SS7 ISUP basic call setup

[Keith Monahan]

I was reading a couple of SS7 tutorials and it appears that I've encountered
a contradiction and I'm searching for the right answer.

The two documents are

http://www.pt.com/tutorials/ss7_tutorial_05_07_01.pdf

http://www.iec.org/online/tutorials/acrobat.pl?file=ss7.pdf

The first document is written by Performance Technologies(PT), the second by
Illuminet/Bell Atlantic(BA).

When a call is placed, the originating switch transmits an IAM to the
destination switch, via the appropriate STP's. The destination switch in
turn sends back an ACM to the originating switch, via the appropriate STP's.
Both documents agree here.

But then,

The PT document says, "The STP routes the ACM to the originating switch,
which rings the calling party's line and connects it to the trunk to
complete the voice circuit from the calling party to the called party"

so this implies that the ringing signal (the noise the caller hears as the
destination phone rings) is generated by the ORIGINATING switch.

The BA document says, "Switch B[the destination switch].....transmits the
ACM .......to Switch A[originating switch]. At the same time, it[the
destination switch] completes the call path in the backwards
direction(towards Switch A), sends a ringing tone over that trunk towards
Switch A, and rings the line of the called subscriber"

so this document says that the DESTINATION switch generates the ringing and
sends it over the backward path.

Which is it? Are both ways actually implemented? Is one more common than
the other? If both ways are used, what sort of negotiation process happens
to ensure than the origination switch AND the destination switch doesn't
both generate it?

Thanks for the help,

Keith M
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[Michael Will]

Keith Monahan wrote:
>I was reading a couple of SS7 tutorials and it appears that I've encountered
>a contradiction and I'm searching for the right answer.

Hell, the specs themselves contain more than a couple of contradictions :)

>When a call is placed, the originating switch transmits an IAM to the
>destination switch, via the appropriate STP's. The destination switch in
>turn sends back an ACM to the originating switch, via the appropriate STP's.
>Both documents agree here.
>
>But then,

[remote vs. local ring tone]

>Which is it? Are both ways actually implemented?

(Assuming U.S. here) The ANSI specs require full backwards path cut-through
on ACM and forward on ANM. (However, forward cut-through before ANM is very
common, except for AT&T call legs, 'cause they'll be damned if you transmit
*any* information before billing commences. <g> )

Now, intercept messages, etc. (conveyed by cause codes in REL messages) are
usually originated as locally as possible.

So, you hear international ringing in the cadence local to that country, and
intercepts in the language most common/official to yours. Works out fairly
well in real life. (Do they alternate languages with each intercept play
in Quebec?)

Now, my first-hand experience is more with LD/international calling, but this
isn't to say that other methods aren't used. I'd think that when the call is
fully within the control of one entity you might see more local ring tone,
say on calls carried fully within a local operating company. Of course, it
could be argued that when working within one entity, both ends are the same.

- Michael

[Deepa Ahluwalia]

Most of the time its the destination switch that feeds the ring back tone.
Simple way to prove this is try calling different countries, you will get
a different ring back tone.

For more information about ISUP call flows refer to the following URL:

http://www.eventhelix.com/ThoughtProjects/Xenon/EventStudioDocuments.htm

You will find message sequence charts (sequence diagrams) describing ISUP
call flow.

Deepa
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