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The Public Switched Telephone Network (PSTN) represents the most successful and long-lived circuit-switched network in history. For over a century, it has connected billions of people across the globe, achieving reliability levels that remain the benchmark for all communication systems.
At its peak, the PSTN comprised over a billion subscriber lines, millions of switching systems, and enough copper and fiber to wrap around the Earth thousands of times. It processed hundreds of billions of calls annually, maintaining 'five nines' availability (99.999% uptime—less than 5 minutes of downtime per year).
Studying the PSTN is not merely historical curiosity. It demonstrates circuit switching at maximum scale, reveals the engineering principles that enable extreme reliability, and shows why this architecture—despite its limitations—continues to influence modern networks. The PSTN is circuit switching incarnate.
By the end of this page, you will understand the PSTN's architecture at every level: subscriber lines, local exchanges, tandem hierarchies, long-distance routing, international connectivity, and the transition to digital switching. You'll see how concepts from previous pages—dedicated paths, connection establishment, resource reservation—combine into a working global network.
The PSTN is organized in a hierarchical architecture that evolved over more than a century. Understanding this structure reveals how local calls, long-distance calls, and international calls are handled through increasingly complex routing mechanisms.
The fundamental layers:
The North American switching hierarchy (historical):
In the United States, AT&T developed a five-level switching hierarchy that persisted for decades:
| Class | Name | Function | Quantity (1980s) |
|---|---|---|---|
| 5 | End Office (Local Exchange) | Connects subscribers | ~19,000 |
| 4 | Toll Center | Toll call switching | ~1,000 |
| 3 | Primary Center | Regional switching | ~230 |
| 2 | Sectional Center | Sectional switching | ~67 |
| 1 | Regional Center | National connectivity | 10 |
Every call could reach its destination by climbing the hierarchy to a common point and descending to the destination—guaranteed connectivity through hierarchical routing.
Modern simplification:
The strict hierarchy has relaxed with:
But the fundamental concept—local access, tandem aggregation, long-distance backbone—persists.
The local loop (also called the subscriber line or 'last mile') is the physical connection between the subscriber's premises and the telephone company's central office (CO). This humble pair of copper wires represents the most expensive and complex part of telephone network deployment.
Technical characteristics:
Physical construction:
Electrical parameters:
| Signal | Direction | Characteristics | Purpose |
|---|---|---|---|
| Line voltage (48V DC) | CO → Subscriber | Continuous DC | Power telephone, detect loop status |
| Off-hook current | Subscriber → CO | 20-50mA DC | Signal that phone is in use |
| Dial pulses | Subscriber → CO | Interruptions in current | Convey dialed digits (rotary) |
| DTMF tones | Subscriber → CO | Dual audio tones | Convey dialed digits (touchtone) |
| Ringing voltage | CO → Subscriber | 90V AC @ 20Hz | Alert subscriber to incoming call |
| Voice signal | Bidirectional | 300-3400 Hz audio | Actual conversation |
Why copper pairs persist:
Despite decades of predictions about fiber-to-the-home replacing copper, local loops remain predominantly copper for several reasons:
Loading coils:
For longer loops, loading coils (inductors) were traditionally added every 6,000 feet to improve voice frequency transmission. These coils:
However, loading coils devastate DSL performance (they filter out DSL frequencies), so deloading has been necessary for broadband deployment.
Digital Loop Carrier (DLC):
To serve distant subscribers, Digital Loop Carriers place multiplexing equipment in neighborhoods:
The local loop represents 80% or more of telephone company capital investment. Each subscriber requires a dedicated physical connection from their premises to the network—no sharing possible for voice service. This 'last mile problem' drives much network economics and explains why telephone infrastructure is considered natural monopoly.
The Central Office (CO)—also called an end office or local exchange—is where subscriber lines terminate and switching occurs. A typical CO serves 10,000 to 100,000 subscribers with highly redundant, continuously operating equipment.
