Managing traditional public switched telephone network (PSTN) 911 services in the large enterprise can be a tedious and tangled process. PSTN PBX administrators have had the duties of negotiating local 911 telecom laws, ordering special trunks for 911, validating and updating location records with the local exchange carrier (LEC), and constantly monitoring the network to meet capacity requirements. These difficulties are further compounded in organizations with branch offices. The advent of voice over Internet protocol (VoIP)-based E-911, however, offers a solution to these challenges.

New E-911 services for the IP PBX use virtual trunks, so administrators can turn them up or take them out of service at will. Location records are provisioned once and then automatically validated and updated in real time. Capacity requirement monitoring is not required, because the virtual trunks can swell to the IP capacity of the facility. Branch offices can open their own virtual trunks or be multiplexed at a single facility. The IP PBX provides administrators with an easier, more economical path to providing 911 services to the enterprise.

To understand the value of using a VoIP-based E-911 solution, a basic understanding of 911 for the PBX is useful. When a user in an enterprise dials 911, the PBX recognizes it has received a request for an emergency services call. The PBX refers to its configuration and determines that it needs to send the call to a specialized LEC switch, called a selective router. Selective routers are switches that can only forward calls to emergency operators within the area they are located.

Emergency operators are located in a facility called a public safety answering point (PSAP). The selective router forwards the call to the appropriate PSAP as a result of configuration values provisioned by the LEC. When the call arrives at the PSAP, the PSAP equipment will query a regional database called automatic location identification (ALI). The ALI will provide the caller's address along with the company information and the callback number, if it is available.

If the caller is unable to speak, the PSAP can dispatch appropriate emergency services to the address contained in the ALI record. Administrators can provision the record in the ALI database by utilizing a device referred to as a PS-ALI appliance. ALI record additions take three to five days, and any error in the record requires restarting the process.

The key benefits offered by VoIP E-911 service in the enterprise are realized through a centralized aggregation point called the VoIP position center (VPC). The VPC maintains connectivity to the vast majority of selective routers and ALI databases. While PSTN-based solutions require separate trunks from each location, VoIP E-911 service allows the enterprise to maintain only a single IP connection from its PBX to service its entire location footprint.

Since the VPC maintains connectivity to each one of the regional ALI databases, the enterprise only needs to maintain connectivity to the VPC in order to support connectivity to all the regional ALI databases. This allows companies with a national footprint to easily scale across regional boundaries. The need to establish countless agreements with a host of different providers is no longer necessary.

The VPC will provision a respective ALI with the user' s record at call time. This allows a user to move from location to location without any meaningful latency in the commitment of address changes. The user can modify his address, manually or through an automatic location device, and then pick up the phone and dial 911.

The VPC provides the ability to modify a subscriber' s address in one of two ways. The first is through a Web-based portal that allows an administrator or user to change a subscriber' s location manually. The second is an automatic location information services (LIS) device. The LIS device contains a table of preprovisioned addresses, with each record corresponding to a switch port. When a user moves from one location to another, the LIS device will automatically update the user' s location.

The LIS also can notify the VPC when a user moves to an unknown location, and the VPC will route 911 calls appropriately. In this scenario, the VPC will typically route the call to a call center where the user will be verbally queried as to his location and the call center will perform a handoff to the appropriate PSAP.

Many companies have a large number of employees either working from home or traveling frequently. The emergence of the VoIP PBX has allowed enterprises the ability to issue these users phones that connect directly to the PBX. The VPC enables these users to have 911 services as long as they remain in the United States. Users simply have to add the location of the phone to the VPC.

In the case that a user attempts to provision a malformed address, the VPC identifies the error and prompts the user to either modify the address or presents a list of alternatives. Mobile VoIP phones can also be provisioned to always route to the call center, regardless of their location.

Scalability is important to IT administrators when they select systems for deployment. The VPC allows companies to scale seamlessly across the number of users or the number of locations. For a large banking company, for example, with retail locations, corporate offices and regional offices, the VPC would serve each location equally. If the company decided to architect its telephone network to be serviced by a single PBX or many PBXs, each user would receive the same services.

Andrew Caughey is a product manager for TeleCommunication Systems, Annapolis, Md.

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