Enhanced 911 service or E911, is a North American telephone network feature that automatically associates the physical address of a caller with the calling party’s telephone number. This is generally done using a form of reverse telephone directory that is supplied by the telephone company. This provides emergency responders with the location of the emergency without the person calling for help having to provide it.
In the late 90’s some states started to require that E911 functionality be provided for when installing PBX telephone systems. Regulations and requirements very by state. In Washington State, PBX phone systems are required to be E911 compatible in certain circumstances when installed in schools, or when installed in multi-tenant office buildings. This enables an emergency services operator to know not only the address of a caller, but also the building (if there are multiple buildings), floor and suite or room number from which a call originated. This information enables faster response to emergencies and helps to eliminate confusion about a caller’s exact location.
The Asterisk Open Source PBX provides a cost effective way of achieving E911 compatibility when used in conjunction with a telephone carrier that offers E911 database management services to its customers. This database management service consists of a reverse directory maintained by the customer, but residing with the carrier, that associates location information to private (DID) telephone numbers that the customer uses.
In order for E911 to function on an Asterisk PBX, each extension phone must be assigned a 10-digit DID number. This DID number is then registered with the telephone carrier (or contracted ALI database service provider) along with the location of the phone. An ISDN PRI link connects the Asterisk PBX to the carrier. This digital link utilizes a dedicated signaling channel (D-Channel) to handle call setup and signalling. It is the D-Channel that enables E911 functionality through the carrier. When a 911 call is placed on the PBX, Asterisk needs to be programmed to set the outbound CallerID (ANI) for the call to the registered DID for the calling extension. When the call reaches the Public Safety Answering Point (PSAP) this ANI number is matched with the DID number and location information which was pre-registered with the carrier (or contracted ALI database service provider). The exact location of the caller is then displayed to the emergency services operator.
The same method may also be used with Asterisk to provide E911 service when subscribing to services from an E911 enabled VoIP Internet Telephony Service Provider (ITSP).
Asterisk offers a cost effective way to implement E911 services, increasing safety and security within an organization. Even organizations that are not legally required to implement E911 can greatly benefit from its implementation.
E911, Emergency Services, Telephone Network, Telephone System