Major CO components:
BORSCHT functions on line cards:
Every subscriber line interface performs these seven functions:
| Letter | Function | Description |
|---|---|---|
| B | Battery | Provides 48V DC power to subscriber equipment |
| O | Overvoltage | Protects equipment from lightning, power crosses |
| R | Ringing | Applies ringing signal for incoming calls |
| S | Supervision | Monitors on-hook/off-hook status |
| C | Codec | Converts analog voice to digital (and reverse) |
| H | Hybrid | 2-wire to 4-wire conversion for full-duplex |
| T | Test | Enables remote testing of line parameters |
The switching process:
When a call is processed at the CO:
Class 5 switches are designed for '5 nines' reliability. They feature redundant processors (active/standby), duplicated switch fabrics, battery power with generator backup, and component-level redundancy. If one component fails, the switch continues operating on the spare with no call impact.
Routing in the PSTN determines the path a call takes from originating to terminating switch. The system must handle local calls, regional calls, long-distance calls, and international calls—each with different routing requirements.
Number Plan Administration:
The North American Numbering Plan (NANP) organizes numbers hierarchically:
+1 (NPA) NXX-XXXX
│ │ │ │
│ │ │ └── Subscriber number (0000-9999)
│ │ └────── Exchange code (200-999)
│ └─────────── Area code (200-999)
└─────────────── Country code
Routing based on number analysis:
| Dialed Prefix | Call Type | Route Decision |
|---|---|---|
| NXX-XXXX (7 digits) | Local | Direct to local CO or same-CO subscriber |
| 1-NPA-NXX-XXXX | Toll/Long Distance | Via IXC (e.g., AT&T, Verizon) |
| 1-800-NXX-XXXX | Toll-Free | Database lookup for actual destination |
| 011-CC-... | International | Via international gateway |
| 911 | Emergency | Route to PSAP (Public Safety Answering Point) |
| *XX or #XX | Feature codes | Local switch feature activation |
Trunk group selection:
When routing requires another switch, the originating switch selects from available trunk groups:
Primary route: Direct trunk group to destination (high-usage route) Secondary routes: Via tandem switches (alternate routes) Final route: Guaranteed path through hierarchy (always available unless network failure)
Least Cost Routing (LCR):
For long-distance calls, LCR algorithms select among carriers:
Local Number Portability (LNP):
Since 1996 in the US, subscribers can keep their phone numbers when changing carriers. This complicates routing:
Equal Access:
In the US, subscribers can choose their long-distance carrier:
A 'simple' phone call may involve number plan analysis, LNP database queries, carrier selection, multiple trunk groups, SS7 signaling across numerous nodes, and coordination among competitive carriers—all completed in under 3 seconds. This complexity is invisible to users but represents decades of engineering evolution.
The long-distance network (also called the interexchange network or IXC network) connects local exchange carriers across cities, states, and countries. This is where the economics of circuit switching—and its evolution—become most apparent.
Historical context: The AT&T Long Lines:
Before 1984, AT&T operated a unified long-distance network:
Post-divestiture structure (1984-present):
After the AT&T breakup:
| Era | Technology | Capacity | Cost per Circuit-Mile |
|---|---|---|---|
| 1950s | Coaxial (L1) | 600 voice channels | High (dedicated copper) |
| 1960s | Microwave | 6,000 channels | Medium (shared spectrum) |
| 1970s | Coaxial (L5) | 10,800 channels | Medium |
| 1980s | Early Fiber | 90,000 channels | Low (massive capacity) |
| 1990s | DWDM Fiber | Millions of channels | Very low |
| 2000s+ | Coherent Optical | Tens of millions | Approaching zero per channel |
SONET/SDH backbone:
Modern long-distance networks use SONET (Synchronous Optical Network) in North America and SDH (Synchronous Digital Hierarchy) elsewhere:
SONET hierarchy:
Network topology:
Long-distance networks use ring topologies for protection:
Interconnection:
Long-distance carriers interconnect at:
Each interconnection point requires complex commercial and technical agreements.
