lined in Table 4-6. Security has been a significant concern since RF transmissions are far more open to interception than those confined to a cable. Security issues will be covered in detail in Chapters 8 and 11. Data link reliability, bit transmission errors resulting from interference and other signal propagation problems, are probably the second most significant challenge in wireless networks, and one technology that resulted in a quantum leap in addressing this problem (spread spectrum transmission) is the subject of the next section. Controlling access to the data transmission medium by multiple client devices or stations is also a different type of challenge for a wireless medium, where, unlike a wired network, it is not possible to both transmit and receive at the same time. Two key situations that have the potential to Table 4-6: The Radio Frequency Networking Challenge Challenges Considerations and solutions Link reliability Signal propagation, interference, equipment siting, link budget. Media access Sensing other users (hidden station and exposed station problems), Quality of service requirements. Security Wired equivalent privacy (WEP), Wi-Fi Protected Access (WPA), 802.11i, directional antennas. Chapter Four Hidden Station: C is hidden from A A will not sense C's transmission and A's transmission to B will fail due to interference from C. Range of station C's transmitter Station A Station B Station C Exposed Station: B is exposed to C B will be prevented from transmitting to A while station C is transmitting, although A would be able to receive successfully. Figure 4-2: Hidden and Exposed Station Challenges for Wireless Media Access Control degrade network performance are the so-called hidden station and exposed station problems. The hidden station problem occurs when two stations A and C are both trying to transmit to an intermediate station B, where A and C are out of range and therefore one cannot sense that the other is also transmitting (see Figure 4-2). The exposed station problem occurs when a