Unique Characteristics of VoIP Traffic

VoIP traffic has several unique characteristics:

VoIP is a significant consumer of bandwidth. Application performance deteriorates when VoIP traffic runs on the same network because VoIP sends data at a fixed rate, with no throttling mechanism.
VoIP has a fair amount of header overhead and sends data continuously in two directions. Depending on several factors, such as the codecs you use and the levels of phone usage, VoIP traffic can really fill up your network. Your TCP applications gracefully back down under conditions of congestion. VoIP can starve them out.
VoIP uses the Real-time Transport Protocol (RTP), which rides on top of the User Datagram Protocol (UDP). RTP applications typically send packets at a fixed rate, and because UDP is a connectionless protocol, there is no retransmission or reordering of data. When a packet is dropped, it is gone. The signal cannot be retransmitted. If a whole group of packets is dropped at once, entire portions of a conversation between two IP phones are lost.

Think of VoIP as an application that is highly delay-intolerant, and whose quality depends on delivery with minimal latency, jitter, and packet loss.