How 9-1-1 Works

The 9-1-1 system allows the public to request emergency services simply by dialing or texting, the numbers 9-1-1.  Exactly how that request reaches a Public Safety Answering Point (PSAP) for answering and sending emergency responders depends on what type of device was used to dial 9-1-1.  This document will explain how the 9-1-1 system works with various telephonic devices.

A 9-1-1 system is the sum of the various components needed to operate the system.  These components include the network, software, applications, databases, personnel, and procedures which are required to provide 9-1-1 service. 

The public has a variety of technologies from which they can choose for telephony service today.  Additionally, members of the public may utilize multiple technologies in a given day.  For example, someone may choose to only utilize wireless service for his/her personal use but may utilize VoIP multi-line telephone service at work.  Each technology works differently with 9-1-1, but all of them utilize some combination of information supplied by the phone, the telecommunications service provider, and location databases.  The PSAP tries to provide the same level of service using the data delivered by the 9-1-1 system, regardless of what technology is used to reach 9-1-1.  The accuracy of the caller location data delivered by the 9-1-1 system does vary depending on the type of technology the caller is using to make the 9-1-1 call, which can impact the level of service the PSAP is able to provide by increasing the time needed to locate the emergency.

Wireline 9-1-1 Service

The 9-1-1 system in place in the metropolitan region today was installed in 1981 and was made available for public use in December 1982.  The system was designed as an Enhanced 9-1-1 (E9-1-1) system to be used with traditional wireline phones.

In an E9-1-1 system, when a wireline call is placed to 9-1-1, the telephone number associated with the telephone being used to call 9-1-1 is sent along with the call to the 9-1-1 system.  The process of using the caller telephone number for 9-1-1 call routing and location retrieval is known as Automatic Number Identification (ANI).  The 9-1-1 system takes the ANI (caller’s telephone number) and submits it to a 9-1-1 routing database to determine to which PSAP that 9-1-1 call should be sent.  The information in the routing database comes from the combination of the PSAP’s defined service area and the telephone service location information provided by the wireline customer’s telephone service provider at the time the customer signs up for service.

When the E9-1-1 call is answered by the PSAP, the ANI is sent again over dedicated circuits to query another database called the 9-1-1 Automatic Location Information (ALI) database.  The ALI database provides the telephone service location information provided by the customer’s telephone service provider when the customer signed up for service and sends that location information back to the PSAP.  This happens almost instantaneously.  E9-1-1 was built to provide 9-1-1 service from fixed location telephones, thereby creating an efficient system utilizing an ALI database.  The customer’s telephone service could not be removed from the service location and moved to another location without assistance from the telephone service provider.  Because the telephone service provider handled the moves and updated the 9-1-1 ALI database whenever there was a move, 9-1-1 telecommunicators always had correct location information with each call. 


How 9-1-1 Works

E9-1-1 benefits 9-1-1 calls placed from wireline phones by:

  • Enabling the 9-1-1 call to get to the correct PSAP via a selective router
  • Providing the answering PSAP the caller’s telephone number (ANI)
  • Providing the 9-1-1 telecommunicator at the PSAP with a text display showing the address of where the calling phone is installed, the name of the subscriber for that telephone number, and the unique grouping of emergency response agencies which service that address.

Though call-back number and location information are provided automatically to the 9-1-1 telecommunicator in an E9-1-1 system, as a best practice telecommunicators ask callers to confirm the information provided by the 9-1-1 system for accuracy, and to ensure the 9-1-1 telecommunicator can call the caller back if disconnected for some reason.  When calling 9-1-1, wireline callers should be prepared to confirm their location information with the telecommunicator.

Wireless 9-1-1 Service

 Today, over 80% of 9-1-1 calls are made via wireless telephones.  These 9-1-1 calls are routed into the 9-1-1 system which was designed to support wireline calls well before wireless telephones were invented.  Because the wireless service providers must utilize the existing 9-1-1 system, which was not designed specifically for wireless service, new 9-1-1 processes had to be developed for wireless calls which added complexity to the 9-1-1 system.

