ional applications such as e-mail pression to suppress the packets sent by the non-speak- and Hypertext Transfer Protocol (HTTP) traffic. ing party in a conversation. As illustrated in Table 3.3-2, a VoIP packet consists of the payload, IP header, User Datagram Protocol (UDP) header, Real-time Transport Protocol (RTP) header, and IP Network protocols to support Layer 2 Link header. At a packetization rate of 20 ms, VoIP packets have a 160-byte payload for G.711 or a 20- voice byte payload for G.729. The IP header is 40 bytes, the UDP header is 8 bytes, and the RTP header is 12 bytes. Voice quality is only as good as the quality of the weakest The link header varies in size according to media. network link. Packet loss, delay, and delay variation all The bandwidth consumed by VoIP streams is calcu- contribute to degraded voice quality. In addition, because lated by adding the packet payload and all headers (in network congestion (or more accurately, instantaneous bits), then multiplying by the packet rate per second. buffer congestion) can occur at any time in any portion of This does not include Layer 2 header overhead and the network, network quality is an end-to-end design does not take into account any possible compression issue. The QoS tools discussed here are a set of mecha- schemes, such as compressed Real-time Transport Pro- nisms to increase voice quality on data networks by de- tocol (cRTP). creasing dropped voice packets during times of network Table 3.3-3 shows the typical bandwidth need for congestion and by minimizing both the fixed and variable common codecs and sampling rates. delays encountered in a given voice connection. Table 3.3-4 Bandwidth consumption with headers included. CODEC Ethernet 14 Bytes PPP 6 bytes of ATM 53-byte cells Frame relay 4 of header header with a 48-byte bytes of header payload G.711 (20 ms sample 85.6 kbps 82.4 kbps 106 kbps 81.6 kbps rate) G.711 (30 ms) 156.5 kbps 54.4 kbps 70 kbps 54 kbps G.729A(20 ms) 29.6 kbps 26.4 kbps 42.4 kbps 25.6 kbps G.729A(30 ms) 19.5 kbps 17