Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Some wireless SIP based features such as Push to Talk may have slightly dif-

ferent specifications for delay.

Burst vs. Average Packet Loss

The average packet loss figures quoted by most legacy telecom sources are not

very meaningful for either data or for voice, and do not accurately reflect the

behavior of IP networks and IP applications, since:

■■ Network congestion or route flaps produce long bursts of packet loss.

■■ Distributed packet loss is, therefore, not meaningful and is also easier to

compensate for when transmitting either data or voice.

Internet voice codecs have a high tolerance for distributed packet loss over

time, but long bursts of packet loss cannot be compensated for and translate

into loss of speech syllables or even entire words and sentences, similar to that

experienced with mobile phones. For this reason, the Extended Report (XR)

has been defined for the RTP Control Protocol (RTCP) in RFC 3611 [3].

Acoustics and the Network

Voice is probably the most demanding real-time application for the Internet,

though networked games are quite close, or even more demanding, when it

comes to delay and packet loss. Figure 18.1 shows the main phenomena [4] at

issue for real-time voice communications.

Figure 18.1 Acoustics and the network

Network (delay and packet loss)

Adaptive Filter

Decoder

4 wire to

2 wire

Far end echo

Near end

echo

(sidetone)

Encoder Σ

304 Chapter 18