In converged networks that contain coexistent voice and data traffic, the volume of either type of traffic is unpredictable. For example, transferring a file using the File Transfer Protocol (FTP) can cause a sharp burst in the network traffic. At other times, the network might have no data.

While most data applications are insensitive to small delays, the recovery of lost and corrupted voice packets is a significant problem. For example, users are not concerned if the reception of email or files from file transfer applications is delayed by a few seconds. In a voice call, the most important expectation is the real-time exchange of speech. To achieve real-time communication, network resources are required for the complete duration of the call. If resources are unavailable or the network is too busy to carry the voice packets, clicks, pops, and stutters are heard at the destination. Therefore, for real-time exchange of speech with adequate quality, a fixed amount of bandwidth is continually required during the call.