5. Conclusions

Finally, some preliminary results from simulations are reported. The experiments employed the Markov channel models as configured in 4.1 and the evaluation module of 3.2 with α = 15%, applied to a transmissions of the well-known Foreman sequence in QCIF format at 9.6 kb/s. The video codec was the H.263 TMN [3], i.e., without error correction and concealment techniques. Fig. 7 shows the above introduced PFFA versus frame number. Fig. 7. Damages to the " Foreman " sequence introduced by channel errors Of course, although the assumed BER of 5,0×10-5 is fairly low for a GSM T-Mode context, the damage of image quality is significant, showing the urgent need for additional error correction and concealment techniques. Fortunately, the test sequence Foreman exhibits lateral camera tilts leading to complete reconstructions of frame sections close to the border, which provides a kind of implicit video error correction after an error occurred. Furthermore, the curves obviously show different characteristics of damaged image quality despite the same overall BER of the GSM case and that of equal error distribution, confirming the assumption that the burst error statistics are of foremost importance. It should be noted, however , that the testbed has to be run many times before comparisons of this sort become representative. In addition, evaluation of the resulting subjective video quality has shown that the PFFA measure aptly describes the subjective impression of quality impairments caused by transmission errors. This paper presented a generic testbed for the evaluation of multimedia terminal schemes over arbitrary mobile channels. The feasibility of our testbed approach was confirmed by a first-hand analysis of a sample video transmission over the GSM BS26 transparent data channel, modelled by a 51-state Markov chain burst model. A novel QoS-measure describing the percentage of failed frame area was introduced and found to be useful in describing video quality impairments caused by the mobile channel. The test-bed will be used in the future development and analysis of MMTS error correction and concealment techniques in the context of present and emerging mobile data channels.