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    Evolving core networks from GSM to UMTS R4 version Ye Ouyang ... Evolving core networks from GSM to UMTS R4 version Ye Ouyang ... Document Transcript

    • Int. J. Mobile Network Design and Innovation, Vol. 3, No. 2, 2009 93 Evolving core networks from GSM to UMTS R4 version Ye Ouyang and M. Hosein Fallah* Howe School of Technology Management, Stevens Institute of Technology, Hoboken, NJ 07030, USA E-mail: youyang@stevens.edu E-mail: hfallah@stevens.edu *Corresponding author Abstract: More than 217 UMTS licenses have been issued by June 2007. Mobile operators, especially those with GSM legacy networks, prefer UMTS R4 technique to evolve their existing 2G GSM networks. UMTS R4 technique provides a smooth path to bridge legacy TDM-based network to an IP-based soft-switched network. This paper describes the basic architecture and topology of UMTS R4 core network and introduces two options in network planning: flat structure or layered structure. To propose an evolution path, the paper then suggests a ‘three- layer structure’ solution to seamlessly converge UMTS R4 core network with legacy GSM core network. The proposed solution approach achieves the all-IP vision and is capable of convergence with IMS and EPC. Keywords: GSM; universal mobile telecommunications system; UMTS; soft-switch; SS; core network; CN; circuit switch; media gateway; MGW; MSC server; MSCS; IP multimedia subsystem; IMS; evolved packet core; EPC; mobile network design. Reference to this paper should be made as follows: Ouyang, Y. and Fallah, M.H. (2009) ‘Evolving core networks from GSM to UMTS R4 version’, Int. J. Mobile Network Design and Innovation, Vol. 3, No. 2, pp.93–102. Biographical notes: Ye Ouyang is a PhD student in the Telecommunications Management Program at Stevens Institute of Technology. His research interest is in communications network technologies and services, focused on communications network planning, convergence, evolution and techno-economic analysis. He has extensive experience in planning, design and implementation of 2-4G networks. He worked in Starent Networks and ZTE Corporation, dimensioning the first nationwide GSM core network for Ethiopia and UMTS core network for Pakistan and Lybia. He holds an MS in System Engineering Management from Tufts University and an ME and a BE in Control Engineering and Information Engineering from Southeast University. M. Hosein Fallah is an Associate Professor of Technology Management at Stevens Institute of Technology in New Jersey. His research interest is in the area of innovation management with a focus on the telecommunications industry. Prior to joining Stevens, he was the Director of Network Planning and Systems Engineering at Bell Laboratories. He has over 30 years of experience in the areas of systems engineering, product/service realisation, software engineering, project management and R&D effectiveness. He holds a BS in Engineering from AIT and MS and PhD in Applied Science from the University of Delaware. 1 Introduction while addressing their 3G deployment requirements, and will not be a ‘forklift’ upgrade from legacy facilities. Over the past 20 years, the way people communicated, Radio access domain is a primary concern of the stayed informed and entertained has changed dramatically. UMTS deployment strategy, as it is closely coupled with the The technical changes in mobile networks are always mobile operators’ most valued asset: spectrum. However, revolutionary, generation by generation, and the deployment equally important, the core network (CN) is also playing of universal mobile telecommunications system (UMTS) an essential role in enhancing mobility, service control, is no exception. The transition from second-generation (2G) efficient use of mobile network resources and a seamless to 3G took several years and we expect the same for the evolution from 2G to 3G/4G. Therefore, the network transition from 3G to 4G. The requirements of smooth evolution calls for a migration to a soft-switch (SS) CN transition drive mobile operators to look for strategies and with a ‘flat’, all-IP and simplified architecture and open solutions that will enhance their existing GSM networks, interfaces which interwork with non 3rd Generation Partnership Project (3GPP) mobile networks. Copyright © 2009 Inderscience Enterprises Ltd.
