Technology companies have created WiFi-based phone systems serving niche markets for several years. Early products were introduced with 802.11b-only wireless subsystems having maximum phone capacity of about five voice calls. These systems often used proprietary signaling and QoS techniques. The wireless phone links wouldn't have been secure, and there wouldn't have been sufficient bandwidth to service data applications and phones simultaneously by current expectations.
Ongoing improvements in 802.11 technologyhigh-rate PHYs, WPA security, and QoS methodspromise to bring Voice over IP (VoIP), VoIP-plus-data, video-plus-data, and VoIP-plus-video-plus-data applications into the mainstream. The growth of VoIP service providers, small and large, suggests business opportunities for applying 802.11 technology to VoIP, or for providing VoIP service over 802.11.
Unfortunately, there are no standard practices for providing VoIP/802.11 service. There are high-level problems to resolve such as billing, call processing, and secure rapid handoff between systems. There are 802.11-level problems to resolve, such as how to support both VoIP and data on the same wireless channel while also optimizing handset battery life.
Emerging VoIP/WLAN systems will be compared with, and may compete with, cellular phone systems. The cellular systems are synchronous; the phones, base stations, and backhaul connections have common timing. Thus, capacity and timing are known and unvarying. There's only one class of service, voice; thus, QoS access methods aren't needed to provide differentiated or guaranteed classes of service. Even when data services are added, they're added in a way that's compatible with the time slots, multiplexing, and management of the voice services. Cellular systems use licensed spectrum and have planned deployments that avoid interference between base stations. For all these reasons, cellular systems are predictable to the microsecond level.
802.11 systems aren't synchronous, are seldom planned, use unlicensed spectrum, can experience significant interference from multiple wireless networks and other non-WLAN devices, and are generally unpredictable at the microsecond level even though they may be robust overall. Talk time, i.e. battery life, is an important point of comparison between cellular telephony and VoIP-over-WLAN. When 802.11 subsystems are added to cellular handsets, they're constrained to use the existing battery system and will be compared directly with the cellular implementation. A well-designed 802.11 subsystem can deliver talk times and power budgets comparable to cellular systems only by placing the 802.11 subsystem into sleep mode between transmitting voice packets, just like the cellular systems.