FINDING A SIGNAL
PBXs also have problems when internal users are running VoIP over an internal Wi-Fi network. Wi-Fi phones are still comparatively rare in most enterprises, but they're used extensively in a few niche industries. Among their largest users are hospitals, where the ability to locate a person or a piece of equipment can be critical.
If the PBX's database only includes Ethernet ports, it will only be able to tell which AP a user is connecting through, not exactly where the user is. In a dense network where every user connects through the closest AP, that may be enough. But the typical Wi-Fi AP has a range of about 340,000 square feet. That's nearly 50 times the area allowed by the FCC's proposed rules.
It's unlikely that any FCC rules governing enterprise PBXs will cover wireless VoIP. The state laws don't mention it at all, and there are already E-911 regulations for cell phones. These are comparatively lax by Wi-Fi standards: Networks that don't place a GPS receiver inside each phone need to get two-thirds of their location fixes right to within 150 meters, so just finding the nearest AP should be accurate enough.
Enterprises that require accurate location data from internal wireless devices need to consider other systems. The leading one for Wi-Fi networks is RF fingerprinting, also known as Wireless Location Signatures. RF fingerprinting measures the strength of a signal and compares it against a reference model. By understanding how radio waves are affected as they pass through and defract around different building materials, the network can calculate a position to within a few feet. The precise accuracy depends on the building's architecture--the more walls and immovable objects, the better--but it can generally determine which room a user is in.
RF fingerprinting's earliest adopters were European cell phone operators. Many use a system from software company Polaris Wireless, which pitches it mainly as a management technology: Knowing where customers are helps carriers decide where to put more cell towers and allows them to bill based on location. Its initial applications in Wi-Fi were similar. For two years, Newbury Networks has been selling an overlay network of hardware sensors that use RF fingerprinting for security and management. For example, it can restrict coverage to particular areas or pinpoint intruders.
Wi-Fi vendors are now using the technology for more general location-based services. For example, Ekahau sells software that can add RF fingerprinting capability to standard IEEE 802.11 networks, tracking the locations of all users and storing them on a Windows or Linux server. Makers of Wi-Fi APs are also beginning to incorporate RF fingerprinting, as most already provide less accurate location databases. The first to market was Cisco, thanks to its acquisition of Airespace. It stores the tracked location data in a dedicated appliance, which applications can access through a SOAP interface.
Wi-Fi telephony is unlikely to become popular until dual-mode phones that also include cellular capability are available. This will give users another option for location tracking, as all cellular networks now need to incorporate positioning capability. The technologies they use often go beyond the FCC's requirements, though precision can vary widely. For example, the GPS receivers used by CDMA providers such as Sprint and Verizon tend to be much more accurate outside, but can be a problem in office environments.