Wireless 9-1-1 calls originate as a wireless radio transmission between the phone and a wireless telephone service provider antenna tower.  The equipment at the tower converts the call from wireless to a wired network connecting the tower to the wireless service provider’s switch.  The wireless service providers utilize their own 9-1-1 routing databases which associate a specific PSAP with each of the wireless service provider’s tower antenna coverage areas.  Wireless 9-1-1 calls are routed based on the tower/antenna that is handling the radio transmission from the caller’s phone. 

Because wireless phones are mobile and can be used across the country, the telephone numbers for wireless phones are not in the traditional ALI database described above for wireline phones.  The wireless service providers created a pool of re-usable pseudo ANI (pANI) telephone numbers that are associated with each individual PSAP in the wireless service provider’s service area.  When a wireless 9-1-1 call is made, the wireless service provider’s switch assigns a pANI to the call and sends the pANI to the PSAP where the wireless 9-1-1 call will be routed.  At the same time, the wireless service provider’s equipment is determining where the caller is really located.  The caller location and the caller’s telephone number are loaded in real time into a re-usable shell record in the traditional ALI database that is associated with the pANI the wireless carrier is using for the 9-1-1 call.  When the PSAP gets the wireless call, the PSAP equipment uses the pANI to query the traditional ALI database which returns the pANI shell record now containing the caller’s location and telephone number. 

Wireless caller location is identified by a set of geographic coordinates (latitude/longitude) which can be plotted to a point on a map showing approximately where the caller is located.  The accuracy of the geographic coordinates can vary depending on the technology used by the telecommunications service provider, terrain, buildings, and the tower locations; generally, locations provided are within 50 – 300 meters from the caller’s location.  9-1-1 telecommunicators ask wireless callers to verify their location by asking for a street address, street and cross street, or by having the caller describe their surroundings if they are outdoors.  Work is currently underway to try to improve the level of wireless 9-1-1 caller location accuracy determination by the wireless carrier’s equipment, including indoor caller location accuracy.  Efforts are underway to utilize the location information most wireless devices contain and use for other purposes, so that device location can be sent to PSAPs and used to route 9-1-1 calls when the device location information is more accurate than the wireless carrier’s equipment is able to determine.   


Text-to-9-1-1 is now available throughout Minnesota.    Texts-to-9-1-1 route similarly to wireless calls, though they utilize different switching and routing systems to reach the appropriate PSAP for the caller’s location.

Given that text-to-9-1-1 relies on location information provided by a wireless carrier, texts are subject to the same location constraints discussed above in relation to wireless 9-1-1 calls.  Text calls also take the PSAP longer to process, which may lead to delays in response.  Text-to-911 should be utilized only when speaking out loud may put the caller in danger or in other circumstances where making a voice call is not possible. 

Remember, “Call if you can, text if you can’t.


VoIP 9-1-1 Service

Voice over Internet Protocol (VoIP) telecommunications service has been growing in popularity.  Many legacy wireline telecommunications service providers are transitioning their networks to VoIP technology.  This type of service would be identified as “fixed” VoIP 9-1-1 service, meaning that the customer cannot move the VoIP equipment to a new location without notification to the service provider.  Fixed VoIP telecommunications service providers continue to enter the customer’s location information into the 9-1-1 location database when the customer signs up for this type of service, as is done in legacy analog wireline service.

Other VoIP providers allow for “nomadic” VoIP telecommunication service which relies on the customer’s high-speed Internet service to transport telephone calls.  Nomadic VoIP telecommunications service providers utilize simple equipment the customer connects directly to the his/her Internet service modem.  The customer can easily move the VoIP service provider equipment and have telecommunications service wherever they connect the VoIP equipment to another Internet connection.  Because the VoIP telecommunications service provider does not know where the customer is using the service, the customer is responsible for providing their service location to enable the 9-1-1 system to work properly.   When the customer signs up for service on the VoIP telecommunications service provider’s Internet website, the VoIP service provider will have a webpage where the customer enters his/her 9-1-1 location information.  If the customer moves the VoIP equipment to a new location, the customer is responsible for going back to the VoIP service provider’s website and updating his/her 9-1-1 location information.  If the customer fails to either provide their location at sign-up or after a move, their 9-1-1 calls will not route properly and will delay emergency response.

