This document provides a summary of integrating IP telephony into the public switched telephone network (PSTN) environment. It discusses the evolution of PSTN from analog to digital networks and the emergence of time division switching. It then outlines the architecture of a soft switch solution for integration, including edge, core, control, and application layers. Finally, it discusses existing practices for integration, such as replacing tandem exchanges, and considers the specific context of integrating IP networks in Bangladesh.

1. Prepared by:
Name-Towfiqul Islam
Student ID-063382056
Course Number-ETE-605
Prepared for:
Dr. Mashiur Rahman
Report On:
Integration of IP Telephony
In PSTN Environment

2. Integration of IP Telephony in P.S.T.N. Environment
1.1 PSTN network evolution history:
1.1.1 Invention of Telephone System:
As known to all, DR. Alexander Graham Bell is the father of earliest version of telephone system. After
his invention for a long period telephone system has been supported merely by manual switching. After
that period machine switching and programmed switching were successively followed. The first
automatic dialing system was patented in 1891 by a Cansas City undertaker. The first coin tel phone was
e
installed in Hatford, Connecticut in 1900.Party lines were soon developed to lower cost of telephone.
With the development of automatic dialing technology, almost 80% telephone of the world was automatic
phone by the end of 1950s.
1.1.2 From analog to digital transformation:
There are two principal means by which telephone signals are transmitted: analog or digital. As the
electronic current that transmits the voice is actually imitating the voice behavior, it has various shapes;
while the digital system transmits only on or off-two binary states. In the early days of telephone history,
the switching system was controlled by analog switches. As technology had advanced enough by the late
1970s where 100% Digital Systems were being developed for use in the telephone industry. Digital
systems sample the analog signal and handle telephone calls internally as binary digits, then convert them
back to analog to be compatible with regular telephones. This again made the switch smaller and more
reliable. It also allowed new technologies to be added faster by using modular techniques (adding systems
to the original system without complete redesign/reinstallation) and by using advanced computer
software.
1.1.3 Emergence of “time -switching ”:
While traditionally, telephone switches connected physical circuits, modern telephone switches use a
combination of space and time division switching. In other words, each voice channel is represented by a
time slot(say 1 or 2) on a physical wire pair ( A or B).In order to connect two voice channels (say A1 and
B2) together, the telephone switch interchanges information between A1 and B2.It switches both the time
slot and physical connection. To do this, it exchanges data between the time slots and connections 8000
times per seconds, under control of digital logic that cycle through electronic lists of the current
connections. Using both types of switching makes a modern switch far smaller than either a space or time
switch could be by itself. The structure of a switch is an odd number layer of smaller, simpler sub
switches, interconnected by a web of wires that goes from each sub switch, to a set of the next layers of
sub switches. In most designs, a physical switching layer will alternate with a time switching layer. The
layers are symmetric, because every call is symmetric (there is a connection in both directions) A space
division sub switch uses digital multiplexers controlled by a cyclic memory. This takes physical space for
the wiring. A time division sub switch reads a complete cycle of time slots into a memory and then writes
it out in a different order, also under control of a cyclic computer memory. This causes some delay in the
signal. The above mentioned discussion very briefly describes how a Circuit Switch based PSTN system
works.
1.1.4 Future of Circuit Switch based PSTN technology:
In 1970’s the telecom industry conceived that digital services would follow much the same pattern as
voice services and conceived a vision of end to end circuit switch based technology known as B-ISDN.
But the B-version technology has been overtaken by disruptive technology of Internet. Many experts
believe that the long term future of PSTN will be just one of the applications of Internet.

