201O Second International Conference on Communication Systems, Networks and Applications

Optimum Migration Scenario From PSTN to NGN

Ahmad A.Almughaless Faculty of Engineering, Sana'a University ,

SANA'A, Yemen Ahmfa2000@yahoo.com

Abstract-Telecommunication is one of the fastest growing industries. Yemen Telecom (YT) always tries to follow the modern technologies taking into consideration the technical issues, economical factors, customers demands and satisfaction , ... etc . This paper illustrates the migration scenarios to migrate Yemen Telecom legacy network into the latest technology, Next Generation Network (NGN).The best migration scenario( hybrid from PSTN replacement and NGN overlay scenarios ) and the migration priority of YT TDM switches to the newest technologies NGN is determined. This paper explains the migration process of the TDM switches in 22 governorate .

key words -NGN, PSTN migration, NGN overlay , PSTN replacement .

I. INTRODUCTION

NGN is a packet-switched network providing a range of telecommunications services, which uses transport technologies for several bandwidths and classes of service, in which service related functions are independent of the underlying transport technologies. NGN covers multiple networks and layers. It is a means of providing services across a range of technologies giving users consistent and ubiquitous service provision and unrestricted access to different service providers ,with supporting general mobility. NGN provides a single platform and integrate the existing separate voice and data networks into a simpler and more flexible network based on IP protocols[10]. The aim of the network operators in the whole world is to migrate their networks to NGN in order to benefit from the NGN features as well as to avoid the drawbacks coming from legacy network beside other financial factors such as reduce capital expenditure (CAPEX) and operation expenditure (aPEX ).Worldwide ,NGN deployments still at an early stage , some operators are planning or still in finalizing their plans to go to NGN, other operators are started to migrate their PSTN to NGN taking different paths and strategies based on their situation and conditions as illustrated in [1], as well as based on other financial factors. Indeed, the economic slowdown that appeared recently in the world due to the World financial crisis is driving many operators to expand their early thinking on how best to migrate and implement the NGN Network. YT is the incumbent telecom operator in Yemen that offering fixed voice services and data for all over Yemen. It has two separated networks one for voice and the other is for Data. The majority of switching technology used is TDM switching. The

978-1-4244-7477-6/1 0/$26.00 ©20 1 0 IEEE 227

Ali M.Alsaih Faculty of Engineering, Sana'a University

SANA' A, Yemen Alimanea@yahoo.com

goal of this paper is to determine the optimum migration strategy for YT from PSTN to NGN. The best migration scenario proposed in this paper is a hybrid scenario that combined from the PSTN replacement and NGN overlay strategies. The criteria used to estimate the priority of either replacement or overlay scenario to be implemented for each local switch will be discussed. Our migration plan aims to achieve smoothly and economically migration to NGN with low cost and minimum risks , taking into consideration saving traditional business , focusing on customers' needs and getting a target network that could be evolved in the future to a full IP Multimedia System (IMS) architecture. The existing network of YT will be described briefly in section 2 . Section 3 will review the NGN architecture. In section 4 the selected migration scenario will be described. Section 5 analyses the evaluation method .Finally the conclusion will be mentioned in section 6.

II. YEMEN TELECOM LEGACY NETWORK

A. Legacy Network Architecture

Y.T legacy network consists of five TDM switching layers International, Transit ,Tandem, local ,and access layers as shown in Fig (1). International Layer: Contains two international gateways used MGWs and soft switches. This layer is managed and operated by another company. Transit Layer : Contains two TDM exchanges installed in the capital ( Sana'a).This layer offers interconnection to tandem switches and interconnect them with international layer. Tandem Layer: contains seven TDM switches offering Intra regional switching between local exchanges and interconnecting

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Figure (I) - Yemen telecom legacy network Topology all regional traffic to and from transit layer . Local Layer: contains forty two TDM switches with the lowest functional level of exchanges. The main role is to offer access and local switching to subscribers. Local level switches are connected to tandem Level exchanges. Access Layer: Contains of 576 Subscriber Units (SU) distributed through 250 sites. And 53 main Optical Line Carrier (OLT) using V5.2 . The access layer is distributed in the rural areas . In the existing topology each local switch is connected to two tandem switches, similarly, each tandem switch is also connected to two transit switches [3].

B. Capacity and Services

Yemen telecom provides All the POTS services using TDM switches, it has around 2 million subscriber distributed in all over Yemen[3].

