Routing in the PSTN: How Call Routing and Switching Works

Call routing and switching in the PSTN is a complex process, but it's actually quite fascinating. The PSTN, or Public Switched Telephone Network, relies on a system of switches and routers to connect calls between different locations.

Each call is assigned a unique identifier, known as a DID, or Direct Inward Dialing number. This allows the network to route the call to the correct destination.

The process of call routing begins when a caller dials a phone number. The call is then sent to a local exchange, which checks the DID number and determines the best route to take. The local exchange acts as a gateway to the wider network, allowing the call to be forwarded to its final destination.

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The call process in the PSTN is a complex but fascinating topic. Each time a call is placed, the destination number is entered by the calling party into their terminal, which generally has two parts: an area code that identifies the geographical location of the destination telephone, and a telephone number unique within that area code.

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The area code is sometimes known as an "NPA", and the area-code and prefix combination is known as an "NPA-NXX." In the United States, prior to 2021, in some locations, if the call was between two terminals in the same local area, then the area code could be omitted.

The destination number is used to determine the optimal path for the call, which can involve multiple exchanges and trunks. Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to.

These routing tables are generated centrally based on the known topology of the network, the numbering plan, and analysis of traffic data. They are then downloaded to each exchange in the telephone operator's network.

In most cases, the area code is required for a call to a regular phone number to complete. Some calls, however, cannot be routed on the basis of prefix alone, for example non-geographic numbers such as toll-free or freephone calling.

Here are the different treatments that may be applied when a call is received by an exchange:

According to Braess' paradox, the addition of a new, shorter, and lower cost route can lead to an increase in overall congestion.

The PSTN components are the backbone of the Public Switched Telephone Network. They work together to facilitate voice communication.

At the core of the PSTN are telephone exchanges, which are central hubs that route calls to their destinations. These exchanges can be local, regional, or international, depending on the call's range.

Connecting these exchanges are various transmission mediums, including copper wires, coaxial cables, and fibre-optic cables, each offering different capacities and speeds.

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The early telephone networks were localised, with operators manually connecting calls using switchboards. These early systems were limited in scale, primarily serving businesses and affluent households.

The demand for broader connectivity drove innovation, leading to the development of automated exchanges by the early 20th century. Automated exchanges used electromechanical switches to route calls more efficiently.

These exchanges eliminated the need for human operators, allowing for faster and more reliable call connections. As networks expanded, they began interconnecting, forming regional and national networks.

Standardisation efforts accelerated this transformation, ensuring compatibility and interoperability across different systems. Despite technological advancements, the core purpose of the PSTN remains unchanged, preserving its foundational role in telecommunications.

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The PSTN is built on a robust framework of components that work together seamlessly. At the core of the network are telephone exchanges, which serve as central hubs that route calls to their destinations.

These exchanges can be local, regional, or international, depending on the call's range. They're the backbone of the PSTN.

Connecting these exchanges are various transmission mediums, including copper wires, coaxial cables, and fibre-optic cables. Each type offers different capacities and speeds.

The local loop, often referred to as the last mile, connects individual telephones to the nearest local exchange. It's a crucial part of the network.

Signalling systems, such as Signalling System No. 7 (SS7), coordinate the routing and management of calls through the network. This ensures that calls are delivered efficiently.

Switching centres power the entire system, employing both digital and analogue technologies to ensure seamless connectivity.

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TDM Voice Traffic is crucial for Telecom carriers, especially CLEC and IXC carriers, who have direct TDM trunks to LECs.

Routing voice calls to the correct LEC tandem or end office is a challenge many carriers face. If routed incorrectly, calls can go to dead-air, reorder, or recordings, or simply not be routed correctly.

LEC tandems require traffic to be segregated, with all Interstate traffic sent to "Access Tandems" and Local / IntraLATA traffic sent to "Local (only) Tandems".

Sending Interstate traffic to a "Local" tandem is a violation of ICA and FCC rules. If Local Traffic is sent over Access Tandem trunks, it won't complete and is also a violation.

Two things are needed to get calls to the right tandem: the DNIS needs to be dipped to determine the LRN, and then the LERG is used to find the proper routing path.

A ported number DNIS call requires dipping the call to see if it's portable and has an LRN, and then querying the LRN in the LERG file to associate it with the appropriate switch CLLI.

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Routing and switching are fundamental processes in the Public Switched Telephone Network (PSTN), ensuring voice calls reach their intended destinations efficiently. The local exchange determines the optimal path for a call based on the dialled number.