    • 94 Y. Ouyang and M.H. Fallah Mobile operators are looking for a best network (MSC) into mobile switching centre server (MSCS or structure to maximise quality of service for users and to MSS) and media gateway (MGW). This is also the minimise the impact on legacy networks. Therefore, a physical embodiment of SS in UMTS R4. The other three new challenge for the UMTS operators is: how to most separations and the common service logic will be achieved efficiently and smoothly evolve their legacy CN to UMTS in UMTS R5 network with the introduction of IMS. and IP multimedia subsystem (IMS)? With this question, In UMTS R4, the separation of control from bearer several considerations need to be addressed: achieves the all-IP vision of NGN and moves the time • Simplified topology division multiplexing (TDM) portion into the edge of the network. That is why, from technical aspect, most mobile A simplified and flattened CN with a possible operators who are operating GSM networks select UMTS reduction of network entities (NEs) involved in service R4, but not R99 as the target network to evolve their legacy processing and data transport enhances the performance facilities. of UMTS. Hence, it is very important to study the convergence of • Evolved backhaul UMTS R4 CN with legacy GSM CN. To answer the With the deployment of UMTS, the transport backhaul question posed earlier and based on four considerations for becomes a key consideration that many resources are network evolution and three characters of NGN, we propose achieving after the fact. It is critical to deploy a CN a network architecture of circuit switched (CS) domain for solution that is flexible enough to offer smooth mobile operators for evolving their legacy networks to migration from centralised (longer backhaul) to UMTS R4 CN. The proposed architecture of UMTS CN distributed (shorter backhaul) CN nodes. consists of three possible layers: local network, tandem network and gateway network. • Enhanced performance Section 2 will give an overview of UMTS CN; Section 3 Obviously, the intent of UMTS is to improve the summarises the current network structure; Sections 4, 5 and performance and efficiency of the legacy GSM 6 describe our proposed UMTS CN structure layer by layer; network. In order to realise the full potential of and finally, Section 7 presents a summary and conclusions. UMTS, it will be important to deploy an appropriate CN structure that can meet the demands generated by increased mobile services and a growing subscriber 2 UMTS CN architecture base, including increasing network capacity As discussed by Britvic (2004) and Vrabel et al. (2007), the requirements, thousands of call volumes per second UMTS network consists of three primary portions: CN, and significant throughput. radio access network (RAN) and user equipment (UE). The • Smooth migration RAN provides all of the functions related to the radio network. CN is the heart of the mobile communication When mobile operators upgrade their networks networks. It processes all the voice and data services in the to UMTS or further to IMS, they need to ensure UMTS core system and also implements the switching and compatibility of the new network with legacy facilities. routing functions with external networks. CN provides This requires the UMTS CN structure to avoid a capabilities to achieve the essential network functions such ‘forklift’ upgrade and address 2G/3G network as: mobility management, call and session control, and requirements, while at the same time, being used billing and security. Logically, the CN can be further for evolution to IMS or evolved packet core (EPC) classified into CS domain and packet switched (PS) domain. network. Shalak et al. (2004), Mishra (2003), Konstantinopoulou et Furthermore, the term SS initially came from the definition al. (2000), Harmatos (2002) and Hoikkanen (2007) of next generation networks (NGN) with a ‘pure IP’ proposed several solutions to plan GSM and UMTS core vision. NGN has ‘three separations and one common’ networks. The solution proposed in this paper is focused on characteristics: CS domain only to evolve the legacy CNs since the PS domain mainly stays the same in the network topology and • separation of call control and media bearer via H.248 the composition of NEs. and signalling transport (SIGTRAN) From 3GPP TS 23.002, 3GPP TS 25.401, 3GPP TS • separation of all features from call control via session 25.415 and Neruda and Bestak (2008), 3GPP has defined initiation protocol (SIP) many versions for UMTS standard, from the include R99 to • separation of subscriber database [home location R4, R5, R6, R7 and R8. R99 is the first version of UMTS. register (HLR) or HSS] from service logic via diameter CN in R99 is also composed of two domains: CS domain and PS domain, both of which remain the same as in GSM • common service logic for all service mechanisms with network in topology and NEs. From GPP TS 29.415, 3GPP subscriber portability. TS 25.413 and 3GPP TS 29.414, there are changes in RAN: The first ‘separation of call control and media bearer via node-Bs and radio network controllers (RNCs) are H.248 and SIGTRAN’, applied in UMTS Release 4 (R4), introduced to replace or co-exist with base stations (BTSs) can be achieved by dividing the mobile switching centre and base station controllers (BSCs).
