This sets the loopback state of this DS3 or E3 port: no loop, payload loop, or line loop. Payload loop tests just the framer/mapping logic on this port while line loop reflects the raw signal back at the line interface, so choosing between them determines exactly how much of this port's actual local hardware you are validating during a test. A technician suspecting a bad line interface unit on this port would use lineloop to reflect the signal at the earliest possible point, while dsx3PayloadLoop would instead be used to rule out just the DS3 framer.