This is the most common method of data transmission used by MWD tools. Downhole, a valve is operated to restrict the flow of the drilling fluid (mud) according to the digital information to be transmitted. This creates pressure fluctuations representing the information. The pressure fluctuations propagate within the drilling fluid towards the surface where they are received from pressure sensors. On the surface, the received pressure signals are processed by computers to reconstruct the information. The technology is available in three varieties: "positive" pulse, "negative" pulse, and "continuous wave".