Electronics Guide

Subscriber Loop Systems

The subscriber loop, also known as the local loop or "last mile," represents the physical connection between individual telephone subscribers and the central office. This critical component of the PSTN typically consists of twisted-pair copper wires that carry analog voice signals from customer premises to the telephone company's switching equipment.

Traditional subscriber loops operate using loop start signaling, where the telephone handset going off-hook creates a DC current path that signals the central office to provide dial tone. The loop must meet specific electrical characteristics, including resistance limits (typically under 1,300 ohms) and minimum current requirements (around 23 mA) to ensure proper operation of both the telephone instrument and central office equipment.

Loop qualification determines whether a particular physical loop can support various services, considering factors such as wire gauge (commonly 24 or 26 AWG), length, bridge taps, and loading coils. Modern DSL technologies have repurposed these same copper loops to deliver broadband data services alongside voice, requiring careful engineering to prevent interference between voice and data signals.

Central Office Switching

Central offices (COs), also called end offices or local exchanges, house the switching equipment that connects subscribers within a local area and provides access to the wider network. These facilities contain not only switching systems but also power equipment, distribution frames, line equipment, and various support systems necessary for reliable telecommunications service.

Modern digital switches replaced earlier electromechanical systems (like crossbar and step-by-step switches) and perform multiple functions: line concentration (sharing trunk resources among many subscribers), call processing (analyzing dialed digits and routing calls), signaling (communicating with other network elements), and service provisioning (implementing features like call waiting and call forwarding).

Digital switches use time-division multiplexing (TDM) to handle multiple calls simultaneously, converting analog voice signals from subscriber loops into 64 kbps digital streams (using pulse code modulation) and switching these digital channels through a high-speed matrix. A typical Class 5 end office switch might serve 10,000 to 100,000 subscriber lines and handle thousands of simultaneous calls.

Trunk Systems

Trunk circuits interconnect switching systems, carrying aggregated traffic between central offices, tandem switches, and toll offices. Unlike subscriber loops that connect individual telephones, trunks are designed for high utilization and typically employ digital transmission technologies to maximize capacity and efficiency.

Digital trunk systems commonly use T-carrier (in North America and Japan) or E-carrier (in Europe and most other regions) hierarchies. A T1 circuit carries 24 voice channels at 1.544 Mbps, while an E1 carries 30 voice channels at 2.048 Mbps. Higher-capacity systems include T3/DS3 (672 channels at 44.736 Mbps) and optical carrier levels like OC-3, OC-12, and OC-48, which can transport thousands of simultaneous calls.

Trunk engineering involves capacity planning to balance service quality against infrastructure costs. Telephone traffic follows statistical patterns described by Erlang models, which help engineers determine how many trunks are needed to achieve acceptable blocking probabilities (typically P.01 or 1% during busy hours).

Tandem Switching

Tandem switches, also called Class 4 switches or transit exchanges, provide interconnection between central offices without directly serving subscribers. These switches create a hierarchical network structure that enables efficient routing of calls through intermediate points rather than requiring direct trunks between every pair of end offices.

In metropolitan areas, local tandem switches aggregate traffic from multiple central offices, providing both routing functions and access to inter-exchange carriers for long-distance services. This tandem architecture significantly reduces the number of required trunk groups—instead of N(N-1)/2 trunk groups needed to fully mesh N central offices, a single tandem reduces this to N trunk groups.

Access tandems specifically handle traffic between local exchange carriers and inter-exchange carriers (long-distance providers), implementing the equal access provisions mandated by telecommunications regulations. These switches perform least-cost routing, selecting the most economical path for calls based on destination, time of day, and available carrier services.

International Gateways

International gateway switches provide the interface between domestic telephone networks and international telecommunications facilities. These critical nodes perform call routing, protocol conversion, billing data collection, and regulatory compliance functions for cross-border communications.

Gateway switches must handle multiple signaling protocols (such as converting between different variants of SS7 used in different countries), implement international numbering plans (E.164), and coordinate with international carriers and submarine cable systems. They also enforce regulatory requirements like call accounting for settlement between countries and lawful intercept capabilities.

International traffic routing often involves multiple carriers and transit countries, requiring sophisticated routing tables and alternative path selection to optimize quality and cost. Gateway switches maintain relationships with international carriers and implement the complex accounting arrangements specified in bilateral agreements between telecommunications operators.

