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Chapter 1
INTRODUCTION
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Introduction
In the 21st century, telephones and other communications devices are being
used in new and innovative ways. This represents a major historical shift for
the telecommunications industry. The telecommunications industry hasgrown dramatically over the past two decades. Worldwide connectivity and
inter operability has revolutionized trade across cultural and geographic
borders. Fax and data services have enhanced the versatility of telephone
devices and networks. The Internet has experienced dramatic growth with
millions of users worldwide. In the last five years, several factors have
driven the telecommunication industry to look at a new way to provide
services to any device, anytime, anywhere.
The traditional public switched telephone networks (PSTN) although werereliable and robust were built on hardware based circuit switching model
that leave the changeability and extensibility for service addition. The
coupling between the hardware and software was very tight making use of
COTS and services extremely difficult to add. A migration to the Internet
and IP (Internet Protocol) networks using the flexible packet switching
technology, in comparison to traditional telephone networks using circuit-
switching technology is a solution but that would have meant the existing
investments in PSTN networks would have been totally wasted. Hence the
need of the hour was to look for a hybrid PSTN/IP environment, which was
found in the form of soft switches. The emergence of softswitches signified
advancements in telecommunication switching architecture by having better
technology and better software architecture. This is limited to the
advancement in software architecture due to the introduction of softswitch.
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Chapter 2
DEFINITION
Definition of a Softswitch
Softswitch is the generic name for a new approach to telephony switching
that has the potential to address all the shortcomings of traditional local-
exchange switches. The softswitch is where all the service intelligence
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resides for the delivery of local telephone services. Softswitch technology
solutions can lower the cost of local-exchange switching, present the means
to create differentiated local telephony services, and ease the migration of
networks to support packet voice end-to-end. Packetized voice involves the
digitizing, compressing, and dividing of voice into packets. These packets
can then be sent from the sender, via various routes, to the receiver,
whereupon they are reassembled. Softswitch, media gateway controller, call
agent, gatekeeper are the varied nomenclatures that have been attached to
the products that perform the functions that are coming to be called
softswitches. Gatekeeper is the ancestor term, derived from VoIP systems in
which gateways converted the voice and signaling from analog PSTN and
SS7 to IP packets. The gatekeeper controlled one or more gateways, guiding
the setup and teardown of voice circuits between the two kinds of networks.
Media gateway controller is an elaboration of gatekeeper, growing out of the
first efforts to standardize the control of media gateways using a protocolcalled media gateway control protocol (MGCP). Call agent is a highly
generic term that attempts to describe all systems that handle call-control
functions.
softswitch
Softswitch (software switch) is a generic term for any openapplication program interface (API) software used to bridge a
public switched telephone network (PSTN) and Voice over Internet Protocol
(VoIP) by separating the call control functions of a phone call from the
media gateway (transport layer).
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Chapter 3
MOTIVATION OF SOFTSWITCH
Motivation for the Softswitch
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By far the most complex part of a local-exchange switch is the software that
controls call processing. This software has to make call-routing decisions
and implement the call processing logic for hundreds of custom calling
features. Todays local-exchange switches run this software on proprietary
processors that are integrated tightly with the physical circuit-switching
hardware itself. The inability of existing local-exchange switches to deal
directly with packet voice traffic, however, is a major barrier to packet voice
migration. In the future, delivery of local telephony will come over a purely
packet-based infrastructure. But for years to come, the migration path to
end-to-end packet voice will require working with a hybrid network
handling both packet and circuit voice.
Current Market trendsThe telecommunications landscape is changing dramatically due to
economic and technology changes. Service providers and vendors alike arerequired to respond to these changes in order to thrive. The points have been
borrowed largely from the reference
Worldwide deregulation of the telecommunications industry is
creating new market opportunities for traditional and innovative
services
Increased competition is emerging in every market - local, national
and worldwide - and geographic limits to competition are
disappearing
Evolution to a single voice/data network is moving rapidly, driven by
the inefficiencies and costs associated with maintaining two networks
(PSTN and IP).
Technology changes
Voice over packet is increasingly reliable and high-quality
'Always-on' solutions are now technically possible
Improved bandwidth and lower costs are available for all types of
communications
Higher-value packet-based services are more customizable, faster to
come to market and easier to deploy and manage, enabling providers
to differentiate their offers and capture high-value customers.
