Example using communication lines

Lines are often shared between processors and can even be shared between operations. Lines are never allocated by z/OS, because they are owned by a communication controller, but you might want to ration the number of file transfer jobs, for example. If you have a number of lines from New York to London, with a total capacity of 256 kbaud, you can define a quantity of 256. If you give a file transfer operation exclusive use of 20 units, for example, that gives the lines a limit of 12 file transfers.

You can protect online and voice systems that use the same lines by giving them a shared allocation of, for example, 50 units each. Because they share units, they do not compete with each other; you can have an unlimited number of operations each sharing 50 units, but this limits the quantity available for file transfers to 206.