Hybrid IMS/TDM Networks
By Richard "Zippy" Grigonis
The initialism “IMS” means “IP-based Multimedia Subsystem”, but the world’s long,
piecemeal migration to this grand, common service architecture for wireless and wireline communications
means that traditional TDM (Time Division Multiplexed) networks will be very much a part of the
picture for many years to come. This shouldn’t be surprising, since IP communications itself was an “outsider”
for nearly a decade, a small fish in a large sea of legacy circuit-switched equipment, both at the network
operator level as well as in the enterprise. Operators deploying IMS technology gradually over time
must support signaling and session control in both NGN and IMS domains while providing connections
to Signaling System 7 (SS7) signaling in the PSTN — but the whole idea of a “hybrid” network is actually
more complicated than just a “patchwork quilt” of TDM, NGN and IMS, as we shall see.
It was, after all, the appearance of VoIP technology that resulted
in the reengineering of Mobile Switching Centers (MSCs) so that
they could handle both packet-switched voice traffic along with
existing circuit switched traffic, a process that led to the disaggregation
of legacy network switches into a control component (the
softswitch) that uses a protocol such as the Media Gateway Control
Protocol (MGCP) to manage the other major component, the
media gateway. The media gateway can handle both packet and
circuit switched traffic and it can convert one form to the other
under the control of the softswitch. this is why 2.5G networks are
able to carry both circuit-switched and packet-switched traffic.
One might wonder how applications and services can function in
such a complex environment. Fortunately, vendors have risen to
the occasion. Take the Cisco Mobile Office solution, for example,
designed so that service providers can offer seamless mobility to
enterprise and Small and Medium-sized Business (SMB) customers
by integrating mobile and enterprise voice and messaging networks
into a single communications environment. For enterprise and SMB
customers, operator services based on Cisco Mobile Office integrate
existing TDM PBXs and IP PBXs with wireless networks including
WiFi, WiMAX, and mobile networks. Providers can now deliver to
their customers fixed-mobile convergence (FMC) communications
services, accessible from any device over any kind of network, and
self-managed by subscribers through a friendly web-based interface.
A business customer’s collection of communication devices are now
be treated by the service provider as a single entity having access to
the converged communication services and call routing capabilities,
independent of the underlying infrastructures. Indeed Cisco Mobile
Office services can be offered simultaneously on a broad range of
network architectures including Signaling System 7 (SS7), VoIP and
of course IMS.
In the world of IMS itself, provision has been made for PSTN
ingress and egress functions that allow interworking with the
PSTN. The physical conversion between TDM and IP for media
is done via media gateways, and signaling gateways perform
translation at the transport layer between SS7 on MTP (TDM
networks) and SS7 on SCTP (IP networks). The Breakout
Gateway Control Function (BGCF) determines whether
PSTN “breakout” will happen in the current network — in
which case the signaling is passed to a Media Gateway Controller
(MGC) which allocates a media gateway port — or else
the SIP-I (ISUP over SIP) is sent to another IP network where
breakout actually occurs (the signaling passes across the IBCF,
or Interconnect Border Control Function, to another BGCF in
the breakout network).
Service intelligence interworking is generally done from the
signaling control plane, by leveraging Signaling System 7 control
with Session Initiation Protocol (SIP) control and then extending
the signaling service framework from the intelligent network
(IN) to both the NGN and IMS domains. In terms of transitioning
networks to IMS while maximizing existing architecture
investments, the incremental migration of the control layer of
SS7 to IP includes deploying SIGTRAN (SS7 over IP) and/or
eventually deploying an independent SIP signaling control plane
in the NGN. Once a SIP Signaling Router (SSR) for session
routing (an idea first championed by Tekelec a few years ago)
and a whole unified signaling layer is put in place to unite various
signaling and control procedures across multiple network types,
hybrid media processing platforms are then brought online to
enable uniform media delivery across the many existing access
technologies. Another step involves linking the application layer
to the control layer by implementing SCIM (Service Capability
Interaction Manager) functions to assist in bridging the transition
from TDM to NGN, and eventually IMS.
