T1 Line Testing Guide

T1 circuits are used for different purposes; they are mostly distributed in a network transport systems as a better way to carry 24 voice channels in single circuit or transmit any 1.544 Mbps worth of data bandwidth. Testing and troubleshooting T1 circuits have been a task for technicians involved in provisioning, installation and maintenance. There are different procedures involved to test a T1 circuit, some of them were developed by large telephone companies and were based on industry testing standards.

I am a bit surprised that when I browsed over the internet to look for any available guide on how to test a T1 circuit. Search results show a slight discussion of how a T1 circuits are being tested and most of them are just in theory. Results also show websites of companies selling test equipment and their specifications.

This guide will show you how to test a T1 circuit in a simpler approach and will give you some ideas on how a circuit is being tested basically in the ‘real’ world,  the common  procedure in doing so and who are involved.

In any common carrier class environment, equipment like telephone switches, transmission equipments, and other network components are normally connected to a central distribution point. Most testing are done in these central point, usually called as cross connection point. These cross connection platforms are applied in different hardware,  the most common nowadays are called Digital System Cross-connect better known as DSX . Testing T1 circuits that are connected to DSX panels are a lot simpler and better. Circuits are tied down to these DSX panel for simple and quick access to the circuits.  If your circuits are not connected in this matter, it is important to know and understand how your circuits are physically connected and you are ready to access or provide whatever the testing needs you to do with the circuit.

In order to make a successful T1 testing, you have to prepare for the procedures that are involved. Understanding of the circuit itself, you need to know where is the limitation of your responsibility and the components involved within your limits. This limit commonly ends  after the demarcation point or “demarc”,  and is usually the reference point between two interconnecting carriers, where each party’s network connection terminates and responsibility ends. These responsibilities were agreed by both parties beforehand and usually shows in the circuit information or circuit order. If the trouble lies after the demarc point and within your network, it’s your responsibility to fix it. Another thing that you need to prepare for is the actual activation of the circuit. Depending on the both parties’ availability, schedules are set to coordinate for the actual activation.

Preparation involves setting up the circuit within your network based on the circuit order before the activation date. It might involve several connections and configurations within your network. You can assume that the circuit is still down or inactive yet unless you have the confirmation from the other party that they  are done provisioning on their side. Otherwise your testing alone will not conclude anything.

Figure 1 below shows a typical local T1 connection of a company having a voice switch located in the 10th floor of a building, manages a pair of M13 multiplexer which extends to the basement of the same building, where normally most carriers maintain their network connections, or point of presence.

The figure also shows different reference points labeled A to D. Understanding the importance of these references is critical in testing or troubleshooting because it will determine the portion where the problem lies during troubleshooting.

During the actual activation and testing, the carrier’s technician will normally ask you to put a loop back towards them in the demarc or hand-off point. A loop back is a diagnostic means which can be applied to virtually any telecom equipments, by connecting the transmit pair to the receive pair of the circuit, thus making a complete loop on a circuit and normally clears error and alarms.  The technician or provisioning engineer might be using a T1 test set, or simply looking at their equipment’s T1 interface port, but either way, they could determine if you are providing them a good loop.

In our figure, it would be point A,  and if they could confirm or “see” the loop back from their location, they will ask you to put  a loop back again to test your extended demarc, which is point D in our figure.  This loop back point which comprise your local network and the actual point before it reach the user equipment, is normally referred to as the “far end loop” in reference to other party’s point of view. Determining the circuit’s far end points is essential in isolation and end to end testing. It would be a good a idea if you provide a bi-directional loop, meaning setting up a loops in A and B,  towards the carrier and your local network, respectively. If  both side confirm the loop ( having the carrier see it, while the switch in the 10th floor could see it as well) then it means that this particular circuit in your network is properly configured and provisioned in that point. If your switch cannot see the loop, then you have to check your wiring and the equipment configurations. Sometimes you just need to “roll” the circuit, meaning correcting the physical connection of the circuit, by making sure that it is cross connected. Configuration issues might be the line coding and framing, and line build out setting.

If everything looks good in that point, you can then proceed and extend the circuit all the way to the intended location. You can move your bi-directional loop in reference points C and D, and you can apply the same test and observe how the remote technician react on your new loop, as well as your far end equipment, which in our sample figure, is the Telephone Switch. If the loops are good, then you can normalize the circuit by removing the loops and tell the technician that you are done testing.

If you have a T1 test set, you might need to initiate a BERT (Bit/Error Rate Test)  and ask the other party to provide you a physical loop back from the far end, means the portion before the other party’s equipment. Figure 2 shows how to test the other party’s far end loop and your own network.

To confirm that the entire circuit is working, your test set is supposed to verify the loop back from the far end, by having a error free BERT results. You also need to ask the other party to break the loop to verify the actual location of the loop back. If everything goes well then you are done testing the transport portion of the circuit and activation in the switch level follows. The other level might be handled by a different group of people, but your test with the circuit will assure that the transport portion is working.

Take note of the circuit identification i.e. Circuit ID, technicians’ names, location, the date and time of the activation, and the telephone number to call whenever there is a problem with the circuit, as these information are required in filing your activation reports. Most of these information shows in the circuit order, but it is still important to keep the records of the actual testing and coordination information that you had.

Testing T1 circuits is fairly simple if you really understand how the circuit runs, know how to isolate and correct the issue within your control.

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