Public Safety Dispatch Centers 102 | 9-1-1 Call Flows
Estimated reading time: 21 minute(s)
Thanks for tuning in for installment #2 in our Public Safety Dispatch Center 100-series blog.
In the 101 post (Public Safety Dispatch Centers 101 – Terminology & Systems) I talked about dispatch center terminology, roles, and systems. In this post, I’ll be talking about the flow of a typical 9-1-1 call.
Before we dive in, in case you’ve wondered about the origin of “9-1-1” being written with hyphens between the digits, as opposed to just “911,” it has to do with every child in US public schools receiving instruction on how to dial for an emergency. When 9-1-1 was first introduced, it was in fact written as “911” without hyphens. But some people read this as “nine eleven,” and children were sometimes confused because there was no “11” key on their phone. So it was changed to be written and spoken as “9-1-1” or “nine one one” (i.e., never “nine eleven”) to avoid confusion.
OK, let’s get started. Figure 1 below illustrates the basic call flow of a typical 9-1-1 call.
1. A 9-1-1 call is generally initiated by on inbound call (or text) from a citizen dialing “9”, “1”, “1” (or in the case of an older PBX, an outside line prefix such as “9” may be needed first, no longer required for new PBXs because of Kari’s Law). Regardless of whether the call is from a wired analog phone, a cell phone, or a VoIP system, the call gets routed to the public switched telephone system (PSTN) where it gets special treatment.
2. This process includes an attempt to locate the rough origin of the call, either by a street address (using the wired phone company’s records), or if wireless, by locating the cell site that the call is coming from. Based on this rough location, a database of Emergency Communications Centers (ECCs, also known as PSAPs) is searched to find the one that most likely covers that rough area. The call is then routed to that ECC.
3. At the ECC, the emergency call taking system begins to ring. If the ECC has Automatic Call Distribution (ACD) the call will be routed to the next available public safety telecommunicator (PST). If the ECC doesn’t have an ACD, then the call rings at every PST’s phone, to be answered by whomever is available. Once answered, the PST’s display shows the rough location of the caller, as well as a call back number.
Typically, 10 seconds after the call, a more precise location is provided for wireless callers (by FCC rule, it is supposed to be within 30 meters of the actual caller 90% of the time). If this more precise location is needed by the PST, they can request a location data update by clicking a button on their screen.
4. The location is typically displayed both textually, and on a map via a Geographic Information System (GIS)
A PST will typically answer a call with “9-1-1, what is your emergency?” They are first trying to verify whether their ECC is the right one to handle this call, and if not, they will transfer the call appropriately. The answering ECC may have received a call that is from an adjacent jurisdiction (cell sites don’t fall on ECC boundaries). Or they may have received a call that is not for the public safety discipline they dispatch (e.g., the primary ECC may only dispatch for law enforcement, and the caller may have a need for fire or EMS). The first ECC to answer a call is the “Primary” ECC (or PSAP). An ECC that receives a transferred call is the “Secondary” ECC.
5. Once the right ECC has been established, the answering PST will ask additional questions of the caller to determine if and what kind of action the reported incident requires. If action is required, it becomes a “dispatchable incident” and the PST will enter the incident into their Computer Aided Dispatch (CAD) system. The CAD system, armed with the incident’s location and type, will then recommend which first responder field units should respond. At this point dispatch can be initiated.
While dispatch is occurring, the answering PST may stay on the phone with the caller to talk the caller through ways to handle or mitigate the emergency. In the case of medical emergencies, the answering PST can give pre-arrival instructions on first aid and CPR to aid citizens on-scene until medics arrive. Sometimes the answering PST will remain with the caller until the dispatched first responders arrive on scene.
6. Dispatch can either occur automatically via data for field units equipped with mobile CAD terminals, or manually via voice transmitted through a Dispatch Console. In the case of voice dispatching, in small centers, the same PST who answered the call, also does the dispatching. In larger centers, there may be PSTs dedicated only to call taking, and others dedicated only to dispatching, and CAD is used to pass the incident information to the right dispatching PST (all PSTs, regardless of duties, will typically have access to CAD). In the case of data dispatching via mobile CAD, the data is typically sent via cellular data service (including present-day FirstNet).
7. Once field units are dispatched, CAD tracks who has been dispatched so that all PSTs know the status of their first responders. First responders actively involved in an incident are removed from the list of available resources and won’t be dispatched again until they clear themselves from the incident. First responders equipped with mobile CAD can clear themselves from an incident. Those without mobile CAD will radio in their status to the ECC dispatch PST.
Often times, law enforcement (LE) responders are by themselves. Dispatch PSTs have a role in ensuring their safety. Every time an LE responder responds to an incident, their status is tracked in CAD. If the LE responder doesn’t respond again within a preset time threshold, their dispatcher is notified and the PST will then try to contact the LE responder to verify they are safe.
Once dispatched, field units are often instructed to use a specific tactical Land Mobile Radio (LMR) channel with which to conduct their incident communications (especially in the Fire/EMS service). This ensures the main dispatch channel remains free for subsequent dispatching.
8. All voice communications, including the 9-1-1 call and the radio dispatch traffic, are recorded on archival logging recorders. This is mandated by law in many states. The recording becomes part of the public record and can be obtained by FOIA (Freedom of Information Act) requests or lawyer subpoenas in court cases. But ECC supervisors also use recordings to survey the performance of PSTs to ensure the ECC’s policies are being met, and for training/coaching purposes to improve future responses.
In addition to the long term archival recordings, various systems also record the most recent voice traffic and make it quickly accessible to the PSTs who were involved. This allows those PSTs to replay voice traffic that may not have been initially understood, and helps eliminate the need to ask for information to be repeated as new voice traffic.
The recording systems can also record “meta data” associated with voice traffic. In the case of 9-1-1 callers, this can include the location and call back number of the caller. In the case of radio traffic, this can include the talker’s radio ID, status and in more advanced systems, the location of the first responder who was originating the traffic.
Increasingly, archival logging recorders are also recording other data, such as CAD keystrokes, screen shots, and other info that can help recreate the sequence of events during an incident. For actual CAD records, the CAD system typically archives that information itself.
That wraps up our Public Safety Dispatch Center 102 post on 9-1-1 call flows. If you have comments, questions, or points you’d like to add please post them.
Stay tuned for installment #3, How Does 9-1-1 Know Where I Am?
By: Randy Richmond
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