The telecommunications infrastructure of a network operator consists of a set of networks (transmission network, switching network, access network, signaling network, mobile
network, intelligent network, management network), each performing a particular function
towards the provision of the service to the customer. With the evolution towards IP-based
network, the circuit switched network is migrating towards a new architecture called Next
Generation Network (NGN) which emulates the behavior of circuit switching. With the advent
of broadband access networks, the core network evolves towards IP Multimedia Subsystem
(IMS) which provides IP-based multimedia services.
The objective of this tutorial is to briefly introduce the telecommunication network structure to
acquire the vocabulary of the field, understand the several types of networks involved in the
operator’s “Telecommunication Network”, and gain knowledge on how these networks
interface and interoperate. The various services supplied by each type of network are also
emphasized. Another important objective of the tutorial is to introduce the evolutions of these
networks and services on the medium and long terms.
A Telecommunication network consists of two parts:
• the “network” (transmission, switching, access, signaling, mobile, intelligent network)
• the “business and technical information system” which consists of OSS (Operating
Support System) and BSS (Business Support system).
1 Public Switched Telephone Network (PSTN)
PSTN is the fixed voice network. It consists of the transmission, switching, signaling and
intelligent networks.
The transmission network enables carrying all kinds of traffic (voice, video, data). It consists
of nodes called multiplexers and links among multiplexers. The goal of the multiplexer is to
multiplex/demultiplex traffic onto/from the link. There exists three multiplexing technologies :
PDH, SDH and D-WDM. The link technology is generally optic fiber but may also be coax,
radio, etc. A transmission network generally consists of hundreds of multiplexers and tens of
thousands of kilometers of optic fiber.
The switching network enables switching the traffic from the sender to the appropriate
destination. A switching network consists of switches. All switches rely on the transmission
network which provides digital trunks. A switch receives traffic from the transmission network
at input ports, applies the switching function which forwards the traffic to output port. Then,
the switch relies on the transmission network to send the traffic to an adjacent switch. The
voice network is using the circuit switching technology which provides voice services.
A switching network operates in a connection oriented mode. That means that prior to
enabling users exchanging their traffic, there is a need of reserving resources on the path
between the sender/caller and the receiver/callee. To reserve resources, all switches on the
path exchange signaling information. Signaling information is data. In the case of circuit
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switching, signaling data is carried over a separate network, i.e., a signaling network called
Signaling System 7 (SS7). This is out-of-band signaling.
The intelligent network is used in the voice network for the provisioning of services such as
freephone, premium rate, virtual private network, account card calling, etc. It consists of a set
of application servers containing service logic and service data.

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