usually their core business. Any customer can be Introduction to Telecommunications Network Engineering connected to the public telecommunications network if he has the correct equipment and an agreement with the network operator. 2.10.1.1 Telephone Network The PSTN is the main public network in use. Sometimes we refer its service to as POTS if we want to distinguish ordinary fixed telephone service from other services provided by telecommunications networks today. In addition to voice communications between fixed telephones, data can be substituted for speech with the help of a voice-band modem. ISDN, introduced later, is considered the next evolutionary step after PSTN. 2.10.1.2 Mobile Telephone Networks Mobile or cellular telephone systems provide radio communications over the local access part of the network. They are regional or national access net- works and connected to the PSTN for long-distance and international con- nections. We introduce mobile networks in Chapter 5. 2.10.1.3 Telex Network This is a telegraph network that allows teleprinters to be connected by means of special dedicated switches. The bit rate of telex is very slow, 50 or 75 bps, which makes it robust. It was once widely used but its importance has been reduced as other messaging systems such as electronic mail and facsimile have reduced its market share. 2.10.1.4 Paging Networks Paging networks are unidirectional only. Pagers are low-cost, lightweight wireless communication systems for contacting customers without the use of voice. Simple pagers just say "beep," but sophisticated pagers can receive large amounts of text and display the e-mail message on a screen. The impor- tance of paging systems has been reduced in countries where penetration on cellular systems, providing text-messaging service, is high. 2.10.1.5 Public Data Networks These networks provide leased point-to-point connections or circuit- switched or packet-switched connections. Leased point-to-point lines are often an economical solution for connections between the LAN