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One possible way to address all this is to create a hybrid device that can
switch voice in both packet and circuit formats, with all the necessary call
processing software integrated into this switch. While this approach may
help address the question of migration, it does not necessarily lower the cost
of local-exchange switching or improves the prospect for differentiated local
voice services.Separation of Functionality: Need of the Hour
The telecom industry appears to have reached broad consensus that the best
answer lies in separating the call processing function from the physical
switching function and connecting the two via a standard protocol. In
softswitch terminology, the physical switching function is performed by a
media gateway (MG), while the call processing logic resides in a media
gateway controller (MGC). There are a number of reasons why thisseparation of functionality is believed to be the best approach:
It opens the way for smaller and more agile players who specialize in
call processing software and in packet-switching hardware
respectively to make an impact in an industry that has been dominated
by large, vertically integrated vendors.
It enables a common software solution for call processing to be
applied in a number of different kinds of networks, including
combinations of circuit-based networks and packet voice networks
using multiple different packet voice formats and physical transports. It allows standardized commodity computing platforms, operating
systems, and development environments to be leveraged, thereby
bringing considerable economies to the development, implementation,
and processing aspects of telephony software.
It allows a centralized intelligence in a service providers voice
network to remotely control switching devices located in customer
premises, a key requirement for the full exploitation of IP telephony in
the future.
Separation between Media Gateway and Media Gateway Controllerrequires a standardized protocol for communication between the two,
and an appropriate standard is now emerging.
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Chapter 4
ARCHITECTURAL DETAIL
Architectural Considerations:
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The key architectural consideration should include:
Interoperability with Telecom equipment and carriers
Services and protocols integration
Bandwidth and external connectivity required
Sizing requirements
Equipment required
Network operations interface(s) required
Operations management requirements
Scale as you grow criteria for services and volume
Service availability requirements
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Architecture of softswitch organization
Functional Planes
The architecture of the softswitch can be seen to be divided into the
following software planes. They represent the separation between the
functional entities in a Voice over IP (VoIP) network. There are four distinct
functional planes employed by the soft switch to describe the functioning of
an end-to-end VoIP network:
Transport
Call Control & Signaling
Service & Application
And Management.
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Transport Plane
The basic functionality served by the transport plane is to handle and
transport call signaling, call and media setup messages across the VOIPnetwork. Now the transport mechanism used could be based on any
technology conforming to the standards e.g. SS7 ANSI or ITU. This plane to
the external world is like an access form where they can enter to use the
services of call control. So often if we look at the implementations the
Transport Plane devices and functions are controlled by functions in the Call
Control & Signaling Plane. The transport plane gets sub-divided broadly into
three domains:
IP Transport Domain Interworking Domain
and Non-IP Access Domain. Let us look at each of them:
IP Transport DomainThe IP Transport Domain comprises of:
The transport backbone and routing/switching fabric
Devices like routers and switches domain.
Devices that provide Quality of Service (QoS) mechanisms and policies for
the transport also belong to this domain.
Interworking DomainThis domain comprises of:
The devices that are primarily responsible for the transformation of
signaling or media received from external networks into a form that can be
sent among the various entities in the VoIP network and vice versa.
It consists of devices like Signaling Gateways (signaling transport
conversion between different transport layers), Media Gateways (media
conversion between different transport networks and/or different media), and
Interworking Gateways (signaling Interworking on the same transport layerbut with different protocols).Non-IP Access DomainThe Non-IP Access Domain applies primarily to
Non-IP terminals and wireless radio networks that access the VoIP
network.
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Consists of Access Gateways or Residential Gateways for non-IP terminals
or phones, ISDN terminals, Integrated Access Devices (IADs) for DSL
networks, Cable Modem/Multimedia Terminal Adaptors (MTAs) for HFC
networks, and Media Gateways for a GSM/3G mobile radio access network
(RAN).
Call Control & Signaling Plane
The Call Control & Signaling Plane is like the centralized arbitrator
exercising control over the major elements of the VoIP network, especially
in the Transport Plane. This is the heart of the system and performs the basic
call processing and signaling. When we say call processing and signaling we
essentially means handles the subscriber requests for setting up and tearing
down the voice circuit, carries out call control based in signaling messages
received, controls components in Transport Plane, ensures digit translationand routing based on directory numbers, maintains trigger detection points
to access the intelligent database, keeps the finite state necessary for
maintaining the call context etc. Summarizing it controls what essentially is
switch has to do for setting up and releasing a call. The Call Control &
Signaling Plane consists of Devices like the Media Gateway Controller
(a.k.a. Call Agent or Call Controller), Gatekeepers and LDAP servers.
