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Design and Capacity Planning of Next Generation Network (NGN)
Ali Amer, IEEE Member.Saudi Telecom Company, Riyadh, Saudi Arabia.
Figure 1: Typical PSTN Network
traditional PSTN uses Class5 and Class4 circuitswitches along with Time Division Multiplexing(TDM) technology to transport voice[2]. It alsouses the SS7 signaling network to handle callsetup and teardown, plus other control functions.Figure 1 depicts a typical PSTN network.
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Eventually, Legacy networks (PSTN) lack thecapability of providing multiservice, and gettingcostly operated, therefore, it has to be migrated toNGN which is considered as a service drivenNetwork[4]. In fact, much has been written,debated, and posited about what exactly is thisNext generation network (NGN). The NGNconcept, defines telecommunication networkarchitectures, and technologies. It describesnetworks that cover conventional PSTN type,and data, as well as new types of service such asvideo. All information is carried in packetswitched form[9]. In addition to that, NGN hasbeen promoted, to Network Operators andservice providers, as a new platform to decreasethe CAPEX and OPEX of their NetworkInfrastructure, and increase their revenues.The lTV defined the Next Generation Networkas a packet-based network able to provide
The current trend of fixed communication is multiservices based on broadband access. This willprovide users with variety of communicationsservices (voice, data, video, and SIP based services),and improve revenues for service providers. The
Abstract- The last decades has seentremendous shifts in the telecommunicationslandscape. Telecom Operators and serviceproviders, are responding by adopting strategiesthat lower their costs of operations and allowthem to offer rapidly new services with betterrevenues.The Next Generation Network (NGN) is anetwork architecture that is ultimately designedfor new service provision, independent of accesstechnology. In addition to that, NGN can greatlyreduce Capital Expenditure (CAPEX) andOperational Expenditure (OPEX), enablessmooth transformation of the legacy networksinto a simpler, but more powerful, while keepingcompatibility to support traditional services.Worldwide, NGN deployment is still at an earlystage, though some Telecom service providersincluding incumbent are in the process offinalizing their plans for deployment of NGN intheir networks. This is likely to be implementedin a phased manner starting with core networkand then the access network, and finally serviceprovision.The first and unavoidable phase of NGNimplementation is migrating the legacyNetworks, starting with the Public SwitchingTelephone Network (PSTN) to NGN.In this paper, we present a cost effective, futureproof solution architecture that can be used forthe design and planning of the NGN networkElements (NEs) capacity and dimensioning, toserve this migration Phase.
I. Introduction
II. NGN Architecture
Figure 2:NGN Architecture Layer Overview
The access plane provides the infrastructure (e.g.access network between the end user andtransport network. The transport plane providesthe communication among the referencearchitecture entities, as well as communicationbetween the neighboring layers of the functionalNGN model. The control plane is responsible forthe network elements and of services control.The service plane provides the features of thebasic services that can be used for thedevelopment of the more sophisticated servicesand applications[18]. The NGN functionalStructure is outlined in Figure 2.
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Telecommunication networks architectures arechanging to meet new requirements for anumber of services/applications (Broadband, IP,Multimedia, mobile, etc.)[20]. New generationequipment (soft switches, databases, Mediagateways, Signaling gateways, new protocols,and interfaces, etc.) and new call/mix trafficcases are going to be introduced in the networks.Standards bodies like lTV, and ETSI areprimarily working on the NGN architecture.The Technical Committee forTelecommunications and Internet convergedServices and Protocols for AdvancedNetworking (TISPAN) of the ETSI, hasmanaged to complete a comprehensivearchitectural specification to date. Thisspecification is referred to as TISPAN NGN Rl.The TISPAN NGN Rl specification wascomposed by leading vendors and serviceproviders, and is expected to cover serviceprovider requirements comprehensively, within
services including Telecommunication Servicesand able to make use of multiple broadband,QoS-enabled transport technologies, and inwhich service-related functions are independentfrom underlying transport-related technologies.It offers unrestricted access by users to differentservice providers. It supports generalizedmobility which will allow consistent andubiquitous provision of services to users[1].Also, for the European TelecommunicationsInstitute (ETSI), the Next Generation NetworksNGN, is a concept for defining and deployingnetworks, which due to their formal separationinto different layers and use of open interfaces,offers service providers and operators a platformwhich can evolve in a step-by-step manner tocreate, deploy, and manage innovativeservices[ll ].In general, NGN can be viewed as all IP(Internet Protocol) or packet-based integratednetworks with a number of