[Crew] UK PSTN In-band signaling??!??!?!
daac2000 at yahoo.com
Tue Sep 5 11:04:47 BST 2006
Point one was not ironic, my question really wasn't
about blue boxing.
Penny-sweets to the boys and girls who recognised 852
hz+1633 hz as being the c-tone on the extended dtmf
pad! My question was prompted by Mab's fantastic paper
on CALEA countermeasures. It seems CALEA hardware
makes extensive use of in-band signaling, including
the use of the c-tone to denote call termination. The
trick is that you can inject a c-tone which is outside
the parameters(volume and/or freq) accepted by the
switch in your local exchange, but within the those of
the CALEA device. Thus the switch doesn't disconnect
you, but the CALEA device thinks the call is over.
Very nice. Implied by this scenario is the idea that
the switch will recognise in-band signaling(namely the
c-tone) I imagine this is a backwards compatibility
thing for PABX's and pre-historic exchanges. My
question is, therefore, do we have a similar situation
in the UK?
Can/should dtmf in the local loop be considered
in-band signaling? When my call hits my local
exchange, is the DTMF still in-band? I could imagine a
fun device, sitting between Alice and her exchange.
Alice calls Bob, but the device filters the DTMF, and
injects DTMF for Eve's number. Alice starts talking to
Eve, thinking she's talking to Bob...
More in-band signaling:
One example of in-band signalling that we all know and
love is CLID from the exchange to the called party.
CLID is sent in-band between between first and second
rings, I think. There's a phrack article on UK CLID
that is worth reading, I forget which issue it's in
Any comments most gratefully recieved.
ps. Is the Fed Test that thing where you have to keep
whistling while your legs are forced apart by the
school bully? :)
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