Signaling System 7 (SS7)

Signaling System 7 represents the global standard for telecommunications signaling, providing the out-of-band signaling infrastructure that controls PSTN call setup, routing, and teardown. Unlike earlier in-band signaling methods that used the same circuit as the voice call, SS7 operates over a separate packet-switched network specifically designed for signaling messages.

The SS7 architecture consists of three main node types: Signal Switching Points (SSPs), which are telephone switches that originate or terminate calls; Signal Transfer Points (STPs), which route signaling messages through the network; and Service Control Points (SCPs), which contain databases and service logic for advanced features like 800 number translation and local number portability.

SS7's protocol stack includes the Message Transfer Part (MTP) for reliable message delivery, the Signaling Connection Control Part (SCCP) for addressing and routing, and various application parts such as the Telephone User Part (TUP) and ISDN User Part (ISUP) for call control. The Transaction Capabilities Application Part (TCAP) enables database queries and advanced intelligent network services.

This sophisticated signaling system enables rapid call setup (typically under one second for domestic calls), supports advanced features impossible with older signaling methods, and provides the foundation for modern telecommunications services including caller ID, call forwarding, and seamless roaming in cellular networks.

In-Band Signaling

Before SS7's widespread deployment, in-band signaling methods carried control information within the same channel as the voice or data. Multi-Frequency (MF) signaling, used between switches, transmitted digit information using pairs of tones from six frequencies (700, 900, 1100, 1300, 1500, and 1700 Hz), similar to but distinct from the Dual-Tone Multi-Frequency (DTMF) system used by subscriber telephones.

Legacy in-band signaling remains relevant for understanding older installations and certain applications. Loop start and ground start signaling methods still control the interaction between analog telephones and switches, while robbed-bit signaling (using the least significant bit of certain DS0 channels for signaling information) continues in some T1 applications.

Number Portability

Local Number Portability (LNP) allows telephone subscribers to retain their telephone numbers when changing service providers or, in some cases, when moving to different geographic locations. This regulatory requirement, implemented in many countries, promotes competition by reducing a significant barrier to switching carriers.

LNP implementation requires sophisticated database infrastructure, typically using SS7 signaling to query centralized databases that map telephone numbers to routing information. When a call is placed to a potentially ported number, the originating switch queries the Number Portability Administration Center (NPAC) database to determine the current Location Routing Number (LRN) associated with that telephone number.

The LRN identifies the switch currently serving the number, enabling proper call routing even when the number has been ported to a different carrier. This process happens transparently to callers but requires coordination among carriers and careful database management to ensure accurate, up-to-date routing information.

Toll-Free Services

Toll-free telephone numbers (800, 888, 877, 866, 855, 844, and 833 in North America) reverse the charging model, billing the called party rather than the caller. These services rely on Service Control Points (SCPs) that maintain databases mapping toll-free numbers to actual destination numbers, which may vary based on time of day, caller location, or percentage allocation among multiple destinations.

Advanced toll-free services support sophisticated call routing strategies: time-of-day routing directs calls to different call centers based on when the call occurs; percentage allocation distributes calls among multiple destinations; geographic routing sends calls to the nearest or most appropriate location; and disaster recovery routing automatically redirects calls if primary destinations become unavailable.

The intelligent network architecture underlying toll-free services includes Service Switching Points that detect toll-free numbers and suspend call processing, STPs that route queries to appropriate SCPs, and SCPs containing the Service Management Systems that allow customers to configure their routing preferences in real-time.

Directory Assistance and Operator Services

Directory assistance provides subscribers with telephone numbers they don't know, traditionally accessed by dialing 411 for local numbers or 1-[area code]-555-1212 for other areas. Modern directory assistance operations use sophisticated databases combining multiple information sources, computer-assisted operator systems, and increasingly, automated speech recognition and text-to-speech technologies.

Operator services, accessed by dialing 0, assist with various call types including collect calls, third-party billing, person-to-person calls, and calling card validation. While automation has reduced the role of human operators, they remain available for complex situations, customer service, and services requiring human judgment.

These services require specialized switch configurations, operator workstations with access to customer account information, and real-time rating systems to calculate charges for operator-assisted calls, which typically cost more than direct-dialed calls.

