The cross traffic intensity of the optical burst switching networks is a key parameter in the network. This parameter restricts the operation of the assembly algorithm in edge nodes. It is revealed that the service time of the control plane in core nodes, closely related with the cross traffic, is a random variable instead of a deterministic variable in the wavelength conversion devices. Two distinct queuing models named M/D/1 and M/G/1 are constructed to describe the behavior of the core node. An estimate model to infer the cross traffic in the control plane of core nodes is proposed based on the observing variable of residual time of the burst. Numerical simulation results verify that the model can reflect the intensity of cross traffic on the control plane of core nodes in time.