The Internet and the Public
Switched Telephone
Network
Disparities, Differences, and Distinctions
This paper discusses the telephone network infrastructure commonly known as the Public
Switched Telephone Network (PSTN) and provides insight into fundamental differences between
the "traditional" telecommunications network and the Internet. The Internet has evolved from a
research computer network to a vibrant communication platform, based on competition, regulatory
independence and transparency, and continuous innovation. Today a significant amount of the
world’s voice traffic travels over IP-based networks and the Internet. However, legacy PSTN
regulations cannot be transferred or applied directly to the Internet, as there is little commonality
between it and the PSTN.
Public Switched Telephone Network (PSTN)
The Public Switched Telephone Network was designed to facilitate ubiquitous two-way voice
communication. Initially, establishing a direct connection between any two points or users was
almost as simple as stringing wire between those points and placing telephones on either end.
The cost of this connection was largely a function of the location of each endpoint or end-user and
the distance and the geography between the two points; cost increased with an increase in
distance and decrease in accessibility.
Over time the telephony network evolved to support more users and endpoints through a network
of switches. Engineers capitalized on the point-to-point nature of telephony network architecture
and the fact that one end user can could only be connected to one other end user at a time. By
placing switching equipment in centralized locations, network engineers were able to interconnect
large numbers of end users via these switches to maximize network access; thus the concept of
circuit switching was born. This revolutionized voice communications and telephony network
design, creating the Public Switched Telephone Network (PSTN) architecture that is still in place
today. The network focus is on circuit-based, or connection-oriented, systems designed for
delivery of one universal service – voice. Essentially, this established a system in which each
usage of the network required a “call-setup” phase in which a connection was set up end-to-end or
between both endpoints and end users, reserving the required resources along the path for the
duration of a call whether the network resources were used or not. This system allowed the
network operator to efficiently route and bill for the calls because each step of the call path is
identified and managed by the network and its switching systems.
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The diagram above shows that by arranging telephony network switching offices in a hierarchical
fashion by the function that they performed and employing dedicated trunk lines between the
networked switches, it was possible to scale the telecommunications network to accommodate a
large number of end users as traffic was managed between the various switching offices
depending on the type of traffic that was to be connected: local traffic, long distance traffic and
international traffic.
PSTN switches operate on a link-by-link basis: each switch within the hierarchy forwards its traffic
along the link or hop to the next PSTN switch based on the call setup process.
Importantly, in order to establish arbitrary connections, intelligence had to be placed in the
network. Initially operators made physical connections for customers followed by stepper switches
that could connect end users based on telephone number prefix, and eventually digital switching
using stored program control systems (i.e., computers) was introduced to add intelligence to the
network for service management. Whether through manual, electro-mechanical, or digital means,
the PSTN operates on the basis of having type or class identified for each line in service (e.g.
Business or Residential), carefully engineered interconnection arrangements between operators,
networks and switches, with service provisioning provided solely by its network and closed
operator group. This PSTN structure provides network operators the ability to physically measure
service between end users and through this mechanism, to charge customers, to exchange
interconnection settlements and to recover network capital costs.
The Internet
The Internet is a both a transit network that moves data and a network of computers, which allow
anyone to access, retrieve, process, and store all manner of information in digital format (e.g.
voice, video, documents, images). In Internet Protocol based (IP) networks, multiple independent
computer networks are joined together into a network of networks through interconnected
gateways or routers using a standard protocol suite –Transmission Control Protocol/Internet
Protocol (TCP/IP) for interworking. Internet traffic has no dedicated path; there is no single
interconnection point or fixed network hierarchy as employed in the PSTN; and there is no
dedicated settlement and billing regime. In fact, PSTN-style telecommunication accounting is
impossible to apply to the Internet because individual data packets may take different routes over
separate networks as they travel to their final destination to be reassembled and delivered to the
application.
Where telephony relies on physical, point-to-point connection, and switch hierarchy, the Internet
does not prescribe topology, but rather simply seeks to deliver data. In this highly distributed
communications system, data is placed in packets with an IP destination address and an IP
source address, much like the “TO” and “RETURN” addresses on a postal envelope. Each packet
is then placed on the network where it is delivered to its destination. In the simplest case, a single
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network connects two computers. In more complex (and more common) cases routers and
gateways establish the exchange of traffic among and between many networks. Regardless, the
endpoints are unaware of network or interconnection complexity and take no special action as a
consequence. This is unlike the PSTN, where the telephone number must be known by the user in
order to establish a connection and the path for the entire communication is decided during the
connection setup.
