ollision detection-no- you've entered the building, you're presumed to be ticing that the signal on the cable isn't the one the station an authorized user. Wireless propagation is not SO is sending-allows the station to detect that another well controlled, SO it is important for the network to station has tried to send a message at the same time it is ensure that a station ought to be allowed access and equally important for the station to ensure that it is transmitting. (The combination of these two techniques is often abbreviated CSMA/CD, for carrier-sense multi- connecting to the intended network and not an ple access with collision detection.) Finally, random impostor. backoff is used after a collision to ensure that stations Medium access control: The wireless medium is shared won't simply continue to collide: after all stations become by all (local) users. Who gets to transmit what when? silent, each station that has a pending packet to send A good wireless network protocol must efficiently randomly chooses a time delay before attempting to send multiplex the shared medium. it. If another collision results, the stations do the same Security: Even if the network refuses to talk to exercise, but over a larger time window. This approach stations it doesn't recognize, they might still be has the effect of forcing the offered traffic to the network listening in. Wireless networks may provide to go down when collisions are frequent, SO that data additional security for the data they carry by transfer continues at a lower rate but the network doesn't encryption of the data stream. It is important to note get tangled up: ethernet networks fail gracefully when that security may also be provided by other layers of they get busy. Thus, Ethernet was an early example of the network protocol stack; the lack of local a MAC layer designed to allow peer stations to share encryption in the wireless link does not necessarily a medium without central coordination. imply insecurity, and encryption of the wireless data CHAPTER W