Gary: Re: RB: Even better reading IMO... If you understand the flow.
This set's the stage for what must come to be with Internet Phone workings.
Good Reading to you,
Jim...
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Achieving VoIP interoperability
internettelephony.com.
A call transaction processor may be the key to providing voice over IP
CHRISTOPHER COUCH
Integrating traditional telephone networks with new voice systems
using Internet protocol (IP) has not been easy. To provide the quality
of service that telephone customers demand, telecommunications
service providers must make data networks and traditional voice
networks function together seamlessly. A software solution linking
heterogeneous networks is finally within reach--and market trends
suggest that voice over IP (VoIP) service providers would be wise to
take advantage of it.
Most analysts agree that the price advantages VoIP has over
traditional public switched telephone networks (PSTNs) will
disappear. The only question is when. MCI WorldCom Inc. has
already introduced 6¢-a-minute circuit-voice service between London
and the United States. The path for VoIP providers is clear: Make the
most of pricing arbitrage now.
What's holding VoIP back is the lack of standards for interconnection
between competing gateways in the data world and the signaling
system 7 (SS7) networks used in PSTNs. SS7 allows advanced
intelligent network (AIN) services including 800 number routing,
automated credit card calling, local number portability (LNP), voice
mail, conferencing, caller ID and other value-added services that
callers now expect as part of the package.
While SS7 is a nearly universal standard among PSTNs, there is no
equivalent call model standard for data networks. Transferring AIN
services over VoIP requires high-level communication between the
SS7 system and any intervening IP networks that carry the call. Until
now, this level of service has been out of reach for most VoIP
providers.
Efforts are underway to establish a unifying protocol for VoIP, but
even if a standard is adopted, implementation will take time. Rather
than waiting, VoIP providers should install a call transaction
processor, a compatibility solution that solves all their problems
today. The call transaction processor is a robust solution that
understands competing call models and signaling systems and
provides immediate interconnection between existing IP gateways
and SS7 networks.
SS7 Call Functionality
To fully understand what VoIP providers need in the form of a call
transaction processor, it helps to establish a basic understanding of
call models and the function they serve within SS7 networks. The call
model is the foundation of all telecommunications. It is the logical
sequence of events that must be carried out to set up a call.
In the days when plain old telephone service (POTS) was the only
game in town, a call was nothing more than a point-to-point
connection, and signaling information was carried "in-band" on the
same circuit as voice traffic. If someone was on the line, you simply
got a busy signal.
As telecommunications service grew in complexity, call models
evolved. In-band signaling took up valuable bandwidth on circuits that
could be carrying toll calls, and it was also vulnerable to fraud. To
provide more service, minimize fraud and increase network
efficiency, telecommunications researchers developed "out of band"
signaling, the cornerstone of today's AINs.
When a customer places a call on a PSTN, the network immediately
queries a database to find out what to do with the call. This query is
carried out of band on a packet-switched network that operates
independently of the circuit-switched voice network. SS7 was
established by the International Telecommunication Union
Telecommunications (ITU-T) as the standard protocol for AIN
communications worldwide.
All SS7 networks contain three basic elements:
signal control points (SCPs), which are computers that hold
databases of customer-specific information used to route calls;
signal switching points (SSPs), which are digital telephone switches
that can query SCPs for information about call routing; and
signal transfer points (STPs), which are packet switches that send
messages between SSPs and SCPs.
When an SS7 call is placed, the SSP creates a query to find out how
the call should be handled. The STP sends the query to the SCP,
which interprets the query based upon information in its database,
and sends a return message via the STP, telling the SSP how to
complete the call. The SCP computer can change the call in any
manner dictated by the database, and this ability is what gives an AIN
its versatility.
VoIP Connectivity
VoIP includes any set of enabling technologies that can transmit voice
signals in packetized format. These technologies include TCP/IP,
asynchronous transfer mode (ATM), frame relay and Sonet on public
or private networks.
VoIP offers distinct advantages over existing circuit-switched
networks. Once voice signals are digitized into packets and placed
on the network, they are indistinguishable from data packets. Carriers
can then build out their network using generic packet-switching
equipment.
In addition, the packet format and connectionless mode of operation
allow for more efficient use of network bandwidth. Finally, by using
open standards such as TCP/IP, VoIP offers significant new revenue
potential by reducing time to market and implementation costs of new
services.
VoIP networks use end points such as terminals and gateways to
permit multimedia communications over a packet-switched network.
Terminals are direct end-user devices such as IP telephones or PCs
with IP phone applications. Gateways provide protocol conversions to
interconnect with other networks, including those with non-VoIP
devices like PSTNs (See Figure 1).
There are a number of gateways on the market that provide protocol
conversion between different encoding schemes and transports.
Current solutions for PSTN interconnection fall into three categories:
subscriber line, in-band signaling or associated signaling.
Subscriber line solutions use analog subscriber lines as
interconnection trunks to a PSTN. Subscriber line solutions are
cumbersome and conspicuous to users, who are required to perform
multistage dialing to complete a call. Subscriber line signaling also
requires expensive gateway hardware.
Overall, it is not a long-term, scaleable solution.
In-band signaling gateways use digital interconnection trunks such as
Wink start, Feature Group D (FG-D) or ISDN. These gateways detect
and transmit control signals over the same circuit used to transmit
voice traffic. Solutions in this category do not require multistage
dialing, but they do require more expensive and specialized gateway
hardware to generate and interpret in-band signals.
Associated signaling uses out-of-band signaling, but it accomplishes
this either by using a single signaling link for each channel of voice
traffic or by converting voice trunk lines to carry signaling traffic. This
is inefficient because each signaling link can handle signaling traffic
for several thousand trunks. There is also a higher cost associated
with deploying additional equipment to handle the large number of
signaling links required in an associated arrangement.
