Enhanced RSVP in new Cisco IOS release enables voice service. Because the access network is usually the slowest segment of the network, it tends to be the most vulnerable to congestion.
Congestion leads to latency, which can impact voice quality.
Unlike with RSVP implementations for data-only networks, resources can now be reserved before a session ever begins. Service providers offering managed VoIP services, then, can configure Cisco routers at the edges of their networks and their customers' networks to ensure that network resources are available for the duration of voice sessions. In a data-only environment, if resources are not available somewhere along the way, data is queued or delayed. This situation usually has little or no noticeable effect on application performance, since most data is not impacted greatly by low levels of latency. In the voice world, however, it is imperative to minimize delay to the degree possible to enable high-quality voice conversations.