The monolithic circuit switches of the traditional PSTN
are well known. There are over 19,000 Class 5 switches
deployed in central offices and over 500 Class 4 tandem
switches deployed in regional offices across the United
States alone. While the annual growth rate of these
voice switches has recently been slow and steady, the
traditional voice switching architecture will soon
undergo an evolution similar to that of the computing
architecture in the early 1980s, in which advanced PC
workstations running third-party software replaced
vertically integrated proprietary mainframes.
Carriers have begun to migrate to next-generation
packet-switched voice networks because of intensifying
competition, significant cost efficiencies, and new
service revenue opportunities. Multiple vendors, both
new and incumbent, who specialize in switching hardware,
call control software, and new applications software are
beginning to offer a dizzying array of different
solutions. Nevertheless, astute choices can be made to
optimize the performance of the packet voice solution
and the return on investment of the purchase decisions.
THE SOFTSWITCH ARCHITECTURE
In an environment of intensifying competition for
customers among incumbent and competitive carriers,
next-generation packet-switched voice networks offer
several fundamental benefits: Significantly reduced
capital and operation costs, less reliance on overlay
networks, simplified service provisioning, and new
services creating additional revenue and
The four key elements in the softswitch architecture
- Media Gateway;
- Softswitch (or Media Gateway Controller);
- Signaling Gateway (functionality may be integrated
in softswitch); and
- Application Server.
The media gateway is an access/trunking switch that
converts and switches TDM voice traffic and packet voice
traffic, and provides interworking between the two types
of interfaces. The packet voice traffic can be either IP
or ATM depending on the capabilities of the media
gateway and the carrier requirements. The media gateway
should be capable of providing interworking between
multiple access protocols: Any pair of TDM, ATM, and IP,
Media gateways must accommodate current traffic as well
as future growth. Media gateways currently range from 2
to 20 Gbps switching capacity. Port capacity for a Class
4/5 switch typically ranges from 10,000 to 100,000 DS0s.
Media gateways must be highly reliable, usually
exceeding five nines availability to be considered
carrier-class. This type of reliability requires
superior components, quality manufacturing processes,
and rigorous product testing. Full fault-tolerance and
redundancy are critical to service availability
assurance. NEBS Level 3 Certification is required and
OSMINE compliance may be required depending on the
Interworking and Interoperability
Carriers may use media gateways and softswitches from
multiple vendors; as a result, interoperable MGCP/Megaco/SIP
protocols are necessary. Bearer interfaces (TDM, ATM,
IP) and signaling protocols (PNNI, BICC) interworking
are also essential.
Media gateway footprint drives occupancy costs. More
scalable products will accommodate denser modules and
typically reduce occupancy costs.
Carriers deploying media gateways will always consider a
cost-per-port comparison. Density is typically an
important factor in lowering cost-per-port.
Most media gateways have been developed initially to
transport voice using either ATM cells or IP packets.
While many industry analysts think that carriers will
migrate to IP/MPLS, for now most carriers rely on ATM to
provide the carrier-class quality of service required to
deliver packet voice services. Either way, a media
gateway that meets industry standards is critical.
The softswitch, or media gateway controller, provides
the intelligent call control for the voice network. The
softswitch also provides another essential function in
the context of packet networks; it provides the
interworking between the different signaling on the TDM
and on the packet portions of the network, e.g., ISUP
and MGCP protocols. The softswitch may have a Class 4
and/or Class 5 feature set depending on its role in the
Important factors for softswitches are:
The number and type of Class 4 and/or Class 5 features
Voice processing capability, typically measured as voice
calls per second or busy hour call attempts (BHCAs), per
system, is a critical factor.
Softswitches must support and interwork among multiple
signaling protocols to provide more flexibility to the
carrier deploying its system of choice.
Softswitches must provide carrier-class reliability.
Full fault-tolerance and redundancy is critical to
service availability assurance. Fault tolerance between
the softswitch and the media gateway is also essential.
Softswitches must interoperate with a wide range of
other network elements, especially other softswitches,
because their role is central to the functionality of
the packet voice infrastructure. Carriers view
interoperability as the top concern in the purchasing
decision for new equipment (Figure 1).
The softswitch can be either physically separate from
or physically integrated with the media gateway. In the
former, a general-purpose server platform provides call
control, and in the latter, an optimized module fits
within a standard slot on the media gateway chassis.
