radio signals, but to transmit data to the network. Most of the invest- ment was already made when wideband cables were installed. This existing medium is especially attractive for providing Internet service to every home connected to a cable TV network. Typically, a data connection made via a cable TV network is shared between many home users; that is, there is no physically separate connection to every home as we have in the case of ISDN or xDSL. This service is has often attractive tariffs because of shared investments, but it may suffer from temporary congestion when many users happen to be active at the same time. 2.10.2 Private or Dedicated Networks Private networks are built and designed to serve the needs of particular organizations. They usually own and maintain the networks themselves. Services provided are a tailored mix of voice, data, and, for example, special control information. 2.10.2.1 Voice Communication Networks Examples of private dedicated voice networks are those used by the police and other emergency services and taxi organizations. They are called private or professional mobile radio (PMR). Railway companies also have private tele- phone networks that use cables that run alongside the tracks. Introduction to Telecommunications Network Engineering 2.10.2.2 Data Communication Networks Data communication networks are dedicated networks especially designed for the transmission of data between the offices of an organization. They can incorporate LANs with mainframe computers feeding information to the branch offices. Banks, hotel chains, and travel agencies, for example, have their own separate data networks to update and distribute credit and reserva- tion information. 2.10.3 Virtual Private Networks It is very expensive for an organization to set up and maintain its own private network. Another choice is to lease resources, which are also shared with other users, from a public network operator. This virtual private network (VPN) provides a service similar to an ordinary private network, but t