**3.3.2 Case 2 – Optimizing for Coder and Packet Loss Level Selection**

The goal of this case is to find the optimal voice coder and the optimal packet loss level given link bandwidth and background link utilization. Table 10 contains the parameters used for case 2 of the simulation which is according to the ITU-T recommendation G.113


Table 10. Codec Parameters for case 2 (ITU-T Rec. G.113 2007)

The OPNET simulation is configured by the above parameters like the codec bit rate and the packet size and the number of voice frames per packet but other values like Ie and Bpl are coming from ITU-T G.107 and G.113 for the mentioned codecs. The simulation was run for 1 hour, 2 hours and 4 hours and for the 3 coders G.711, G.729 and G.723 with different values of packet loss ratio. For the 3 coders G.711, G.729 and G.723 with different values of packet loss ratio (0.5 %, 1 %, 1.5 %, 2 % and 5 %) knowing that the maximum allowable ratio is 2% but the simulation was run for PL% equal 5% to observe the network behavior in case of big crisis as shown in Figure 7.The test was run with a link speed of 1.544 Mbps. The maximum number of calls was 29 calls. G.723.1 with packet loss of 0.5% was the combination chosen and the same combination was chosen till packet loss of 1.5 %. When packet loss ratio reached 2 %, G.723.1 became not feasible as its R value is less than 70 and G.729 with packet loss 2% was the combination chosen. For packet loss more than 2 % G.723.1 and G.729 became not feasible and the only feasible coder is G.711.G.711 with packet loss more than 2 % was the combination chosen.

Design and Analysis of IP-Multimedia Subsystem (IMS) 89

decrease in R is due to the fact that as utilization values approach 100%, the delay becomes unbounded, which negatively affects the R value. It is noticed that at 90% background Link utilization that all coders give the same number of calls, so the selection in this case is based on the R-Value which is the highest for G.711. It is also noticed that the most affected parameter in this case is the Id which is expected as the Background Link Utilization is affecting the Delay (Id) parameters as shown in Figure 10.The optimization Result for case

Fig. 8. R Value, Background link Utilization % vs. Coder – case (3)

Fig. 9. Background Link Utilization % and Id vs. Coder – case (3)

Table 12. Results of E-Model Optimization Case (3)

(3) is listed in Table 12

Fig. 7. R Value and PL % vs. Coder – case (2)

The optimization Results for case (2) is listed in Table 11 from the results in table 7we can notice that:



Table 11. Results of E-Model Optimization Case (2)
