e system, physical access to telephone lines or to a private branch exchange (PBX) in an office is required in order to intercept voice traffic. The physical security of these systems means that the encryption of voice traffic over a conventional telephone system is only justified for highly security-sensitive organisations. In the case of VoWLAN, voice traffic going outside the organisation will pass over the completely unsecured Internet, and it becomes relevant for many more organisations to provide the same degree of security for voice for data traffic. VoWLAN services will be susceptible to the range of security attacks described in Chapter 8, and will be particularly sensitive to Denial of Service attacks in view of the delay sensitive nature of voice traffic. Specific VoWLAN/VoIP security considerations are discussed in the Section "VOWLAN and VoIP Security, p. 239". A further security concern for VoWLAN/VoIP services is the possibility of unsolicited bulk messages being broadcast to Internet connected Implementing Wireless LANs phones. As IP telephony becomes more widespread, SO called spam over internet telephony (SPIT) could become as pervasive as spam e-mail. Technology to counter this threat is under development, based either on filtering and deleting unwanted calls based on characteristics such as call frequency and duration, or identifying authentic calls by enabling these to be digitally signed and checking signatures against an authorised caller list. This page intentionally left blank CHAPTER Wireless LAN Security The Hacking Threat The flexibility of wireless networking is bought at the price of an increased need to think about security. Unlike a wired network, where signals are effectively limited to the connecting cables, WLAN transmissions can propagate well beyond the intended operating area of the network, into adjacent public spaces or nearby buildings. With an appropriate receiver, data transmitted over the WLAN may be accessible to anyone within range of the transmitter. Although the spr