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23. ISDN Connection Anomaly – Strange Behavior   9/28/12 by Dave Immer

Here’s an issue I believe is unique to ISDN: Facility A, using L2Mono128, cannot establish a framed (locked) 2-line connection with facility B, also using L2Mono128, regardless of who calls who, despite each having no problem with other facilities. However, when facility C connects with A or B the link is framed. I see this circumstance from time to time.

One work-around is to use a bridge. An alternate approach is to try a 1-line connection using an algorithm like MP3 or AAC albeit at a slightly compromised audio quality. This sometimes works when the problem is related to the 2-channel bonding that is used in the CCS/Telos 2-line mode.

But the question remains: What is going on here? A short list of areas where things can go wrong includes:
1. Your Long Distance Provider 
2. Your Local Loop including  the CO and the circuit 
3. Your Equipment.
But these possibilities diminish in light of your successful connections with other facilities.

I would love to impart some nugget of wisdom here, but I too am stumped. This is one of those mysteries that occupy the shadow-world of ISDN’s dark side.  Fortunately, this behavior is rare.

Let me know if you have experienced this and any ideas about why this happens.   Thanks,

-Dave