Re: [openss7] [General SS7 question] -- OPC and DPC concept.

["Brian F. G. Bidulock" <bidulock@openss7.org>]

Patrick,

Patrick Lam wrote:                        Tue, 10 Jul 2001 11:41:28
>
> 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.

Well, no.  Switches are not usually equipped with the transfer
function, and will usually discard a messsage which is received
which does not have the correct point code as a 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
> ...

Well, not unecessary, just trivial.

> 
> 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.

s/unnecessary/trivial/

> 
> 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.

The discrimination function is less trivial in an STP.  Remember
also that the discrimination function discriminates between MTP
management and MTP users, and between MTP users on the basis of
SIO.  Because you are only considering ISUP, the discrimination
seems more trivial.

> 
> 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)?

Normally in the Access Tandems, trunk groups are provisioned and the
Point Code of the switch at the other end of the trunk group is
datafilled in some manner.  When the switch goes to signal on the
trunk group, it uses the CIC for the selected trunk member (circuit)
and signals using its point code to the point code assigned to
the trunk.  It has no concept of a "final" DPC.

> 
> Thanks very much in advance again,
> 
> 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 ¦