The nomadic VoIP telecommunications service providers send all 9-1-1 calls from their customers to a VoIP Positioning Center (VPC).  The VPCs maintain a location database that holds all the location information provided by the nomadic VoIP service customers.  The VPCs also have the PSAP service area boundaries, which they utilize to determine to which PSAP to send a nomadic VoIP call.  VPCs plot the service address supplied by the nomadic VoIP customer and see in which PSAP service area the location falls.  This works well when users confirm their actual location.  However, the 9-1-1 community has many examples which show the problems that can occur when the customer does not update their service address.  For example, if a nomadic VoIP user confirmed being in Minneapolis when they signed up for service, but has now moved to Honolulu, his/her 9-1-1 call will route to the Minneapolis Emergency Communications Center rather than the Honolulu PSAP. Alternatively, there have been cases where users have input shortened city names, which have caused problems.  Several years ago, a VoIP user input his city as “Chan” rather than Chanhassen.  He called 9-1-1 because his house was on fire.  The call went to a national call center because the VPC didn’t recognize “Chan” as a city in Minnesota; the national call center further delayed the call because it didn’t know that “Chan” meant Chanhassen, MN and that the call should be routed to Carver County Sheriff’s Office.  While the call was eventually sent to the correct PSAP, it was too late to save the caller’s home.

Multi-line Telephone Systems 9-1-1 Service

Multi-line telephone systems, sometimes known as Private Branch Exchange (PBX) systems, have been installed by businesses and governments all over the country.  Some are also installed in residences, such as apartment buildings or other multi-tenant buildings.  PBX systems are often used by a business or government with many telephones within a building or when they have large numbers of telephones at multiple locations as a way of containing costs.  For example, Hennepin County, MN has multiple building facilities each with many telephones in different cities, but all operate on one phone system which is owned or leased by Hennepin County.  Under Minnesota law, PBX system owners have the responsibility for providing accurate location information for each of the telephones attached to the system when one of those telephones is used to call 9-1-1.  Depending on the type of PBX system, 9-1-1 calls route according to the wireline or VoIP methods previously described; how calls are routed to 9-1-1, and the ability of first responders to find a caller within a large building or in a multi-building configuration, is dependent upon the accuracy of the location information the PBX owner provides to the 9-1-1 system.

Conclusion to Next Generation 9-1-1

At the beginning of this document, it was stated that the metro region E9-1-1 system was installed in 1981.  Though the system was originally designed for wireline service, solutions have been made to allow for wireless and VoIP calls to work with E9-1-1 systems across the country, although those solutions are not ideal.  Given this, some might ask “What will 9-1-1 look like in the future?”

The Metropolitan Emergency Services Board is working closely with the State of Minnesota to determine design requirements for and transition to a Next Generation 9-1-1 (NG9-1-1) system over the next five years.  NG9-1-1 systems are IP-based and provide for more functionality and flexibility to support technology changes.  At some point during the transition to NG9-1-1, 9-1-1 calls will route based on Geographic Information Systems (GIS) data.  For wireless 9-1-1 calls, this means calls will route based on the actual latitude and longitude (x/y) coordinates (and later z-coordinate – elevation) of the wireless device.  Additionally, the x/y coordinate will display in the telecommunicator’s call information and map, making it easier for emergency services to find callers. 

When wireless carriers have the capability, NG9-1-1 systems will also support the ability of callers to send images or streaming video of emergency scenes to 9-1-1.  This information can be forwarded to law enforcement, fire, or EMS responders to give them a better understanding of the emergency.  NG9-1-1 systems will allow customers to provide additional information they believe will help emergency responders, such as building floorplans, photos for identification of suspects or victims, or even their own medical history pertaining to allergic reactions medicines, for example. 

Even though many things are changing in the 9-1-1 system, the goal of the MESB remains the same; it will continue to work with PSAPs, telecommunications service providers, and emergency responders to deliver the best emergency response possible for the people who live, work, and travel through the ten-county Minneapolis/St. Paul metropolitan area when they dial 9-1-1.