 is defined Assuming a large number of independent voice calls, according to Palm's theorem (Theorem 6.1) the call arrival process can be modeled as a Poisson process, which also implies that the call interarrival times (the time between two consecutive calls) are exponentially distributed. In addition to model the user behavior during a call conversation, a codec model is also required to characterize the properties of the applied speech codec. In UMTS, the Adaptive Multi-Rate (AMR) speech codec is adopted as the standard speech codec by 3GPP [3GP99a]. More information of AMR is given in Appendix A.6. Table 5.2 summarizes the parameters for this voice traffic model with AMR 12.2 voice codec. Voice Traffic Model Parameter Distribution and value Voice Codec Adaptive Multi Rate (AMR) 12.2kbps Silence period Exponential distribution, mean = 3 seconds Speech period Exponential distribution, mean = 3 seconds Session duration Exponential distribution, mean = 120 seconds Table 5.2: Traffic model for voice [TWP*02] In addition, there are also a number of alternative voice traffic models. A. Schieder [Sch03] applied G.723.1 speech codec and a voice traffic model from Brady [Bra69] for modeling VoIP applications. It is also an ON-OFF model where activity and silent periods are generated by an exponential distribution. But the mean value for active period is set to be 1.35 seconds and for silent period is 1.65 seconds. And in [VRF99] a model for voice is described for mobile communications, which includes not only the ON-OFF behavior but also the effect of the voice encoder, compression device and air interface characteristics in mobile communications. It uses a four-state model, where each state defines the generated packet size, the probabilities that a packet is of certain size, and the burst duration with the Weibull distribution. In the framework of this thesis, the air interface and UMTS protocol layers are modeled separately. Therefore, a voice traffic model only describing the voice user behavior is required. The tr