## 4 Methodology
### Congestion Resilience  
With respect to congestion resilience, consider Figure 11 in which a single video frame has been assigned equal bit length slices. The geometric space taken up by any slice is dependent on the coding complexity of the content. Shaded MBs in Figure 11 represent intra-coded MBs. Each slice was further partitioned in source coding space into up to three partitions. As mentioned already, it is possible that partition-B may occasionally be absent (the top slice in Figure 11) if no RIMR MBs are allocated to a slice and if no naturally intra-coded MBs are assigned to the slice by the encoder. (Naturally encoded intra MBs are inserted if an encoder can find no matching MB in a reference frame or as a way of improving the quality.) Similarly, it is possible that if, for example, an extremely high bit rate was allocated to the stream, partition C might not be present, as the encoder could afford the luxury of encoding all MBs with intra coding. However, the simple packet scheduling scheme for data-partitioned streams is independent of source-coding allocations of partitions and slices.  
Figure 12 demonstrates the results of two alternative packet scheduling regimes. A frame interval is shown, which is 1/30 s at 30 frame/s. In the default case, scheduling is at equal intervals in time. We are aware that packet scheduling may never follow this ideal regime in a processor even with a real-time operating system present. However, the regime acts as a point of comparison. In the simple scheduling scheme proposed, packets are allocated a scheduling point according to their relative size within a frame's  
Figure 11: Example video frame from _Stefan_ split into equal bit length slices, showing each slice partitioned into up to three data partitions (DPs) (A, B, C).  
(or multiple frames') compressed size. Thus, for any one packet indexed as \(j\) with length _l'_, its scheduling time allocation is:  
\[t_{alloc}^{j}=\frac{l^{j}}{\sum_{i=1}^{n}l^{l}}f \tag{3}\]  
where \(f\) is the fixed frame interval, and there are \(n\) packets in that frame. The denominator of Equation (3) sums the lengths of the packets within frame \(j\) and allocates a time interval relative to its size (numerator of Equation (3)) relative to the total length of the packets within frame \(j\).  
The proposed scheduling method obviously preserves the original average bitrate, though there is a single video-frame latency while the frame's packet sizes are assessed and the scheduling takes place. In [56], it is pointed out that IEEE 802.21 Media Independent Handover (MIH) services (IEEE 802.21 WG, 2008) provide a general framework for cross-layer signaling that can be used to achieve the scheduling. In IEEE 802.21, a layer 2.5 is inserted between the level 2 link layer and the level 3 network layer. Upper-layer services, known as MIH users or MIHU communicate through this middleware to the lower layer protocols. For mobile WiMAX and later versions of WiMAX, another WiMAX-specific set of standardized communication primitives exists as IEEE 802.16 g.  
Figure 13 identifies the place that AL packet scheduling takes place with the processing cycle and also summarizes the processing cycle as a whole. After compression through the video codec, rateless channel coding is applied at the application layer on an intra-packet basis at the byte level. A simple form of application-layer H-ARQ serves to operate the adaptive form of channel coding. AL packet scheduling now takes place for the source packets. After packetization at the upper protocol stack layers, the PHY layer is responsible for a series of protection measures already mostly described. Data interleaving of each PHY FEC block in an OFDM-based system acts to map adjacent bits across non-adjacent sub-carriers. As previously stated, PHY layer H-ARQ was not enabled in the evaluation of Section 5.  
Figure 12: Example of packet scheduling regimes: (**top**) Equally spaced in time; (**bottom**) Packet ordering preserved but scheduling time allocated according to packet size.