    • Evolving core networks from GSM to UMTS R4 version 95 Figure 1 UMTS R4 CN architecture (see online version for colours) R4, the second version of UMTS technique, introduces UMTS radio networks as well as all existing interfaces with SS technology into CS domain in which bearer function is legacy network elements. separated from control function. The MSC in GSM/UMTS R99 version is split into two NEs in UMTS 4 version: MSCS and MGW. As a result, the logical separation of 3 Current network structure traffic bearer and call control is achieved based on this The flat (full meshed) structure is mostly selected in physical division. It also evolves the CS domain to an all-IP building the legacy 2G CNs by mobile operators whose structure and enables voice and SIGTRAN to be separated. network size is not large. However, the flat structure will no Compared to a single TDM bearer mode in R99, CS domain longer fit in the new environment with growing traffic in R4 supports various bearer modes: IP, ATM and TDM. volume and more new NEs. So, there are two options in From Figure 1, UMTS R4 CS domain consists of three planning the UMTS R4 networks: flat structure or layered NEs: the MSCS (or MSS) or gateway mobile switching structure. centre server (GMSCS); MGW or gateway media gateway (GMGW) and visitor location register (VLR) which is Figure 2 Flat structure of switching network (see online version physically integrated in MSCS. The HLR is a common for colours) entity that both CS domain and PS domain can access. UMTS R4 PS domain includes such NEs as serving GPRS support node (SGSN) and gateway GPRS support node (GGSN). Below is a short description of the NEs existed in CN domain of UMTS R4 CN. The core of the CN in UMTS R4, MSCS is a functional entity that implements mobile call service, mobility management, handover and other supplementary services. Due to the philosophy of separation of control function from bearer function in UMTS CN, it is actually the MGW that establishes call routes between mobile stations (MSs) via interface Mc. The MSCS also serves as an interface between UMTS and circuit switching networks such as public switched telephone network (PSTN) and integrated services digital network (ISDN). Furthermore, it also manages SS7, auxiliary radio resources and mobility management between RNS and CN. In addition, to establish The flat structure enables each NE to connect with every call routes to MSs, each MSCS needs to function as a other NE in the network either physically, for example, GMSCS. through leased lines, or logically, for example, through T1 An MGW in UMTS R4 implements bearer processing or E1. The flat structure (full meshed structure) is similar functions between different networks. It implements UMTS to the point to point structure in the internet, bypassing voice communication, multimedia service, CS domain data the tandem routes and communicating directly between service and interworking between PSTN and UMTS and two NEs. The flat structure possesses a feature of high between 3G and 2G networks. It also supports GSM and redundancy and simple connectivity. But its network
    • 96 Y. Ouyang and M.H. Fallah topology will become more and more complicated when the Table 1 compares the flat with layered structure. Based on network keeps expanding. For example: if 51 2G visiting the same traffic, the layered structure, compared to flat MSCs are distributed in the legacy GSM network, there are structure, saves the link resources for local exchanges via at least 51 * 50 = 2,550 routes to be configured to deliver traffic converging and forwarding. The flat structure has a the signalling message or carry the TDM-based traffic for lower CAPEX due to no investment on the tandem network the non-local calls. With a flat structure, the number of elements. However, the reduced CAPEX does not guarantee connection for the least route is given by: to offset its higher cost of OPEX. Least route number = N × ( N − 1) (1) Table 1 Comparison between flat and layered structure where N is the number of network elements in the network. Characters Flat network Layered network Preferred The layered structure does not require direct links Network Depends on Depends on No between all the network elements, but provides some processing subscriber subscriber difference tandem elements (class 2–4 switches) in the tandem layer to capability number or traffic number or traffic connect all the local exchanges in the local access layer. model model In this scheme, the traffic or signalling routing between the Number of N * (N – 1) K*N+K+K* Layered NEs takes place either directly if they are connected or links (K – 