PHASE 1 & PHASE 2
WIRELESS CALLS

Prior to the FCC order, the effectiveness of calling 911 from your wireless phone had backtracked to the days of relying on the caller to relay the proper information. This presented an additional challenge because now the caller may not even be at a fixed address - he or she could be on the side of the road, in the woods, or in an unfamiliar place. Obtaining accurate location information from the system is critical because getting it from the caller is even more unreliable.

The first phase of sending this information to the correct PSAP is for the callback number and cell tower of origination to be relayed to the PSAP.

So what happens? When a wireless subscriber initiates a call, the closest tower picks up the signal. The wireless service provider's network also has a switching center that works much like the switches on wireline calls. It reads the digits and forwards the calls accordingly.
The wireless service provider must first program its tower to immediately send any 911 call to the appropriate 911 tandem.

First, there is the Pseudo ANI (PANI). This number identifies the cell face (up to three per tower) or just the tower itself. This can narrow down the location of the caller to several hundred square meters at best, but more often several square kilometers. In addition, there is the Wireless Subscriber's callback number, which is sent along the signal. From the 911 tandem, the PANI, callback number and the voice are forwarded on to the appropriate PSAP. In most Phase I deployments, the callback number is part of the ALI response message.

However, number portability presents a challenge to the system. Today, with the explosion of phone numbers needed for wireless service, pagers, fax and Internet access, the use and number of area codes is in abundance. In addition, wireless phones can now roam all over the country.

Remember the eight-digit number used in wireline calls? There were only four Numbering Plan Digits equaling four area codes. This is no longer adequate.
The capability to use the actual area code and a new signaling protocol between the 911 tandem and the PSAP has required new software to be installed in our 911 tandem switches.
Phase II of bringing wireless 911 to the public means the PSAP has to receive fairly accurate location information from the wireless subscriber. How accurate? Depending on the technology, anywhere from 50 square meters to 300 square meters.
The responsibility of getting this information from the callers falls to the wireless service providers.