Carrier Services Infrastructure (CSI) is the foundation of telephony communications. It carries both public and private communications for all VoIP providers. When a business chooses a VoIP solution, it will then have its own sub-network of a larger CSI.

There are 5 different CSI types: PSTN, DS, OC, HFC and Wireless. Here we take a look at what characterises these 5 types and how they are owned. Finally we look at choosing a VoIP provider.

Public Switched Telephone Network (PSTN)

In 1876 Alexander Graham Bell made the world’s first voice transmission over a physical wire. When combined with switches and telephone exchanges, the PSTN began to grow as bundles of wire running between exchanges. PSTN supports VoIP over a Primary Rate Interface (PRI) line or over broadband DSL.

Digital Service Network (DS)

DS is a dedicated network dating from 1964. When a business took out a DS line, it had its own dedicated line – known as type T1 or T3. A T3 line would carry high-speed data, voice and video at 45 Mbps. DS carries VoIP over private dedicated network (PDN) channels.

Optical carriers (OC)

OCs are another type of dedicated network, introduced in the 1980s using fibre optic cable. Like DS, they support VoIP using PDN channels, but with OC3 or OC12 transports – OC12 carries 622 Mbps.

Hybrid fibre-cable (HFC)

HFCs were also introduced in the 1980s, using a mix of optical and coaxial cable to carry broadband content. HFCs support VoIP using broadband cable modems over dedicated networks.


Wireless CSI was introduced in the 2000s, providing customers with mobile telephony. Like PSTN, wireless is switched, rather than dedicated, although using dedicated frequency spectrums and channels. There are several options for VoIP using wireless CSI, such as VoIP over Wi-Fi, and VoIP over WiMax.

Ownership of CSI

There is no single owner of the CSI, rather there are multiple carriers, each owning a part, and leasing to and from one another at wholesale rates. Carriers then sell provision to their customers, the VoIP providers.

Voice over Internet Protocol (VoIP)

VoIP is the transmission of voice (and multi-media) over Internet networks. The same physical or wireless network is used as for digital telephony, but the information is broken into packets before transmission and then re-assembled at the destination. The data is also encoded using audio and video codecs to optimise transfer.

For example, a global business may use a combination of OC, DS and wireless technologies to build VoIP communication systems connecting their locations around the world.

Choosing a voice termination provider

Today’s businesses know that VoIP improves employee productivity and reduces costs. Choosing a provider means looking hard at service quality, reliability and value. Global businesses will also look for excellent connectivity across their locations.

At IDT, we provide wholesale and retail call termination as part of a range of VoIP services. We provide connectivity with almost 50 countries around the world. Why not contact our experts to see how we can help you?