ant step toward wireless Internet. When a Web page containing an applet is fetched, it is automatically executed in the client machine. This creates security risks. There will always be people around who enjoy designing applets that cause damage by, for example, reformatting a hard disk or searching confidential information and transmitting it to the Internet. In the design of Java security risks are consid- ered but they are not entirely solved yet [3]. 6.6.12 Voice over IP (VoIP) The vast majority of information exchanged over the public telecommunica- tions networks has been voice. The present voice communications networks, public telephone and ISDN, use the circuit-switching principle. Circuit switching provides good quality service and it does not require a complicated Introduction to Telecommunications Network Engineering encoding algorithm. A simple waveform coding scheme such as PCM, as dis- cussed in Chapter 3, is sufficient for a circuit-switched connection that pro- vides constant-bit-rate service. Charging for voice services has been straightforward-we simply pay for the duration of a call. This has been a relevant approach because each call reserves a certain data capacity whether there is speech on the line or not. 6.6.12.1 Voice Communications over Circuit- and Packet-Switched Networks The characteristics of data transmission are different from waveform-coded speech, and the data networks that were developed to provide data services utilize packet-switched technology. These technologies include LANs, Inter- net, frame relay, and ATM. Packet-switched networks utilize network resources more efficiently than circuit-switched networks because the capac- ity in the network is dynamically shared among all users. If there are no data to be transmitted between two users, their share of the data capacity is avail- able for other users. This difference in the operating principle makes a packet-switched network superior to a circuit-switched network when the data rate per user is not constant. When all transm