In the simple world of analog radio, we must contend with problems associated with multipath interference. Radio waves travel from the transmitter to the receiver over different paths based on the terrain over which the waves travel. Clearly these multiple waves will interfere with each other at the receiver.
In the digital world the problem of interference can be complicated. When digital information is sent, it is coded into symbols, which are then modulated on the radio carrier. With multiple paths, the receiver gets multiple copies of the symbol. The quandary for the decoder is to determine which of these symbols is the correct one. If they are close it is not a real problem, but as the delay spread grows so does the problem. The result is that effective transmission speed will decrease.
In 802.11b, the maximum delay spread is 500 nanoseconds. However, to achieve 11 Mbps, the delay spread must not exceed 65 nanoseconds. Problems associated with multipath propagation are the reason that 802.11b WLANs advertise speeds of up to 11 Mbps with no guarantees that it will work at all.