CAS backhaul - Why backhaul

Hluchyj, Mike mhluchyj at SONUSNET.COM
Tue Nov 10 13:45:15 EST 1998

Ray has raised some excellent points that address some fundamental
issues worthy of careful consideration.

There seems to be a basic, underlying assumption that the
centralization of all gateway control provides the most cost-effective,
scaleable and flexible solution to the building of media gateways.  In
my opinion, this moves the pendulum too far and doesn't take advantage
of a number of important technical lessons learned over the past few

There has been a natural evolution in communications equipment
architectures to distribute the workload among multiple processing
elements as a means of achieving scalability.  In particular, the trend
has been to centralize "global" functions while distributing "local"
functions.  We have seen this trend on the data side with centralized
routing and forwarding being replaced by centralized route computation
and distributed (among the line cards) forwarding.  We have seen it in
the circuit-switching world with signaling and, in some cases, call
control functions being distributed among the line cards supporting the
corresponding bearer channels.

This approach of distributing purely local functions provides a natural
system scaling - add more line cards, get more processing with
inexpensive, low-end processors.  Furthermore, there is no compelling
reason to keep all of the detailed state information for all end points
in one place - the switch creates connections among end points and end
points which are not connected do not need to share state information.
In fact, on a truly global scale, this distributed processing model is
what has allowed us to build worldwide telephony and IP networks.  The
principles of distributed processing at a network level work equally
well in building scaleable switching and media processing.

So, how does this principle apply to our current situation? In order to
answer this question, we have to understand what we are trying to
accomplish.  Specifically, carriers/service providers would like to be
able to rapidly deliver services on a large scale using hardware and
software from multiple suppliers (hence the need for open interfaces
and interoperability).  The separation of the media controller (MC)
from the media gateway (MG) seems to be a basic tenet of how this
should be accomplished, and I have no fundamental disagreement with
this.  However, I believe that we have placed too much functionality in
the MG and that this may work against the scalability.

In particular, with the exception of SS7 (which I'll address later),
signaling is a function that can and should be distributed among the
line cards.  When one configures a line or a trunk on the PSTN side,
inherently each end of the line/trunk is configured for the bearer
channel and the associated signaling protocol that both ends
understand.  Basic to the signaling is the ability to collect address
(i.e., the dialed number) and other information for incoming calls.
Basic to CAS and ISDN interfaces are protocol state machines to handle
all this routine processing.  This first stage of processing does not
vary from call to call - it's configured so that each side can
understand the other.

The interesting stuff comes after this initial information is
collected, and this is where an external control function like the MC
can play a significant and important role.  What makes a gateway "dumb"
should not be the lack of signaling stacks or even call state, but
rather what to do with a call when it arrives to the gateway.  Each
arriving call should trigger a message to the MC that basically says
"here's the information that I collected for this arriving call, what
should I do?"  The MC may determine that it is a simple tandem call for
which the MC will perhaps do some authorization on the calling number
and then determine a route (by maybe consulting a gatekeeper or other
backend function).  Alternatively, based on the information collected
by the signaling function, the MC may determine that it's a calling
card call and requires a second stage of dialing.  Whether tandem,
calling card or other service, the response to the MG is a script (or
some form of call processing instructions) from the MC that tells the
MG how to process the call.  In this way, the MG is "dumb" with respect
to how to process each arriving call, yet assumes the signaling and
call processing burden that is most cost-effectively performed by
processors in the MG.  The MC is now more of a pure IT environment with
lots of transaction processing and database queries best suited for a
general purpose computing environment.  New services can be introduced
by developing new scripts.

Now what about SS7?  I think that early thinking on how SS7 can be used
for providing PSTN interconnection for ISP remote access servers via
IMTs led to the current trend of piling lots of signaling and call
processing into the MCs.  As many others have convincingly argued, it
makes much sense to terminate SS7 A and F links on an external
signaling gateway (to preserve scare SS7 point codes, to allow one to
separate a significant and complex development with many country
variations and certification requirements into a separate component,
etc.)  But an ISUP IAM message signifying an incoming call for a
particular CIC (DS0 on a trunk group) can just as easily be processed
by a processor on the trunk card that supports that DS0 (as though the
signaling arrived via CAS or ISDN).  This then makes it consistent with
how CAS and ISDN trunks are served per the above discussion.