Calling Features

The PSTN supports numerous enhanced calling features, many enabled by SS7 signaling and intelligent network capabilities. Call waiting alerts subscribers to incoming calls while they're already on a call, allowing them to place the first call on hold and answer the second. Call forwarding redirects incoming calls to another number based on various conditions: unconditional forwarding, forwarding when busy, forwarding when no answer, or selective forwarding based on caller ID.

Three-way calling allows a subscriber to conference three parties together, while caller ID delivers the calling party's number (and sometimes name) to the called party before the call is answered. These features require coordination between the switch, the subscriber line interface, and in the case of caller ID, special equipment or a compatible telephone that can receive and display the information.

More advanced features include speed dialing, which stores frequently called numbers for quick access; distinctive ringing, which assigns different ring patterns to different incoming numbers on the same line; and call blocking services that prevent calls from specific numbers or categories of callers.

Billing Systems

Telecommunications billing systems capture call detail records (CDRs) generated by switches, process these records to calculate charges, and produce customer bills. Each completed call generates a CDR containing information such as calling and called numbers, call start time and duration, trunk groups used, and any special features or services activated.

Modern billing systems must handle complex rating scenarios: different rates based on time of day, day of week, and call distance; special rates for specific calling plans; bundled minute packages with overage charges; and charges for features and services beyond basic call completion. The system must also handle billing adjustments, credits, payment processing, and regulatory requirements like universal service fund contributions.

Mediation systems collect CDRs from multiple switches and different vendors' equipment, normalizing the data into consistent formats. Rating engines apply tariff rules to determine charges, while customer care systems provide interfaces for service representatives to view account information, make adjustments, and address billing questions.

Traffic Engineering

Traffic engineering optimizes network capacity to meet service quality objectives while controlling costs. Engineers collect traffic measurements from switches, analyzing busy hour call attempts, call completion rates, trunk occupancy, and blocking probabilities to determine where network augmentation is needed.

Erlang B and Erlang C models provide the mathematical foundation for trunk sizing, helping engineers determine how many circuits are needed to handle expected traffic loads with acceptable blocking probabilities. Erlang B assumes blocked calls are cleared (the caller gets a busy signal), while Erlang C assumes blocked calls wait in queue, applicable for different scenarios like trunk groups versus call center agents.

Traffic engineering must account for both routine variations (daily and weekly patterns, seasonal fluctuations) and special events (holidays, emergencies, popular television events that generate post-show calling spikes). Network management systems provide real-time monitoring and control, allowing operators to reroute traffic, activate additional capacity, or implement calling restrictions during unusual conditions.

Network Reliability

The PSTN achieves exceptional reliability through redundancy at multiple levels, rigorous maintenance practices, and carefully designed failure recovery mechanisms. Central offices typically maintain dual-redundant power systems with battery backup and diesel generators, ensuring operation during commercial power outages. Critical switching equipment uses redundant processors and components with automatic failover capabilities.

Network reliability metrics include "five nines" availability (99.999%), representing less than 5.26 minutes of downtime per year. Achieving this requires not only redundant equipment but also careful operational procedures, regular maintenance, software quality assurance, and rapid fault detection and repair processes.

Diverse routing strategies protect against facility failures by establishing primary and alternate paths through physically separate cables and switch locations. Network synchronization systems (using GPS or atomic clocks) ensure timing accuracy critical for digital transmission, while network management centers monitor thousands of network elements, responding to alarms and coordinating restoration efforts when failures occur.

ITU-T Standards

The International Telecommunication Union's Telecommunication Standardization Sector (ITU-T) develops global standards for telephony. Key recommendations include the Q-series (covering switching and signaling, including Q.931 for ISDN and Q.700-Q.799 for SS7), the G-series (covering transmission systems and media including G.711 for pulse code modulation), and the E-series (covering telephone network operations including E.164 numbering plans).

These standards enable interoperability between equipment from different manufacturers and telecommunications systems in different countries, crucial for international calling and global telecommunications commerce.

North American Standards

In North America, industry standards supplement ITU-T recommendations with region-specific requirements. The North American Numbering Plan (NANP) defines the structure of telephone numbers across the United States, Canada, and several Caribbean nations, using the format NPA-NXX-XXXX where NPA is the area code and NXX identifies the central office.

Telcordia (formerly Bellcore) standards, now maintained by iconectiv, specify technical requirements for network equipment, operations systems, and interconnection. GR-series Generic Requirements documents cover topics from SONET transmission systems to operations support systems interfaces.

