1.6 Types and Features of 9-1-1 Systems: Basic, Enhanced & NG9-1-1

1.6 Types and Features of 9-1-1 Systems: Basic, Enhanced & NG9-1-1

Quick Answer: 9-1-1 systems range from Basic 9-1-1, where a call reaches a PSAP with voice only, through ANI-only and ANI/ALI systems, to Enhanced 9-1-1 (E9-1-1), which adds selective routing based on the caller's telephone number, and NG9-1-1, which routes IP calls by location and policy and delivers text, video, and Additional Data. Core features include selective routing, alternate routing (to a backup PSAP when the primary is busy or unavailable), default routing (for calls the network cannot route selectively), call transfers (selective, fixed, and manual), forwarding the caller's ANI with a transfer, router-to-router transfers, and in NG9-1-1, geospatial and policy-based routing.

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1. Types of 9-1-1 Systems

"Enhanced 9-1-1" generally means selective routing combined with ANI/ALI. Some areas historically had ANI/ALI without selective routing, which is why the Body of Knowledge lists these types separately.

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2. Features of Legacy 9-1-1 Systems

Selective Routing

NENA describes selective routing as routing a 9-1-1 call to the appropriate PSAP based on the caller's location. In E9-1-1, the selective router uses the incoming ANI (or a pseudo-ANI for wireless and VoIP calls) to look up the ESN in the selective routing database (SRDB), then sends the call over the trunk group to the PSAP that serves that ESN (1.1 and 1.3).

Alternate Routing

Alternate routing sends calls to a designated alternate PSAP when the primary PSAP cannot take them:

• Busy condition: All trunks to the primary PSAP are in use.
• Unavailable condition: The PSAP is out of service, evacuated, or has activated a "make busy" or night-transfer arrangement.

Alternate routing is set up in the selective router for each PSAP. It must be tested regularly, and the alternate PSAP needs procedures, maps, and agency contact lists for the area it may suddenly serve.

Default Routing

Default routing sends a 9-1-1 call to a designated default PSAP when the call cannot be selectively routed, for example because of an ANI failure or a missing routing record. In legacy networks, each incoming trunk group to the selective router is assigned a default PSAP, so the default depends on the trunk group the call arrived on. Wireless and CLEC trunk groups that serve large areas need especially careful default assignments.

Call Transferring

ANI Resend: Forwarding ANI With Transfers

The ENP Body of Knowledge lists this feature as ANI resend. When the 9-1-1 network transfers a call, it can pass along the caller's ANI (and, where supported, location data) so the receiving PSAP can display the number and query ALI itself instead of relying on a verbal relay. Feature names and capabilities vary by network and equipment, so PSAPs should confirm with their 9-1-1 system service provider which data follows a transferred call, and whether a new ALI query or wireless location update (rebid) is needed after transfer.

Router-to-Router (Tandem-to-Tandem) Transfer

PSAPs served by different selective routers can transfer fully enhanced calls only if the routers are connected by inter-tandem trunks and translations. With router-to-router transfer, the ANI and ALI travel with the call across router boundaries. Without it, the transfer becomes an ordinary 10-digit call and the receiving PSAP loses the caller's automatic information.

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3. Features of NG9-1-1 Systems

Geospatial Routing

The ECRF compares the caller's location (civic address or geodetic coordinates) with authoritative GIS boundary polygons and returns the PSAP that serves that point (4.2 and 8.2). Routing follows the caller's actual location rather than a telephone number record.

Policy-Based Routing

The ESRP applies rules from the Policy Routing Function (PRF) and policy store that the 9-1-1 authority controls. Rules can consider:

• PSAP state: normal, overloaded, closed, or evacuated.
• Queue state: how many calls are waiting.
• Time of day and day of week: for example, overnight consolidation.
• Call type: voice, text, or video.
• Incident conditions: diverting a flood of calls about one known incident.

Policy-based routing makes alternate routing dynamic and adjustable without switch translations.

Transfers and Default Routes in NG9-1-1

NG9-1-1 transfers commonly bridge the receiving agency into the call, and location, Additional Data, and incident information (EIDO) can follow (4.4). When location is missing or the ECRF cannot return a PSAP, the ESRP uses a default route defined in policy (8.2).

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4. Legacy-to-NG9-1-1 Feature Map

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5. Planning Considerations for 9-1-1 Managers

1. Document routing tables: Keep current records of primary, alternate, and default PSAPs for every trunk group and ESN.
2. Test alternate routing: Schedule tests with the 9-1-1 system service provider and the alternate PSAP, and log the results.
3. Maintain transfer lists: Verify selective and fixed transfer destinations whenever agencies or boundaries change.
4. Coordinate across router boundaries: Confirm router-to-router transfer capability with neighboring PSAPs, and update procedures during NG9-1-1 transition.
5. Write NG9-1-1 policies deliberately: Policy routing rules are powerful, so they need change control, testing, and documentation.

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6. Operational Traps & ENP Exam Watch

• Alternate vs. Default Routing: Alternate routing applies when the PSAP cannot take the call; default routing applies when the network cannot determine where the call should go.
• Basic 9-1-1 Has No ANI or ALI: Voice-only delivery is the defining limitation.
• E9-1-1 Means Selective Routing Plus ANI/ALI: ANI/ALI alone does not make a system fully enhanced.
• Selective vs. Fixed vs. Manual Transfer: Selective transfer depends on the ESN, fixed transfer always goes to one place, and manual transfer requires dialing.
• Policy Routing Is Dynamic: In NG9-1-1, the 9-1-1 authority changes routing behavior through policy, not switch translations.