III. NGN STRUCTURE:

NGN implementation for Y.T has been planned into many phases. The first step was mainly to build up the Multiprotocol Label Switching(MPLS) backbone in many cities , this MPLS will be the infrastructure of the NGN , it will be used for convergence of the voice and data network. This phase has already started and it will continue to reach other new sites and cities .The following section will show the three main NGN layers represented in YT NGN.

A. Edge/Core network

Edge/Core network build in Y.T uses label switching technology MPLS with the ability to set up connection­oriented paths over a connectionless IP. This part of NGN network includes : Ethernet Service Switch (ESS), Service Router (SR) and Routing Management (RM). This part of the migration process has been completely finished by YT and it will not be mention here in this paper. We will focus on the migration mechanism of TDM switches to NGN.

B. Access Network:

The access network consists of different types of equipments that help the end user to interconnect with the NGN such as : Multi-service Access Nodes (MSAN) which able to deliver different services Including telephony ,high speed internet access and video services using the same copper pair .Different access gate ways (AGWs) will be installed in the customers campuses for access to NGN.

C. Control layer

The control layer controls all the calls and bearer in the network. It consist of : Soft Switch (SS) is the telephony signaling platform which contains four functional blocks: 1. Residential Gateway Controller (RGC). 2. Call and Session Controller (CSC). 3. Subscription Location Server (SLS). 4. Media Gateway Controller (MGC). Media Gateway (MGW) is the equipment that converts the voice traffic from TDM into VoIP. Media Gateway offers services such as: VoIP Termination, VoIP Trunking, TDM to

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Primary Rate Interface (pRJ) grooming, Private Branch Exchange (PBX) Access. MGC does all signaling for MG. Management Center is the management application converging all management elements from VoIP subsystem and OSP (Open System Platform). It gives centralized access from single server to all Graphic user interface GUI used by operation application.

IV. MIGRATION SCENARIOS

There are different strategies to migrate the legacy PSTN to the NGN. Chosen one of these strategies depends on many factors such as network situation, economic aspects, market prospective, competitions .. etc. The three mainly alternatives scenarios for network migration are as follow : Upgrade TDM with NGN capabilities, Replace TDM by NGN, Overlay TDM with NGN as described in [1,8,9]. Whatever the selected Scenario is, it should achieve the convergence as outlined in [11]. Below is a short description for these scenarios.

A. Overlay scenario

In this scenario the TDM network will be kept and a New NGN network will be implemented in parallel. It is the best NGN implementation strategy that offers little risk and plenty of opportunities. It also protect the investment in the TDM, requiring no major initial investments and yet guaranteeing comprehensive and immediate availability of services that can be readily deployed. Taking cost and performance as a decision criteria Some studies done on [2,4,5] show that the overlay is the optimal strategy to migrate PSTN to NGN since it has the lowest CAPEX , lower OPEX and little payback time.

B. PSTN replacement Scenario

In the replacement scenario the TDM switches are replaced with IP based equipments that have all the edge functionality required now and in future. It achieves the principal goal of optimizing network structure and reduce the OPEX of the TDM network, but has a high risk and a high upfront investment to build new access gate way to temporary support the POTS services. After the migration this access gateway will not be used any more . British Telecom (BT) is one of the most operators that used PSTN replacements strategy . Many other operators in many countries are also following this strategy . For example , Italia Telecom has completed their IP trunk layer with saving around 75% of their OPEX, Bell Canada's as well follow this strategy[2].

C. Upgrading TDM with NGN capability

This scenario aims to upgrade the trunk functions in existing TDM switching by IP function , by inserting or replacing the TDM trunk function with IP cards to provide IP connections with others entities . High CAPEX is needed to upgrade all the existing TDM switches which most of them have reach their End of life. It also has implications on the the stability and performance of the existing network[2,4].

D. Selected Scenario for IT

A mixture of replacement and overlay scenarios is selected for Yemen Telecom PSTN Migration to NGN. This scenario will be able to satisfy most of YT recommendations and conditions such as network situation, market demands,

business requirements, regularity requirements, lower cost, higher reliability and minimum risk or failure . . . etc .The following addressed requirements should also be achieved by

the selected migration scenario .

• Build a reliable target NGN network .

• Reduce CAPEX and OPEX.

• SimplifY the network architecture to be IMS ready

• Saving cash flow and achieve investment payback Profitability.

• Increase the income of YT.

• Get benefits along the migration line.