The call is then routed through a series of exchanges, including local, trunk, and international exchanges, depending on the distance and location of the recipient. This process is managed by signalling protocols, such as SS7, which handle call setup, management, and termination.

Each exchange uses switching equipment to connect the call to the next segment of the network. Modern PSTN systems primarily use digital switching, which allows for faster and more reliable connections compared to older, analogue systems.

The destination number generally has two parts: an area code, which identifies the geographical location of the destination telephone, and a telephone number unique within that area code. The area code may be subdivided into a prefix that identifies a more specific geographic location or a telephone exchange.

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The telephone number may be subdivided into a prefix and a number unique within that prefix. Sometimes, if the call is between two terminals in the same local area, the prefix may be omitted.

There are two treatments that may be applied when a call is received by an exchange: either the destination terminal is directly connected to that exchange, or the call must be placed to one of the neighboring exchanges through a connecting trunk for onward routing.

Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to. These tables are generated centrally based on the known topology of the network, the numbering plan, and analysis of traffic data.

Some calls cannot be routed on the basis of prefix alone, for example non-geographic numbers such as toll-free or freephone calling. In these cases, the Intelligent Network is used to route the call instead of using the pre-computed routing tables.

Here are some key factors to consider in determining routing plans:

The network planner must take these factors into account when designing routing paths to ensure efficient and reliable call routing.

The Modern PSTN is a complex network that relies on routing to connect calls between different areas. It's made up of many different switches and nodes that work together to direct calls.

Each call is assigned a unique address, known as a telephone number, which is used to identify the caller and the destination of the call. This address is used to route the call through the network.

The Modern PSTN uses a variety of routing techniques, including circuit switching and packet switching, to connect calls. Circuit switching is used for voice calls, while packet switching is used for data calls.

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Modern PSTN applications have come a long way from their traditional voice-centric roots.

The integration of Voice over Internet Protocol (VoIP) technology has enabled PSTN to support multimedia services, such as video conferencing and online gaming. This has significantly expanded the scope of PSTN applications.

With the advent of cloud computing, PSTN services can now be delivered from the cloud, reducing infrastructure costs and increasing scalability. Cloud-based PSTN solutions have become increasingly popular.

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PSTN applications are also being used in the Internet of Things (IoT) space, enabling devices to communicate with each other and with the cloud. This has opened up new opportunities for PSTN-based IoT applications.

The convergence of PSTN and mobile networks has led to the development of advanced mobile services, such as voice over LTE (VoLTE) and voice over Wi-Fi (VoWiFi). These services have improved the overall mobile experience.

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The PSTN is undergoing a significant transformation with the transition from traditional analogue systems to digital and IP-based technologies. This shift enhances the efficiency and capacity of the network.

One of the primary benefits of this transition is the ability to integrate with modern communication platforms like Voice over Internet Protocol (VoIP). This allows for better communication and collaboration.

Advancements in fibre-optic technology are also being leveraged to upgrade existing infrastructure. This provides higher data transmission speeds and improved reliability.

The integration of artificial intelligence and machine learning is also on the horizon, potentially offering smarter routing and enhanced security. This will enable the PSTN to adapt and thrive in a rapidly changing digital world.

These technological improvements aim to extend the life and functionality of the PSTN, ensuring it continues to meet the demands of contemporary communication.

Hybrid routing is a game-changer for network subscribers, allowing for the colocation of switches using different routing schemes in the same area code.

This means that switches using a deterministic routing scheme can be paired with switches using a non-deterministic routing scheme, such as flood search routing.

Routing tables are constructed with no duplicate numbers, enabling direct distance dialing service to all network subscribers, which may require the use of ten-digit telephone numbers.

Hybrid routing allows for the colocation of switches using different routing schemes in the same area code.

This means that switches using a deterministic routing scheme can coexist with switches using a non-deterministic routing scheme, such as flood search routing.

Routing tables are constructed with no duplicate numbers to support direct distance dialing service to all network subscribers.

This may require the use of ten-digit telephone numbers to ensure unique identification of network subscribers.

The PSTN is a vital part of our communication infrastructure, and its core use cases are numerous.

One of the most common use cases of PSTN is for traditional voice calls, which are still widely used today.

PSTN is also used for faxing, which is a critical function for many businesses that rely on paper-based documents.

In addition, PSTN is used for emergency services, such as 911 calls, which are a matter of life and death.

PSTN's reliability and ubiquity make it an essential tool for businesses that require a high level of communication uptime.