3. 1.2.1 Development of IP Network from a historical perspective:
Until 1990, all efforts were concentrated on the development of voice service over legacy
telecommunication network. Afterwards an effort was successful to send data, voice and video over a
single line by introducing Integrated Services Digital Network technology. A separate packet switched
based data network was evolved only for transmission of data. But ISDN is a circuit switched digital
network and DSL variants are packet-switched data services over laid on the PSTN. Telephone systems
have been slowly adopting the “IP” (Internet Protocol) technology over the last decade’s .Early
experiments proved that voice can be converted to digital packets and sent over the internet. The packets
would be collected and converted back to analog voice. The quality of the calls was not great but it
showed that it could be done. The major problem was packet loss which is common with TCP/IP
connections. By the early 2000’s IP telephony had improved. Using classes of service, reliable
connections could be obtained and packets loss reduced to minimum levels. Business systems started
using VOIP technology in their PABX.
Development of network
• The 1 st generation Fixed-line network
-Analog switching
• The 2 nd generation Fixed-line network
-TDM and digital switching
• The 3 rd generation Fixed-line network
-Based on IP network and soft switch 3G
Technology
Service: Voice/Data/ Mult
1G 2G imedia
Service: Voice
Service: Voice Device: soft switch ,data
Device: Manual Device: Digital Switch Switch/ Router, Application
Connection, step switch server, SHLR, etc.
Crossbar Switch, Analog
Service Development in network evolution
Service evolution(tone service-)
PSTN tone/data-) data/multi-media service
plays pointy)
Data All in IP evolution
Network Circuit switching-) packet switching-) All in
IP
PLMN
Broad Separation of service and transportation
band
access
1990 1995
Multi-service 2000 2010
Multi protocol
Simple Integration

4. 2.1. Integration of IP in PSTN domain:
2.1.1 Critical Evaluation of existing network:
At present PSTN network exists for voice communication and packet network exists for data
network. A critical review of the two networks is given below:
PSTN Network :
Voice is real time, delay-sensitive in nature. Here the communication is connection oriented
Advantages and disadvantages:
PSTN offers highest quality of speech (high intelligibility).It is also reliable because of no delay,
no traffic loss. It also provides rich telecom features. PSTN has the disadvantage that it offers
fixed bandwidth; bandwidth remains idle if there is no traffic. PSTN is not basically engineered
for long holding of a call such as internet call.
Data network :
It is mainly designed for data traffic. Data is rate-sensitive, on-real in nature. Data
communication is connectionless.
Advantages and disadvantages
Data network supports variable bandwidth facilities to integrate data, voice and video. It
provides true end to end delivery of IP packets. Data network is subjected to delay and jitter. It is
vulnerable to packet loss
2.1.2 Dynamic behavior of existing market players :
Following changes are being observed in existing networks:
• Data traffic is increasing faster than voice
• Terminals are becoming more intelligent
• Network is becoming more passive
2.1.3. Changes in customer demands :
• Requirement of higher bandwidth
• Integrated access for voice, data and video
• Requirement of variety of sophisticated services, multimedia, web conferencing
• Requirement of guaranteed quality of service and low price.
2.1.4 Changes in network operators requirements :
• New revenue streams generation for new opportunities
• Rapid service provisioning
• Open interface to support multiple vendors
• Reduced capital expense for future expansion

5. 2.1.5 March towards NGN:
With the sustainable growth of data services, the current network framework is evolving to Data
Networks. Therefore, both the traditional telecom carriers relying mainly on voice services and
the emerging data network carriers pay much attention to the new generation Telecom Network
that converges voice, video, data and integrates the PSTN circuit switching with packet
switching technology.
The network integration oriented soft switch architecture is a total solution of the new generation
multimedia services. It implements the interconnection of various service networks including
PSTN/ISDN, PLMN, IN and Internet etc.By optimizing the network architecture, it realizes
network integration, and more important, the service integration, thus enabling the inheritance of
rich service functions of the traditional Circuit Switching Network, meanwhile providing new
services that are hard to be implemented in the traditional networks all over the new- generation
networks.
2.1.6 Reasons to choose NGN as a means to integrate IP in PSTN domain:
At present, two totally independent networks exist: the PSTN network and data network, which
provide the voice service and basic data service respectively. Network separation and isolation of
operation and maintenance have been keeping the costs on a high level and furthermore, a
network cannot provide complicated convergence services, although network convergence has
been an inevitable trend.
Since a traditional voice network is a closed network with monopolized resources, it has become
a common understanding in the telecom industry that the packet network with the advantages
such as open architecture, low costs and large scale, will replace the PSTN to become the basic
frame of the next generation of convergence networks and the construction of the next generation
of networks will be based on current packet networks.
2.1.7 Considerations for converged networks:
It is necessary for carriers to consider resource utilization and investment protection during
construction of future networks. On one hand, carrier should trace the latest technologies; and on
the other hand, they should try to utilize existing technologies and resources. Thus, carriers can
provide users with large numbers of services economically and rapidly to make the highest
profits, without the need of large-scale network alteration. The solution of smooth transition
from existing networks to the new generation of networks is the key to the problem. The soft
switch solution based on soft switch technology is just a mainstream solution to smooth network
evolution.
3.1 Architecture of Soft Switch Solution:
Hierarchical models are adopted for the soft switch based converged network .The entire
network can be divided into four layers: Service Layer, Control Layer, Core (Transport Layer)
and Edge Access Layer.

