Subject: [Sigtran] AS with different DPC in SUA without GT

From: "HyeonSeung Jang" <[email-masked]>
Date: Mon, 6 Jun 2005 08:38:30 +0900

I have some questions related to SUA without GT.  

We have been developing SMSCs over SUA.  GT is *not* used in the
network and the customer want all the legacy systems ( MSCs, HLRs) not
to be changed.  Is the following transition possible with SUA spec,
especially to SG ?

SG   : DPC=1
AS1(SMSC1) : DPC=10, SSN=8
AS2(SMSC2) : DPC=11, SSN=8

here, 
  (1) SG's DPC and ASes' DPC are different.
  (2) Legacy MSCs send UDT( SCCP Cd_addr:SPC=10 and MTP3 DPC=10) to reach AS1(SMSC1), for example.

After I go through SUA spec,

 "
 1.3.1.1.  SG as endpoint

    In this case, the connectionless SCCP messages are routed on point
    code (PC) and subsystem number (SSN).  The subsystem identified by
    SSN and Routing Context is regarded as local to the SG.  This means
    from SS7 point of view, the SCCP-user is located at the SG.

 1.3.1.2.  Signalling Gateway as relay-point

    A Global Title translation is executed at the signalling gateway,
    before the destination of the message can be determined.  The actual
    location of the SCCP-user is irrelevant to the SS7 network.  GT
    Translation yields an "SCCP entity set", from which an Application
    Server can be derived.  Selection of the Application Server is based
    on the SCCP called party address (and possibly other SS7 parameters
    depending on the implementation).

 1.5.3.  Address Mapping Function at a Relay Node

    The relay function is invoked when:

    -  Routing is on Global Title
    -  Routing is on Hostname
    -  *Routing is on SSN and PC* or SSN and IP Address and the address
    presented is not the one of the relay node
 "

I thought that SG should work as relay-point for above configuration
but it says, "A Global Title translation is executed at the signaling
gateway ... "

If we do not use GT and want to use different DPCs for ASes, does it
break the SUA spec ?  or can we just interpret 1.3.1.2 as " A GTT *may* be
executed at the signaling gateway .... " ?

I tested Cisco ITP and it worked as we expected.  but another SG, a
local vendor, requires that legacy MSCs should send UDT with DPC =
SG(1) and SPC = AS1(10) to reach AS(SMSC1) which means that all the
legacy MSCs must be changed. That local vendor argues that in order to
use different DPC for AS, GT must be used as SUA spec.  I believe the
legacy systems(which do not use GT) should be same even we apply the
SIGTRAN(SUA). Can I ask here your expert opinion about this ?

Thanks in advance.

Jang.
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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Sun, 5 Jun 2005 22:46:43 -0600

HyeonSeung,

Where the SUA RFC says "GTT", read "Address Translation" or
"Address Mapping Function (AMF)": actually having a GT is not
necessary -- PC + SSN is a valid address.

I cannot help you argue with a vendor, and this is supposed to
be a vendor-neutral forum, so please try to avoid brand names in
your emails here.

Hevertheless, here are a couple of things to think about:

* Support for relay mode is optional.

* In your configuration it is not necessary to have the SG PC
  be different from the AS point codes.  The AS point codes can
  "reside" on the SG if the SG is capable of supporting it.

* Your other piece of equipment might prefer the latter due to
  not supporting the former... (?)

* It might be a better idea to look at ETSI TS 102 143 V1.1.1
  (2003-05) or a successor document for GSM specifics.

* Your operator is foolish.  GT is the preferred way of routing
  messages in the SS7 network.  They would be well advised to
  convert their SS7 network to GT before it is too late.

Administrative difficulties (well, more like nightmares) will
always prevail when GT is not used in anything but the smallest
of networks (consider an IP network without DNS)!  As the
reluctance to change demonstrates their lack of knowledge of
SS7, I sympathize with you.  You might have more success asking
the second vendor for help rather than trying to roll the RFC
into a tight rod to beat them with. ;)

--brian

HyeonSeung Jang wrote:                           (Mon, 06 Jun 2005 08:38:30)

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

---

From: Andrew Booth <[email-masked]>
Date: Mon, 06 Jun 2005 12:47:33 -0400

It doesn't sound to me like Relay Node funtionality is needed here.

The SG in this case is terminating the traffic: it's sending the traffic 
to an SCCP User.  To support your configuration the SG must allow 
multiple PCs on the platform, similar to an Alias PC or SPMC 
configuration.  I think it's fair to say that not every verndor will 
support this feature.

Possibly of interest: Q.704 section 2.2.1, when a message is received 
from MTP says:

   Even if an SPC is present in the "called party address" parameter,
   it shall not be used by SCRC.

My interpretation is that AS should be selected using the DPC from the 
MTP Routing Label and the SSN from the CLD Address, but others may have 
a different interpretation.

Regards,
Andrew

-- 
Andrew Booth
Signaling Systems Group
Performance Technologies
www.pt.com
+1-613-237-4344
fax: +1-613-237-5277

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