U
unidirectional path switched ring (UPSR).

Path-switched SONET rings that employ redundant, fiber-optic transmission facilities in a pair configuration. One fiber transmits in one direction, and the backup fiber transmits in the other. If the primary ring fails, the backup takes over.

unnumbered link.

A link that does not have an IP address assigned to it.

User-to-Network Interface (UNI).

A specification of the interface between an end system and a backbone system. An example is the specification of an Ethernet interface that connects a switch at the customer site and a router at the provider site.

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