The Internet was developed such that the packet transport and network routing mechanisms are
neutral with regard to the application traffic carried by the packets, making it a very generalpurpose network. This has allowed competition and tremendous innovation by end-users. With the
Internet, innovation at the edge of the network is not dependent on network providers for anything
other than transporting packets across their domains. In the Internet it is possible for any user to
connect to it, build new parts of it, and study it overall. In the Internet, no one is just a consumer of
content or services, but anyone can be a contributor – produce content on existing services,
invent new applications or services, put up a server (an Internet node), and connect new
networks.
Critical Comparison of the PSTN and the Internet
The unchanging properties of the Internet system have included features of the underlying
networks, technologies and standards, as well as emergent properties that impact users and uses
of the Internet. These can be compared and contrasted with the PSTN.
•
Global Reach
Internet: Any endpoint of the Internet can address any other device or endpoint connected to
it, without any requirement for a call-setup phase. The specific path of the packet flow between
the endpoints and thus, the intermediaries, is not prescribed and can dynamically change, or
be different for parts of the flow, since it is a meshed topology. There are no geographic
boundary limitations – many enterprises have nation-spanning networks as part of the Internet.
Moreover, the Internet looks and acts the same from anywhere a user connects to it.
PSTN: While the PSTN has a global footprint, connection between two (2) endpoints requires
intermediation through a specific control point in the hierarchical switching system.
•
General purpose
Internet: Capable of supporting a wide range of demands for its use. While some networks
within it may be optimized for certain traffic patterns or expected uses, the Internet service
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provides a best effort forwarding of data packets which does not place inherent limitations on
the applications or services that make use of it.
PSTN: Optimized for voice services as defined and globally agreed for deployment.
•
Supports innovation without requiring permission (by anyone)
Internet: Any person or organization can set up a new service that adheres to open and
collaborative standards, and make it available to the rest of the Internet, without requiring
special permission. The best example of this is the World Wide Web – which was created by a
researcher in Switzerland, who made his software available for others to run, and the rest, as
they say, is history.
PSTN: Only telecoms companies can define and deploy new services within their networks.
•
Accessible
Internet: It’s possible to connect to it, build new parts of it, and study it overall: Anyone can
“get on” the Internet – not just to consume services, but also to contribute applications and
services, and attach new networks.
PSTN: Limited to licensed telecoms providers, and restricted by technical complexity, as well
as through geographic boundaries of regulatory regimes.
•
Based on interoperability and mutual agreement
Internet: Networks exchange traffic based on simple non-formal agreements and open
standards for interoperability. The basis for the exchange is the “Internet service” – best effort
forwarding of packets from source to destination.
PSTN: Connections are made based on bilateral agreements and standards that have been
authorized by regulatory authorities.
•
Collaboration
Internet: The defined Internet standards development paradigm seeks the best solutions to
new issues that arise from willing collaboration between stakeholders. These are sometimes
competitive business interests, and sometimes different stakeholders altogether (e.g.,
technology and policy).
PSTN: Regulation defines the requirements for cooperation, as well as the boundaries
separating roles.
•
Technology
Internet: Reusable building blocks: Technologies have been built and deployed on the Internet
for one purpose, only to be used at a later date to support some other important function.
Furthermore, operational restrictions on the generalized functionality of technologies as
originally designed have an impact on their viability as building blocks for future solutions.
PSTN: Vertically integrated, closed solutions, whose purposes have been defined, and
technology specified strictly to achieve those defined purposes (services).
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•
Legacy Systems
Internet: There are no permanent favorites. While some technologies, companies and regions
have flourished, their continued success depends on continued relevance and utility, not
strictly some favored status.
PSTN: Stability of the network is core to the PSTN’s health and functioning. Innovation is
restricted by defined requirements to support existing network components and billing
requirements, with existing requirements defined and controlled by network operators who are
more focused on regulatory recovery than modernization.
•
Network Planning and Growth
Internet: Historical data or estimated traffic growth on links or parts of the network is used for
deciding the installation of additional capacity. Different type of end-points connected to the
network will have completely different traffic profiles, so it’s meaningless to calculate an
average usage for any connected endpoint on the Internet.
PSTN: Functions with mathematical and statistical foundations are used to determine the
capacity requirements and configuration of equipment based on the amount of connected
users.
•
Costs and Prices
Internet: The service providers must calculate their costs based on overall network operation
expenses and investments. The price paid by a connected endpoint, enables it to send and
receive packets through any link, interconnection or gateway available on the network.
PSTN: The costs can be isolated and associated with individual calls. The telecommunications
providers can establish settlements for individual interconnection links charging each other
based on their users’ calls. The users can pay based on each requested call and the calling
user will take care for all the costs involved to establish each call.
Internet Society
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bp-internetPSTN-201206-en