Our focus in this book is primarily on the so-called public networks, which are
networks operated by service providers, or carriers, as they are often called. Carriers
use their network to provide a variety of services to their customers. Carriers used
to be essentially telephone companies, but today there are many different breeds
of carriers operating under different business models, many of whom do not even
provide telephone service. In addition to the traditional carriers providing telephone
and leased line services, today there are carriers who are dedicated to interconnecting
Internet service providers (ISPs), carriers that are in the business of providing bulk
bandwidth to other carriers, and even virtual carriers that provide services without
owning any infrastructure.