What Is POTS, PSTN, and VoIP?

Understanding voice communications technologies is key for anyone working with phone systems. POTS, PSTN, and VoIP are essential concepts.

POTS stands for Plain Old Telephone Service. This refers to the original analog telephone systems that used copper wiring to connect home and business phones to the telephone network. POTS delivers analog voice calls over dedicated circuits.

The PSTN, or Public Switched Telephone Network, is the infrastructure that interconnects POTS lines and central offices.

The PSTN provides the backbone for circuit-switched telephone calls. Originally an analog system, the PSTN evolved to incorporate digital backbone networks.

VoIP stands for Voice over Internet Protocol. VoIP allows sending voice calls over IP data networks instead of dedicated circuits. VoIP converts analog voice signals into digital packets for transmission. It provides an alternative to legacy POTS and the PSTN.

As communications evolve from analog circuits to global digital networks, understanding these core concepts is an essential foundation. This article will provide an overview of POTS, PSTN, and VoIP along with how they relate.

Plain Old Telephone Service (POTS)

POTS refers to the original analog phone systems used for most of the 20th century. Let’s look at some key attributes of POTS:

Analog system – POTS transmits voices as analog electrical signals over copper wiring without any digitization.
Dedicated circuits – Each POTS line is a dedicated circuit from the phone to a central office switch. No other phones share the circuit.
Low frequency – POTS only operates at frequencies between 300 to 3400 Hz, limiting call quality.
Power from central office – POTS phones do not require their power supply. They get power from the phone line.
Limited features – POTS supports very basic call features like call waiting, 3-way calling, and voicemail.
PSTN connection – POTS provides connectivity from homes and offices into the PSTN telephone network.

POTS delivered universal telephone service for decades. But it lacks flexibility and advanced capabilities.

As networks evolved, POTS is being replaced by digital voice technologies.

Public Switched Telephone Network (PSTN)

The PSTN is the infrastructure that makes up the public telephone network. Here are some key PSTN concepts:

Interconnected switches – The PSTN consists of telephone switches interconnected by trunk lines. This allows routing calls.
Last mile circuits – Local POTS lines connect subscribers to their central office switch on the PSTN.
Intelligent call routing – Switches use SS7 signaling to look up numbers and route calls across the PSTN.
Toll-free services – Services like 800 numbers introduced flexible inbound call routing and billing.
Digital evolution – While originally analog, the PSTN became increasingly digital through the late 20th century. Digital switches replaced electromechanical ones.
TDM trunks – The backbone connections between switches evolved from analog to digital T1 and T3 TDM trunks.

The PSTN enabled ubiquitous public phone service, supporting billions of subscribers across the globe.

It still serves an essential role today despite the rise of newer IP networks.

Voice over IP (VoIP)

Voice over IP, or VoIP, transmits voice calls as digital data packets over IP networks instead of analog signals over dedicated circuits. Here are some key aspects of VoIP:

Packetized voice – VoIP encodes voice into binary packets that are transmitted over data networks and the Internet.
IP protocols – VoIP calls use networking protocols like SIP, RTP, and STUN that are designed for connectivity over the Internet.
Softphones and IP Phones – Endpoints for VoIP include software and hardware IP phones along with mobile VoIP clients.
Interoperability – Gateways between VoIP and PSTN allow users on different networks to call each other.
Advanced features – VoIP enables a wide range of advanced features and integrations like video calling, CRM lookup, and conferencing.
Scalability – VoIP systems can scale to large numbers of concurrent calls across distributed hardware.
Cloud deployment – VoIP environments can be deployed on cloud infrastructure for elasticity and global reach.

VoIP delivers modern voice solutions built on top of ubiquitous IP data networks. It breaks free of the limitations of legacy analog POTS and circuit-switched networks.

