ure Rate 20-Aug 3-Sep 17-Sep 1-Oct 15-Oct 29-Oct 12-Nov DL TBF est Failure Rate 20-Aug 3-Sep 17-Sep 1-Oct 15-Oct 29-Oct 12-Nov Figure 2.91 Sample uplink and downlink TBF failure rates Radio Network Planning and Optimisation UL TBF Retainability 3-Sep 17-Sep 1-Oct 15-Oct 29-Oct 12-Nov 26-Nov DL TBF Retainability 3-Sep 17-Sep 1-Oct 15-Oct 29-Oct 12-Nov 26-Nov Figure 2.92 Sample uplink and downlink TBF retainability in this example that the uplink TBF failures are much higher than the downlink TBF failures. There are three possible reasons for this: 1. Link loss and performance on the uplink is worse than on the downlink (uplink limited). 2. There are a significantly larger quantity of uplink TBFs due to shorter and more frequent TBFs, increasing the likelihood of failures. 3. Uplink TBF failures result in persistent re-tries (up to five for each failed attempt), SO initial TBF failures are inflated due to compounded re-tries. All three of the above causes must be considered when optimizing the performance of the TBF success rate. TBF Drops TBF drops are harmful to EGPRS network performance since they result in throughput degradation and increased application latency or even lead to application stalling. The following trends outline the uplink and downlink TBF completion rates (1-TBF drops) of four sample networks (Figure 2.92). This example shows uplink TBF retainability (completion rate) ranging from around 98 % to 99.5% and downlink TBF retainability ranging from 94% to just over 96 % (significantly lower than uplink). The uplink TBF Radio Network Optimisation completion rate is better than the downlink TBF completion rate in this example due to many short uplink TBFs successfully completing along with the inherent TBF drops (lost TBFs) that occur during cell reselection. Note the tradeoff between the TBF setup success and the TBF completion rate (drops): TBF failures can shift between the setup and transfer phase of the packet call. In this example, additional uplink TBF failures occur in the earlier setup phase