As a quick summary of a VoIP call, a VoIP voice call will provide the following main items to the user. In this section, a PC can also be a laptop computer. Figure 19.1 is presented with VoIP functionality on a laptop computer. VoIP implementation architecture should provide the following elements:
1. User acoustic voice interfaces
2. Dialing options
3. Processing that need not be directly visible to the user
4. Network interfaces and Internet network connectivity
Audio interfaces and sound cards internal to the PC are typically used for acoustic interface. In some computers, a built-in microphone and speaker are available. These interfaces are reused for VoIP calls. These built-in devices often create severe echo problems; hence, separate external acoustic interfaces and headsets are typically used with a PC. A PC keyboard will be used to enter digits for dialing. PCs have a significant amount of processing power that can
Figure 19.1. PC-based VoIP.
be used to take care of the processing needed for multiple channels of voice and associated network functions.
PCs often have several interfaces to get access to Internet connectivity. The popular network interfaces on a PC are Ethernet, USB, and wireless local area network (WLAN). A PC can also use an internal or external modem for dial-up network connectivity. These network interfaces all connect a PC to an Internet service. In practice, the PC will be connected via the LAN interfaces to digital subscriber line (DSL) and cable modems or directly to Ethernet or WLAN in residential or corporate offices.
Several PC-based applications are available, including MSN messenger, Skype, and Google talk, that provide PC-based VoIP voice functionality. These applications use the PC ‘s processor to provide voice and network functions and to deliver voice on the available audio interfaces and extended interfacing devices.
PC as a Fax Machine and Internet-Aware Fax (IAF)
Figure 19.1 is a functional representation of PC based VoIP calls. As shown in the figure, the PC has a telephone RJ-11 (two-wire) foreign exchange office (FXO) interface that can be directly connected to the PSTN office [foreign exchange subscriber (FXS) interface]. This RJ-11 FXO interface is used like a dial- up modem for Internet connectivity. The PC can also work as a fax machine on the same FXO interface. The PC software will have drivers to work with the internal modem. Any printable document on a printer can be sent as a fax page by the application. A software application on a PC will provide a graphic user interface (GUI) to dial and to observe the fax status. The application detects the ring voltage on the FXO interface and answers the call to receive the fax data. Received fax pages are typically stored in tagged image file format (tiff) in specified default file folders. In summary, the PC operates as a fax machine through the fax software application. Note that when the PC RJ-11 interface is used for a dial-up modem, the same interface cannot be used for other applications.
When a PC is connected to the Internet through Ethernet, USB, or WLAN, a PC software application can also send fax pages over the network interfaces. In this situation, the PC works as a combination of fax machine and VoIP fax adapter. This functionality can be treated as an internet-aware fax (IAF). A separate VoIP adapter and fax machine are not required. Although a single PC could provide both fax and VoIP functions, it is often convenient to have a separate VoIP adapter so that the PC does not need to be running constantly to allow voice calls.