2.1 Wireless 9-1-1: Phase I, Phase II & Location Accuracy

2.1 Wireless 9-1-1: Phase I, Phase II & Location Accuracy

Quick Answer: Wireless 9-1-1 evolved under Federal Communications Commission (FCC) Docket 94-102 across three regulatory milestones: Phase 0 (basic voice connection to a default PSAP without callback or location, mandating transmission of calls from unsubscribed handsets), Phase I (caller callback number [MIN/MDN] plus cell site and sector identification), and Phase II (geodetic latitude and longitude meeting statistical accuracy benchmarks of 50m at 67% and 150m at 90% for handset-based solutions like A-GPS). To deliver dynamic coordinates across legacy circuit-switched networks without cost-prohibitive infrastructure overhauls, the industry standardized on Non-Call Associated Signaling (NCAS). NCAS decouples the voice path (steered to the Selective Router via a temporary Emergency Services Routing Key [ESRK] pseudo-ANI) from the out-of-band data path, where the PSAP queries the carrier's Mobile Positioning Center (MPC) or Gateway Mobile Location Center (GMLC) via the regional ALI database across the J-STD-036 E2 interface.

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1. Regulatory Genesis: FCC Docket 94-102 & PSAP Prerequisites

During the early 1990s, the explosive consumer adoption of cellular telephony created an unprecedented life-safety crisis for public safety communications. Unlike traditional wireline telephones, which were permanently tied to dedicated copper pairs documented in carrier billing databases, wireless handsets were mobile, nomadic, and untethered. Millions of cellular 9-1-1 calls poured into Public Safety Answering Points (PSAPs) with neither a callback telephone number nor an address. Panicked, injured, or disoriented callers navigating unfamiliar highways had no means of identifying their physical location, and telecommunicators had no capability to reconnect if calls disconnected.

The FCC opened CC Docket 94-102 in 1994 and adopted its landmark wireless E9-1-1 Report and Order in 1996. The order promulgated federal regulations compelling Commercial Mobile Radio Service (CMRS) cellular carriers to engineer network and handset interfaces capable of bringing wireless emergency calling to technical parity with wireline Enhanced 9-1-1 (E9-1-1).

+-------------------------------------------------------------------------+
|                   FCC DOCKET 94-102 REGULATORY TIMELINE                 |
+-------------------------------------------------------------------------+
| PHASE 0 (1996 Mandate):                                                 |
| - Deliver all wireless 9-1-1 calls to a designated default PSAP         |
| - Free of toll or airtime charges to the subscriber                     |
| - Mandate connection of Non-Service-Initialized (NSI) handsets          |
| - Zero callback number; zero caller location data                       |
|                                                                         |
| PHASE I (1998 Mandate):                                                 |
| - Deliver 10-digit callback number: Mobile Directory Number (MDN)       |
| - Deliver originating cell site location and antenna sector face        |
| - Coarse sector-based routing to appropriate local PSAP                 |
|                                                                         |
| PHASE II (2001+ Phased Implementation):                                 |
| - Deliver 10-digit callback number plus geodetic coordinates (Lat/Long) |
| - Statistical accuracy: 50m (67%) / 150m (90%) for Handset (A-GPS)     |
| - Statistical accuracy: 100m (67%) / 300m (90%) for Network (U-TDOA)    |
| - Introduces dynamic pseudo-ANIs (ESRK) and carrier location gateways   |
+-------------------------------------------------------------------------+

The Three Implementation Prerequisites

To balance carrier technical feasibility with municipal public safety readi­ness, the FCC originally established three conditions before a CMRS provider was legally obligated to implement Phase I or Phase II service within a given 9-1-1 service area:

1. A Valid Request by the PSAP: The local 9-1-1 governing authority was required to submit a formal, written request to the wireless carrier petitioning for Phase I or Phase II service deployment.
2. PSAP Technical Capability: The PSAP was required to certify and demonstrate that it had procured, installed, and tested call-handling equipment (CHE), Computer Aided Dispatch (CAD), and ALI database links capable of receiving, parsing, and displaying the callback number, cell sector ID, and geodetic coordinate records.
3. Cost Recovery Mechanism: A cost recovery mechanism was initially required under state or local law allowing carriers to recoup capital expenditures. However, because cost recovery disputes created multi-year deployment stalemates across dozens of states, the FCC subsequently eliminated the cost recovery prerequisite, making carrier compliance mandatory upon a valid PSAP request.

