PC Magazine defines VoIP as "a digital telephone service that uses the public Internet and private backbones for call transport." Comcast's Xfinity voice service uses Voice over Internet Protocol, but the service runs under Comcast's private network and is not transferred over the Internet like many other VoIP services.

What Is VoIP?

VoIP is a type of digital telephone calling service that uses a computer network to handle calls. VoIP calls can be bridged with the public switched telephone network to maintain backwards and forwards compatibility, according to PC Magazine. Although the first VoIP call was made back in 1973, VoIP did not appear as a software-based option for the public until 1995. Cable TV and Internet providers like Comcast are opting to use VoIP as part of bundled packages, as it runs on the same infrastructure as the cable TV and Internet instead of the public switched telephone network.

Public and Private Networks

VoIP calls are usually transmitted over the public Internet via computer software or hardware conversion. Comcast's VoIP service runs on the company's broadband network through a private data network. While VoIP conversations being transmitted over the Internet are usually encrypted, it is possible that an unauthorized person could intercept the conversation, decode it and effectively eavesdrop on the call. Comcast's VoIP runs over a private network, greatly diminishing the chance that calls can be intercepted.

Comcast VoIP Service Advantages

One advantage Comcast's private network has over the Internet is that Comcast can isolate and dedicate bandwidth for phone calls, whereas software-based VoIP through a computer has to compete with all other Internet traffic for bandwidth when in use. VoIP generally provides cheaper rates for local and long distance calling service compared to analog lines. Additionally, Comcast customers can save on service when bundling VoIP with Internet and TV services. According to Comcast, even though the company's VoIP service doesn't run on the public switched telephone network, new subscribers can transfer their existing phone number to the service.

VoIP Service Caveats

For analog telephones to work with VoIP, they need to be connected to an analog telephone adapter instead of a wall outlet to work. Comcast uses specialized cable modems with built-in ATAs to connect older analog phones to the VoIP network. VoIP phone calls may suffer from lower audio quality than traditional analog calls. Additionally, analog phone lines have a separate power source that can continue working in the event of a power outage; VoIP does not. Finally, VoIP users need to have broadband access whereas analog phone systems do not.