3)/2 structure indirectly through the tandem NEs (3GPP TS 25.415). The Data Heavy work Easy to operate; Layered layered structure simplifies the network topology and configuration load; one individual structure reduces the link resources. In a UMTS R4 CN, tandem and element revised, revisal mobile switching centre server (TMSCS) or call mediation maintenance all others revised node (CMN) can be built in a tandem layer to converge and CAPEX No investment More CAPEX Not clear forward the signalling messages such as bear independent on tandem layer, on tandem call control (BICC) message or integrated services but link budget network digital network user part (ISUP) message between two is higher elements, saves link budget MSCSs. Similarly, tandem MGW, if needed, may also be provisioned to forward the traffic between any two visiting OPEX Maintenance Simplified Layered MGWs. For example: if a pair of TMSCS or CMN are built scale larger; structure; lower structure higher OPEX OPEX in the tandem layer t to connect the 51 local MSCS in a UMTS R4 network, there are at least 2 * 51 + 1 = 103 links (< 51 * 50 = 2,550 links) configured to forward the BICC signalling messages. With a layered structure, the least route 4 The architecture of local network number is calculated as follows: 4.1 Integration mode ⎧K × N K = 1 The integration mode in Figure 4 has been widely applied ⎪ Least route number = ⎨K × N + 1 K = 2 (2) in GSM CNs. It strictly complies with the administrative ⎪K × N + K + K × (K − 3) × 2 K ≥ 3 division. All the NEs achieve localised deployment. The ⎩ MSC, HLR, short message centre (SMC) and BSC are where N is the number of network elements in the network distributed at the same physical location. In signalling (tandem elements excluded). K is the number of tandem transmission, ISUP protocol is adopted. The signalling elements. messages delivered between MSCs; mobile application part (MAP) is delivered between MSC and HLR and between Figure 3 Layered structure of switching network (see online HLR and SMC. In voice transmission, TDM-based E1 or T1 version for colours) is selected to carry traffic between MSCs. Figure 4 Integration mode of local network (see online version for colours)
    • Evolving core networks from GSM to UMTS R4 version 97 In introducing R4-based NEs into the legacy local network, networks in the end layer. Through interface Nc, the MSCSs the easiest way is to directly replace the current 2G NE communicate with each other via ISUP messages carried by (MSC) with the new 3G NEs (MSCS and MGW). We do TDM links. The interface Mc between MSCS and MGW not need to modify the legacy network topology, but only is the only portion that has achieved the IP transport via need to replace the NEs in the network and allocate more the newly built IP private network which may extend to link resources to accommodate increased traffic in 3G interface Nc or Nb according to the respective plans of phase. However, it does not help achieve the all-IP target wireless carriers. Figure 6 shows the topology of TDM in evolving the legacy networks since the transmission option which achieves IP transport in interface Mc. medium is still based on TDM not IP or ATM in the integrated mode. Figure 6 TDM option of detached mode (see online version for colours) 4.2 Detached mode An alternative is detached mode which centralises the MSCS while distributes the MGW locally. The MSCS is detached with MGW in the end layer and MSCS up into the tandem layer. Since MGW and MSCS are newly deployed into the legacy network, it is more convenient, compared to the existing links in legacy network, to achieve IP connections between new MGW and MSCS. Consequently, the centralised deployment of MSCS enables the wireless carriers to first set up a new IP-based private network for the interface Mc between MSCS and MGW. If detached mode is adopted, the evolution to all-IP actually starts from interface Mc. As per Figure 5, two options are available for the interface Mc between MSCS and MGW: IP/ATM over E1/T1 or IP/ATM private network. If current TDM transmission resources are still sufficient, it is suggested to select IP/ATM over current E1/T1 transmission network as an interim step before the IP/ATM private network is available for the interface Mc. Meanwhile, IP/ATM over Figure 7 IP option of detached mode (see online version for colours) E1/T1 also does not impact the ongoing development of IP/ATM private network to interface Mc. Figure 5 Private IP network for interface Mc between MSS and MGW (see online version for colours) There are three options for voice bearer in the detached mode: TDM, IP or ATM bearer. The wireless carriers make their decisions to select a bearer medium for their networks With IP or ATM option, MSCSs are centralised, while by considering such factors as current TDM resources, MGWs are distributed to deploy respectively. Due to the physical bearer preference, the schedule to deploy IP/ATM availability of IP or ATM bearer, the MSCSs are able to private network for MSCS and MGW, CAPEX and OPEX. apply BICC to substitute ISUP protocol, in which a circuit With TDM option, MSCSs are moved upward to locate identification code (CIC) is specific to TDM, in interface in the tandem layer, while MGWs are distributed into local Nc via the IP private network. Defined by ITU-T Q1901
