Picture quality optimization in ABR video services

In transmission of bursty traffic such as video and data, a network with a fixed capacity may have some unused bandwidth. The available bit-rate (ABR) scheme in ATM networks can make use of this spare channel capacity as a lower priority service. ABR has been developed initially for delay-tolerant services such as data. In this paper we demonstrate how with the help of a video transcoder delay-sensitive services such as video may be transmitted over ABR networks. The transcoder further compresses the video bit stream according to the rates dictated by the network with minimum degradation to the picture quality. We have optimised the video quality by combining the classical minimum cell rate (MCR) with the subjectively acceptable minimum quality (SAMQ). The performance of two different coding formats for ABR sources, namely the constant bit-rate (CBR) and variable bit-rate (VBR) video input to the transcoder are compared. We also present techniques for further reduction of congestion at the multiplexing switches to improve video quality and report on the impact of the forward and backward link delays on the ABR performance with a video transcoder.