What is back pressure?

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Back pressure is generally a measure of fluidic resistance and pressure in a pipe. A fluidic substance, such as a heavy liquid or gas, will move through a tube at the highest speed allowed by its condition. When tube conditions change, such as becoming narrower or bent, the fluid in the tube will slow to adapt to the new environment. In computing, back pressure is an accumulation of data on a network switch.

The resistance-based version of back pressure is more common and also very misleading. The term real refers to the resistance generated when a tube changes configuration, not the momentary change in pressure behind it. The pressure change typically goes by the name of a fluidic hammer.

When a fluidic substance moves through a tube, it accumulates a lot of kinetic energy and inertia. When this energy hits a wall, it bounces back like waves when they reach the rim of a cup. This has two main effects. First, it momentarily increases the pressure in the tube when the fluidic substance first encounters the change. The second effect is to slow down the movement of all the fluid in the tube from the change to the source.

The first effect is commonly called a hydraulic hammer, or water hammer, when it comes specifically to water. When the fluid encounters the change, it bounces back into the fluid trying to pass through the tube. This will increase the fluid present in that small section of the tube and thus increase its pressure. This is a momentary change that ends as soon as the fluid begins to flow naturally around the bend in the tube and does not happen again at that bend as the fluid flows continuously.

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The second effect is a deceleration of movement in the tube, the true meaning of back pressure. Deceleration in a narrow or bent spot prevents the fluid from building up so much momentum. This loss is translated back into the tube, reducing the dynamics of the entire flow. This phenomenon is used to slow down a substance as it leaves a tube or to keep a fluid in a tube longer.

In the computer world, back pressure has a very different meaning. When a network switch or routing system fills its buffer with data and can’t take it anymore, it sends signals back to the transmission source that it’s full. The data accumulated at the transmission source is called back pressure. This is an undesirable situation as the accumulated data is easily corrupted when finally sent.

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