    • 98 Y. Ouyang and M.H. Fallah Series Q and ITU-T Q1902.1 to 5 Series Q, BICC is an actual tandem layer existing in the network. However, developed to be interoperable with any type of bearer. It has the visiting MSCSs and MGWs in the end layer play the no knowledge of the specific bearer technology which is tandem function as well. referenced in the binding information (Cho and Kim, 2008). The layered structure is preferred if either the network Either IP or ATM option achieves the non-TDM (IP or size or the O&M load exceeds the threshold of flat structure. ATM) transport in interface Mc, Nc and interface Nb which An appropriate opportunity to separate the tandem layer enables the MGWs from different local networks to deliver from end layer is at the time of building the IP-based the voice traffic via IP/ATM transport. The only exception SS MGW in the legacy network. The tandem NEs are exists in interface E between the MGW and legacy 2G advised to be provisioned with the deployment of IP MSC, which only allows TDM bearer for voice delivery, (soft-switching) based MGW at the end layer. but does not support the evolution to IP or ATM bearer. The CMN in Figure 9 relays BICC protocol. From Figure 7 shows the topology with IP option which achieves Van Deventer et al. (2001), the CMN may be useful in a IP transport in interface Mc, Nc and Nb. large-scale BICC network with a large number of interface serving nodes (ISNs), where the CMN would route the Table 2 Summaries of the integration and detached mode BICC messages. In this paper, the ISN denotes MGWs. Therefore, it is concluded that, with the independent tandem Detached Detached Integration mode mode with mode with NEs provisioned in tandem layer, the signalling links TDM option IP/ATM option between MSCS and CMN (or TMSCS) are available to deliver BICC messages for the long distance (non-local) call Networking Integrated Detached Detached characters deployment with deployment deployment triggered by a soft-switching MGW in the local network. TDM bearer with TDM with IP Tandem MGWs are also built with CMNs or TMSCS to bearing voice bearing voice forward the IP voice media stream between two visiting CAPEX and Typical SS Separation Higher cost MGWs. OPEX architecture in control from in initial CMN can be co-configured with TMSCS in markets local layer; high bearer; investment, with relatively fewer soft-switching MGWs or with lower integrability balanced but benefits traffic in the local layer. CMN can also be independent from capability in for the TMSCS when the number or the traffic of MGWs keeps local level; long-term growing. Based on the independent structure that CMN optimised resource separates from TMSCS, the independent CMN is only distribution responsible for relaying BICC message, while TMSCS is R4 Change to IP Change to IP One step responsible for delivering ISUP messages only. To achieve evolution interfaces; huge interfaces evolution this independent structure, extra signalling links and modifications routes are configured between the new CMN and visiting to the current MGWs in local networks. The MGWs from different network topology local networks, but under the same MSCS have direct QoS No difference Not much Differentiated connections. The MGWs from different local networks and with existing 2G difference service under different MSCS communicate with each other via the network with existing (DiffServ); tandem MGW in the tandem layer. 