This paper focuses the video coding for real time applications and its transmission over synchronous or asynchronous networks, with special interest in ATM (asynchronous transfer mode) switching and multiplexing. Video coders compress the information of a sequence producing a bit stream whose rate, in real time applications, should be controlled to fit the transmission resources. In this work we analyze the main control proposals from a general and comprehensive point of view, pointing out the observed variables, the prediction mechanisms, the action and periodicity of the control. The control action is to switch between different quantization modes. Actually, the coder must decide before coding a unit. A prediction of the data volume to be generated is helpful to avoid ungraceful degradation. The tradeoff in the periodicity of the action is the other key point. We propose a general strategy, compatible with MPEG standards, that is the real buffer evolution shaping. We observe the real buffer and impose a desired level variation. The buffer shaping unit can be a frame, slice, or macroblock. A method of prediction of the data volume to be produced in a unit is proposed. Using the proposed approach we study constant quality and variable bit rate coding, well suited for asynchronous networks. We compare the buffer evolution and the quality of coding under different strategies. In ATM environments the policing functions are taken into account as restrictions to be respected. methods to adapt coding with known policing functions are proposed.