Although most vendors use external server platforms,
a few vendors use the optimized softswitch modules.
Physically integrated softswitches provide multiple
advantages over these general-purpose platforms: Simpler
interoperability, higher call capacity, smaller
footprint, better redundancy, lower cost, and more
Another important feature of certain softswitches is
the capability to provide call control for multiple
media gateways distributed across the packet voice
network. While solutions from many vendors require
collocation of a softswitch with every media gateway, a
more efficient and cost-effective approach is for remote
softswitches to control bearer voice traffic on multiple
media gateways located in separate central offices.
The signaling gateway function communicates directly
with the signaling transfer point (STP) in an SS7
network via SS7 signaling messages in order to provide
authorized services and route voice traffic through the
bearer traffic network. In some vendor solutions, the
signaling gateway is a separate network element, but
ideally is integrated into the softswitch, which offers
lower cost, simpler operations, and a smaller footprint.
The application server is a product that provides
enhanced services that may include "follow me" roaming,
automatic language translation services, and data-driven
voice services. Until now, carriers have relied heavily
on the feature set of the traditional circuit switch. In
a softswitch environment, however, carriers will have
the flexibility to offer new services without replacing
or updating the entire packet voice platform.
ALPHABET SOUP: SIGNALING PROTOCOLS
How the primary network elements in a softswitch
architecture communicate with each other is not an easy
discussion, because there are a plethora of
standards-based and proprietary protocols. Nevertheless,
the total number of used protocols is shrinking, and can
be segmented based on the specific interfaces (Table
or Application Server
PNNI (private network-network interface) is the
standards-based protocol used by media gateways and ATM
switches to share routing and signaling information to
establish and maintain real-time voice and data
connections. The BICC (bearer independent call control)
signaling protocol, which is a modified SS7 ISUP
protocol, correlated with ATM and PNNI signaling, is
used as a signaling interworking function.
MGCP (Media Gateway Controller Protocol) is the
signaling protocol between the media gateway and
softswitch used until now for carrier deployments. Most
vendors' MGCP implementations are proprietary. Megaco
(H.248) is the compromise ITU-T communications protocol
that will eventually replace MGCP. Megaco is still under
development by most vendors and has not been widely
tested for interoperability. Consequently, Megaco is
still too new for carrier deployments.
ISUP is an SS7 protocol for signaling the parameters
and procedures to set up and tear down circuit-switched
voice calls between a softswitch/signaling gateway and
an STP. TCAP is used to exchange control-related
messages between a softswitch/signaling gateway and
application databases (SCP, or signal control point).
SIP (Session Initiation Protocol) is an IETF protocol
for transporting call control, authentication and other
signaling messages among softswitches and other devices
through an IP network.
EVOLUTION OF THE SWITCHES
The migration to packet switched voice networks will be
an evolution, not a revolution. The most likely scenario
is for carriers to pursue a "cap and replace" strategy
for Class 4 tandem switches, followed later this decade
by Class 5 switch replacement. With so many emerging
network elements and communications protocols, the
vision of the New Public Network can be somewhat
daunting. But similar to the revolution from mainframes
to PC workstations, significant capital cost and
operational savings will prove to spur carriers and
service providers to make the migration. With the range
of packet voice infrastructure products currently being
developed and launched, this segment is definitely one
of the most exciting in telecommunications.
Purchase recommendation: Select a media gateway
that offers scalable port density, has a high
initial non-blocking switching capacity, offers
multiple physical interfaces, has proven
reliability, has proven interoperability with
external softswitches, occupies a small footprint,
and begins with ATM while supporting IP/MPLS
Purchase recommendation: Select a softswitch that
can meet call processing capacity, has desired
feature set, has proven reliability, has proven
interoperability with media gateways, other
softswitches, and application servers, and
incorporates the signaling gateway functionality.
A physically integrated softswitch capable of
supporting call control with multiple distributed
media gateways offers distinct performance and
Purchase recommendation: Select an application
server that provides attractive new services,
scales to meet customer needs, and has proven
interoperability with other softswitches and
is product marketing manager at Oresis
Communications. Oresis Communications is a developer
of next-generation switching systems that provide
integrated multiservice data switching functionality and
media gateway/tandem softswitches.
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