## 5 Evaluation
### Error Resilience  
This section concentrates on the impact of the Gilbert-Elliott channel model rather than any packet losses from buffer overflow. As previously outlined, there are two types of channel packet errors: those caused by error bursts of sufficient intensity in which complete packets are dropped and those bursts errors resulting in the corruption of packets. As previously mentioned, in the event of a failure to decode a corrupt packet, a repair is attempted by an ARQ request for additional rateless data if an attempt at decoding fails. If the additional redundant data still fails in allowing successful packet recovery, that packet is then also regarded as dropped. If not, it is regarded as corrupted but suffers from the additional delay required to request redundant data. Upon receiving that additional data, the packet's data is then decoded again. For this set of experiments, two different video traces were employed for input to ns2 in the WiMAX downlink tests. The first one was the _Paris_ sequence (also used in the congestion resilience experiments). The second one was _Football_ (high temporal coding complexity) sequence. Both sequences had Variable BitRate (VBR) encoded in Common Intermediate Format (CIF) (352 \(\times\)288 pixels/frame) at 30 frame/s. The RIMR level was set to 5% in the IPPP.... GOP structure (where I is an I-frame followed by all P-frames denoted as PPP...). In these experiments, a single slice per frame was allocated. The experiments determined the effect of changing the percentage of redundant rateless data or alternatively adopting the adaptive scheme, which initially calculates the amount of redundant data sent through Equation (2). To make a comparison with the adaptive scheme, fixed amounts, 5% or 10%, of redundant data were added to the packets prior to transmission over the WiMAX link. In Figure 15, it is apparent that increasing the redundancy from 5% to 10% and then changing to the adaptive scheme of Section 4.1 has an effect on packet drops, which impacts video quality, as FEC cannot be applied to dropped packets. However, video content has little impact even though _Football_ is much more active than _Paris_. In contrast, QP has a significant impact because the reduction in size of partition C packets as the QP is lowered (refer back to Figure 10) results in a reduction in the number of larger packets. Comparing the number of corrupted packets in Figure 16, one sees a dramatic increase in their number, with only 5% redundant data but their elimination with adaptive FEC. Thus, for adaptive FEC, the mean delay of corrupted packets (the mean delay including retransmission of all corrupted packets) is zero because there are no corrupted packets. However, in Figure 17 the reconstructed objective video quality, Peak Signal-to-Noise Ratio (PSNR), for 5% and adaptive redundant data is much the same. There is, of course, a penalty from not adapting the redundancy rate according to the channel conditions, because the total delay in retransmitting the corrupted packets (Figure 18), would build up if only 5% rateless coding was applied, leading to larger start-up buffers and discouraging interactive video applications. The higher percentage of dropped packets at low QP (high broadcast quality) and larger packet sizes leads to unacceptable video quality, with the redundant 10% data leading to a drop in quality at QP = 25. There are small variations in the mean end-to-end packet delays across the QPs (Figures 18 and 19), whose variations are influenced by packet sizes and their propagation times. Therefore, adaptive rateless coding reduces delay considerably over using 5% redundant data but achieves better video quality than  
Figure 15: Dropped packets (%) for streaming (**a**) _Football_ and (**b**) _Paris_ with 5%, 10% and adaptive redundant data. when using 10% redundant data. Naturally, delay is an important issue in a real-time service, as it governs buffering provision, enables interactive applications, and if not checked can lead to freeze frame effects.