On Jul 15, 2013, at 9:09 PM, SIP Learner <rfc3...@foxmail.com> wrote:
> I think there is one benefit *when the caller dialed a wrong number!* > If overlap signallig were used, the caller might be informed by the proxy > immediately. In en bloc signalling, however, the caller will only be altered > after he dialed the complete wrong number watsting time and resources. That isn't really useful though - your proxy has no idea what the right digits are for a number, except those it controls/manages. So it would work for things like PBX scenarios with private extension dialing, or detecting you didn't dial the right trunk prefix (the '8' or '9' or whatever dialed to make external calls), but not for general cases. It was originally intended to handle the PSTN-interworking cases where ISUP overlap signaling is used, which is basically never in the USA, but does exist somewhat in Europe. I think Europe used it in ISUP years ago because some of their countries use an open numbering/dialing plan, whereby there's no fixed number length for called party DIDs. The carriers only know about a certain number of digits, and the PBXs know about more digits. For example, a German carrier assigns your company the E.164 number "+49-12345678", but then you can add more digits at the end to create specific DIDs, like "+49-1234567890". The carrier only knows about the 8-digits "12345678", and your PBX knows that there are really 10 digits. So when someone else dials your number, the carriers couldn't create a sufficiently accurate digit-map to detect when they're done dialing the number, because they don't know whether the full number is a 8-digit, 9-digit, 10-digit, etc. So instead of relying on dialing timeouts, they used overlap signaling as soon as they had the number of digits they knew about. So they'd send the 8-digit call to the PBX, and the PBX would know it needed 2 more digits. Having said all that, you'd be crazy to expect overlap signaling in SIP to work. It will work in very specific carriers in very specific scenarios, but in general most SIP servers and SIP service providers do not support it. -hadriel _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
