In Optical Burst Switching (OBS) networks, many packets are aggregated into larger bursts, which exhibit different traffic characteristics in the bufferless optical core networks compared to the packet-level traffic in the conventional electronic switching systems. However, many OBS studies often neglect this issue by assuming the inputs to be bursts when analyzing the system-level performance. This paper demonstrates that the loss performances of optical bursts are dramatically different when considering burst assembly process at the edge nodes and the connectivity of the core nodes. We develop some analytical models for the JET and JETseg networks, which can model the loss performance more accurately than the traditional methods. Our performance evaluation reveals that the connectivity and burst assembly process affect the loss performance significantly, particularly for a WDM network with a small node-degree and a large number of wavelength channels. Our model provides an in-depth understanding on the OBS scheme.