characteristics. NGNdoes not only cover network characteristics butalso service characteristics which provide newopportunities to network operators, serviceproviders, communications manufacturers andusers. On the architecture level, NGN providesan open architecture by uncoupling applicationsand networks, and allowing them to be offeredseparately. In this context, the applications canbe developed independently regardless of thenetwork platforms being used. With an openarchitecture, standardization becomesincreasingly important, but this allows networkoperators to choose the best products availableand a new application can be implemented in amuch shorter period time than for the legacyNetworks such as PSTN for example[7]. Also,third parties, can participate and developapplications and services for end users. Serviceproviders, may package one or moreapplications into a service offering orapplications may be utilized by users on a peerto-peer basis.The NGN architecture is commonly structuredaround the following four major layers (Planes)of technology[l 2]:
• Access Layer• Transport Layer• Control layer• Service/Application layer
its defined scope. The Architectural highlightsofTISPAN NGN Release 1 include[10]:o Support for SIP-based and non-SIP-based
applicationso IP Multimedia Subsystem (IMS) for
conversational SIP-based applicationso PSTN Emulation Subsystem for supporting
PSTN/ISDN services over NGNo Access agnosticismo Support for complex commercial modelso Roadmap to fixed/mobile convergence
based on IMSo Reuse and collaboration with other
Standards Development Organizations(SDO), including 3GPP, DSL Forum and theMulti-Service Forum (MSF)
The TISPAN Release 1 architecture is based onthe 3GPP IMS Release 6 architecture.Different solutions/network architectures can betaken into account for a smooth transition fromexisting network infrastructures (PSTN/PLMN)towards NGN. But a typical NGN network willbe based first on a softswitch solutionarchitecture that can separate call control fromthe physical bearer and service platform fromcall control[8]. Figure 3 summarizes a typicalNGN architecture.
sCWaS-anaLna .......y. As- ~l&u.itHI$4f_. ss- Soft:rwlle~ MRs-~j,RnoI,Il'U Set,"" TOW- Trunkana GIt~.,.AGYI- A((~n p1eW'l"
Figure 3: Typical NGN Architecture
III. Migration Strategy to NGN
NGN deployment worldwide is still at an earlystage, though some Telecom service providersincluding incumbent Operators, are in theprocess of finalizing their plans for deploymentofNGN in their networks. Others, like KPN (inNetherlands), or BT ("BT21 'CN project") havealready announced an all-embracingreconstruction of their network[10]. This islikely to be implemented in a phased manner
starting with core network and then for accessnetwork and finally service provision. Networkmigration to NGNs should adopt the concept of"Evolution" rather than "Revolution". Evolutionto NGN is a process in which parts of theexisting networks are replaced or upgraded tothe corresponding NGN components providingsimilar or better functionality, while maintainingthe services provided now by the originalnetwork. The lTV recommended that To deploythe NGN, complete replacement of existingnetworks is not considered to be either advisableor possible. So, a phased approach should beconsidered for evolution of existing networks toNGN."[13]. The evolution of networks to NGNsmust allow for the continuation of, andinteroperability with, existing networks while inparallel, enabling the implementation of newcapabilities. NGN deployment strategies willexperience a long process, which can be dividedinto several phases like building up the IPbearer network (the IP/MPLS Core Network),migrate PSTN, extend broadband access, delivermultimedia services and finally realize the FixMobile Convergence (FMC) .
In fact, two strategies have been considered forsuch migration. The first, is called overlayStrategy (Revolutionary) where a new NGNnetwork is deployed in parallel with the existingtraditional switched network. The second one, iscalled Replace Strategy (Evolutionary) whichconsist of replacing legacy Networks withNGN. Each has its advantages anddisadvantages.Our proposed solution, belongs to theevolutionary strategy, where we replace thelegacy network gradually by a new NGNnetwork. Obviously, the PSTN is considered tobe the prime candidate for such evolution toNGN [15].
IV. Generic Solution ArchitectureOverview
Most of the Telecom Operators divide theirPSTN in Regions or Domains based on theNetwork topology, Operation & Maintenance(O&M) needs, and the geographicalTransmissions connectivity. Each region/domainhas certain number of Subscribers, Local
DomalnfN
exchanges, Transit Exchanges, Transmissionsrings, etc",) [2]. Our solution builds on the sameconcept of region or domains, and assumes that,the network is divided in N domains. Theproposed solution is a SoftSwitch based solutionthat aims to build a new NGN network (physicaltopology) and migrate the legacy network(PSTN). The migration starts first with Class4the transit exchanges (TEX), then Class5 (LE).The solution can be evolved in the future to afull IMS/TISPAN architecture. The solutionassumes the existence of an aggregation, Edge,and backbone IP/MPLS network capable ofhandling the traffic generated with the adequateQuality of Service, and the required interfaceswhich are mainly based on Ethernet. Figure 4outlines the proposed solution architecture.
