20000 -16.0 CLASSIC 2.2.8 Traffic Types and Propagation Models We can represent real time services (e.g. speech and CS data services) by generating calls according to a Poisson process assuming a mean call duration of 120 S. The speech would an on-off model, with activity and silent periods being generated by an exponen- tial distribution. The mean value for active and silence periods is 3 S and independent of the up and downlink, and both are exponentially distributed. For circuit switched data services, we can assume a traffic model with constant bit rate model and 100% activity. 2.2.8.1 Packet or Non-real Time Services We can represent non-real time services by a WWW browsing session consisting of a sequence of packet calls. We only consider the packets from a source, which may be at either end of the link but not simultaneously. A subscriber may initiate a packet call when requesting an information entity. During this call several packets may be gener- ated, which means that the packet call constitutes a sequence of packets bursts [20,21]. Figure 2.10 illustrates the bursts during the packet call typically seen in fixed network packet transmissions. Instances of packet Last packet arrival to arrivals to BS buffer BS buffer A packet call A packet service session First packet arrival to BS buffer Figure 2.10 Packet service session characteristics. The behaviour of Figure 2.10 can be modelled through the following parameters: Session arrival process Poisson process Number of packet calls per session Npc Geom(UNpc) Reading time between packet calls Number of datagrams within a packet call N E Geom(und) The UMTS Network and Radio Access Technology - Inter arrival time between datagrams (within a packet call) Dg E Geom(H pd) - Size of a datagram, Sd Pareto distribution with cut-off: f.(x) F;(x)=1-(A),xxx (2.54) Packet_Size is defined by the following formula: Packet_Size = min(P,m), (2.55) where P is the normal Pareto distributed random variable (a = 1.1,k = 81.5 bytes) and m is the maximum allowed packet size, m =