1.2 Legacy Location Infrastructure: MSAG, ANI, ALI & ESN/ESZ

1.2 Legacy Location Infrastructure: MSAG, ANI, ALI & ESN/ESZ

Quick Answer: Legacy E9-1-1 location delivery relies on a tripartite data infrastructure: the Master Street Address Guide (MSAG), Automatic Number Identification (ANI), and the tabular Automatic Location Identification (ALI) database. The MSAG establishes valid civic address ranges and binds them to Emergency Service Numbers (ESNs), which define unique Emergency Service Zones (ESZs). When a 9-1-1 call arrives, the PSAP receives the caller's ANI via CAMA or PRI signaling and issues an out-of-band query to the regional ALI database (via PAM, Bisync, or TCP/IP). The ALI database returns the caller's pre-provisioned static record (for wireline) or dynamic record (for wireless/nomadic), displaying the exact civic address, ESN, and designated primary and secondary responding agencies.

---

1. The Master Street Address Guide (MSAG)

The Master Street Address Guide (MSAG) is the authoritative relational database that defines valid street names, address number ranges, community names, and emergency response assignments across a 9-1-1 authority's territory. It serves as the primary validation standard against which telephone service orders must verify before addresses receive automated emergency routing.

MSAG Schema and Data Structure

An MSAG record does not store customer names or telephone numbers. Instead, it defines road segments using geographic and jurisdictional attributes:

• Low Address Number: Lowest numeric building number in the road segment.
• High Address Number: Highest numeric building number in the segment.
• Parity Indicator (Odd / Even / Both):
  • O (Odd): Applies exclusively to odd-numbered addresses.
  • E (Even): Applies exclusively to even-numbered addresses.
  • B (Both): Encompasses both sides of the roadway within the numeric range. Operational Significance: Parity splitting is required when thoroughfares form political boundaries. If a roadway divides two municipalities, even numbers may belong to City A (ESN 101) and odd numbers to City B (ESN 102). Without exact parity coding, calls from one side of the street misroute to the wrong agency.
• Directional Prefix / Postfix: Leading and trailing navigational indicators (e.g., N, S, SW).
• Street Name and Suffix: The standardized thoroughfare name and abbreviation (e.g., MAIN ST, PARK AVE).
• MSAG Community Name: The public safety community name. Exam Warning: The MSAG community frequently differs from the United States Postal Service (USPS) mailing city. Postal cities reflect mail delivery hubs, whereas MSAG communities designate legal political boundaries, fire taxing districts, or municipal dispatch zones.
• Emergency Service Number (ESN): The numerical code establishing agency routing.
• Exchange (NXX): The serving Central Office exchange code.

---

2. Emergency Service Zones (ESZ) & Emergency Service Numbers (ESN)

Emergency Service Zones and Numbers define the core routing logic of legacy E9-1-1 networks:

Emergency Service Zone (ESZ)

An Emergency Service Zone (ESZ) is a defined geographic area that shares an identical, homogeneous combination of emergency response agencies. Within a single ESZ, every address is assigned:

• The exact same Primary PSAP
• The exact same Law Enforcement agency
• The exact same Fire Protection district
• The exact same Emergency Medical Service (EMS) provider
• The exact same Secondary PSAP (if applicable)

If any single responding agency changes—such as a boundary where ambulance coverage transitions to an alternate hospital service—a new, distinct ESZ must be established.

Emergency Service Number (ESN)

An Emergency Service Number (ESN) is the 3- to 5-digit numerical code representing an ESZ. ESNs function in two operational roles:

1. Administrative ESN: Stored in the MSAG and ALI database. When an ALI record appears on the telecommunicator's console, the administrative ESN automatically populates the text fields indicating the specific police precinct, fire station, and ambulance unit responsible for dispatch.
2. Routing ESN: Programmed into the Selective Router's translation matrix. The router references the routing ESN to select the outgoing trunk group connected to the designated Primary PSAP.

Primary vs. Secondary PSAPs and Selective Transfer

• Primary PSAP: The initial facility terminating the emergency call from the Selective Router.
• Secondary PSAP: A specialized center (such as a regional fire dispatch or university police agency) receiving transferred calls.
• Selective Transfer: E9-1-1 switches enable automated one-button transfers. When a primary call taker presses a dedicated "FIRE" button, the Selective Router references the call's active ESN and automatically transfers the voice audio and ANI to the specific fire dispatch center assigned to that ESN.

---

3. Automatic Number Identification (ANI) Mechanics

Automatic Number Identification (ANI) is the calling party's billing telephone number generated by the originating Class 5 Central Office and transmitted across dedicated voice trunks.

In-Band CAMA Multi-Frequency (MF) Transmission

Over dedicated CAMA trunks, ANI is transmitted in-band using Multi-Frequency (MF) tones following a strict handshake:

1. Trunk Seizure: The Central Office closes the loop on an idle trunk.
2. Wink Start: The Selective Router returns a reverse-battery pulse (140 to 290 ms) acknowledging readiness.
3. MF Digit Pulsing: The Central Office pulses audio tones: KP + Information Digit + ANI + ST.
4. Voice Path Cut-Through: The circuit establishes bidirectional audio.

