The Value of U
U or RU stands for rack unit. 1U represents 1.75 inches, and is a standardized height for equipment that is designed to be mounted in data or audio/video racks.
A standard 24 or 48 port network switch will typically come as a 1U device, meaning that it will be 1.75 inches tall so it will fit into a single U slot. Larger switches, routers, UPS equipment, etc. will come in multiple U sizes. A UPS, for example, may have a body that is 4U so that it can accommodate the batteries that go inside it. Or a large modular switch may take up 15U.
The Rack and U
Racks come in different heights. A full rack is normally 42U. So it can hold up to 42 1U devices and stands a little over 6 feet tall. You can also get half sized racks around 22U if you want something the size of a small desk filing cabinet. There are even smaller racks that hold only 2U to 10U. These are normally wall mounted racks and are used for small distribution points, where only a hand full of data cables terminate.
Half-Rack vs U
Sometimes you will see equipment that will say something like: “This is a 5U half-rack form factor”. What is being said here is that the device, whatever it is, will be 5U tall, but will take only use up half of the 19 inch width of rack. So the half-rack here is referring the width of the device, not the height.
Racks usually come with either threaded-holes or square-holes in them. Threaded are still common for 2-post data closet type racks while square-hole are more common in 4-post server focused racks. When it comes to “U” though, the type of hole doesn’t matter much, but the layout of the holes does. Both threaded and square holed racks have the same layout with the holes, so the following points apply to both.
If you look closely at the holes in a data rack you will notice a pattern to the hole layout. There will be two holes close together, one hole by itself, then two holes close together again. At least, that is how I saw the pattern. Well, a “U” in the rack will begin between two close together holes, run past the hole by itself, and end between the next two close together holes. I can’t tell how annoying it can be to come across equipment mounted between 2U in a rack. The equipment doesn’t screw in correctly so it isn’t secured well and it fills up 2 precious “U” when it should only be using up 1U. So, be good and mount equipment in the correct way in your racks!
Well that was short as sweet! Do you have any other questions about U or racks? Drop the question into the comments so it can get some love and attention.