TISPAN (Telecoms & Internet converged Services & Protocols
for Advanced Networks), situated in the ETSI standards body, has long worked at standardizing converged networks
around the IMS core architecture. This means re-jiggering
fixed network access so that it interfaces to IMS and, although
the original main thrust of the work was to figure out a way to
move existing PSTN functionality onto an IP core, it was soon
joined by the matters of interworking, PSTN emulation and
producing PSTN equivalence services (malicious call trace,
lawful intercept, etc.) service.
All this eventually leads us to the matter of service mediation,
which enables new IMS subscribers to immediately leverage
existing services, since operators no longer need to replicate
their services in the IMS domain, thus allowing them to minimize
costs while providing both pre-IMS and IMS services.
Here again, a well-designed SCIM can not only orchestrate
applications, but also mediates multiple services across various
technologies, and allows for seamless service blends that can be
offered to any subscriber at any location.
Realizing that an instant cutover to an IMS-based network
is usually impractical and that service providers must continue
to engage their NGN and TDM networks during the
migration process, Tekelec (a developer of high-performance
network applications for next-gen fixed, mobile and packet
networks) and HP in 2007 combined their expertise in SS7
Signal Transfer Points (STPs) and Home Location Register
(HLR) platforms to form an “Open IMS” vision, to help
network operators migrate to IMS. Their joint Open IMS
solution supports the Call Session Control Function (CSCF),
Media Resource Function (MRF) and HSS functionality, plus
multiple application capability, on a single, standards-based
platform. It delivers service enablers such as MRF, presence
server, electronic numbering (ENUM), group list management
and voice call continuity (VCC).
Around the same time, Tekelec unveiled SIP Signaling Router
functionality on their TekCore Session Manager product.
TekCore SSR enables operators to expand their NGNs, at the
same time maintaining a seamless migration path to the IMS.
The Tekelec SSR provides centralized session routing for softswitches
and enables core signaling capabilities that inter-work
between TDM, NGN and future IMS domains. Subscribers
can take advantage of all these capabilities, enjoying existing,
next-gen and future IMS-based services regardless of access
type; thus, consistent service delivery over hybrid networks is
assured as they migrate to all-IP environments.
The non-trivial process of incrementally converging disparate
networks and providing multi-protocol support to maintain
seamless connectivity between TDM, VoIP and IMS has also
been taken up by Veraz Networks, whose ControlSwitch User
Services Core (CS-USC) is an IP softswitch built to smooth
the migration path to IMS. With the CS-USC, providers
can deliver multimedia services via any mix of traditional
and IP-based voice networks. The platform is based on a
sophisticated, modular, programmable, distributed, scalable,
high-availability architecture with open interfaces to media
devices, application servers and backoffice systems. CS-USC
is consistent with 3GPP IMS, TISPAN and MSF standards.
The CS-USC supports web – IMS integrated services via
Veraz Portal Connect, and linkages to nomadic/mobile users
and devices are maintained in conjunction with the multiservice
and multi-access IMS core. The Veraz CS-USC also
has programmatic interfaces including TL1 and SOAP/XML
for integration into the service provider’s existing OSS/BSS,
thus streamlining business operations.
Veraz Networks has had some of its new equipment in trials for
about a year and is rapidly becoming a major global provider
(50+ countries) of IP softswitches, media gateways and digital
compression products that enable voice, video and other
multimedia services. As the IMS service architecture slowly
percolates throughout the world, service providers have found
that products such as those from Veraz allow them to quickly
and efficiently migrate from traditional voice networks to all-
IP, fixed-mobile and multimedia networks consistent with the
emerging IMS standards.
Operators Saddled with Saddles
The hybrid network will be with us for at least 15 more years,
during which time network operators must deliver seamless
services and support service mediation and interaction between
legacy, mobile, VoIP and IMS networks, delivering older
“siloed” or “stovepipe” services alongside newer IMS-based
applications. As we’ve seen, technologies exist to help them. At
some point, however, the dawn will come, and the final buildout
to IMS (or whatever we call it then) will occur.
Richard Grigonis is Executive Editor of TMC’s IP Communications Group.
Companie's Mentioned in this Article:
Cisco
www.cisco.com
Hewlett-Packard
www.hp.com
Tekelec
www.tekelec.com
Veraz Networks
www.veraznetworks.com
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