Service & Application Plane
The Service & Application Plane provides the controls the logic and
execution of feature servers and other applications like intelligent networks
which are mean to provide various services to the subscriber. The devices in
this plane control the flow of a call based on the service execution logic.
They achieve this by communication with devices in the Call Control &
Signaling Plane. The Service & Application Plane consists of devices like
Application Servers and Feature Servers. The Service & Application Plane
can also perform the control of specialized bearer components, such as
Media Servers, that perform functions like conferencing, IVR, toneprocessing etc.
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Management Plane
The Management Plane is responsible for providing functions such as
subscriber and service provisioning, operational support, billing and other
network management tasks. It can interact with any or all of the other three
planes through industry standard (e.g. SNMP) or proprietary protocols and
APIs. That is to say it forms the operation and maintenance zone. The
functional entities are the logical entities of a VoIP network. This plane
caters to the on the fly needs of the switch in expansion and modification of
networks and entities.
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Chapter 5
MAJOR COMPONENTS
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Major Components of a Softswitch:
A Softswitch can consist of one or more of these components. These
functions can reside in one system or span across several systems.
1. The Gateway Controller is one of the key functional units of theSoftswitch. The Gateway Controller holds the call processing rules, but uses
the Media Gateway and Signaling Gateway to perform the job. There are
many types of protocol media managers that need to be controlled those that
access the softswitch through Transport plane. It is the responsibility of the
Signaling Gateway to perform call set-up and teardown and media gateway
basically controls and coordinates the operations. An example could be
sending data related to call set up to signaling media manager and receiving
finite state messages in response. The gateway controller here is like an
arbitrator for the signaling interface and the corresponding stack utilizing itsservice. In addition, it interfaces to the OSS and BSS systems. Often this
unit is referred as Call Agent or Media Gateway Controller interchangeably.
Sometimes the Call Agent by itself is referred as a Softswitch. This
component communicates within other parts of the Softswitch and also the
external networks using different protocols.
2. The Signaling Gateway serves as the gateway between the SS7Signaling network and the nodes managed by the Softswitch in the IP
network. A Signaling Gateway requires physical connectivity to the SS7network and must be aware of the required protocol suites. There is a
standardized implementation of SS7 signaling protocol both ANSI and ITU-
T which is managed by the gateway. The services of the stack are given to
external world via the gateway which takes care of API formation and
retrieval for incoming and outgoing messages and presenting the extracted
data to the external world which may not need to worry of the stack PI
3. The Media Gateway handles the voice media data payload (the
digitized samples of speech during conversation). In the future, video datawill also need to be supported. In the current model, the Media Gateway
must support connectivity to a TDM bus carrying voice media data to the
Telco switch side. Applicable voice data encoding, decoding and
compression are also performed by the Media Gateway. It also supports
legacy telephony (PSTN) interfaces and protocols such as CAS and ISDN.
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4. The Media Server performs peripheral functions, to enrich theSoftswitch with media capabilities. If required, it supports digital signal
processing (DSP) resources. If IVR functionality is needed (eg: to prompt
voice responses), those tasks would be performed by the Media Server. AMedia Server when applicable would also serve video Media.
5.The Feature Server provides all the revenue generating features andservices such as billing, multi-party conferencing, etc. The Feature Server
uses the resources and related services located on other components of the
Softswitch. It may also support certain services to meet certain
implementation necessity e.g.: Gatekeeper
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Figure 1 Figure 2
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Chapter 6
BENIFITS
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Benefits of the Softswitch:
The distribution of functionality will enable the benefits of improved feature
development and delivery as well as lower costs. Distributing functionality
means that switches will be simpler, more efficient, and cheaper. Switcheswill be able to focus on switching, allowing other components to provide
network control and service logic. Distributed service logic means that
application development will not be constrained to centralized creation,
control, and delivery of services. Instead, services can be created and
deployed at various places through an extended network.
The benefits of the Softswitch approach will include: New revenue stream
for service providers and operators
Flexibility, which supports the development of highly programmable
telephony equipment
Several types of e-business, accessible using newer devices
Unified messaging
Enhanced customer services that it reduces time to market
Easy integration of dissimilar networks and network components
Lower cost of development, deployment and ownership.