Neither subscriber line nor in-band signaling facilitate or use SS7 or
AIN services. Associated signaling has SS7 facilities and can
provide AIN services. However, without a "centralized" call model,
associated signaling solutions must employ some proprietary
signaling overhead to stay in sync.
Providers who chose this option may limit their choices when it
comes to software vendors and hardware manufacturers. Perhaps
most important, each of the three solutions requires a large capital
investment for expensive, limited-function gateways that may have to
be replaced to meet future VoIP standards.
Installing a centralized signaling solution like a call transaction
processor makes more sense (See Figure 2). With this approach,
providers can realize certain cost efficiencies immediately because
their networks will require fewer signaling links. In the long term,
centralized signaling and call control provides even greater
advantages.
If call control is implemented in individual gateways, each gateway
must be upgraded every time a new service is added. Centralized
signaling and call control allows new services to be added more
rapidly and at a lower overall cost. System interoperability also allows
providers to realize extensive capital savings because they can
ignore compatibility issues and purchase gateways based on
reliability and price.
Designing a Superior Solution
To interconnect VoIP and SS7 networks, providers must address a
host of issues including both circuit and trunk management, SS7
message parsing and message generation, backoffice interaction,
call routing, SS7 call control and the generation of call detail records.
Moreover, providers need a system that can grow with their business.
They need a scalable solution that incorporates logical components
and an adaptable plug-and-play architecture--a system that makes
use of current AIN components and can be modified to address
multiple VoIP protocols.
A call transaction processor should be based around a core set of
functionality that addresses the management and service needs
shared by all call models, regardless of type. Outside of the core is a
framework of adapters, each of which provides interconnection with
any network device or signaling protocol, including SS7, H.323, SIP,
SGCP, IPDCP, LDAP (and other directory services), and proprietary
gateways or gatekeepers. Use of an exposed set of application
programming interfaces (APIs) would allow additional adapters to be
developed and deployed quickly (See Figure 3).
In this configuration, the call transaction processor provides a
centralized point for SS7 interconnection and call control within an IP
network. This type of architecture provides a robust and scaleable
solution that maximizes interconnection flexibility while reducing costs
for infrastructure and maintenance.
A call transaction processor should:
act as an SSP in the SS7 network, performing message translation
and mediation between the SS7 networks and IP end points. Use of a
centralized signaling point reduces costs associated with signaling
system infrastructure and maintenance because it requires fewer
signaling links and less processing equipment than associated or
in-band signaling.
facilitate 1+ dialing, allowing a VoIP service provider acting as an
interexchange carrier (IXC) to assign its network as a subscriber¹s
presubscribed interexchange carrier (PIC).
allow for dial around (10XXX) service.
use out-of-band signaling and thus provide better security than
solutions based on in-band or subscriber line signaling.
reduce the number of low-quality lines (e.g. subscriber lines) in the
connection, thus increasing the quality of service (QOS).
allow interconnection to virtually any other signaling network.
Advantages of a Call Transaction Processor
Because the call transaction processor sits between various
networks and call models, every network "thinks" it is interacting with
a sister network that is using an identical call model. To this extent,
there is no impact on individual networks because they require no
modifications or upgrades to work with the call transaction processor.
Furthermore, since the call transaction processor is involved only in
call signaling--and not protocol conversion--it can physically reside
almost anywhere within the various networks it serves.
The architecture of the call transaction processor should also be
conducive to distributed processing. This allows it to be physically
implemented across various nodes while functioning as a single
logical element. Moreover, the adapter framework, coupled with the
open APIs of the core, allow the system to be deployed on
commercially available platforms, proprietary or embedded platforms,
off-the-shelf hardware or specialized hardware platforms.
A call transaction processor using this sort of design could be
deployed in a number of settings. It might function as an IP/PSTN
interconnection for IP telephony providers or as an IP call controller for
gateways or switches. The system could also operate as a signal
system for remote access gear or as an interconnection to AIN
services located on an IP backbone.
IP telephony is the current rage because it provides an opportunity to
create new services more rapidly with lower complexity than in
existing PSTNs. However, because PSTNs make extensive use of
SS7 for call setup and control, a VoIP network must be able to
interconnect seamlessly into existing SS7 networks to provide
world-class, carrier-grade service.
The most efficient way to provide this interconnection is with a call
transaction processor that bridges networks by mapping, mediating
and managing the transactions for the different aspects of the
heterogeneous call models and signaling.
Christopher C. Couch is an Emerging Technologies Analyst in the
R&D division of Evolving Systems, Inc., Englewood, CO.
Visit the Evolving Systems website.
--------------------------------------------------------------------------------
From our Archives
March 1, 1999 InFocus
The brave new world of IP networks
New IP services are creating new areas of competition between ISPs
and carriers. In this brave, new world, where does usage-based
billing fit in?
LIMOR SCHWEITZER
November 16, 1998 The Analyst's Corner
VoIP: Are we there yet?
DAN TAYLOR
November 1998 Global Telephony
IP Impact
Carriers, pay attention! The protocol promises to bring voice and
more to the Internet, though you may have some time before the big
explosion
By PAT BLAKE
April 1998 Global Telephony
The Internet: Changing the face of communications
The Internet has changed from an annoyance to a possible threat to
telcos worldwide
By Wayne Walley
Additional Resources on the Net
Voice on the Net
An excellent page from Pulver.com
An SS7 tutorial
A Primer on the H.323 Series Standard
IETF PINT Working Group Homepage
The PSTN/Internet Interfaces (PINT) WG addresses connection
arrangements through which Internet applications can request and
enrich PSTN (Public Switched Telephone Network) telephony
services.
The International Telecommunication Union
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