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Knowledgebase


T1 Loopback plug for testing both Line and CSU/DSU
Article Id #: 435


Whenever you are experiencing connectivity issue on the e1t1/ethernet a great tool to test if the ports are working fine would be the loopback cable.

A loopback cable redirects the output back into itself. This effectively gives the network interface the impression that it is communicating on a network, since its able to transmit and receive communications.

Therefore on any Smartnode,when the loopback cable is connected on any port and we enter "show port e1t1/ethernet (Port#)" on the CLI or even click on status of the port on the Web GUI, it should show as "Active" on that port.

 

 

1 Steps:

Materials needed


 

You'll need a Cat5 cable end, some Cat 5 cable (just a few inches needed), and a Cat 5 cable crimper & cutter.

 

2 Steps:

1. Cut off about 2 to 4 inches of a Cat 5 cable 
2. Take off the casing 
3. Separate 2 of the 8 wires for use... usually same colors (brown and brown/white, etc.) 
4. Identify the pin order on the Cat 5 end: hold it with the movable/press-side of the clip away from you and the metal contacts facing left. If you are looking at it this way, the bottom slot is pin 1 and the top is pin 8. 
5. Put one end of the first wire (either one will do) into pin 1 and bend it in half and put the other end of it into pin 4. 
6. Put one end of the second wire into pin 2 and bend it in half and put the other end into pin 5. 
7. You should now have 2 wires "looped" in the cat 5 blank end with pin 1 connecting pin 4 and pin 2 connecting pin 5 
8. Put the Cat 5 end into the crimper and verify all of the wires are pushed all the way into the pin slots and then crimp it down. 
9. Test it on a "known good" router with a CSU/DSU (T1 interface). Place the loopback plug into the T1 port and you should see the unit "synch" to itself by indicating either "Link" or having whatever is used for the WAN interface to show the unit now has sync. If it does, you've successfully built the plug. If it doesn't work, you've either built it wrong or the CSU/DSU isn't working properly.

 

3 Steps:

How to use the loop plug

1. Let your ISP know you have a loopback plug. 
2. They will likely want you to plug it into the "smart jack" (the final hand-off point from the local phone company. 
3. The loopback plug will allow the local phone company to test THROUGH THE SMART JACK and verify if the service is working properly through that point. 
4. Once service is verified through the Smart Jack, you should see if you can plug the loopback plug at the end of your Internal (or Inside) wiring. If you have a buscuit jack at the end of an extension from the smart jack, you can plug the loopback plug into it and the ISP can tell you if you are "clean" through the Inside Wiring.

 

4 Steps:

Exception

It IS possible for the test to be good through the loopback plug and there STILL BE AN ISSUE with the local phone company's wiring. In rare cases, the copper pairs may be "rolled." This means that the 4 wires used to provide the T1 service are wired in reverse where Transmit 1/Receive 1 are actually wired to Transmit 2/Receive 2 somewhere along the route. In this case, the loop test will perform properly, but the T1 service will not work. If you suspect this, you should ask the ISP to dispatch the local phone company to "Prove through the demarc/Smart Jack." This will force the local phone company to dispatch to your site and test head-to-head with the local Central Office to prove the service is good. There sometimes may be a charge associated with this dispatch IF IT IS FOUND THAT YOUR EQUIPMENT OR WIRING IS THE CAUSE OF THE OUTAGE.

 

Conclusion

Ultimately, the ISP is "responsible." However, having this simple piece of network test gear can speed up the process of a T1 repair and troubleshooting. In the end, restoring your service as quickly as possible is the ultimate goal and this will help you achieve that goal!

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