# Video over DSL with LDGM Codes for Interactive Applications
## 4 Findings
### Comparison across Channel Codes  
As described in Section 3, packet error bursts were simulated by a Gilbert-Elliott model, in which a worst-case PER for ADSL was taken to be 1% [8], _i.e._, the bad state of the Gilbert-Elliott model was entered on average about 1% of time. Bursts occurred randomly (Uniform distribution). Burst lengths in time are around 8 ms [8] according to the Repetitive Electrical Impulse Noise (REIN) model for ADSL channels. The packet error burst lengths (\(Ln\)), which depend on the transmission rate, were set to 15, 20, and 25. The simulation ranged over the video data from a two-hour video movie. We have concentrated on packet bursts because, as remarked previously, it is known [27] that burst errors are more harmful to multimedia streams than are isolated errors. Moreover, a good number of studies have reported that packet error bursts are common in various types of network. A study of a campus network [33] reported that error bursts were not only common on heavily-utilized links but appeared to occur with time lengths of less than 5 s (the resolution of the packet loss detection method) on under-utilized links. UDP transport is more relevant to low-latency and interactive multimedia streams. In [34], time-stationary traces amounting to 76 h of Internettraffic were examined. The minimum sampling resolution for those traces for which a 2-state Markov model was a good fit was 160 ms, while 80 ms, 40 ms, and 20 ms resolution traces were not a good fit. However, it can be remarked that while packet loss correlations were detected, they may well have come from buffer overflows rather than the REIN noise presumed to occur in ADSL channels. In fact, [35] considered drop-tail routers to be the most likely cause of Internet packet loss bursts of about 0.1 \(\times\) the round-trip-time (RTT). RTTs varied between 2 ms and 200 ms in [36]'s study. Turning to home access networks, ADSL and cable, the empirical study of [36] mainly reported on through put and latency. However, there were a few observations on packet loss. For example, for users with the same Internet Service Provider (ISP) and service plan, one user consistently had greater throughput of several hundred kB that was probably due to packet losses. The authors of [37] tested their interleaving scheme on the PlanetLab network testbed in respect to the wired Internet and over satellite links. In the wired Internet experiments, 45% of flows experienced no losses, while 10% of packet flows experienced over 50% packet losses. Packet losses of over 100 packets occurred in 8% of the packet flows. In general, packet losses occurred in bursts of less than 25 packets. In the satellite tests, packet loss conditions varied greatly over time. Burst lengths were found to be strongly peaked at 2 packets and below. In our experiments, the ADSL packet size was set to 50 bytes (B). That size is close to the 53 B cell size of the Asynchronous Transfer Mode (ATM) [38] predominantly employed at the data-link layer over ADSL. (Only 48 bytes (B) form the payload in ATM; the remaining header bytes are heavily protected.). ADSL \"Fast Track\" [39] was turned on but packet interleaving to reduce further the impact of error bursts was turned off so as not to introduce additional latency. ADSL transmission rates vary according to the version of ADSL. For downstream transmission a maximum of 8 Mbps is achievable in the earliest version, rising to a projected 52 Mbps in the recent ADSL2\(++\). However, it is the upstream transmission rate that is limiting for interactive video streaming and this correspondingly ranges from a maximum of 1 Mbps to a projected 5 Mbps. In this evaluation, as previously mentioned, an erasure channel was assumed. In practice, groups of erasures would need to be detected at the application-layer if that layer was used to identify which packets forming a video bitstream had been lost. MPEG2-Transport Stream (TS) [40] (a standard way of encapsulating multimedia data) has a packet size is 188 bytes, of which four bytes form a header.