6. Soft switch
Traditional
Network
Network
Service
Service
Call control
Call control
Transportati on
Transportati on
APPLICATIO
N SERVER
IN MULTIMEDIA Management
MENT
OSA/PARL
A
SNMP
ST SS7
SS
SS
ACCE
H248 ATM/MPLS CORE SS
TDM NETWORK GATE
MGW MGC WAY/
PLMN P
PSTN
BROADBAND
ACCESS
MEDIA
SERVER H323/SIP
ARCHITECTURE
SIP/H323

7. 3.1.1 Edge Access Layer:
The edge access layer refers to various access gateways and new types of access terminal devices
related to the current network. It implements inter working with the existing various types of
communication networks and provides access of various communication terminals (eg., the
DTMF phone, SIP phone, PC phone visual terminal and intelligent terminal ) to the IP core
layer.
3.1.2 The Core Layer:
The Core (Transport) Layer refers to a packet switching network composed of backbone
transmission equipment such as IP router or broadband ATM switch, which is the bearer basis of
a soft switch network.
3.1.3 The Control Layer:
The Control Layer refers to soft switch control units, which completes integrated control
processing functions such as call processing control, access protocol adaptation, and
interconnection and inter working and provides an application support platform for the entire
network.
3.1.4 The Application Layer:
The Application Layer provides a network with various applications and services, client-oriented
integrated intelligent services and service customization.
3.1.5 Interface and relation between layers :
Standard interfaces are used between layers. Under the control of core equipment (i.e., the soft
switch control equipment) and based on division of labor and cooperation of work, the related
NE equipment implements various service functions of the system. In soft switch architecture,
the soft switch control equipment is the core, which is independent of the bottom- layer bearer
protocols and implements functions such as call control, media gateway access control, resource
allocation, protocol processing, routing, routing, authentication and accounting. The soft switch
control equipment can provide all basic call services, supplementary services and point-to-point
multimedia services a PSTN can provide. Furthermore, with the cooperation of the service layer
equipment (SCP) and application server, the equipment also can provide users with traditional
intelligent services, value-added IP services, diverse third-party value added services and new
intelligent services.
3.1.6 NGN elements:
Soft Switch:
• Provides the intelligence and call control and the signaling for the NGN
• Control of media gateways
• Bridges the traditional TDM and voice over packet networks
• Communication with various networks (SS7, BICC, SIP-T, H.248, INAP etc)
• Support of billing /accounting by providing call details
• Control of subscriber feature capabilities

8. Media gateways:
• Terminates voice calls from the TDM, compress and pocketsize the voice data, and
delivers the compressed voice packets to the packet network
• Supports VOIP functionality as well as all other bearer connections such as fax over IP
• Controlled by the soft switch using an IP protocol such as H: 248
• Can be located remotely from the soft switch or can be co- located with it.
Packet network:
The core connectivity layer is a packet based network that shall provide the voice traffic
bearer connectivity between the gateways and the soft switch.
3.1.7 Protocols of NGN:
The main advantage of NGN over conventional circuit switch is that NGN are layered structure
with open protocols between layer to layer and also between the elements of the same layer. A
summary of the protocols are presented below:
Layer/ Elements Protocols
Soft Switch-TGW H.248/MGCP
Soft Switch-IAD/AG H.248/MGCP
Soft Switch-IP Phone SIP
Soft Switch-Soft Switch SIP-T/BICC
SS7-IP network Sigtran
Soft Switch-Application Parley/SIP
Server
Soft Switch-AAA Server RADIUS
Soft Switch Routing Server LDAP
Routing Server- Routing TRIP
Server
Soft Switch- Signaling M3UA/SCTP
gateway
Soft Switch-Gatekeeper or H.323
H.323 gateway
Signaling gateway-IN INAP
Signaling gateway-SS7 TUP/ISUP
Soft Switch-NMS SNMP
4.1. Integration of IP in PSTN domain- existing practices:
4.1.1 Replacement of Tandem Exchange:
Before discussing on Tandem Excha nge replacement we must first provide a simplified
architecture of a PSTN system. In the given figure we have provided a very simplified
architecture of a PSTN system. In this system if a subscriber from exchange 1 wants to make a
call to a subscriber of exchange 3 it must pass through tandem exchange A. In the PSTN system
in addition to tandem exchange, TAX (Trunk automated exchange) and ITX (International trunk
exchange) are used to carry voice traffic between inter-district and inter- country calls .For
tandem replacement, trunking gateway, signaling gateway and media gate controller together
will replace the tandem switch. The local exchange will be connected trunking gateway by TDM