2G network multiprotocol Below is a summary for the scenarios in which label switching (MPLS) signalling travels through CMN node. If the called party registered in the IP-based MGW in market B while the calling party belongs to the IP-based MGW in market A, the signalling messages will be 5 The architecture of tandem network forwarded via the independent CMN 2 in market B. The The tandem network is responsible for converging signalling routes follow this path: local MSCS in market A and forwarding the voice traffic and signalling messages to CMN 1 in market A to CMN 2 in market B to local between two visiting MGWs, two MSCSs or two MSCs. MSCS in market B. The voice traffic goes through this way: As mentioned in Section 3, the tandem network can be local MGW in market A to tandem MGW to local MGW in organised into either a flat structure when the network market B. The red and blue curve in Figure 9 denotes the size (represented by the number of NEs in the network) is signalling and traffic path respectively for this scenario. small or a layered structure if the network volume keeps If the called party belongs to the TDM-based MSC in expanding. In addition, another factor impacting the tandem market B while the calling party registered in the IP-based network structure is the operation and maintenance (O&M). MGW in market A, the routing will be pointed to TMSC It is suggested that the mobile operators estimate the server 2 which handles ISUP messages. The signalling allowable tolerance of flat networking structure from both routes follow this path: local MSCS in market A to CMN 1 the O&M aspect and network size aspect. in market A to TMSCS 2 in market B to local TDM MSC in Based on flat structure in Figure 8, any two MSCSs or market B. The voice traffic goes through this way: local visiting MGWs have direct connection. There is no longer
    • Evolving core networks from GSM to UMTS R4 version 99 MGW in market A to tandem MGW to local MGW in If the calling party registered in IP-based MGW in market B. market B while the called party registered in TDM- If the calling party registered in IP-based MGW in based MSC in market A, the signalling routes follow this market B while the called party registered in IP-based path: local MSCS in market B to CMN 2 in market B to MGW in market A, the signalling routes follow this path: TMSCS 1 in market A to local TDM MSC in market A. The local MSCS in market B to CMN 2 in market B to CMN 1 voice traffic goes through this way: local MGW in market B in market A to MSCS in market A. The voice traffic goes to tandem MGW to local MSC in market A. through this way: local MGW in market B to tandem MGW to local MGW in market A. Figure 8 Flat structure of tandem network (see online version for colours) Figure 9 Tandem network structure (see online version for colours)
    • 100 Y. Ouyang and M.H. Fallah 6 The architecture of the gateway network and converges the gateway functions used to play by the individual gateway NEs such as GMSCS, GMGW and GSS In 2G, 2.5G and R99 phases, the gateway structure at distributed at the borders of UMTS and NGN network. CN side is not so complicated that the gateway NEs such as This option helps the network actually achieve the IP gateway mobile switching centre (GMSC) stands at the structure in the gateway layer. Compared to the separated border of network to exchange MAP message with gateway structure in Figure 10, the integrated gateway HLR and ISUP message with visiting MSC in the CN or centre in Figure 11 provides the integrated signalling exchange ISUP and telephone user part (TUP) messages process capability to support both IP and SS7 signalling, with PSTN side. Meanwhile, regarding the voice integrated media intercommunication capability to complete transmission, GMSC is also the gateway to exchange the the conversion between multiple voice media streams TDM-based G.711 voice stream between GSM side and such as G.711, AMR, G.729 and G.723, and integrated PSTN side. Gateway NEs in UMTS R4 network, split into interconnection capability to provide multiple interfaces to GMSCS and GMGW, and are also physically distributed at different access networks such as TDM interface for PSTN the border of the CN to achieve the functions of signalling and GSM network, ATM or IP interface for UMTS and conversion and traffic transition between PLMNs, between NGN networks. PLMN and PSTN, between PLMN and IMS or between As per the integrated structure shown in Figure 11, it PLMN and NGN. How to deploy the gateway NEs which is suggested that integrated the SS server in the gateway interconnect with NGN, IMS and PSTN network, to some centre supports the multiple signalling protocol conversion extent, decides whether or not the wireless carrier is able to function. SIP-I/T, as an extension of SIP protocol, is achieve fixed mobile convergence (FMC). advised to apply between ISS server and SS. SIP I/T is also Take the interconnection between UMTS and the basic protocol in IMS, so it helps the NGN side to NGN/PSTN as an example: in Figure 10, a pair of GMSCS converge with the IMS network provisioned from UMTS and GMGW provisioned at the border of UMTS CN R5. On the other side between ISS and