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T1 CAS BOS Testing

T1 signals can be sent back and forth in-band within a bearer channel, or out of band in the D-channel. In in-band signaling, signals for a traffic channel are always associated with that channel. In-band signaling is also known as channel associated signaling (CAS) and less commonly line signaling.

With out of band signaling, signals for a particular channel are associated with a different, dedicated signaling channel -- the D-channel.

One form of CAS is bit oriented signaling (BOS). BOS is also known as bit robbed signaling due the least significant bit of every 6th frame being used for in-band signaling. In super frame signaling (SF), 12 frames make up a SF. When the super frames are numbered 1 to 12, the LSB of the sixth superframe is the "A" bit. The LSB of the twelfth superframe is the "B" bit. SF signaling is also known as D4 signaling.

In extended superframe signaling, (ESF), two superframes make up an ESF and there are four robbed bits; A, B, C and D taken from the LSB of frames 6, 12, 18 and 24 respectively.

T1 Analog Trunk and FX Emulation

By toggling the "A" and "B" bit from frame 6 and 12, the high-low transitions of analog trunks and foreign exchange service could be emulated. This made possible T1 line signaling of ground start, loop start, DID E&M (aka tie trunk); wink start, immediate dial and delay dial. Foreign exchange (FX) service could also be provided via loop start and ground start emulation.


My task was to verify that a new T1 board correctly emulated analog trunks when framing was configured for SF or ESF and line coding configured to AMI or B8ZS. AMSI/TIA/EIA RS-464 compliance was not within the scope of effort. In order to avoid completely reexecuting tests for different framing and line coding. The default configuration was to be ESF and default line coding B8ZS

The T1 board and software were developed in Santa Clara, California. I was to fly there and begin testing until basic call types were possible at which point I was to return to Boca Raton and test detailed call scenarios.

In California, there were quite a few problems with administration software, switch software and board loadware. Eventually, we got through the problems and I returned to Boca where I continued testing call processing interactions with the T1 card.

Testing concluded with load testing. Initial load testing was done by programming an individual call into a Sage box and setting it for repeat. Final load testing was done via bulk call generators.

Upon completion of testing in Boca Raton, my test setup was incorporated into the base lab setup. The lab dial plan as well as test specification was updated and placed under configuration control.

At that point, anyone in our lab area could originate and receive all supported T1 BOS calls. For call originations via T1 BOS, each emulated call type was possible in accordance with every valid digit string defined in the North American numbering plan (NANP) which I seem to recall at the time numbered about 60 unique class of calls.

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