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Re: [openss7] [General SS7 question] -- OPC and DPC concept.
Brian:
Thanks very much for your help. I really appreciate it.
I still have a problem with this concept though. The problem comes from the message
discrimination function in MTP3. What that message discrimination function of a
particular switch does is basically to find out whether the DPC of the recieved message
is the the PC of the switch itself. If not, it will pass the message back to MTP2 for
transfering to the PC that will match the DPC.
With the model that you described (I am sure it's correct, but somehow my mind just can't
get straight on this part.), when a switch sends out a message, the DPC will always be
the "next" switch in the path. Therefore, when the message reaches the "next" switch,
the DPC will always match that "next" switch's PC. That makes the message discrimination
function sound unnecessary ...
Going back to my example, AT1 sends a message to IXC with a DPC = B. When the message
gets to the IXC, the message discrimination function compares the DPC with its own PC,
and of course it will "always" find a match. And the same thing will happen in all the
switches along the path. The message discrimination will be unnecessary in this case.
My question is that when will the message discrimination function be useful? Will there
be some "intermediate" switches (like STPs) between an OPC and an DPC that the current
swtich (AT1) won't see (and so won't place their PC in the DPC)? If so, it seems like
the PC-PC-CIC triple will be significant for more than two switches.
If the above assumption is true, how are the routing databases usually constructed in
the Access Tandems (like how do they determine the "next" DPC versus the "final" called
party's DPC)?
Thanks very much in advance again,
Patrick.
"Brian F. G. Bidulock" wrote:
> Patrick,
>
> Yes. Each leg of the call can be uniquely identified by the triple
> PC-PC-CIC (where it doesn't matter what order the point codes are
> in the triple). This triple uniquely identifies a call leg in an
> entire SS7 network.
>
> --Brian
>
> Patrick Lam wrote: Mon, 09 Jul 2001 17:51:53
> > Thanks Brian.
> >
> > So, according to what you said, if we have the following example:
> >
> > AT1: PC = A
> > IXC: PC = B
> > AT2: PC = C
> >
> > where AT = Access Tandem and IXC = inter-exchange carrier.
> >
> > To make an interLATA call, we will have the following connection:
> >
> > AT1 -> IXC -> AT2
> >
> > Would I be right to describe the OPC-DPC-CIC along the path as the following:
> >
> > A-B-3 -> B-C-4 -> C-X-5 (X is the PC of the EO)
> >
> > Thanks again,
> >
> > Patrick.
> >
> > "Brian F. G. Bidulock" wrote:
> >
> > > Patrick,
> > >
> > > The later in both cases. Normally for ISUP, the OPC/DPC/CIC in the
> > > ISUP message refers to a specific circuit between two adjacent switches
> > > which have a trunk between them. For example, for PC A, PC B, and CIC
> > > 3, the ISUP message from A to B is labeled A-B-3. However, for messages
> > > related to the same circuit in the opposite direction, the ISUP messages
> > > are labelled B-A-3.
> > >
> > > CIC has no significance throughout an entire call when used in the header
> > > of an ISUP message.
> > >
> > > Hope that helps.
> > >
> > > --Brian
> > >
> > > Patrick Lam wrote: Mon, 09 Jul 2001 16:47:38
> > > >
> > > > Hi:
> > > >
> > > > This is a general SS7 question. I think these are very basic SS7
> > > > concepts, but somehow I am just a little confused with the OPC, DPC and
> > > > CIC concepts.
> > > >
> > > > 1. Do the OPC and DPC represent the point codes of the Calling End
> > > > Office and Called End Office specifically (i.e. the OPC and DPC are
> > > > constant within the whole switched path of a call, and is significant
> > > > all the way from the calling EO to the called EO)?
> > > > Or do they just represent the point codes of two adjacent switches
> > > > exchanging messages (e.g. an Access Tandem and an IXC) (i.e. the OPC
> > > > and DPC keep changing along the switched path, and is only significant
> > > > for 2 adjacent switches)?
> > > >
> > > > 2. Similar question for CIC: Is the CIC (Circuit Identification Code)
> > > > constant (and significant) throughout the whole switched path of a
> > > > call? Or is it only significant between two switches (i.e. keep
> > > > changing whenever it "step by" a switch)?
> > > >
> > > > Thanks very much in advance,
> > > >
> > > > Patrick.
> > >
> > > --
> > > Brian F. G. Bidulock ¦ The reasonable man adapts himself to the ¦
> > > bidulock@openss7.org ¦ world; the unreasonable one persists in ¦
> > > http://www.openss7.org/ ¦ trying to adapt the world to himself. ¦
> > > ¦ Therefore all progress depends on the ¦
> > > ¦ unreasonable man. -- George Bernard Shaw ¦
>
> --
> Brian F. G. Bidulock ¦ The reasonable man adapts himself to the ¦
> bidulock@openss7.org ¦ world; the unreasonable one persists in ¦
> http://www.openss7.org/ ¦ trying to adapt the world to himself. ¦
> ¦ Therefore all progress depends on the ¦
> ¦ unreasonable man. -- George Bernard Shaw ¦