In Exchange A, a set of 155/622H equipment is installed to transmit 32* 2M to Exchange B and then transfer to Exchange C. The equipment used by the user is CISCO2600 router, which is interconnected with SS42PD2DB0 tributary unit of 155/622H via V.35-G.703 (120ohm) converter ASM40 of RAD Corporation.
The user complains that there is no loopback information at Port S0 (2M) of Exchange A when 2M hardware loopback is made at Exchange B, yet the router shows that there are normal loopback information at Port S0 when 2M port of ASM40 is self-looped using the jumper in Exchange A.
The transmission is normal, no TA-LOS alarm occurs, and TD-LOS alarm occurs occasionally but soon disappears.
When the router is looped back at the local from the 155/622H tributary unit using NM, Port S0 of the router shows that the loopback is sometimes normal and sometimes otherwise. When the tail fiber of single NE is self-looped for the transmission equipment, the router shows that the loopback is not set up. Check the output impedance of 2M tributary unit and find it is normal. Check the joint grounding of the router, 155/622H and DDF and find it is OK. Check the configuration file and find nothing abnormal. Directly self-loop on DDF the 2M receiving and sending used by the router with four jumpers (120ohm, equivalent to self-loop of the corresponding 2M channels of the tributary unit), and find unexpectedly that the loopback of the route Port S0 still fails to be set up! Once 2M jumpers of 2M interface of DTE are removed from DDF and self-looped with four jumpers, the loopback of the router Port S0 can be set up. It appears that the problem arises as long as the router is connected to DDF.
Carefully observe DDF and find that the equipment buyer, for the convenience of adding services, wrap 32 pairs of 2M jumpers 15m long on DDF beforehand, coil the excessive part and place under the conductive floor, and unbind the jumpers to connect the equipment if necessary. Upon that, it is suspected that 2M jumper from DDF to the router is excessively long (15m). And the network cable (without shielding layer) is used; moreover, a large number of 220V sockets and power cords are placed under the floor of equipment room (bank), with serious 50Hz power supply pollution. 2M jumpers are polluted by the AC power supply and introduce interference level, resulting in the failure of loopback setup of router Port S0.
Remove the original 15m jumpers from DDF, and use the short jumpers to directly connect the router with DDF. Then, the local loopback and remote loopback at the remote succeed and the fault is cleared.