Map of the 911 System | From Call to Response

National Action Teams ReImagine911
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Map of the 911 System

Content compiled by Floria Chan


Vera.org released the full report, A Review of the Literature as it Relates to Policing, and a shorter fact sheet on the topic. The following diagram is a simple generic overview of the 911 call processing system. Across jurisdictions, there are differences in the system which are not captured here.


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Acronym and Terminology Key

ALI | Automatic Location Identification / Information

  • The automatic display at the PSAP of the caller’s telephone number, the address/location of the telephone and supplementary emergency services information of the location from which a call originates.

ANI | Automatic Number Identification / Information

  • The telephone number associated with the call origination, originally associated with the access line of the caller.

CAD | Computer Aided / Assisted Dispatch

  • A computer-based system, which aids PSAP Telecommunicators by automating selected dispatching and record keeping activities.

Caller Location

  • A location provided in an ALI Response (in E9‑1‑1) or a PIDF-LO (in NG9‑1‑1), represented as a civic location or geodetic location, that identifies the location of the caller. In NG9‑1‑1, this is often the same as the routable location but may change during a 9‑1‑1 call.

MSAG | Master Street Address Guide

  • A database of street names and house number ranges within their associated communities defining Emergency Service Zones (ESZs) and their associated Emergency Service Numbers (ESNs) to enable proper routing of 9‑1‑1 calls.

Phases of Wireless Implementation

  • Wireless Phase 0 There are 3 stages that are referred to in implementing Wireless 9-1-1. The most basic of these, sometimes unofficially called Wireless Phase 0, simply means that when you dial 9-1-1 from your cell phone a call taker at a public safety answering point (PSAP) answers. The call taker may be at a state highway patrol PSAP, at a city or county PSAP many miles away, or at a local PSAP, depending on how the wireless 9-1-1 call is routed.

  • Wireless E9-1-1 Phase I is the first step in providing better emergency response service to wireless 9-1-1 callers. When Phase I has been implemented, a wireless 9-1-1 call will come into the PSAP with the wireless phone call back number. This is important in the event the cell phone call is dropped, and may even allow PSAP employees to work with the wireless company to identify the wireless subscriber. However, Phase I still doesn’t help call takers locate emergency victims or callers.

  • Wireless E9-1-1 Phase II To locate wireless 9-1-1 callers, Phase II must have been implemented in the area by local 9-1-1 systems and wireless carriers. Phase II allows call takers to receive both the caller’s wireless phone number and their estimated location information.

PSAP | Public Safety Answering Point

  • An entity responsible for receiving 9‑1‑1 calls and processing those calls according to a specific operational policy.

PSTN |Public Switched Telephone Network

  • The network of equipment, lines, and controls assembled to establish communication paths between calling and called parties in North America.

SR | Selective Router

  • The Central Office that provides the tandem switching of 9-1-1 calls. It controls delivery of the voice call with ANI to the PSAP and provides Selective Routing, Speed Calling, Selective Transfer, Fixed Transfer, and certain maintenance functions for each PSAP.