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2. Phased Architecture Breakdown: Phase 0, Phase I & Phase II

For the Emergency Number Professional (ENP), mastering the functional capabilities and operational limits of each wireless phase is essential:

Phase 0: Basic Wireless 9-1-1

Phase 0 represents the foundational requirement for CMRS providers. Calls are routed across standard public switched telephone network (PSTN) trunks to a single default PSAP—typically a state police dispatch barracks or regional sheriff's communications center—based strictly on the originating cell tower location.

• Delivered Data: None. The call-taker receives voice audio only. The telecommunicator's console displays an administrative central office trunk number or pilot number, but no caller telephone number and no caller location.
• Non-Service-Initialized (NSI) Handsets: Phase 0 established the federal requirement that carriers must complete all 9-1-1 calls from any cellular telephone capable of accessing their radio frequency spectrum, regardless of whether the handset has an active subscription, valid SIM card, or paid service plan. These "911-only phones" (often donated by community organizations to domestic violence survivors) transmit an unassigned pseudo-MIN (such as 911-000-0000 or the handset's Electronic Serial Number [ESN] / International Mobile Equipment Identity [IMEI]). Critical Operational Trap: If an NSI caller disconnects or is incapacitated, the telecommunicator cannot initiate a callback, because no dialable telephone number exists on the public network.

Phase I Wireless 9-1-1

Within six months of receiving a valid PSAP request, carriers were required to deploy Phase I capabilities. Phase I provides two critical data elements:

1. Callback Number: The subscriber's 10-digit Mobile Identification Number (MIN) or Mobile Directory Number (MDN), enabling telecommunicators to call the subscriber back immediately if the call drops.
2. Cell Site and Sector Identification: The physical location of the cell tower and the specific antenna sector (azimuth/face, typically representing a 120-degree beam on a standard three-sector cellular array) capturing the radio transmission.

• Operational & Routing Impact: Phase I introduced coarse sector-based routing. Rather than routing every call from a multi-county cell tower to a single centralized answering point, the wireless carrier's Mobile Switching Center (MSC) can route Sector 1 (facing north toward City A) to the City A PSAP, and Sector 2 (facing south toward County B) to the County B PSAP.

Phase II Wireless 9-1-1

Phase II represents true geodetic emergency location. In addition to the Phase I callback number and cell tower address, Phase II delivers the caller's geodetic coordinates: latitude, longitude, an uncertainty radius, and a confidence percentage.

• Operational Reality: Phase II allows modern CAD and Geographic Information System (GIS) mapping engines to instantly project a visual point-and-error circle onto the telecommunicator's console, enabling immediate dispatch even when callers cannot articulate their surroundings.

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3. FCC Horizontal Accuracy & Statistical Confidence Standards

When promulgat­ing Phase II rules in Docket 94-102, the FCC recognized that mobile positioning technologies operated under different physical constraints. Consequently, the FCC promulgated a dual accuracy standard bifurcated by technological architecture:

Mathematical Principles: Uncertainty and Confidence

Every Phase II ALI coordinate payload delivered to a PSAP contains statistical metadata describing the precision and mathematical validity of the position fix:

• Uncertainty Radius ($h$-accuracy): Expressed in meters, uncertainty defines the estimated geographic spread of the error boundary. In basic implementations, it is formatted as a circular radius $r$; in advanced systems, it is represented as an error ellipse defined by a semi-major axis, semi-minor axis, and orientation angle.
• Confidence Percentage: Expressed as a percentage (typically standardized at 90% by public safety conventions), confidence represents the statistical probability that the mobile device is physically located inside the defined uncertainty boundary.
• Operational Reading: If a Phase II ALI record displays LAT: 32.7767, LON: -96.7970, UNC: 35m, CONF: 90%, the telecommunicator knows with 90% mathematical probability that the caller is within a 35-meter radius of that coordinate point.

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4. Location Determination Technologies: Handset-Based vs. Network-Based

To satisfy Phase II mandates, carriers deployed two distinct engineering paradigms:

[HANDSET-BASED: ASSISTED GPS (A-GPS)]       [NETWORK-BASED: UPLINK TDOA (U-TDOA)]

         🛰️ GNSS Satellites                           Tower 1          Tower 2
             ╲      ╱                                    ╲            ╱
              ╲    ╱                                      ╲          ╱
               ▼  ▼                                        ▼        ▼
             [📱 Phone]                                   [LMU 1]  [LMU 2]
                 │                                            │        │
          Cellular Tower                                      ▼        ▼
       (In-Band Assistance Data:                       [Central Location Server]
       Ephemeris, Almanac, Timing)                    (Hyperbolic Multilateration)

Handset-Based Architecture: Assisted GPS (A-GPS)

Standalone Global Positioning System (GPS) chips in commercial phones face severe constraints in emergency situations: downloading satellite orbital data (ephemeris) directly from satellites requires 30 to 60 seconds of uninterrupted line-of-sight signal at 50 bits per second (Time To First Fix - TTFF). Furthermore, satellite signals are extraordinarily weak (-160 dBm) and attenuate heavily through building walls, roofs, and dense urban foliage.

