|
Security for Vo802.11
Aug 23,2007 00:00
by
admin
Security for Vo802.11 Although a chapter was devoted earlier in this book to security on 802.11 networks, it is important to examine how security applies specifically to Vo802.11. The misperception is that, because the conversation is transmitted over the airwaves, the voice stream is susceptible to interception, that is, eavesdropping. Although such an occurrence is not entirely impossible, it would be extremely difficult to tap into such a conversation. Case Study for Security of Vo802.11: SpectraLink's Secure Radio Technology Vo802.11 telephone systems provide additional measures of security through sophisticated radio technology and proprietary signal encoding. Many Fortune 500 companies use Vo802.11 telephone systems in their most secure areas, such as executive offices, data centers, and network control centers. Vo802.11 telephone systems employ digital spread spectrum transmission and a pseudorandom hopping sequence against radio eavesdropping. Digital Spread Spectrum Transmission The Vo802.11 telephone systems use a proprietary implementation of frequency hopping spread spectrum radio transmission, a radio technology originally developed by the military for secure and covert communications. Spread spectrum takes a discrete signal, such as a digitized voice conversation, and spreads it over a wide range of frequencies rather than transmitting at a single carrier frequency. Vo802.11 phone systems use frequency hopping to spread the signal by changing the carrier frequency once every 10 milliseconds (100 times every second). Because the carrier frequency changes rapidly, a radio scanner or narrowband receiver cannot be used to recover the information. These systems use 25 to 50 different frequencies in the hopping sequence, so a narrowband scanner has access to less than 4 percent of any conversation. Proprietary Pseudorandom Hopping Sequence A proprietary pseudorandom hopping sequence provides additional security. If a potential eavesdropper went to the expense of developing a scanner that could change frequencies 100 times a second, he or she would then have to attempt to determine the pseudorandom sequence to know when to monitor which frequency. Furthermore, each base station is transmitting on a different frequency, so multiple scanners would be required to follow a conversation as it was handed off from base station to base station. Vo802.11 systems use digital transmission, meaning that the analog voice signal is converted to a digital signal. This digital signal is scrambled to improve transmission, further complicating the ability to interpret an intercepted signal. Finally, Vo802.11 systems use Time Division Multiple Access (TDMA) to provide multiple speech channels from a single base station. The frame format and signaling for a Vo802.11 TDMA signal are proprietary and would have to be determined to identify discrete conversations on the radio link.[14] |