When encoding variable-bit-rate content, additional bandwidth is temporarily required in order to handle content that is challenging to encoding algorithms, such as high motion and highly detailed content. By using the additional bandwidth, the encoder can use more key frames and capture more detail, which improves the content quality. In the same encoded stream, simple content will require less bandwidth for compression. Thus, the overall quality of the content achieved by using variable-bit-rate encoding often exceeds the quality of the same content encoded by using a traditional encoding scheme.
When streaming variable-bit-rate content, the Windows Media server must be able to reallocate bandwidth based on the requirements of the stream, or it must be able to buffer the higher bit rate content on the Player in order for the Player to render it correctly. Therefore, when delivering variable-bit-rate content, it is recommended that both Fast Cache and Fast Start be used.