Assisted GPS (A-GPS) overcomes these obstacles by coupling the handset's GPS receiver with the cellular network:

1. The carrier maintains a nationwide terrestrial network of continuously operating reference GPS receivers.
2. When a subscriber dials 9-1-1, the cellular base station injects assistance data (precise satellite ephemeris, almanac, clock drift corrections, and expected Doppler shifts) directly into the handset over the high-speed cellular control channel.
3. Armed with assistance data, the phone's GPS receiver knows precisely which satellites to look for and where they are in the sky, locking onto weak signals and resolving pseudo-ranges in under 2 to 5 seconds.
4. The handset computes its own coordinates (handset-based) or transmits raw pseudo-ranges back to a carrier server to calculate coordinates (handset-assisted).

Network-Based Architecture: Uplink Time Difference of Arrival (U-TDOA)

Network-based architectures require no specialized GPS chips or software inside the caller's phone, ensuring complete backwards compatibility with 100% of legacy mobile devices:

• Mechanism: Specialized Location Measurement Units (LMUs) are installed at cell tower sites. When a mobile phone transmits radio frequency bursts on the uplink channel during a 9-1-1 call, three or more geographically separated LMUs timestamp the signal's arrival using GPS-synchronized timing references.
• Hyperbolic Multilateration: The system calculates the time difference of arrival between tower pairs. Each time difference defines a hyperbolic curve; the mathematical intersection of three or more hyperbolic curves resolves the horizontal position of the handset.
• Physical Limitations: Network solutions depend heavily on tower geometry. If cell towers are arranged in a straight line (along an interstate highway) or if the signal is received by fewer than three towers, geometric dilution of precision (GDOP) degrades, and multilateration fails.

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5. Signaling Architectures: CAS vs. Non-Call Associated Signaling (NCAS)

Deploying wireless E9-1-1 presented an immense telecommunications dilemma: how could carriers pass a 10-digit callback number and geodetic coordinates across legacy Centralized Automatic Message Accounting (CAMA) analog trunks engineered in the 1950s to carry only 7 or 8 Multi-Frequency (MF) tones?

Two competing signaling architectures emerged:

Call Associated Signaling (CAS)

In Call Associated Signaling (CAS), voice audio and signaling data travel together across the same integrated trunk groups. To transmit data, the carrier network utilized either 20-Digit CAMA Signaling (pulsing two successive 10-digit MF tone bursts: KP + 10 digits + ST followed by a second KP + 10 digits + ST) or upgraded digital ISDN-PRI circuits carrying data on the D-channel.

• Why CAS Failed to Dominate: CAS was cost-prohibitive, required expensive hardware replacements at every legacy PSAP, added call setup delay while the long MF digit string pulsed across the trunk, and could not easily accommodate dynamic Phase II coordinate refreshes. It was largely abandoned in North America in favor of NCAS.

Non-Call Associated Signaling (NCAS)

Non-Call Associated Signaling (NCAS) became the universal standard for wireless 9-1-1 in North America. NCAS solved trunk limitations by decoupling the voice path from the data path:

                      [Mobile Subscriber Dials 9-1-1]
                                     │
                                     ▼
                      [Mobile Switching Center (MSC)]
                                     │
         ┌───────────────────────────┴───────────────────────────┐
         ▼ (Voice Path: Legacy TDM Trunks)                       ▼ (Data Path: IP Data Links)
  [Selective Router]                                     [Carrier Location Center]
  - Ingests 10-digit ESRK pseudo-ANI                     - MPC (CDMA) or GMLC (3GPP)
  - Routes voice to PSAP based on ESRK                   - Requests Phase II fix
         │                                               - Stores record indexed by ESRK
         ▼ (Voice + ESRK over CAMA/PRI)                          │
  [PSAP CHE Console]                                             │ (J-STD-036 E2 Interface)
  - Answers voice audio                                          │
  - Extracts ESRK from incoming trunk                            ▼
  - Launches out-of-band ALI query ──────────────► [Regional ALI Database Server]
                                                   - Identifies ESRK as wireless pool
                                                   - Queries carrier MPC/GMLC via E2
                                                   - Formats and returns Phase II record