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Highly Flexible Architecture
A soft-switch-based system provides network design engineers with a highly
flexible, fully distributed, open standards-based network architecture. A
well-architected softswitch separates call-control intelligence and services
software from media hardware with open, standards-based interfaces between the different layers. Typically, call-control and signaling
intelligence resides within the softswitch, while services intelligence is
distributed among the softswitch and application servers that reside at an
upper layer. The interface between the softswitch and different types of
media hardware is provided by open protocols, such as Media Gateway
Control Protocol (MGCP), H.248/Megaco and Session Initiation Protocol
(SIP). The interfaces between the softswitch and application servers are also
open and standards-based, and use protocols such as SIP and H.323, and
tools like Extensible Markup Language (XML) and Java in advancedintelligent networks. This gives network designers the flexibility to choose
best-of-breed components from multiple vendors for their media hardware
and software needs. The fully distributed architecture allows network
topologies with media devices located throughout the network, while the
softswitch provides centralized call control and services. Such a network can
be quickly scaled without the need to deploy expensive circuit switches in
each new market. Furthermore, by enabling voice transport and services
over packet-switched networks while seamlessly inter-working with the
public-switched telephone network (PSTN), softswitches play a critical role
in enabling the design of a common packet network for converged voice anddata services.
Qualities Imparted:
Loose Coupling
Better Extensibility
Higher Changeability
Ability to Interoperate with COTS
Better Scalability
Open architecture
The most important characteristic of the next-generation network is an open
architecture. A softswitch utilizing an open architecture provides best-of-
breed vendor selection, a platform for third party development of new
services and service provider empowerment. Service providers are able to
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select best-of-breed products that allow them to harness innovation
regardless of vendor. The softswitch is the engine powering the next
generation network, controlling edge devices such as media gateways and
broadband Integrated Access Devices (IADs) and interfacing to third party
feature servers and back office systems, including billing. Rapid deployment
of converged services will be made possible by open softswitch architecture.
Third party software companies will revolutionize data and voice
communications. By using standards-based protocols and open APIs such as
SIP, JAIN, XML or even H.323, service providers will be able to harness
converged services from third party vendors within a fraction of the time and
cost compared with today's PSTN.
Qualities Imparted:
Changeability Extensibility
Interoperability
Portability
Distributed architecture
The next-generation architecture requires a transition from a mainframe-type
telephony system to a distributed system, which will drive modular systems,
and provide cost-effectiveness. The switch is decomposed into three layers
i.e. layered architecture. The softswitch will be the strategic component in
service providers' networks because it has the potential to function as thenetwork operating system of the new public network. The softswitch gives
the service provider the freedom and flexibility to add ports where it needs
to by simply adding to an existing media gateway or by rapidly bringing a
new media gateway online. In addition to the quick turn-up time, service
providers can also penetrate new markets without the multi-million dollar
class switch investment. Service providers can add a media gateway with a
low port count to a new market. As business grows, more ports or additional
media gateways can be added. The softswitch controls the media gateway
via a media gateway protocol such as the Media Gateway Control Protocol(MGCP). To reduce operational training and back office complexity,
functionality can be located remotely, controlling media gateways across an
entire region. The call control elements can be collocated with the media
gateways or distributed across service providers' backbones.
Qualities imparted:
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Ease of creation
Ease of Maintenance
Localization of changes
Layered Architecture
Simplified Design
Fault-tolerant architecture
The softswitch architecture is fully modular and has the ability to distribute
modules in the network any way the service provider wants. It is a fully
distributed, highly scalable and fault-tolerant architecture. Faults are easy to
locate, and their impact is localized. The fixes do not have any global
significance and maintenance becomes easier.
Qualities imparted:
Reliability
Modularity
High Availability
Maintainability
Existing infrastructure support
Interoperability with the existing Public Telephone Network is critical
during the transition to the new public network. The softswitch interfaces to
media devices and feature servers and receives call signaling from the PSTN
over SS7 links. Additionally, a softswitch can utilize backhauled channelassociated signaling for interfacing with customerpremises equipment. Withthis combined capability, a service provider obtains a complete solution for
optimizing its circuit network and migrating traffic to the packet network. A
softswitch with these capabilities offers a cost-effective way for service
providers to migrate to next-generation networks, while they continue to use
their legacy class 4 and 5 TDM switch equipment until those reach their end-
of-life.Qualities imparted:
Cost Efficiency Reduced Time to Market
Reuse of Existing Investments
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Chapter 7
USAGE
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USAGE OF A SOFTSWITCH
Softswitches can support a wide variety of services. It is important to
understand the business focus of the switching applications in order to select
the most appropriate suite of services for the environment. This allows thebusiness to maximize its return on investment in Softswitch technologies. In
the telephone industry, there are many services that can be given but require
better switching facilities. Many popular enhanced services include call
forwarding, call hold, call transfer, call waiting, caller ID, three-way and
multi-party conferencing. The advent of Internet usage over telephone
system has resulted in new services such as click-to-dial and Internet call
waiting. Several companies currently offer these features.
