3Com S330 Using Loopbacks, Points should help you identify and resolve the problem, These steps

Using Loopbacks 179

Using Loopbacks	Loopbacks allow you to check circuit continuity between one point and another.
	You should use the PathBuilder S330/S310 loopback feature to check continuity to
	the nearest point first, and if the circuit is valid to that point, then loop to the next
	point. If a circuit has been compromised, isolating the problem between two
	points should help you identify and resolve the problem.
	To access the loopback modes supported by the PathBuilder S330/S310, follow
	these steps:
	1 From the Main menu, select [3] Fault Management to display the Fault
	Management menu shown earlier in Figure 133.
	2 Select [5] Loopback from the Fault Management menu to display the Loopback
	menu, shown in Figure 136.
	Figure 136 Loopback Menu

3Select the card type for which you want to conduct the loopback.

The Loopback menu for the selected port appears. Figure 137 shows the Loopback menu for the OC3/STM-1 port.

If you select a card type for which the PathBuilder S330/S310 does not support loopbacks, a message appears, informing you that no loopback feature is available for that card type.

If you select a multiple-port card, you must also select the port for which you want to configure the loopback.