Step-by-Step NCAS Call Flow

1. Call Initiation: A mobile caller dials 9-1-1. The local cell tower captures the call and signals the Mobile Switching Center (MSC).
2. ESRK Allocation: The MSC queries its location engine—the Mobile Positioning Center (MPC) for CDMA or Gateway Mobile Location Center (GMLC) for GSM/LTE/5G. The MPC/GMLC assigns an Emergency Services Routing Key (ESRK) from a dynamic pool reserved for that specific PSAP.
3. Voice Route: The MSC outpulses the call across dedicated TDM/CAMA trunks to the regional E9-1-1 Selective Router, delivering the ESRK as the calling party number (ANI).
4. Selective Routing: The Selective Router references the ESRK in its routing tables and directs the voice call to the designated PSAP.
5. Data Staging: Simultaneously, the MPC/GMLC creates a dynamic record indexed by the ESRK, storing the caller's true callback number (MDN) and cell site sector (Phase I), while initiating a Phase II A-GPS or U-TDOA positioning fix.
6. PSAP Answer & ALI Query: The PSAP answers the call, extracts the ESRK from the voice trunk, and sends an out-of-band ALI query to the regional ALI database.
7. The E2 Gateway Query: The regional ALI server recognizes the ESRK prefix as a wireless pool and launches a real-time query across the standardized J-STD-036 E2 Interface to the carrier's MPC/GMLC.
8. Data Presentation: The MPC/GMLC transmits the callback number, cell sector address, and geodetic coordinates to the ALI server, which delivers a standard NENA ALI record to the telecommunicator's console.
9. Teardown & Reclamation: When the call terminates, the voice path drops, the dynamic database record is flushed, and the ESRK is returned to the idle pool for reuse on subsequent 9-1-1 calls.

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6. Wireless Core Network Elements & Pseudo-ANI Taxonomy

Core Network Nodes: MPC vs. GMLC

• Mobile Positioning Center (MPC): The authoritative location gateway deployed in ANSI-41 / CDMA wireless networks. It coordinates location determination requests, interfaces with Position Determining Entities (PDEs), and interfaces with public safety ALI databases via the E2 protocol.
• Gateway Mobile Location Center (GMLC): The standardized 3GPP location gateway deployed across GSM, UMTS, LTE, and 5G networks. The GMLC interfaces with the Serving Mobile Location Center (SMLC) or Location Management Function (LMF) to retrieve positioning data and services incoming E2 queries.

Pseudo-ANI (p-ANI) Classification: ESRD vs. ESRK

A Pseudo-ANI (p-ANI) is a 10-digit number conforming to North American Numbering Plan (NANP) formatting that is non-dialable from the public telephone network. In wireless 9-1-1, p-ANIs act as database routing keys:

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7. PSAP Operational Realities: ALI Re-Bids & Call Transfers

For the practicing telecommunicator and ENP supervisor, wireless location delivery is dynamic, not instantaneous:

The ALI Re-Bid Process

When a wireless 9-1-1 call lands at a PSAP console, the initial ALI display frequently presents Phase I data only (the cell tower address and sector). This occurs because the initial call routing must occur within 2 to 3 seconds, whereas carrier A-GPS or U-TDOA positioning calculations require 10 to 25 seconds to resolve.

• Operational Standard: Telecommunicators must execute an ALI Re-Bid (pressing the Re-Bid or Refresh key on the call handling console). The CHE console re-queries the regional ALI database, which interrogates the MPC/GMLC over the E2 interface. If the carrier's location engine has completed its calculation, the ALI display dynamically updates from Phase I coarse sector data to Phase II precise geodetic coordinates.
• Tracking Moving Callers: If a caller is traveling in a vehicle or fleeing a perpetrator, telecommunicators must execute periodic ALI re-bids every 30 to 60 seconds to pull updated coordinates from the carrier network.

Call Transfer Constraints Over Legacy Trunks

When a wireless call must be transferred to a neighboring secondary PSAP (such as county EMS or state highway patrol) over legacy CAMA trunks, the out-of-band data link does not automatically transfer. Only the 10-digit ESRK transfers across the voice trunk. The secondary PSAP's CHE must independently launch an ALI query to retrieve the location record. If the secondary PSAP lacks a direct E2 data link to that wireless carrier's MPC/GMLC, the transfer will result in an ALI failure, forcing the transferring telecommunicator to verbally convey coordinates over the voice bridge.