This approach seems to lead to a scaleable decomposition of the
problem.  Carriers are free to choose best-in-class components
including: MGs with appropriate cost/performance/scaleablity/reliablity
tradeoffs, service creation environments (for creating the standards-
based scripts), MC for providing the intelligence to determine services
and route calls, and SS7 gateways for dealing with the complex and
varying legacy in PSTN signaling.  As always, vendors are free to group
components together. But if we don't carefully select the appropriate
model of decomposition, then service providers may be needlessly locked
into vendor relations because of the close and hidden coupling of what
could have been separable components.  This includes the close and
hidden coupling of software elements within the MC based on the current

Mike Hluchyj
Sonus Networks, Inc.
5 Carlisle Road
Westford, MA 01886 USA
phone: +1-978-692-8999 x227
fax: +1-978-392-9118
email: mhluchyj at

-----Original Message-----
From: Graham, Gregory [RICH6:B917-M:EXCH]
[mailto:ggraham at]
Sent: Thursday, November 05, 1998 8:32 PM
To: sigtran at BayNetworks.COM
Subject: RE: CAS backhaul - Why backhaul


It is true that if ISUP and Q.931 signaling is backhauled to the MC,
the software on the MC will be complex, but it is better to have the
complex software run on a computer that is separate from the gateway
devices. You want the complex software to run on off-the-shelf
computing equipment where processing power is inexpensive and
scalable. Keep the special purpose gateway equipment as simple as

I do agree that you want some layering in your call processing
software to separate protocol specific processing from higher-level
service logic, but I prefer both layers to run on the MC rather than
splitting them between the MG and MC and having to standardizing a
protocol between them.

Greg Graham
ggraham at
(972) 684-5218

> -----Original Message-----
> From: Zibman, Ray [SMTP:izibman at]
> Sent: Tuesday, November 03, 1998 8:22 AM
> To:   'David R. Oran'; Mauricio Arango; 'Christian Huitema'
> Cc:   sgcp at; sigtran at BayNetworks.COM
> Subject:      RE: CAS backhaul - Why backhaul
> I agree that if the call agent (MC) really needs all the rich call
> semantics present in Q.931 then the easiest approach is to backhaul
> whole thing.  What I haven't seen is much discussion of the premise of
> this
> proposition.  What does the call agent need to do its mission?  What
> its
> mission?  If there are pointers to good discussion on this topic
> please point me.
> These questions are equally relevant to backhaul from an SS7 SG to the
> If we our goal is to build any variety of full-featured end office or
> tandem
> switch out of SGs, MGs, and MCs then we need to get to (and from) the
> every bit of information out of any protocol used for control
> A
> more modest and possibly more achievable goal is to try to
> the
> kind of call control scenarios we want the MC to deal with.  Then we
> define some relevant abstractions of call state and determine what
> information reduction can take place inside an SG (or the equivalent
> of
> an MG for CAS, ISDN, ...) so that the call agent gets and sends what
> relevant and the MC is protected from the irrelevant.  I worry about
> complexity of call agent based service logic that needs to deal these
> rich protocols.  I expect that if we don't do the information
reduction in
> the SG or MG that it will take place anyway in some layer of the call
> agent
> before the information reaches service logic, but service
> (e.g. feature interactions, portability) will suffer.
> Intelligent Network reduced the several thousand states of a modern
> to call models with between 2 to 32 points in call.  It is recognized
> the simpler call models do not support all the services of a class 5
> switch,
> but they support enough to be useful to service developers for a
> but
> limited set of services.  (I don't suggest using an Intelligent
> call
> model for a IP-based service model.  I think there are differences in
> requirements.)
> Let's define the mission for a call agent and the reasons for backhaul
> before trying to define the details.
> -------------------------------------------
> Ray Zibman                            GTE Laboratories Incorporated
> Senior Technologist                   40 Sylvan Road
> Office: (781) 466-2291                        Waltham, MA 02454
> Fax: (781) 890 9320                   <mailto:izibman at>

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