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Figure4:Proposed NGN Architecture
The solution Architecture consists of:
o Two Softswitches per Domain.
o Two Signaling Gateways
o Two Media Resource Server perDomain.
o Each Class4 switch (Tandemfunction) is replaced by TrunkingGateway (TGW).
o Class5 switches are replaced byAccess Gateway (AGW) based on thenumber of subscribers to be migrated.
All NGN NEs have carrier class reliabilitydesign (1+1 redundancy). The softswitch, also
known as Media Gateway Controllers (MGC),Call Servers (CS), and Call Agents, is the coredevice in the NGN [8]. The Softswitch handlescall control, signaling functions, and interactswith Application Servers (AS) to provideservices that are not directly hosted onSoftswitch. The Media gateway (MGW) usedhere as Trunking gateway (TGW), residesbetween the circuit switched (CS) network andthe IP network. It converts TDM Traffic to IPmedia flow. The MGW can connect withdevices, such as the PSTN exchange, privatebranch exchange (PBX), access network devicesand base station controller (BSC). A SignalingGateway (SGW) provides seamless signalingbetween the IP and TDM Networks, under thecontrol of the Soft switch[7]. Media ResourceServer (MRS) Under the control of theSoftSwitch, provides medium resource to packetnetwork. Unlike the traditional peripherals basedon circuit technology, MRS is directly based onpacket technology and eliminates the mediastream conversion between TDM and IP,resulting in high quality medium flow on IP.Access Gateway (AGW) acts as the line sideinterface to the core IP network and connectssubscribers with analogue subscriber access,integrated services digital network (ISDN)subscriber access, V5 subscriber access, PABXand digital subscriber line (xDSL) access.
The proposed solution, covers most of Telecomoperators requirements for a softswitch-basedcall control NGN platform, it can be used formigrating and evolving PSTN network into aNGN infrastructure, and provides PSTNEmulation System (PES). In the long run, theproposed network can evolve into a full IMSsolution achieving the operator's ultimate goalofproviding Fixed Mobile Convergence (FMC).
The solution assumes the existence of anaggregation, edge, and backbone IP/MPLSnetwork capable of handling the trafficgenerated with the adequate QoS, and therequired interfaces, which are mainly based onEthernet for connectivity to the NGN networkelements. In general terms, the TGWs and theSoft Switches connect to the co-locatedaggregation nodes of the IP/MPLS network.
psrn
The proposed solution also is designed on a 1+1Configuration in addition to the dual homingprinciple as illustrated in figure 5, andsummarized as follows:o The TGW is connected to Two carrier class
(Real Time traffic) Provider edge router(PE) in the IP core network.
o Soft Switch (A) and Soft Switch (B) supportbackup role for each other
o If Soft Switch (A) fails, TGW(X) wouldregister to Soft Switch (B) automatically.
o The switching process is transparent tosubscribers, and there is no change in theload of Trunks.
o Soft Switches detect the status of the peerend switch by checking the heart beatsignals.
._---_._---_._-_._._._._._._._._._---_._._._._._._._--._._-_._._._._._-_._._._._._._._._._.,NGN Domain # X
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Heart bit - • - Signaling Traffic - Bearer Traffic -
Figure 5: Dual homing of the NGN Solution
The dual homing solution is a network reliabilitysolution proposed to reduce the operation risksince, the risk of having larger capacity ofsoftswitch is very high where a larger number ofservices can be interrupted, revenue are lossbeside the heavy penalties for service by localauthority. Architecturally speaking, redundancyis again the often approach of choice. Systemreliability is addressed by implementingredundant systems often operating insynchronization but geographically separated
with redundant, diversely routed links providingthe interconnection.
v. Capacity Planning
Successful deployment of Next GenerationNetworks depends on a cost effective designand planning of the target network. Identifyingthe location, the number ofNGN NEs, and thelink capacity between NGN NE is the crucialpart in any Network planning and dimensioningprocess. In our solution, we start by dividing thelegacy Network in domains (regions), then wedeploy one pair of softswitch , one signalingGateway, and one Media Resource Server pereach domain. In addition to that, the TGWreplace the Class4 switches on a one to one basis.LEs are replaced by AGW based on the numberof subscribers to be moved. Some operators areusing the Multiservice Access Node (MSAN) asan AGW. Once the location and the nodes areidentified, the required capacity of the links(bandwidths) can be calculated/dimensioned.Figure 6 outlines the main links betweendifferent NGN NEs, and their correspondingbandwidth (B1 to B6).