PSTN to VoIP Migration

The rise of VoIP is allowing migration away from legacy POTS lines and PSTN networks. Here are some key trends in this shift:

Consumer broadband adoption – As households adopted broadband Internet with adequate QoS, consumer VoIP services became viable.
IP PBX adoption – Businesses switched from legacy PBXs to VoIP PBX systems, avoiding PSTN trunk costs.
Cable company offerings – Cable operators like Comcast now offer VoIP home phone services over their broadband networks.
Mobile VoIP integration – Cellular networks integrated VoIP internally, and smartphones enabled VoIP apps.
Number portability – Regulations enable keeping your existing PSTN phone number when switching to VoIP services.
PSTN switch decommissioning – With less traffic, telecoms have been decommissioning outdated PSTN switches and analog infrastructure.

While the PSTN still carries a subset of voice traffic, its role is diminishing as networks and subscribers migrate to VoIP with its greater flexibility and features.

Limitations of POTS, PSTN, and VoIP

1. Limitations of POTS (Plain Old Telephone Service)

Only supports voice calls, no data transmission
Limited call capacity on each copper line
No mobility – phones are tied to a fixed location
Susceptible to noise and interference on copper lines
Requires extensive wiring from the central office to each premise

2. Limitations of PSTN (Public Switched Telephone Network)

Built for analog voice, not suitable for digital data
Expensive to upgrade from copper to fiber optics
Not scalable due to the use of circuit switching
Limited built-in security features
Lacks many modern features like caller ID, call waiting, etc.

3. Limitations of VoIP (Voice over Internet Protocol)

Quality of service is not guaranteed on the public Internet
Sensitive to network congestion and bandwidth constraints
Can have interoperability issues between vendors
More vulnerable to power outages than traditional PSTN
Emergency call routing is complicated

The Future of POTS, PSTN, and VoIP

1. The Future of POTS

POTS is a legacy technology and has been largely replaced by newer systems
Very few plain old telephones remain in service
The last remnants of POTS will likely disappear as networks transition to all IP

2. The Future of PSTN

Many providers are phasing out PSTN in favor of all IP networks
AT&T plans to retire PSTN by 2025
Upgrading PSTN to support new digital services is not economical

3. The Future of VoIP

VoIP adoption will continue to increase globally
Quality and reliability will improve with new protocols like WebRTC
Shift from hardware to software-based VoIP phones and services
Integrated with video calling and conferencing over IP networks
Emergence of unified communications blending VoIP, messaging, and collaboration

In summary, the limitations of legacy telephone networks are driving the adoption of new all-IP solutions.

POTS will soon be extinct while PSTN faces retirement or upgrade. VoIP will become the predominant technology for voice calling as it integrates with other IP-enabled services.

Frequently Asked Questions (FAQ)

Ques 1. What is the difference between POTS and PSTN?

Ans. POTS refers specifically to the original analog telephone lines running to end users’ homes and offices.

The PSTN is the larger public switched telephone network infrastructure interconnecting local and long-distance wiring, switches, and trunks.

Ques 2. Does VoIP require an Internet connection?

Ans. Yes, VoIP needs an Internet connection or private IP network to transmit call data packets between phones, servers, and gateways.

Broadband wired or WiFi provides the best quality. VoIP can also work over 4G/5G cellular data with some quality limitations.

Ques 3. Can you use a VoIP phone without a VoIP provider?

Ans. No, a VoIP phone requires a VoIP PBX system or hosted VoIP provider to work.

The VoIP provider handles registration of the phone, call routing, security, and other server-side functionality. VoIP phones cannot directly call each other.

Ques 4. Is VoIP more expensive than traditional phone service?

Ans. VoIP is typically cheaper than legacy analog POTS service in most cases. However, providers may charge extra fees for features, so costs depend on the service plan.

VoIP saves money by utilizing existing broadband connectivity and avoiding PSTN charges.

Ques 5. Will VoIP replace the PSTN entirely?

Ans. The PSTN still carries a significant portion of voice traffic globally. However, its role is declining as networks and subscribers migrate to VoIP.

It is unlikely the PSTN will disappear entirely soon. It still serves unique purposes like ubiquitous emergency calling, law enforcement access, and backup for VoIP outages.