2.3. Key Issues: Voice over Data: Many Conversations, One Network
VoIP can replace traditional telephony, but quality-of-service measures are required in order to make it as reliable as old-school gear.
The OSI network model breaks down VoIP in terms of layers. The networking aspects run at the lower layers, and the application aspects run at the higher layers .
VoIP media streams are delivered by connectionless UDP datagrams, and not TCP packets. This is because, in telephony and other real-time media applications, there's no point in error correction. VoIP administrators would rather strive for full error abatement . This means designing an IP network to carry voice, not just data.
Most IP phones allow simple calls to be made directly to each other, dialed by IP address, without the need for a VoIP PBX server as an intermediary. The job of the server, among other things, is to provide a human-friendly addressing scheme and other features that the phones alone can't provide.
Traditional telephony networking is characterized by client/server or mainframe-like tendencies. VoIP networks are characterized by distributed or fat-client tendencies.
Most IP endpoints sit at the proverbial "edge" of the network, where PCs and printers also reside.
Pure IP voice systems don't use any legacy interfacing or protocolssuch as POTS or T1. Rather, they support only VoIP protocols and offload the media conversion required for such interfacing to other devices.
IP-enabled, or hybrid IP, voice systems offer server-based interfacing for legacy links while also providing VoIP signaling, usually in one server chassis.