The original telecommunication system was developed for PSTN (Public Switch Telephone Network) for voice communications. But now communication networks include all types of voice, video and data communication over copper wire, optical fibers and wireless medium.With the inception of Internet, increasingly numbers of computer networks are now connected via the Internet. So now the concept of telecommunication system has increased complexity significantly. These systems can be divided into different types by the information they carry. With this kind of categorization, communication systems can be categorized as long-haul point-to-point links, distribution networks, MAN (metro area network) and WAN (wide area network).Now let’s examine them one by one.1. Public Switched Telephone Network (PSTN)This network connects billions of users by switches, routers or other dedicated connections. It was designed to carry voice only at first, but with enhanced services, they can carry digital data information as well.Computers with modems can transmit and receive digital data via the PSTN network with enhanced data services such as DSL.2. Cable TV systemsCATV is still the biggest player for television broadcasting. But now they are facing competitions from Satellite, DSL video on demand. CATV providers also entered into data communication market and VoIP services.3. Data NetworksWith modern telecommunication systems, all information are transmitted in digital form. These digital networks can be categorized based on network topology, switching technology and the communication protocols.The switching technologies used in modern networks can be categorized as circuit switching and packet switching. Circuit switching is used mostly for voice networks but not very efficient. Packet switching can route data packet according to the least busy path and it provides a virtual connection.