visiting MSCS, connects with a pair of SS and PSTN switch at NGN side BICC protocol is applied to comply with the same protocol via a back to back format in which the medium to carry adopted in interface Nc between MSCSs in UMTS network. traffic and transit signalling is still TDM-based E1 or T1. Therefore, the most important requirement on the ISS server Therefore, this option does not achieve the all-IP structure is to support the protocol conversion between SIP I/T and on the gateway level. Figure 10 also displays the typical BICC/ISUP. The integrated gateway is required to achieve position of gateway NEs in UMTS and NGN networks. the codec conversion between different voice formats and The alternative is to build an integrated soft-switch between different video formats. For example: it needs to (ISS) gateway centre to achieve direct intercommunication support the conversion of G.711/G.729/G.723/AMR voice between UMTS and NGN networks. Including ISS server stream and H.263 video stream. and integrated MGW, the ISS gateway centre integrates Figure 10 Gateway NEs between UMTS and NGN network (see online version for colours)
    • Evolving core networks from GSM to UMTS R4 version 101 Figure 11 Integrated gateway structure for UMTS and NGN network (see online version for colours) 7 Conclusions and future work a radical way, is not an optimal strategy for the mobile operators to evolve their legacy networks. As discussed in Mobile operators, especially those with GSM legacy this paper, the layered design philosophy does not mean to networks, need to evolve their existing 2G GSM networks place the different NEs in a hierarchy in the network at to an all-IP network. This transition is a process that needs once, but to help the mobile operators steadily transit from to be managed effectively over a period of time. The paper traditional circuit-based network to IP-based network step first gave an overview of UMTS network including its by step and layer by layer. The proposed architecture with architecture and topology, and then described two network its evolution path, as discussed in this paper, has been structures for legacy network evolution: flat structure and partially deployed in some tier 1 mobile operators in layered structure. The pros and cons of the two structures Asia and the Pacific area. We are continuing our study of are compared so that mobile operators can adopt an converging with IMS and SAE networks. appropriate strategy to plan the architecture of their UMTS CN. Based on the theoretical considerations, the paper proposed a three-layer structural network for CS domain References of UMTS R4 CN. A detail description of the architecture, topology and intercommunication of local layer, tandem 3GPP TS 23.002, Technical Specification Group Services and layer and gateway layer is provided. Systems Aspects; Network Architecture. The current literature is focused more on RAN and 3GPP TS 25.401, Technical Specification Group Radio Access overlooks the CN. A lot of design philosophy and proposed Network: UTRAN Overall Description. architecture have been applied in the plan of UMTS radio 3GPP TS 25.413, Technical Specification Group Radio Access network. However, not much effort, however, has been Network: UTRAN Iu interface Radio Access Network Application Part (RANAP) Signaling. made on how to evolve UMTS CN. This may be explained by two facts that CN in either logical or physical structure is 3GPP TS 25.415, Technical Specification Group Radio Access Network: UTRAN Iu interface user plane protocols. more complicated than RAN and the internal throughput or traffic in CN may vary by different vendors’ NEs. This 3GPP TS 29.414, Technical Specification Group Core Network and Terminals: Core Network Nb Data Transport and paper explored both RAN and CN to provide a proposed Transport Signaling. solution for evolving a legacy network to an all-IP-based 3GPP TS 29.415, Technical Specification Group Core Network network with IMS and system architecture evolution (SAE) and Terminals: Core Network Nb Interface User Plane capable. Protocols. The discussion of NGN, FMC or voice over IP (VOIP) Britvic, V. (2004) ‘Steps in UMTS network design’, Electro- eventually boils down to ‘pure IP’ or ‘all-IP’, which is the Technical Conference, 2004. MELECON 2004. Proceedings vision of every wireless or wire line operator. The evolution of the 12th IEEE Mediterranean, 12–15 May, Vol. 2, from TDM to IP is a lengthy process, but never just a pp.461–464. simple task of replacing the circuit-based NEs in the legacy network with new IP-based NEs. Consequently, forklift, as
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