The mud column is the only channel from downhole to surface

There is no cable across the thousands of metres between the tool and the rig floor: every bit of data travels as a pressure wave in the drilling fluid column. That channel is narrow, noisy, and it keeps changing with depth and fluid properties. The hard part of MWD telemetry is to raise throughput, shorten transmission time and keep the data correct under exactly those conditions.

How the pulse is made, and where each type fits

• Positive pulse — A valve needle throttles the flow cyclically and raises pressure inside the drillstring. Mechanically simple, strong signal, tolerant of a wide range of fluids — by far the most widely used today. The valve port is the restriction, so sand and lost-circulation material erode it faster and the service interval has to be shortened to match.
• Negative pulse — A bypass valve briefly vents to the annulus, producing a pressure drop. Relatively tolerant of solids content, but it needs a vent port through the collar wall, and signal amplitude depends on annulus conditions.
• Continuous wave — A rotary valve turns continuously to produce a near-sinusoidal carrier, and the information rides on its phase or frequency. It offers the highest usable bandwidth and makes the highest demands on flow stability and mechanical reliability.

Encoding is what sets the effective rate

The same mechanical pulser can differ several-fold in effective rate depending on the encoding scheme. The trade-off runs between rate, noise immunity and error-correction overhead: widening the symbol interval makes decoding more robust but lowers the rate; adding error correction cuts packet loss in high noise but consumes bandwidth; raising the modulation order lifts the rate but makes it more sensitive to signal-to-noise ratio.

A bps figure quoted on its own, without the well conditions, therefore means little. What has to be established is the rate that can be held reliably at the actual flow rate, mud weight, depth and noise level of the well in question.

Measured data