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Class 4 PRI Protocol Testing

Following the completion of my T1 CAS BOS testing, I was assigned to do ISDN PRI layer 2 and layer 3 protocol testing because it was running behind schedule and the schedule was fixed in stone due to advanced booking of PRI conformance testing at the ATT test lab in Washington, DC.

The reason for conformance testing was because a PBX is a form of customer premesis equipment (CPE). The telephone companies require that any CPE connected to the public network must first pass telephone company conformance testing.

The Hicom PBX supported both class 4 and class 5 PRI. A Class 4 central office is a toll tandem switch used by long distance carriers such as ATT. A class 5 central office is one that provides local landline service. You cannot connect your phone directly to a class 4 switch but you can to a class 5 switch.

ATT's 4ESS was their flagship toll switch. Another long distance telephone company MCI used the Nortel DMS250's but no code had been changed for the DMS250 during the releases, so anything beyond regression testing of the DMS250 wasn't required.

Testing the PRI for the 4ESS PRI should have been relatively easy because Siemens had undergone compliance testing during prior releases and the ATT compliance test specification was a public document. The complete messaging sequences were part of a published specification.

What I discovered was that at the Siemens test lab in Boca Raton, was that only an incomplete suite of scripts existed for the 4ESS PRI. Whatever the reason, a complete suite of compliance scripts was needed. In the case of DMS250, a similar situation existed but since no budget had been allocated for DMS250 work, it would have to wait.

So, with the ATT 4ESS PRI compliance test specification in hand, I began writing scripts.

Through long days and short weekends, I had a complete suite of layer 2 and layer 3 scripts written, debuged, executed and placed under configuration control. All that was left was to prepare the equipment for transporation to the ATT facility.

One item of considerable concern to management was how to load patches onto the PBX during conformance testing. The PBX had no floppy drive or tape drive and the application that installed software and software patches onto the PBX resided on a unix workstation. The unix workstation also had no floppy or tape drive.

The public internet was in it's infancy and most web surfing was still done via dialup modems. I had the network support guys take our department managers laptop, install a 56 kbps PCMCIA modem on it and configure an ethernet connection between the laptop and and workstation.

How it all worked was that a call would be placed from the laptop to an analog access port in California. From there, I could log onto our unix network, telnet to wherever I needed to go and ftp the patches back to the laptop. Once the patches were on the laptop, I would move them to the workstation and from there copy them onto the PBX.

The ATT lab was located in Washington, DC not far from the capitol building. The testing went smoothly and consisted of layer 1, layer 2 and layer 3 tests. Layer 1 testing was outside my scope but as part of my L2 and L3 tests, I did verify while testing the layer 2 and layer 3 protocol that the PBX would synchronize to a network clock, upon loss of network clock revert to island mode (internal clock) and upon restoration of the network clock resynchronize to it.

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