One of the drawbacks of step-by-step and other early systems is that the preference for choosing trunks or selector links is fixed, and the most preferred links are used more often, with the result that the same faulty hardware blocks repeated call attempts until it is removed from service. 5XB markers were designed to rotate preferences in such a way that it is highly unlikely that the same circuit elements will be used in the next call. Thus, if a call encounters an equipment problem, a second try will probably succeed.