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T1 Basics – a Primer (cont’d) – Testing April 15, 2010

Posted by TelUS Consulting Services in Data Networking catagory.

Loopback Testing

In order to stress test a T-1, layer 1, you will need a test set that can do at least 2 things; loop various devices on the T-1 line and send a BERT (Bit Error Rate Test) test.  Before we can go any farther step 1 must be discussed.  The first step in testing, no matter what level of circuit you are testing is to obtain permission from the customer to intrusively test the circuit.

(BERT)  Bit error rate test is a test comprised of various patterns such as QRSS, 3in24, 1:7, ALL 1’s, ALL0’s, etc.


  • This pseudorandom sequence is based on a twenty bit shift register and repeats every 1,048,575 bits. Consecutive zeros are suppressed to no more than fourteen (14) in the pattern and it contains both high and low-density sequences. QRSS is the most commonly used test pattern for T-1 maintenance and installation. This pattern will stress timing recovery, ALBO and equalizer circuits, but it also simulates customer traffic on the circuit. The QRSS pattern can be used framed or unframed and will force a B8ZS code in circuits optioned as B8ZS.

3 in 24

  • This is a 24 bit pattern which contains three ones. The longest string of consecutive zeros is fifteen (15), and the average ones density is 12.5%. This pattern is used primarily to test timing (clock) recovery and may be used framed or unframed for that purpose. This pattern will force a B8ZS code if transmitted through equipment optioned for B8ZS.


  • This is an eight bit pattern which contains a single one. This pattern is used primarily to test timing (clock) recovery and may be used framed or unframed for that purpose. When transmitted unframed the maximum number of consecutive zeros is seven (7) and the pattern will not force a B8ZS code. When transmitted framed, framing bits force the maximum number of consecutive zeros to eight (8), and the pattern will force a B8ZS code from equipment optioned for B8ZS.
  • This pattern is composed of ones only, and causes line driver circuity to consume the maximum amount of power. In a circuit with repeaters, this pattern will verify that the dc power is regulated correctly. When transmitted unframed, an all ones pattern is defined in some networks as an Alarm Indication Signal (AIS). An unframed all ones signal may also be referred to as a “Blue Alarm”and is sent forward by a device that has lost its input signal. Framed all ones is often used as an idle condition on a circuit that is not yet in service. Thus, all ones is the most common pattern found on a circuit during installation. This pattern will not have a B8ZS code present, even if the circuit is optioned for B8ZS.

All Ones

All Zeros

  • This pattern is composed of zeros only and must only be used on circuits optioned for B8ZS. The pattern verifies that all circuit elements are optioned for B8ZS and should be used whenever a B8ZS circuit is under test.

NOTE – A battery of each of these patterns should be run on a T-1 for 3 to 5 minutes each to truly stress the facility and ensure circuit integrity.

First thing to do once the circuit is split is to send a test signal straight away, doesn’t matter what pattern, towards the direction that you are testing to ensure there are no loops currently present on the line.  If your signal syncs up, then you are looking at loop and now you need to find the source of the loop.  If your test pattern does not sync up, then testing can begin.

Begin testing with loop testing.  There are various devices on a T-1 that are software loopable by a code that is sent down the T-1 interwoven in a test pattern, such devices are; NIU, CSU’s, and repeaters.  When trying to loop a device, it is good to note that if the loop fails the first time, try it again with a different framing.  If you tried SF, and it failed, try ESF, if that fails then try it unframed.

Always begin loop testing with the device that is furthest away from the point at which you split the circuit.  On a typical customer T-1 that device would be the customers CSU (channel servicing unit).   If that device cannot be looped up, try to loop the Bell NIU (Network Interface Unit).  If the Bell NIU cannot be looped up, stop troubleshooting and call the LEC (Local Exchange Carrier) who owns the copper, give them your test results and open a trouble ticket.

If either the CSU or the NIU device responds to a loop up code, now it is time to run your battery of tests mentioned above to the device that is looped up.  During the test, be sure to insert a bit error and make sure that you receive it back.  When you insert an error facing a loop, the error will come back to you under normal operation.  If you do not receive the error back when you inject one, there is a problem.

Straight Away Testing

In straight away testing the dependencies are different than in loopback testing.  When you test straight away you are not going to be sending a loop up code to a loopback a device, you will be sending a test pattern to another test set at the other end of the circuit.  So as you might have guessed, this type of testing requires a certain amount of coordination and planning to ensure that you have an individual at the other end of the circuit with a test set with the same capabilities as yours.

When you are testing straight away, or end-to-end as it is commonly called, both parties at each end of the circuit must send the same test pattern relatively simultaneously.   One of the ways to ensure that you are testing end to end and not seeing a loopback is to send a bit error.  When testing straight away and you send a bit error the test set on the other end will receive the error.  If it comes back to you, then you are looking at a loop and not the other test set.

Next time, more on T1’s with more detailed troubleshooting.

Joe Buck, N.C.E.

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