Technical Helpweb

- more articles

Diva PRI Loop-back Test

When to use this test

Use this test in the following circumstances:

1. If you want to confirm a Diva card with one or more PRI interfaces is working and correctly installed, but you don't have a PRI line to test it on.

2. If the Line Check reports a Layer 1 error (or the red LED on the back of the card is lit) when you plug the Diva PRI into the PRI line, and you wish to confirm that the problem is not with the Diva Media Board itself.

Note:  This test only confirms that the Diva Media Board is working - it does not check that the configuration is correct for your line.

Making the loop-back cable

A cable with bare wires at one end and an RJ-45 connector at the other end is normally shipped with the Diva PRI.  The pin assignments on this cable are :

You can perform a simple loop-back test by connecting pin 1 to pin 4 and pin 2 to pin 5. 

What happens when you plug this cable in

1. The red LED on the Diva PRI card should go out, showing that Layer 1 is now active.

2. A Line Check should now say 'Layer 2 failure' rather than 'Layer 1 failure'.

3. An XLOG trace should look similar to this:

0:0130:180 - D-X(003) 00 01 7F
0:0130:182 - D-R(003) 00 01 7F
0:0131:181 - D-X(003) 00 01 7F
0:0131:182 - D-R(003) 00 01 7F
0:0132:181 - D-X(003) 00 01 7F
0:0132:182 - D-R(003) 00 01 7F
0:0133:181 - D-X(003) 00 01 7F
0:0133:182 - D-R(003) 00 01 7F
0:0134:180 - MDL-ERROR(G)
0:0134:181 - SIG-EVENT FFFF 0A

0:0135:181 - D-X(003) 00 01 7F
0:0135:182 - D-R(003) 00 01 7F
0:0136:181 - D-X(003) 00 01 7F
0:0136:182 - D-R(003) 00 01 7F
0:0137:181 - D-X(003) 00 01 7F
0:0137:182 - D-R(003) 00 01 7F
0:0138:181 - D-X(003) 00 01 7F
0:0138:182 - D-R(003) 00 01 7F
0:0139:180 - MDL-ERROR(G)
0:0139:182 - SIG-EVENT FFFF 0A

You will see this sequence repeating. 

Analysis

1. The PRI card is transmitting SABME (LAPD Control field = 7F) and expects to receive UA (73) but instead receives the same looped-back SABME.

2. After retrying a further 3 times, this fails with the error MDL-ERROR(G) and starts again.  Layer 2 never comes up because the right response is not received.