9. link and the TGW will be the interface of PSTN and NGN. In this way the huge installation of
cable network of PSTN system as well as local exchanges are retained. Thus it is a cost efficient
gradual replacement of outdated circuit switches.
Exchange Exchange Exchange Exchange
A2 A1 B3 B1
Exchange Tandem Tandem Exchange
A3 Exchange A Exchange B B2
Tandem
Exchange C
Exchange
Exchange C3
C1
Exchange
C2
Existing Structure of PSTN system
Tandem replacement solution

10. 4.1.2 Bangladesh Context- special reference to network of BTTB:
While Bangladesh is struggling to meet the demand of providing basic telecommunication
services, question can be raised whether we should implement NGN projects. In Bangladesh,
BTTB is the major PSTN operator. Still it has a huge number of unmet subscriber demands .So
there is argument that BTTB should concentrate on PSTN business. But BTTB also provides
data and internet service and it has also installed a nationwide data network. In the present
scenario BTTB has to maintain separate networks for voice and data. Converging these networks
will reduce the cost of mantling two separate networks. One important thing to add is that
manufacturers of conventional switches are gradually closing the production of pure circuit-
switch equipment and shifting to produce NGN equipment. The hybrid type solution to have old
PSTN exchanges with conventional fixed phone at lower levels and IP backbone at core should
be an appropriate choice. This will also allow BTTB to use the benefit derived from information
super highway connectivity through submarine cable. Meanwhile as an interim arrangement to
offer value added service BTTB has installed fixed intelligent network platform. This can be
considered as an intermediate arrangement to offer many rich services before adopting full scale
NGN.
The newly introduced
Intelligent Layer system
(Computer and
data base system)
TCP/IP IN Separates service control function
From switching function
Signaling Layer
(CCS7&TCP/IP
Conversion The existing network
Resources
CCS7
Switch &
Transmission Layer
4.1.3. General rules to be followed for migration:
• Migration from traditional network infrastructure should be carried with the strategy so
that past investment is protected.
• Established operators have to find some ways to provide the customers with sophisticated
services till the conversion to full scale NGN.
• Migration should follow the speed so that established operators can compete the new
operators to retain their customers.
• Formulate an integrated management of legacy exchanges and the NGN soft switches in
order to smoothly route increasing amount of traffic on NGN.

11. References:
1. International Telecommunication Union (ITU): NGN 2004 Project description-version
3”, February, 2004.
2. Organization for Economic Co-operation and Development (OECD): Next Generation
Network Development in OECD Countries, Document No.
DSTI/ICCP/TISP(2004)4/FINAL, January, 2005
3. Geok-Leng, Dr. Tan: Are you ready for the Next Generation Network (NGN)?APEC
Telecommunications and Information Working group 29th meeting, Hong Kong, March
2004.
4. Feit, Sidnie : TCP/IP : Architecture, Protocols and Implementation, McGraw-
Hill,Inc.,1993
5. ZXSS10 SS1 Training Materials, ZTE University,Dameisha,Yantian District,
Shenzen,P.R. China
6. Teletech, a journal of BCS Telecom Samity, Vol. XIV, May 2005
7. Wikipedia on Next Generation Network
8. Technical Documents of Huawei Technologies Co. Ltd. about N.G.N.

12. APPLICATIO
N SERVER
IN MULTIMEDIA Managemen
t
OSA/PARL
A
SNMP
ST SS7
SS
SS
ACCE
H248 ATM/MPLS CORE SS
TDM NETWORK GATE
MGW MGC WAY/
PSTN P
PSTN
BROADBAND
ACCESS
MEDIA H323/SIP
SERVER ARCHITECTU
SIP/H323
TERMINAI S