussian Noise Channel (AWGN) with cumulative noise handled by InterferenceHelper. The abstract class WifiChannel is designed to model the radio signal transmission. Yan- sWifiChannel is the only implementation modeling an IEEE 802.11 channel.?
