Back pressure is usually a measure of fluidic resistance and pressure in a pipe. A fluidic substance, such as a liquid or heavy gas, will move through a pipe at the highest speed allowed by its condition. When the conditions of the pipe change, such as becoming narrower or bending, the fluid in the pipe will slow to match the new environment. In computing, back pressure is a buildup of data in a networking switch.
The resistance-based version of back pressure is more common as well as very misleading. The actual term refers to the resistance generated when a pipe changes configuration, not the momentary pressure change behind it. The pressure change typically goes by the name of a fluidic hammer.
When a fluidic substance moves through a pipe, it builds up a lot of kinetic energy and inertia. When that energy hits a wall, it bounces back like ripples when they reach the edge of a cup. This has two major effects. First, it momentarily increases pressure in the pipe when the fluidic substance first encounters the change. The second effect is slowing down the movement of all of the fluid in the pipe, from the change all the way back to the source.
The first effect is commonly called a fluid hammer, or water hammer when specifically talking about water. When the fluid encounters the change, it bounces back at the fluid attempting to come through the pipe. This will increase the fluid present in that small section of pipe and, thereby, increase its pressure. This is a momentary change that ends as soon as the fluid begins to flow naturally around the pipe bend and doesn’t happen again at that bend as long as the fluid flows continuously.
The second effect is a slowing of movement in the pipe, the actual meaning of back pressure. The slowing at a narrow or bent spot prevents the fluid from building as much momentum. This loss is translated back up the pipe, reducing the momentum of the entire stream. This phenomenon is used to reduce the velocity of a substance when it leaves a pipe or to keep a fluid in a pipe for longer period.
In the world of computers, back pressure has a much different meaning. When a network switch or routing system fills its buffer with data and cannot take anymore, it sends signals back to the broadcast source that it is full. The data built up in the broadcast source is called back pressure. This is an undesirable situation, as the built-up data is easily corrupted when finally sent out.