Information Digits (Class of Service)

The digit transmitted between the Key Pulse (KP) and the telephone number identifies the line class:

• 0: Standard single-party residential or business line (valid ANI).
• 1: Multi-party line (ANI cannot be automatically verified; requires manual inquiry).
• 2: ANI failure (switch malfunction or noise corrupted digit collection).

Common ANI Failures

• ANI Failure (000-000-0000 or 911-0000): Displayed when trunk hardware fails to wink, noise corrupts tones, or an unassigned line dials 9-1-1.
• PBX Billing Number Slip: Enterprise PBX systems lacking direct station identification transmit the corporate Billing Telephone Number (BTN) instead of the individual office extension, obscuring the caller's specific room location.

---

4. Automatic Location Identification (ALI) Database Architecture & SOP

The Automatic Location Identification (ALI) database is a high-availability tabular repository correlating telephone numbers (ANI) with validated civic addresses, subscriber names, and ESNs.

Service Order Processing (SOP) & Electronic Bonding

The database is maintained by the 9-1-1 System Service Provider (incumbent local exchange carriers or third-party database managers) through Service Order Processing (SOP):

1. When a subscriber establishes new landline service or relocates (Move, Add, Change - MAC), the carrier enters the order into its provisioning system.
2. Carrier provisioning systems transmit records daily to the 9-1-1 Database Management System (DBMS) via automated Electronic Bonding batch files.
3. MSAG Validation: The DBMS automatically verifies each address against the MSAG. The house number must fall within an established high/low range, parity must align, street spelling must match, and the community name must exist.
4. Commit or Kickout: Validated records are written to the operational ALI database with assigned ESNs. Records failing validation are rejected as kickouts.

MSAG Kickouts & Error Resolution

Kickouts represent a serious vulnerability. When a record kicks out, it does not enter the active ALI database but enters an exception queue. The MSAG Coordinator must work with carrier database specialists to inspect county GIS data, tax parcels, or addressing records to reconcile discrepancies. If a subscriber dials 9-1-1 while their service order is trapped in a kickout queue, the telecommunicator receives an ALI NOT FOUND error, displaying only the telephone number without an address.

---

5. Out-of-Band ALI Retrieval Lifecycle & Transport Protocols

While voice and ANI travel over dedicated trunks, ALI data is retrieved out-of-band across independent data circuits:

1. The PSAP call-handling equipment captures the incoming ANI from the voice trunk.
2. The equipment formulates an out-of-band data query to the regional ALI database.
3. The ALI database retrieves the record and returns a formatted text block.
4. The telecommunicator's screen displays the caller's name, address, ESN, and responding agencies.

Transport Interfaces

• PSAP ALI Modem (PAM): Early systems utilized analog Bell 202T modems operating at 1200 baud over dedicated 4-wire conditioned analog leased lines using Frequency Shift Keying (FSK). Query responses typically took a few seconds.
• Binary Synchronous Communications (Bisync / BSC): An IBM character-oriented synchronous protocol on leased multi-drop lines offering improved error verification.
• Private TCP/IP WAN: Modernized legacy networks utilize redundant IP connections (MPLS or private T1 lines) running TCP/IP sockets, delivering sub-second query retrieval.

NENA Standard Record Formats

ALI records are built from NENA-standardized data exchange formats (such as NENA 02-010, Standard Data Formats for 9-1-1 Data Exchange & GIS Mapping), and ALI displays present structured fields: Record Type, Telephone Number, Class of Service, Customer Name, House Number, Street Name, MSAG Community, State, ESN, Agency Identifiers, and Special Remarks.

---

6. Static vs. Dynamic ALI Records

• Static ALI Records (Wireline): Wireline records are pre-provisioned. Address, name, and ESN reside permanently on database storage long before an emergency occurs, updated only upon a new service order.
• Dynamic ALI Records (Wireless & Nomadic): Because wireless handsets and nomadic VoIP devices move continuously, pre-provisioning fixed addresses is impossible. The network creates a transient, dynamic ALI record bound to a temporary pseudo-ANI (such as an ESRK). Real-time cell site or geodetic coordinates are held in memory during the call and purged upon call termination.

---

7. Operational Traps & ENP Exam Watch

• MSAG Community vs. Postal City: The MSAG community is established by local public safety authorities to reflect dispatch boundaries, not USPS postal delivery routes.
• Trunks Carry Audio and ANI; Data Links Carry ALI: Trunks carry only audio and the calling telephone number. The PSAP equipment must launch an independent out-of-band data query to fetch the ALI display.
• MSAG Errors Cause Systemic Failures: An incorrect street range in the MSAG causes all subsequent carrier service orders on that street to kick out, leaving callers without automated address displays.