## 4 Methodology
### Error Resilience
The FEC redundant data (Raptor code data) are also included (refer to Figure 9) within the packet payload along with checksums. If additional FEC data are requested over the uplink, that data also is included in the packet payload. As NALU aggregation was not enabled, each NALU is assigned to a separate network packet as its payload. Additionally, it is important to notice that, for our experiments, each video frame formed a single slice. Figure 6: Decomposing a video frame into partitions within slices. Figure 7: H.264/AVC data partitioning in which the partitions of a single slice are assigned to three NAL units (types 2 to 4). As described in the text, the relative size of the partitions varies with the QP setting of the encoder and is not fixed, as shown in the schematic representation of this figure. Reconstruction of the other partitions is dependent on the survival of partition-A, though that partition remains independent of the other partitions. Constrained Intra-Prediction (CIP) [54] (Ref. [54] introduces standardized intra CIP while proposing non-standard inter-CIP) was set in order to make partition-B independent of partition-C. When only partition-A survives, its motion vectors can then be employed in error concealment at the decoder using motion copy. When partition-A and partition-B survive, then error concealment can combine texture information from partition-B when available, as well as intra concealment when possible. When partition-A and partition-C survive, to reconstruct it requires partially  
Figure 8: H.264/ Advanced Video Coding (AVC) data flow for combined slicing with data partitioning. Figure 9: WiMAX packets (MSDUs) from BS to an MS containing video source data from higher layers with additional application-layer-forward error correction (AL-FEC) added by the streaming application. Packet x + 1 contains additional piggybacked AL-FEC data requested to aid in recovery. The Raptor code data overhead is adapted to channel conditions. The amount of code data added is schematic only, as in practice around 5% would be present. Not shown are possible cyclic redundancy checks (CRCs) added by the application and Internet Protocol (IP)/User Datagram Protocol (UDP)/Real-time Transport Protocol (RTP) headers that would also be present in the source packet payload in addition to the Network Abstraction Layer units (NALU). discarding partition-C data reliant on missing partition-B MBs and then reconstructing either of them using partition-A MVs or partition-A with partition-C texture data, when it is available. The relative size of the data-partition packets is determined by the quality of the video, which in turn is governed by the QP of the MBs. The QP is set in the configuration file of the H.264/AVC codec, prior to compression. Figure 10 is a comparison between the relative sizes of the partitions according to QP for the two diverse reference video clips used later in Section 5. For the purposes of assessing the impact of the QP, Variable Bit-Rate (VBR) was encoded in Section 5.1's evaluation. In VBR video, the QP remains constant in order to preserve video quality. However, many broadcasters prefer Constant Bit-Rate (CBR) video, as it allows transmission bandwidth and storage to be predicted. Transmission jitter is also reduced. For that reason, CBR video is tested in Section 5.2, though the QP value may vary a little [55] in order to maintain a constant bit-rate. Rateless Raptor code is an effective form of FEC that provides protection to the data partitioned video stream with reduced computational complexity and capacity approaching overhead. Rate adaptation of AL FEC is performed in order to match wireless channel conditions. The code was applied at the level of bytes within a packet in the interests of reduced latency rather than at the usual packet level. Thus, the byte forms the block symbol size and only bytes passing the CRC at the PHY layer are accepted at the application layer channel decoder.