The integration of the telephone network with wireless networks and cable
networks presents major opportunities to offer value-add services. These
types of services are best supported by the Softswitch approach. MostSoftswitch services are usually related to revenue, in which billing becomes
an important function. There are many variations in call plans, group plans,
and business arrangements with external service providers, applicable rates
and the type of connection circuits. It is required to work with the existing
Operations Support System (OSS) that handle many types of operational
details, as well as the Business Support System (BSS) which also performs
the actual billing. The flexibility of Softswitch technology allows a provider
to support a wide variety of billing options. Softswitches can support
traditional features offered to telephone systems customers such asconsultation hold, call forwarding, conferencing, etc., as described in the
local telephone directory.
Other services that can be given by the Softswitch are:
Emergency calling (911)
Toll-free 800 calls
Calling cards and pre-paid calling cards
Virtual call center
IP Centrex
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Chapter 8
AN EXAMPLE
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subscriber places the request after dial tone is fed to him and the digits are
carried to the call control plane. It is now the duty of the control plane to
ensure the circuit set up and tear down and for this the media controller
which resided in the Call Control and Signaling Plane would hand over the
message in the required format for the corresponding media manager e.g.
SS7 signaling media manager which will direct it to the stack which is a part
of the Transport Plane. The signaling media manager based on the response
from the SS7 stack would handle and complete the required operations like
allocating the circuit and stuff.
Consider the case that the customer wants to call the number 1-800-PIZZA-
HUT. The intention of the subscriber and the nature of the number demand
that the nearest Pizza hut be connected. Here is where the Services and
Application Plane comes in to the picture. The call control analyzes the
directory number of the subscriber and has triggers defined for different
nature of numbers. Here the trigger for an intelligent service which is basedon say number 1-800 invokes the services of an intelligent relational
database handler Service Control Point (SCP) which resides in the Service &
Application Plane. The Call Control would forgo the control at this moment
and the service and application plane takes over returning the new number
based on the data supplied by the call control plane. This is the nearest Pizza
Hut directory number and the call control plane once again on getting the
correct destination number proceeds with the rest of the routing sending the
messages to the stack to set up the call and allocate a circuit the details of
which are beyond the scope of this document. The service and application
frame makes addition of the services easier since they can be creating
without disturbing the call control plane. Newer services can be added to the
switch by putting most of the logic in SCP. Similarly for mobile and other
applications there could be other feature servers, which can take care of easy
addition of newer services, which are required for in a competitive market.
To see the functionality of the management plane lets consider that the
telephone company wants to see if the call control plane or the transport
plane operations are working fine. This is done utilizing the functionality of
the Management Plane, which can be designed using the fault tolerance
patterns mentioned. The Management Plane also takes care of the billingdata of the subscriber, provides the operator interface for maintenance,
performs functions like invoking alarms and diagnostics, monitoring of the
switch functionality collects traffic data, creates, deletes and modifies entries
like say addition of Data Link Card or new hardware alarm monitor or ATM
concentrator.
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CONCLUSION
CONCLUSION
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This describes a migration path for broadband packet-voice access: a
migration moving from a transport-only solution that relies on a
conventional local-exchange switch toward a full-fledged local-exchange
softswitching and access solution that delivers packet voice dial tone.
By far the most important ingredient in this migration path is to the
softswitch technology that forms the basis of the MGC. The softswitch is
where all the service intelligence resides for the delivery of local telephone
services. A far higher level of capability is required for this type of
softswitch than for todays tandem softswitch applications where
functionality is limited to Interworking between PRI, SS7, and VoIP
signaling. For local telephony, the softswitch must not only deliver a critical
mass of local telephony features but also provide a feature creationenvironment that enables service providers to develop differentiated
services.
The development of a local-exchange softswitch that is truly capable of
meeting these requirements is a major undertaking. But for those who
succeed in reaching this goal, an awesome prospect lies ahead: nothing less
than the total transformation of the local telephone network.
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