VoIP Voice and Fax Signal Processing

10.3 VoIP PACKET IMPEDIMENTS

In VoIP, network packet drop, packet fixed delays, jitter (also called delay variation or reorder), packet errors, and fragmentation are very common. End VoIP systems and interfaces also introduce a certain amount of these impediments. The jitter buffer works on packet impediments and tries to deliver steady packets to the decoder with a minimum possible buffering delay. Depending on the end-to-end packet flow and adaptation dynamics, jitter buffer (JB) may miss the delivery of some packets. Jitter buffer and PLC will try to improve voice quality in the presence of packet impediments. At the first level, jitter buffer optimally buffers packets and tries to deliver the maximum available packets to the voice processing modules. Jitter buffer cannot replace any missing, error, fragmented, and packets arriving at unacceptable delays; it is the role of the PLC algorithm and decoder to improve voice quality beyond jitter buffer. PLC synthesizes the missing voice samples during the packet erasures without creating the noticeable artifacts. Architecturally, PLC resides on part of the decoder. Voice quality improvements in PLC vary with the compression codec, packet drop characteristics, and on the selected PLC algorithm used with the decoder.

10.3.1 Sources of Packet Impediments and Helpful Actions

This section describes sources of packet impediments. An overview on packet impediments and voice quality is given in reference [Nagireddi (2006)]. Depending on ...