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This paper presents the development of propagation models for wireless sensor networks for landslide management systems. Measurements of path loss in potential areas of landslide occurrence in Thailand were set up. The effect of the vegetation and mountain terrain in the particular area was therefore taken into account regarding the measured path loss. The measurement was carried out with short-range transmission/reception at 2400 MHz corresponding to IEEE 802.15.4 wireless sensor networks. The measurement setup was divided into two main cases, namely, the transmitting and receiving antennas installed on the ground and 1-m high above the ground. The measurement results are shown in this paper and used to develop propagation models suitable for operation of short-range wireless sensor networks of landslide management systems. The propagation model developed for the first case was achieved by fitting the averaged experimental data by the log-normal model plus the standard deviation. For the second case, the model was derived from the ray tracing theory. The mountain-side reflection path was added into the model which contained the reflection coefficient defined for the soil property. Furthermore, the resulting propagation models were employed in order to realistically evaluate the performance of wireless sensor networks via simulations which were conducted by using Castalia. In the simulations, the sensor nodes were placed as deterministic and random distributions within square simulated networks. The comparison between the results obtained from the deterministic and random distributions are discussed.

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Perceptual VoIP quality is an issue for VoIP applications/services because VoIP applications require realtime support. Not only network factors (e.g. packet loss, packet delay and jitter) but also codec selection that affects VoIP quality. Therefore, this study has been conducted, focusing on the perceptual VoIP quality and codec selection. This paper presents a study of the perception of native Thai subjects to the popular codecs, G.711, G.722, G.723.1 and G.729 using ACR listening opinion tests and conversation opinion tests. It has been found that native Thai users ranked VoIP quality from G.723.1 at 5.3 kbps as the worst, and no statistically significant difference from G.729 (8 kbps), G.711 (64 kbps) and G.722 (64 kbps). Then, the paper proposes voice quality -bandwidth tradeoff analysis approach for codec selection, based on perceptual VoIP quality and bandwidth consumption of voice-payload of codecs. It has been found from this approach that G.729 at 8 kbps is the best choice for VoIP codec selection for Thai users in Thailand, compared to the other codecs.

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