A ‘carrier’ is a telecommunications service provider (TSP) that enables the public to connect to the public switched telephone network (PSTN) or mobile networks. Examples of carriers include BT, AT&T, Sprint, T Mobile and Vodafone.
PSTN is the name for the combined telephone networks of the world. It is a primarily digital network, which also forms a large part of the infrastructure of the internet.
This overlap in technology is part of the reason carriers have been ideally placed to double up as internet service providers (ISPs). Carriers are often providers of other adjacent services such as cable and satellite television.
How Does Carrier Technology Work?
A phone number is both a destination for a signal and a set of instructions for how the call should be routed along switching stations between the caller and the recipient. Calls are routed through multiple switches at the local, national, and international scale.
The original mass implementation of telephone networks required switchboards and operators to manually connect calls along individual copper wires. This practice had ended almost entirely by the late 1980s, at which point calls began to be digitised. This enabled automatic switching and a much greater number of calls per line.
More recently, fibre-optic technology has meant that thousands of calls can travel along the same line.
In office environments, internal calls are routed through a private branch exchange (PBX). This is essentially a switching station that is administered by the company, allowing it to allocate phone numbers and make internal calls free of charge.
A trunk line connects the PBX to the carrier and the wider telephone network.