for cellular subscribers and is connected to the PSTN/ISDN network at the trunk exchange level. Internet users are connected to the global Internet via the hosts of their ISPs. Networks of national ISPs are connected and this interconnection is extended to the networks of ISPs of neighboring countries, and these networks together make up the global Internet. Figure 2.20 shows two main methods for accessing the Internet. A telephone or ISDN network is used for dial-up connections and ADSL provides permanent higher rate Internet service. Some different means of accessing telecommunications networks are also shown in Figure 2.20. Digital PBX/PABX is connected to a local exchange with a 1,544/2,048-Kbps digital line that has the capacity of 23/30 simultane- ous calls. This connection is called the primary rate interface in the case of ISDN. PBX/PABX is a dedicated small exchange that provides telephone serv- ice to the personnel of a company. Analog PBX/PABX uses analog telephone lines, one for each simultaneous external call. Each analog line (twisted pair) carries one telephone call with signaling. This analog signaling is close to the ordinary analog subscriber loop signaling that we described previously. Introduction to Telecommunications Network Engineering The corporate-wide PBX/PABX service can also be implemented with- out any equipment investments in the company, that is, without physical PABX equipment. Network operators provide a service called Centrex and for that the public exchange is programmed to behave as a PBX/PABX. One of the subscriber lines is set to operate as a switchboard line and the others make up a user group with abbreviated dialing and other PBX/PABX services. For data communication via an analog network or digital network with analog subscriber interfaces, a modem is required. The term modem comes from modulator/demodulator and it transmits data through a speech channel in voice frequency tones. If a subscriber has ISDN service, which is fully digi- tal, no modem is needed and an end-to-