PSTN to IP Transition

Telecommunications networks worldwide are transitioning from circuit-switched PSTN technology to packet-switched IP-based systems. This evolution offers numerous advantages: more efficient use of network resources through statistical multiplexing, easier integration of voice with data and video services, lower equipment and operational costs, and greater flexibility for implementing new features and services.

However, the transition presents significant challenges: ensuring service quality and reliability comparable to the PSTN's proven track record, maintaining regulatory compliance including emergency services (E911) and lawful intercept capabilities, preserving universal service while managing the costs of maintaining legacy infrastructure, and coordinating the migration of millions of customer connections.

Gateway devices and soft switches provide interoperability between PSTN and VoIP networks during the transition period. These systems convert signaling protocols (such as SS7 to SIP) and media formats, enabling calls to traverse both circuit-switched and packet-switched portions of the network seamlessly.

Regulatory Considerations

The PSTN operates under extensive regulatory frameworks that vary by country but generally address universal service obligations (ensuring affordable access to basic telephone service for all citizens), interconnection requirements (mandating that carriers connect their networks), number administration (managing the allocation and use of telephone numbers), and service quality standards.

Regulatory bodies like the Federal Communications Commission (FCC) in the United States and similar organizations worldwide oversee PSTN operations, resolve disputes between carriers, and adapt regulations to technological changes. As the industry transitions to IP-based communications, regulators face challenges in applying traditional telecommunications regulations to new technologies while ensuring continued service quality and consumer protection.

Legacy Infrastructure Management

While new installations increasingly use IP-based technologies, substantial PSTN infrastructure remains in service and will likely continue operating for years. Telecommunications providers must maintain expertise in both legacy and modern systems, managing the gradual retirement of older equipment while ensuring service continuity.

This involves parts inventory management for equipment no longer manufactured, maintaining staff expertise in older technologies, and strategic planning for when to migrate customers from PSTN to newer platforms. Some specialized applications, such as certain alarm systems and industrial control systems, may depend on PSTN characteristics and require special consideration during migration.

Enterprise Telephony

Businesses connect to the PSTN through various interfaces depending on their size and requirements. Small businesses might use individual subscriber lines or basic PRI (Primary Rate Interface) connections, while large enterprises typically employ T1 or PRI trunks connecting their private branch exchange (PBX) systems to the PSTN.

Understanding PSTN interfaces remains relevant even as enterprises migrate to IP-based systems, since many maintain hybrid environments with both traditional and IP telephony. Knowledge of trunk types, signaling methods, and numbering plans helps in designing effective telecommunications solutions.

Emergency Services

The PSTN provides critical infrastructure for emergency services (911 in North America, 112 in Europe, etc.). Emergency call routing uses specialized databases and network configurations to direct calls to the appropriate Public Safety Answering Point (PSAP) based on the caller's location, delivering automatic number identification (ANI) and automatic location identification (ALI) to emergency dispatchers.

Enhanced 911 (E911) systems provide more precise location information, particularly important for mobile callers. Maintaining emergency service capabilities during the transition to IP networks requires new technical solutions like NG911 (Next Generation 911) that can handle calls from various technologies while preserving location accuracy and service reliability.

Common Issues

PSTN troubleshooting requires systematic approaches to isolating problems among subscriber equipment, loop facilities, central office equipment, and interoffice trunks. Common subscriber loop issues include noise (often from poor connections, water infiltration, or electromagnetic interference), opens (complete circuit breaks), shorts (unwanted connections between wires), and grounds (unwanted connections to earth).

Test equipment like Time Domain Reflectometers (TDRs) help locate physical problems by measuring reflected signals, while transmission test sets measure loss, noise, and other electrical characteristics. Central office technicians use switch maintenance terminals to examine call processing, analyze traffic patterns, and diagnose equipment problems.

Testing and Verification

Proper PSTN maintenance includes regular testing of critical systems, preventive maintenance of equipment, and prompt response to service-affecting failures. Test call generators verify end-to-end connectivity and measure call quality parameters like noise, distortion, and echo. Protocol analyzers examine signaling messages to diagnose interoperability issues or routing problems.

Database auditing ensures accuracy of critical information like number portability data, toll-free routing, and customer service profiles. Discrepancies between network databases and switch configurations can cause call routing failures or billing errors, making regular verification important.