Measurement and Analysis of Skype VoIP Traffic in 3G UMTS Systems

throughput [kbps]403530252015p loss= 10%10p loss= 50%5senderp loss= 70%receiver00 100 200 300 400 500time [s]Figure 3: Using dummynet, Skype reacts on packet loss by increasing the sender throughputdummynet scenario, the random variable L dummy follows a geometric distribution shifted byone: L dummy ∼ GEOM 1 (q) with parameter q = µ−1µand a mean measured distance µ. Thenumber of consecutive packets without any packet loss is denoted as K. It holds: K = L − 1.Figure 4 compares the inter packet loss distance for the dummynet and the traffic shapingrouter scenario. In the dummynet scenario, the packet loss probability was set to 50 percent. Inthe traffic shaping router scenario, the bandwidth on the link was restricted to 16 kbps whichresults in a packet loss ratio of 48 percent. The dummynet software leads to a geometric distributionwith q = 0.5 in Figure 4(a). This means each packet gets lost with propability q. Thetraffic shaping router, however, leads to a completely different behaviour. Packets do not get lostindependent of each other, but in bursts, which can be seen from Figure 4(b). Due to the nature0.70.60.5measuredGEOM 1(q)10.8measuredprobability0.40.3probability0.60.40.20.10.200 5 10inter packet loss distance(a) Dummynet01 2 3 4 5 6 7 8 9 10 11 12inter packet loss distance(b) Traffic shaping routerFigure 4: Distance between two consecutive packet losses4