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Figure 6: B/W links between NGN NEs
For link capacity between NGN element, BusyHour Call Attempt (BHCA), and The CallAttempts Per Second (CAPS) have to be definedfirst:
BHCA= Total Number of users X User traffic(Erl) X 36001Average Call Duration.CAPS= Total Number of Subscriber or TrunksX Traffic in Busy Houri Average Call Duration.B1= Bandwidth (BW) between the twoSoftswitches used for of SIP-T protocol.B1= CAPS X Number of Messages for each callX (Number of payload bytes for each message+ Number of overhead Bytes for each message)X 8 bits I Bandwidth Redundancy factor (BRF),for carrier Grade class the BRF= is taken as70% or 0.7.For back-up softswitch, each ISUP call needs 9SIP-T messages, and 600 byte for each message.So, total byte number of message is 9 X 600 =5400 bytes. Total byte number of networkoverhead is 9 X 66 = 594 bytes. So, signalingbandwidth between the two Soft Switches is:Bl= CAPS X (5400+594) X 8 170%.B2= BW between SS and SGW (Bandwidth ofSIGTRAN M3UA Protocol).B2 = CAPS X Number of messages for each callX (Number of payload bytes for each message +Number of overhead Bytes for each message) X8 bits IBRF. (Generally, the average number ofmessages for the ISUP call of each PSTN is 6;the number of messages for each V5 call is 21;the number of messages that each PRA callneeds is 33).B3= BW between the SS and the MRS(Bandwidth of H.248 Protocol). For MRS, itadopts MGCP protocol. Each call needs 8messages, and average 73 byte per message. So,total byte number of message is 8X73 = 584byte. Total byte number of network overhead is8X66 = 528 bytes.B3 = CAPS X Number of messages for each callX (Number of payload bytes for each message +Number of overhead Bytes for each message) X8 bits IBRF.B4= BW between SS and TGW (Bandwidth ofH.248 Protocol).B4 = CAPS X Number of messages for each callX (Number of payload bytes for each message +Number of overhead Bytes for each message) X8 bits I BRF.B5= BW between SS and AGW (Bandwidth ofH.248 Protocol) .AGW CAPS= Number of Subs X Avg. Traffic
per Sub I Average Holding time
B5 = CAPS X Number of messages for each callX (Number of payload bytes for each message +Number of overhead Bytes for each message) X8 bits I Bandwidth Redundancy factor.B6= BW between TGW and IP Core(Bandwidth between IP core and TGW).B6 = [Total Number of trunks X AverageTraffic per Trunk (Erl) X Payload flowbandwidth (the service)] IBRF.Hence, the Bandwidth required between TGWand the core IP Network (PE) is depending onthe Total Number of Trunks managed by eachTGW.
VI. Case Study: Saudi Telecom Network
Saudi Telecom Company (STC) is theincumbent Telecom operator in the Kingdom ofSaudi Arabia, and one of the largest TelecomService provider in the Middle East. STC isconsidered as a technology smart followercompany. It has recognized the need formigrating its network infrastructure to an NGNplatform that can support the provision of moreadvanced services efficiently and costeffectively to its customers. STC has decided tostart migrating its PSTN network to NGN, inorder to construct a unified service platform tomeet fixed & mobile customer's requirement inthe near future, and to increase revenue andcustomer's loyalty. The same solution presentedin this paper has been adopted and it's in therollout phase now. Figure 7 outlines the STCNGN architecture.
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Figure7: STC NGN Network Architecture
VII. Conclusion
NON deployment is still at an early stage,though some Communications serviceproviders, including incumbent Operatorshave started migrating their legacy networksto NON. The salient driver behind thismigration is to reduce the costs of buildingand operating a number of separatenetworks. In this paper, we have presented asoftswitch based solution, that can be usedto migrate the PSTN to NON. Also theNetwork capacity planning (Location,Number of NON NEs, and signaling Linkcapacity ) has been presented. Also, theproposed solution has been adopted bySaudi Telecom company.
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