tom@pdx.mentor.com (Tom Ace) (08/10/89)
A couple of years ago, an acquaintance in Los Angeles told me about a problem GTE experienced after modernizing one of their COs down there. They had replaced a mechanical exchange with a digital one (if memory serves me correctly, a NEC switch), and some kids found an interesting problem with it. Three subscribers with call forwarding would create a forwarding loop, e.g. A forwards his calls to B, B to C, and C back to A; once that was set up, calling any of the three numbers from another location would, after a short pause, cause the entire switch to go down (completely go out of service). There was evidently code in the switch to recognize two-subscriber forwarding loops, but with three or more, it would hang. The only fix they had at the time was to disable call forwarding for customers served by that switch. I can't vouch for the accuracy of this story, it was just something I'd heard. Has anyone heard of anything similar? What happens with Bell ESS switches? What happens if two customers in different areas try to set up a loop? (I'd try, but I don't have the feature and don't know people who do.) Tom Ace tom@sje.mentor.com ...!mntgfx!sje!tom [Moderator's Note: We covered this topic rather extensively some time back in the Digest. There are now absolute limits on the number of loops which can be made. Maybe someone will respond who knows the specifics. PT]
ellisond@ncar.ucar.edu (Dell Ellison) (08/18/89)
In article <telecom-v09i0286m01@vector.dallas.tx.us>, sje!tom@pdx.mentor.com (Tom Ace) writes:
-> .... Three subscribers with call forwarding would create
-> a forwarding loop, e.g. A forwards his calls to B, B to C, and C back
-> to A; once that was set up, calling any of the three numbers from
-> another location would, after a short pause, cause the entire switch
-> to go down (completely go out of service). There was evidently code
-> in the switch to recognize two-subscriber forwarding loops, but with
-> three or more, it would hang. The only fix they had at the time was
-> to disable call forwarding for customers served by that switch.
-> [Moderator's Note: We covered this topic rather extensively some time back
-> in the Digest. There are now absolute limits on the number of loops which
-> can be made. Maybe someone will respond who knows the specifics. PT]
With a GTE switch, the 'absolute limit' is now seven. After the call
has been forwarded (or diverted) seven times (even if it's in a loop),
the originating party will get Reorder tone (fast busy).