Fiber optic technology transformed long-distance economics. Where capacity was once scarce and expensive (driving efficiency obsession), it became abundant and cheap. This abundance enabled the shift from strict circuit efficiency to packet-based flexibility—long-distance fiber has so much capacity that efficiency matters less than features.
Connecting the world's telephone networks requires international coordination on an unprecedented scale. The PSTN's global reach demonstrates both the power of standardization and the complexity of cross-border communications.
ITU-T and international standards:
The International Telecommunication Union (Telecommunication Standardization Sector) coordinates:
Country code system (E.164):
International format: +CC NNN...N
CC = Country code (1-3 digits)
NNN...N = National number (up to 15 total digits)
Examples:
+1 = North America (NANP)
+44 = United Kingdom
+91 = India
+86 = China
| Route | Technology | Capacity | Latency |
|---|---|---|---|
| US ↔ Europe | Transatlantic cables (TAT-14, etc.) | Multi-Tbps | ~70ms |
| US ↔ Asia | Transpacific cables (TPE, Unity) | Multi-Tbps | ~100-150ms |
| Europe ↔ Asia | SEA-ME-WE, FLAG | Multi-Tbps | ~150ms |
| Domestic long-haul | Terrestrial fiber | Multi-Tbps | ~50ms/5000km |
| Remote/developing | Satellite | Lower Gbps | ~500-700ms |
International switching hierarchy:
International Switching Centers (ISCs):
Transit networks:
Settlement and accounting:
Historically, international calls involved complex financial settlements:
Challenges in international routing:
The ability to dial any telephone in the world from any other telephone—with consistent quality and sub-minute setup time—represents one of humanity's greatest infrastructure achievements. The PSTN's global reach preceded and enabled the internet's connectivity, demonstrating that worldwide standardization is achievable.
The PSTN is not static—it is undergoing a fundamental transformation from circuit switching to packet-based Voice over IP (VoIP). Understanding this transition reveals both the enduring value and limitations of circuit switching.
Drivers of the transition:
Technical architecture of VoIP transition:
Soft switches and media gateways:
VoIP-to-PSTN interworking uses:
IMS (IP Multimedia Subsystem):
3GPP's IMS provides carrier-grade VoIP:
The 'all-IP' network:
Full VoIP transition eliminates TDM entirely:
VoIP has not yet matched PSTN reliability. Internet outages affect VoIP calls; power outages disable VoIP phones (no battery backup like PSTN); and 911 service depends on accurate location data that VoIP complicates. The transition is technically feasible but operationally challenging.
The PSTN's remarkable reliability didn't happen by accident. Decades of engineering evolution produced principles that any critical system can learn from.
Design principles that enabled 'five nines':
Reliability metrics:
The 'nines' framework:
| Availability | Annual Downtime | Common Name |
|---|---|---|
| 99% | 3.65 days | Two nines |
| 99.9% | 8.76 hours | Three nines |
| 99.99% | 52.6 minutes | Four nines |
| 99.999% | 5.26 minutes | Five nines |
| 99.9999% | 31.6 seconds | Six nines |
PSTN targets: Five nines (99.999%) has been the traditional target, allowing less than 5.3 minutes of downtime per year. Critical components (9-1-1 processing) may target six nines.
How the PSTN achieves this:
The PSTN's reliability principles inform modern SRE (Site Reliability Engineering) practice. Concepts like error budgets, redundancy, canary deployments, and operational runbooks all trace lineage to telephone engineering. When building critical systems, study how the PSTN achieved its legendary reliability.
We have comprehensively explored the Public Switched Telephone Network—circuit switching's greatest achievement. Let's consolidate the essential insights:
What's next:
Having examined the PSTN in depth, we'll conclude our circuit switching module with a comprehensive analysis of Advantages and Disadvantages. You'll develop a nuanced understanding of when circuit switching is the right architectural choice and when packet switching better serves your needs.
You now possess deep understanding of the Public Switched Telephone Network—the most successful and longest-lived circuit-switched network in history. This knowledge provides context for modern networking decisions and demonstrates how engineering principles can create systems of extraordinary reliability and reach.