VoIP Voice and Fax Signal Processing

18.2 IPQoS AND QUEUING JITTER ON AN INTERFACE

IPQoS creates improved voice quality conditions. Several methods and options are available in the implementation of the IPQoS for voice and data applications [Black (1999), Ferguson and Huston (1998), URL (Cisco-QoS), Nichols et al. (1998), Blake et al. (1998)]. The main function of IPQoS for VoIP voice packets is to assign high priority to the voice packets. IPQoS eliminates packet drop or reduces the packet drop at scheduling, which assumes available bandwidth is at least more than the voice traffic requirements. IPQoS reduces fixed delay, delay variations, and takes care of dynamic conditions of data, voice, and other priority media. IPQoS in its basic form will not take care of handling the delays created from previously scheduled large packets. To help, based on the available link rate, fragmentation and interleaving are used with large packets.

18.2.1 Fragmenting the Packets for Lower Jitter

The purpose of fragmentation of large packets is to reduce the delay variations created by the large packets [LaBarba (2000)]. In fragmentation, large packets are split into smaller packets. The fragmentation size requirements vary with the available link rate, and fragmentation may be used dynamically based on the traffic and link bandwidth conditions. Most VoIP deployments target for delay variation in the range of 5 to 10 ms. It is good to achieve 1- to 2-ms delay variations, but this is not possible unless upstream bandwidth supported ...