t of a telephone network is involved in the call. The second application example in Figure 6.58 illustrates a customer surfing the Internet and a Web service provider with enhanced WWW serv- ice using VoIP. People surfing the Web can connect to a company's call cen- ter by clicking a Call button located on the company's Web page. Users can communicate with a customer service group, ordering department, or help desk by using their Web browser and a personal computer equipped with a compatible speech encoder. This is an important new feature as commercial use of the Internet expands. The third example shows a company with locations in multiple sites and an intranet. Intranet connections between office A and B use the IP net- work. The IP network may carry secure VPN connections where IP packets are tunneled and ciphered, although firewalls are not shown in the figure to keep it simple. Now transmission capacity is available between offices and in addition to data VoIP can carry voice via the same VPN connections. There is no need to lease separate channels from a PSTN service provider for speech only. In office B no PABX equipment is required if the PCs contain suitable sound cards, headsets, and software for VoIP. External calls can be done via PABX in office A. Another application that PSTN operators use is to replace their con- ventional trunk network with an IP network. Modern telephone exchanges contain VoIP functions and are able to establish calls alternatively via the IP network instead of conventional 64-Kbps trunk network channels. However, emergency calls are usually routed via a circuit-switched trunk network because it has much higher reliability. 6.6.12.3 VolP Protocols As mentioned earlier the quality of VoIP service is worse than that of PSTN service. The two main reasons are delay and jitter (variable delay). Typically 20 ms of speech is encoded into one IP packet and encoding, packetization, packet handling, and buffering will often lead to an overall delay of well over 200 ms, which disturbs our 