1.4 Network Design: Originating Service Providers, Local Loops & Transport Facilities

1.4 Network Design: Originating Service Providers, Local Loops & Transport Facilities

Quick Answer: Every 9-1-1 call starts with an originating service provider (OSP): an incumbent or competitive local exchange carrier (ILEC or CLEC), a wireless carrier, or a VoIP service provider (VSP). The call travels over a local loop (copper, coaxial cable, fiber, or radio) to the provider's switch or softswitch, then over dedicated 9-1-1 trunks or IP links to a selective router or NG9-1-1 core. Transport facilities include copper, fiber optic, microwave, and coaxial cable, each with different capacity, power, and failure characteristics. Numbering changes such as area code overlays and splits, the N-1-1 codes, and quality-of-service and security threats (TDoS and DDoS) all affect how reliably calls reach the PSAP.

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1. Originating Service Providers (OSPs)

The ENP Body of Knowledge groups the companies that originate 9-1-1 calls under originating service providers. Each type connects to the 9-1-1 network differently and falls under different rules.

Interconnected VoIP and 9-1-1

The FCC's 2005 VoIP 9-1-1 order required interconnected VoIP providers (services that use a broadband connection, need IP-compatible equipment, and let users both receive calls from and place calls to the public telephone network) to deliver 9-1-1 calls with the caller's callback number and Registered Location. In legacy networks, a VoIP Positioning Center (VPC) assigns an ESQK pseudo-ANI so the selective router can route the call and the ALI database can return the location. Under the rules implementing RAY BAUM'S Act Section 506, fixed VoIP must now deliver dispatchable location automatically, and non-fixed (nomadic) VoIP must deliver dispatchable location or an approved alternative.

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2. The Local Loop and Numbering

The Local Loop

The local loop (the "last mile") connects the customer's premises to the provider's central office or network node:

• Copper twisted pair: Traditional plain old telephone service (POTS). The central office supplies roughly -48 volts DC over the loop, so a corded phone keeps working during a commercial power outage as long as the central office has power.
• Coaxial cable: Hybrid fiber-coax cable plant carries voice as VoIP, and the modem at the home needs local power.
• Fiber: Fiber to the premises carries voice through an optical network terminal that also needs local power.
• Fixed wireless: Radio links to the home, which likewise depend on local power.

Because fiber, cable, and fixed wireless home voice service is not line powered, FCC rules require providers of those services to offer subscribers backup power options (an 8-hour option and a 24-hour option) and to explain that service will not work in an outage without backup power. PSAP public education should explain this limitation, especially to people who rely on a home phone.

Area Code Overlays and Splits

When a numbering plan area (NPA) runs out of numbers, regulators add capacity in one of two ways:

• Geographic split: The region is divided, and customers in one part receive a new area code. Their telephone numbers change, so ALI records, selective routing tables, and callback data must be updated carefully.
• Overlay: A new area code is added over the same geography. Existing customers keep their numbers, but 10-digit local dialing becomes necessary because two area codes serve the same place.

9-1-1 is still dialed as three digits in both cases. The larger risks are database and equipment updates: PSAPs should verify that CAD, ALI displays, callback functions, and speed dials handle the new area code. The 2020 designation of 988 for the Suicide and Crisis Lifeline forced 10-digit local dialing in areas that used 988 as a local exchange prefix, a reminder that numbering decisions ripple into PSAP systems.

N-1-1 Abbreviated Dialing Codes

The FCC assigns several N-1-1 codes that PSAPs deal with regularly:

Directory assistance (4-1-1) and repair service (6-1-1) are carrier conventions rather than FCC assignments.

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3. Transport Facilities

The physical media that connect customers, central offices, selective routers, ESInets, and PSAPs have distinct strengths and weaknesses:

Designing for Diversity

A single cut should never isolate a PSAP. Common design practices include:

1. Route diversity: Two paths that do not share conduit, poles, bridges, or building entrances (Chapter 12).
2. Media diversity: Pairing fiber with licensed microwave or with another carrier's facilities.
3. Trunk distribution: Splitting 9-1-1 trunk groups across separate facilities and, in NG9-1-1, across geographically separate core sites.
4. Monitoring: Circuit alarms with clear escalation, backed by the FCC's rules requiring providers to notify PSAPs of 9-1-1 outages.

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4. Trunking, Switching, and Routing Configurations

Several network configurations appear on the exam and connect to other sections:

• Direct trunked: Each end office sends 9-1-1 calls straight to one PSAP over dedicated trunks, common in Basic 9-1-1 (1.1).
• Tandem (selective router): End offices trunk to a 9-1-1 tandem that selects the PSAP by ESN. Older analog tandems relied on MF signaling; digital tandems support SS7 and digital trunks.
• End office routing: The end office itself selects the PSAP, which works only when office boundaries match PSAP boundaries.
• Grade of service: Trunk groups are sized for a blocking probability, typically P.01 (1 call in 100 blocked in the busy hour), using Erlang B (1.1 and 6.1).
• Wireless and PBX interconnection: Mobile switching centers and MLTS/PS-ALI systems use their own trunking and database arrangements (Chapter 2).

Common practice is to provision at least two 9-1-1 trunks from each originating office, over diverse facilities where possible, so one failure does not block all calls.

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5. Quality of Service and Network Security

Quality of Service (QoS)

Once calls move onto IP networks, voice quality depends on delay, jitter, and packet loss. ESInet designs mark voice traffic for priority queuing and use carrier service-level agreements to bound delay (3.1).

Telephone Denial of Service (TDoS) and DDoS

• TDoS floods 9-1-1 or administrative lines with automated or malicious calls so legitimate callers cannot get through. In October 2016, a malicious link that made iPhones dial 9-1-1 repeatedly disrupted PSAPs in several states.
• Distributed Denial of Service (DDoS) overwhelms IP systems, such as ESInet interfaces, CAD servers, or public websites, with traffic from many sources.
• Defenses include carrier-level filtering, call-pattern monitoring, BCF rate limiting and anomaly detection in NG9-1-1, overflow routing to partner PSAPs, and incident response plans (3.2 and 9.3). Because federal rules protect calls to 9-1-1 from caller ID-based blocking, mitigation must avoid dropping legitimate emergency calls.

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6. Operational Traps & ENP Exam Watch

• CLEC vs. IXC: CLECs compete for local service after the 1996 Act; IXCs carry long-distance traffic between LATAs.
• Overlay vs. Split: Overlays keep existing numbers but require 10-digit local dialing; splits change some customers' area codes.
• Line Power: Only traditional copper POTS is powered from the central office; fiber, cable, and VoIP phones need local backup power.
• Interconnected VoIP: The E9-1-1 obligation applies to interconnected VoIP (two-way calling with the public network), not to every internet voice app.
• 7-1-1 Is Not a 9-1-1 Substitute: Relay users may reach PSAP administrative lines through 7-1-1, but PSAPs must still provide direct TTY and text access to 9-1-1.