From: deej@cbnewsf.cb.att.com (david.g.lewis)
Newsgroups: comp.dcom.isdn
Subject: Re: Use of HLC in the US
Date: 6 Oct 92 17:26:19 GMT
Organization: AT&T

In article <1992Oct6.125349.5359@nntp.nta.no> finns@hal.nta.no (Finn Stafsnes FBA) writes:
>Use of HLC in the North America.
>
>I am participating in ISDN terminal standardization in Europe, 
>within ETSI. So far we have been told that HLC cannot be used for  
>calls to North America because the North American networks will not 
>support the transmission of HLC. I must admit that I do not know 
>the origin of this information.
>
>However, a posting from deej@cbnewsf.cb.att.com (david.g.lewis), 
>Message-ID: <1992Oct5.132657.19023@cbfsb.cb.att.com>, suggestes 
>that the above mentioned assumption is not valid.
>
>D G Lewis writes:
>
>>>Bellcore requirements (specifically TR-268 and TR-444) specify 
>>>that HLC should always be passed end-to-end.  AT&T's network 
>>>supports transport if it's received from a LEC.  I don't know 
>>>implementation information about any switches, however.
>
>And later:
>
>>>It is supposed to be passed transparently by the switch, which 
>>>means it *should* be consistent across switch types, if only 
>>>because the switch ignores it.  Of course, then you've got to 
>>>make sure the CPE on both ends agree.
>
>To me this says that an HLC-element which is sent from the 
>originating ISDN terminal, will be presented at the destination 
>interface if we have end-to-end ISDN. This applies even if the 
>destination is in the US.
>
>Will anybody please confirm my assumption (or tell me it is wrong)?

Well, I'll start by expanding a little bit in three areas.

First, quoting from TR-NWT-000444, Issue 2, November 1990.  (Page 3-7,
Section 3.1.1.1-I, Access Transport (parameter).

"The SS7 Access Transport Parameter (ATP) is shown in Section B.1 in
Appendix B.  It is used to carry information elements received from an
originating ISDN access transparently through the SS7 network to the
terminating SPCS.

"The ATP should be included in the SS7 IAM if
  - information elements are delivered from the ISDN originating access to
be delivered transparently (i.e. unchanged and without SS7 processing) to
the terinating SPCS, and
  - (i) the call does not involve an IXC, or (ii) the call involves an IXC
and the selected IXC is allowed to receive the access transport parameter.
(See Section 3.3.1).  [typist's note: 3.3.1 refers to office parameters, 
specifically in this case to parameters permitting per-IXC setting of 
whether or not ATP should be passed]

"When the ATP is included in the IAM the information elements allowed for
inclusion in the ATP depend on the services being offered at the originating
SPCS to the ISDN access subscriber.  These requirements support SS7
interworking with the ISDN access procedures in TR-TSY-000268.  For
supporting hte requirements in (TR-268), the following information elements
should be included in the SS7 ATP if delivered to the SS7 network from the
originating ISDN access and the selected IXC, if any, is allowed to receive
the ATP:
  - called party subaddress
  - calling party subaddress
  - low layer compatibility
  - high layer compatibility
  - progress indicator."

Page 3-15, Section 3.1.1.3-A.1.e

"Information elements received in the SS7 Access Transport Parameter (ATP)
should be delivered [in the SETUP message] to the terminating access for
processing.  Information elements that could be received in the ATP as a
result of these requirements being used at an originating SPCS are called
party subaddress, calling party subaddress, low layer compatibility, high
layer compatibility, and progress indicator."


Second, what this means:  This means that Bellcore recommends that switches
implementing ISDN and Signaling System 7 ISUP for purchase by the Bell
Operating Companies should exactly map IEs, including HLC, from the the
originating access to ISUP ATP, and from ISUP ATP to the terminating access.

Third, what this does not mean:  This does not necessarily mean that (a)
switch vendors have developed according to these recommendations; (b) these
switches have been purchased and installed by the Bell Operating Companies;
(c) the services specified in TR-268, specifically transport of HLC, are
being offered by the Bell Operating Companies; (d) ISUP interconnection to
IXCs is being offered including support of ATP transport; (e) International
Carriers are accepting ATP over international trunk signaling links and
delivering it to BOCs, or vice-versa.

I'm not implying that none of those are occurring today; merely, that the
Bellcore TR by stating its requirements does not necessarily indicate that
they *are* occurring.  Actual information as to the state of deployment
would require contact with a knowledgeable party within the appropriate
Local Exchange Carrier.

So the answer to your question is an unqualified "maybe".

Disclaimer: my own opinions, not AT&Ts, etc., etc.
Newsgroups: comp.dcom.isdn