Based on the previous requirements, it is clear that , the migration scenario for YT is recommended to be characterized by the initial retention of the existing TDM switches , which means that the NGN should rolled out in parallel. This can be achieved by overlay scenario for class 5 switches, allowing YT to migrate from TDM network to NGN in small steps and maximize the return on the capital invested in TDM technology. However, the simplification of the network architecture and reducing the OPEX could not be achieved by Overlay scenario. PSTN replacement scenario can achieve it instead. As a result NGN new services and VOIP can be fast and easily deployed which yield to increase the Average Return Per Unit (ARPU ), and increase YT income as well.

By achieving the features of both PSTN replacement and overlay scenarios as in [2,4,5], the best mixture scenario can be compound to implement NGN .In our method the migration will be started from the upper plane switches (e.g. Transit, Tandem Switches ) and reach the lower planes .

The attraction of replacement strategy is saving OPEX and CAPEX and simplifYing the network architecture, while the risk disadvantage will be avoided by a good plan for implementation as in [14]. Overlay strategy guarantee that the availability of an NGN is not tied to a particular location . As will as it is the best strategy that offers little risk and plenty of opportunities. The overlay approach characterized by protecting the investment in the TDM, requiring no major investments.

Replacement of the TDM switches in the transit and the tandem Layers will reduce the network complexity and OPEX , as well the replacement of the very old switches will also reduce the risk of network failure and OPEX. Overlay is suitable for the sites in which the TDM local switches are doing well and the demands on NGN service's are in slow growing. Based on the above requirements the outlines of the proposed

migration scenario comprise the following steps: • Reduce network complexity by Replacing the Transit

and Tandem layers with two NGN soft switches ,six MGWs , two SHLR and, two Media Servers MS for playing tones. The Quantities decided based on the calculations and redundancy as illustrated in [13].

• Test the NGN equipments performance before commercial lunching Based on ITU-T specification

and the test item listed in [ 6,7,8,12] as well as based on YT and vendors recommendations .

• Connect all the Local End (LE )Switches to the

MGWs to be controlled by Soft switches.

• Replace the very old switches that have a very low efficiencies starting by the larger capacity and more important ones .

• Build new overlay access gateways for NGN new services where needed . ( market demand).

• Install and implement NGN OS�

• Create a rich variety of applications over the NGN.

• Install WLL(wireless local loop) access if needed

• Replace the LE which most of its subscribers have already migrated into the NGN overlaid Access gateways.

• Migrate TDM customers from existing networks to NGN (market demands )

• Undertake the technical migration of customers to NGN if necessary due to particular technical circumstances (network provider driven).

• Replace older switches as migration allow and so on.

V. NGN IMPLEMENTATION PRIORITY

After finishing the replacement of the tandem and transit switches, the migration of the Local switch should be started.

In the Migration of the local switches the important issues is to decide wither replacement or overlay scenarios will be applied for each switch ,and what is the capacity to be installed. To make this decision accurate, an evaluation for TDM switches should be done and analyzed based on different criteria. Efficiency, Consequential value and On Site Survey are the most important characteristics used to evaluate each switch. Efficiency is estimated by different technical characteristics namely switch technical status , EOL (end of life), spare parts, maintenance ability and support. An evaluation marks are suggested for every parameter. Efficiency evaluation will be applied for each TDM switch as shown in table (1).

On the other hand the consequential value of each switch is estimated based on other criteria the most important once are locations , traffic ratio , capacity , numbers of ADSL subscribers .... etc. An evaluation marks are suggested for every criteria. The marks given to each criteria depend on the switch status, switches statistic data , and technical opinions. The Consequential value evaluation parameters shown in table(2) are implemented on each local switch, then the total results will be used to determine the consequential value of each switch. The On Site Survey is an important factor that determine the availability of the space , rooms and buildings in both Central switch sites and remote sites.

In case if there is no enough space or new buildings are needed to install NGN equipments , the replacement of such switches will be delayed and overlay is carried according to the

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market demands and the space availability. Indeed, this evaluations is a bridge for migration from PSTN fixed network to NGN, since based on them the best migration plan and design can be done according to the real situation .

TABLE I : EFFICIENCY V ALUE DETERMINATION

Item Mar

Description Switch type & name

ks Central Remote 15 Operation life span x x

15 Reach E)(Jlire EOL x x

Switch 10 Spare parts x x

10 End of repair x x

15 Stability & x x

Reliability Support and 10 Vendor x x

ma intenance 15 Local staff x x

CPU load 10 CPU load x x

Total xx xx

TABLE-2 SWITCH CONSEQUENTIAL VALUE EVALUATION

Item Marks Switch name

Central Remote

NO. of subscriber 25 x x

Switch location 30 x x

Number of ADSL lines 20 x x

Total traffic 25 x x Total x x

The results found from these evaluations are analyzed and studied carefully to determine which scenario ,replacement or overlay, is suitable currently for each switch. Till now we have three evaluation results , one is based on the Consequential values of the switches , the other is based on the switches efficiencies and the last one is based on the On Site Survey of the rooms and space availability. The three preceding evaluations are an important to make a good migration plan according to the real situation and far from theoretical concepts.

