On Mon, 17 May 2004 10:32:32 +0200 Iljitsch van Beijnum <iljitsch@muada.com> wrote: | If they knew the difference between a busy signal and a congestion | signal they probably would... Er, no. Congestion signal normally means that there are no circuits and the phone network has handled that situation without any issues. But that's not the primary threat that the switches have to handle When the call demand far exceeds the number of circuits/operators available, repeated busy or congestion tones will cause callers to make repeat attempts. Local (originating) switches handle this just fine, and then send forward a C7 call set-up request to the switch that handles inbound for that number range (I guess, that's their equivalent of an "MX" host). And that's where things go wrong. Digital circuit-switches such as AXE10, DMS100/250 etc are far more vulnerable to high levels of call-set-up traffic, which would cause their processors to be overloaded. Again, the IP analogy is obvious. Multiple-repeat-attempts at call setup to the same number (and same destination switch) from numerous originating switches, cause the processor at the destination switch to be overloaded and to crash. That's doesn't result in busy or congestion signals - that results in NO signals (not even dialtone in that exchange's local area). The telcos' priority therefore is to block the call-setup-attempts at the edges of their network (i.e. originating /early transit switches). This is known as "call-gapping" and is not without some controversy. -- Richard Cox