Rationale for VoIP
VoIP is a special area because it bridges the old with the new; that is, time-division multiplexing (TDM) and IP. VoIP can be deployed end-to-end using IP phones at each end, or it can be phased in over a period of time. The latter is usually referred to as a migration strategy, and is beginning to get a lot of traction with service providersnot just for voice service, but for others such as data and video. If a phased approach is used, legacy phones can be used in conjunction with special-purpose hardware. The latter can be PBX-based, if necessary, to extend the lifecycle of the PBX equipment.
An important point about VoIP deployment is that it can be configured to use WAN links as far as possible, spilling over into PSTN links only when all allocated WAN capacity has been exhausted. This arrangement allows for toll savings up to the limit of the WAN allocation.
VoIP may also be deployed for functional rather than financial reasons. Many large enterprises can easily negotiate reduced tariffs for telephone service. But VoIP can allow for useful applications to integrate with the telephony function. This is the rich area of computer-telephony integration (CTI). Examples of applications are call centers that allow customer details to be accessed by agents while they're still on the phone. All the applications use the LAN as the transport mechanism.
Let's take a look at some of the components of VoIP QoS.