The decision for which switches should be replaced first and which should be overlaid is based on the manipulating of the evolutions results using (1). The switch which have a high importance and have a very low efficiency should be replaced first. The higher of the differences between the consequential and the efficiency values of the local switch, The higher the priority of replacement the switch .

Priority = Consequential value - Efficiency . (1)

VI. RESULTS AND DISCUSSIONS:

The optimal migration scenario is combined from replacement and overlay scenarios to fulfill our migration plan motioned above. The evaluation of the TDM switches is carried out for all the TDM switches. Fig.( 4-a,4-b) plots the efficiency and consequential value for 43 local switch respectively. High value means the corresponding switch has high efficiency or high Consequential value .The replacement and overlay scenario periorty is shown in Fig.( 4-c). The

negative sign (-) priority means that the corresponding switch number has a low priority for replacements, since it has a good efficiency and low importance, so no need to be replaced at once it can be overlaid for some time. YT will install new NGN access equipments in each switch that have a priority for

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60 �----------��------------��r_ Overly pel'iority Replacement p�riority

LO

20

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[S'lVitdl NO I -40 �-----------------------------------

Figure (4-c) Switch replacement and overlay priority

overlay with a capacity proportional to the ADSL subscribers in the switch or according to the market demands, since ADSL subscribers number give an index to estimate the use of the new introduced NGN services.(i.e. If the ADSL subscribers number are high in a certain TDM switch ,it indicates that these subscribers are educated well or rich and they are more likely to buy the NGN services). Once the majority of subscribers are migrated to NGN, the remaining TDM subscribers can be supported by means of an IP Access Gateway (lP AGW).The point at which this threshold is reached is one of simple economics and management decision . Once the NGN is supporting the entire user base, the TDM can be switched off. The continued

existence of the TDM switches , the protection of investment achieved in this manner. As the TDM network will not be expanded the replacement parts can be drawn from the stock of the decommissioned equipments, investment can be scaled back and money is used for investment in NGN. the replacement priority is shown in Fig.( 4-c) , the right side show that the corresponding switches have a high priority for replacement , for example switch Number 43 has the highest priority for replacement. The left side of Fig.( 4-c) plot the overlay priority .The switches in the most left side will be overlaid for longer time , since the are still performing well and better than others.

VII. CONCLUTION :

This paper presents an optimal Migration scenario that mixed from both PSTN replacement and NGN overlay scenarios to migrate Yemen Telecom YT Legacy network to the Next Generation Network NGN. Taking into considerations the current situation for YT network, regularity requirements , market demands ,customer needs '''., as well as other financial factors. Since the world recently has been suffering from the known World financial crisis , operators should plan well to save their cash flow , save old investments and reduce their CAPEX and OPEX . A detailed method was explained to evaluate wither the replacements or overlay scenario will be implemented for each switch .

VIII. REFERENCES

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[2] Shabnam, Ladanand, Alireza, Yari "Evaluation of Migration Scenarios toward NGN Considering Economic Aspects." IEEE 978-1-4244-2324-8/08 -2008 .

[3] YT" Yemen Telecom evaluation DATA" 2008-2009 .

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[4] Bodo Jacobs "Economics of NGN deployment scenarios: discussion of migration strategies for voice carriers. " Deutsche Telekom Allee 7, 64295 Darmstadt, Germany -IEEE T-Systems Enterprise Services GmbH.

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[10] Vasco N. G. J. Soares, Paulo A. C. S. Neves, and Joel J. P. C. Rodrigues "Past, Present and Future of IP Telephony" - International Conference on Communication Theory, Reliability, and Quality of Service 978-0-7695-3190-8/08 $25.00 © 2008 IEEE DOl 1O.1109/CTRQ.2008.12

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[12] Zohreh Ayatollahi , Saeede Sarukhani, Fatemeh Fayazi , Zahra Askary , Roknabady, Afsane Madani "Interoprability Problems In Next Generation Network Protocols" (IEEE-92-61-12441-0ICFP0838E - 2008 lTV).

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