IJMNDI030204 FALLAH

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Int. J. Mobile Network Design and Innovation, Vol. 3, No. 2, 2009 93

Copyright © 2009 Inderscience Enterprises Ltd.

Evolving core networks from GSM to UMTS R4version

Ye Ouyang and M. Hosein Fallah*

Howe School of Technology Management,

Stevens Institute of Technology,Hoboken, NJ 07030, USA

E-mail: [email protected]

E-mail: [email protected]

*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 SoutheastUniversity.

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

Over the past 20 years, the way people communicated,

stayed informed and entertained has changed dramatically.The technical changes in mobile networks are always

revolutionary, generation by generation, and the deployment

of universal mobile telecommunications system (UMTS)

is no exception. The transition from second-generation (2G)

to 3G took several years and we expect the same for the

transition from 3G to 4G. The requirements of smooth

transition drive mobile operators to look for strategies and

solutions that will enhance their existing GSM networks,

while addressing their 3G deployment requirements, and

will not be a ‘forklift’ upgrade from legacy facilities.

Radio access domain is a primary concern of theUMTS deployment strategy, as it is closely coupled with the

mobile operators’ most valued asset: spectrum. However,

equally important, the core network (CN) is also playing

an essential role in enhancing mobility, service control,

efficient use of mobile network resources and a seamless

evolution from 2G to 3G/4G. Therefore, the network

evolution calls for a migration to a soft-switch (SS) CN

with a ‘flat’, all-IP and simplified architecture and open

interfaces which interwork with non 3rd Generation

Partnership Project (3GPP) mobile networks.



94 Y. Ouyang and M.H. Fallah

Mobile operators are looking for a best network

structure to maximise quality of service for users and to

minimise the impact on legacy networks. Therefore, a

new challenge for the UMTS operators is: how to most

efficiently and smoothly evolve their legacy CN to UMTS

and IP multimedia subsystem (IMS)? With this question,

several considerations need to be addressed:

• Simplified topologyA simplified and flattened CN with a possible

reduction of network entities (NEs) involved in service

processing and data transport enhances the performance

of UMTS.

• Evolved backhaul

With the deployment of UMTS, the transport backhaul

becomes a key consideration that many resources are

achieving after the fact. It is critical to deploy a CN

solution that is flexible enough to offer smooth

migration from centralised (longer backhaul) to

distributed (shorter backhaul) CN nodes.

• Enhanced performance

Obviously, the intent of UMTS is to improve the

performance and efficiency of the legacy GSM

network. In order to realise the full potential of

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

base, including increasing network capacity

requirements, thousands of call volumes per second

and significant throughput.

• Smooth migration

When mobile operators upgrade their networks

to UMTS or further to IMS, they need to ensure

compatibility of the new network with legacy facilities.This requires the UMTS CN structure to avoid a

‘forklift’ upgrade and address 2G/3G network

requirements, while at the same time, being used

for evolution to IMS or evolved packet core (EPC)

network.

Furthermore, the term SS initially came from the definition

of next generation networks (NGN) with a ‘pure IP’

vision. NGN has ‘three separations and one common’

characteristics:

• separation of call control and media bearer via H.248

and signalling transport (SIGTRAN)

• separation of all features from call control via sessioninitiation protocol (SIP)

• separation of subscriber database [home location

register (HLR) or HSS] from service logic via diameter

• common service logic for all service mechanisms with

subscriber portability.

The first ‘separation of call control and media bearer via

H.248 and SIGTRAN’, applied in UMTS Release 4 (R4),

can be achieved by dividing the mobile switching centre

(MSC) into mobile switching centre server (MSCS or

MSS) and media gateway (MGW). This is also the

physical embodiment of SS in UMTS R4. The other three

separations and the common service logic will be achieved

in UMTS R5 network with the introduction of IMS.

In UMTS R4, the separation of control from bearer

achieves the all-IP vision of NGN and moves the time

division multiplexing (TDM) portion into the edge of the

network. That is why, from technical aspect, most mobile

operators who are operating GSM networks select UMTS

R4, but not R99 as the target network to evolve their legacy

facilities.

Hence, it is very important to study the convergence of

UMTS R4 CN with legacy GSM CN. To answer the

question posed earlier and based on four considerations for

network evolution and three characters of NGN, we propose

a network architecture of circuit switched (CS) domain for

mobile operators for evolving their legacy networks to

UMTS R4 CN. The proposed architecture of UMTS CN

consists of three possible layers: local network, tandem

network and gateway network.

Section 2 will give an overview of UMTS CN; Section 3summarises the current network structure; Sections 4, 5 and

6 describe our proposed UMTS CN structure layer by layer;

and finally, Section 7 presents a summary and conclusions.

2 UMTS CN architecture

As discussed by Britvic (2004) and Vrabel et al. (2007), the

UMTS network consists of three primary portions: CN,

radio access network (RAN) and user equipment (UE). The

RAN provides all of the functions related to the radio

network. CN is the heart of the mobile communication

networks. It processes all the voice and data services in the

UMTS core system and also implements the switching and

routing functions with external networks. CN provides

capabilities to achieve the essential network functions such

as: mobility management, call and session control, and

billing and security. Logically, the CN can be further

classified into CS domain and packet switched (PS) domain.

Shalak et al. (2004), Mishra (2003), Konstantinopoulou et

al. (2000), Harmatos (2002) and Hoikkanen (2007)

proposed several solutions to plan GSM and UMTS core

networks. The solution proposed in this paper is focused on

CS domain only to evolve the legacy CNs since the PS

domain mainly stays the same in the network topology and

the composition of NEs.

From 3GPP TS 23.002, 3GPP TS 25.401, 3GPP TS

25.415 and Neruda and Bestak (2008), 3GPP has definedmany versions for UMTS standard, from the include R99 to

R4, R5, R6, R7 and R8. R99 is the first version of UMTS.

CN in R99 is also composed of two domains: CS domain

and PS domain, both of which remain the same as in GSM

network in topology and NEs. From GPP TS 29.415, 3GPP

TS 25.413 and 3GPP TS 29.414, there are changes in RAN:

node-Bs and radio network controllers (RNCs) are

introduced to replace or co-exist with base stations (BTSs)

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

SS technology into CS domain in which bearer function is

separated from control function. The MSC in GSM/UMTSR99 version is split into two NEs in UMTS 4 version:

MSCS and MGW. As a result, the logical separation of

traffic bearer and call control is achieved based on this

physical division. It also evolves the CS domain to an all-IP

structure and enables voice and SIGTRAN to be separated.

Compared to a single TDM bearer mode in R99, CS domain

in R4 supports various bearer modes: IP, ATM and TDM.

From Figure 1, UMTS R4 CS domain consists of three

NEs: the MSCS (or MSS) or gateway mobile switching

centre server (GMSCS); MGW or gateway media gateway

(GMGW) and visitor location register (VLR) which is

physically integrated in MSCS. The HLR is a common

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, italso manages SS7, auxiliary radio resources and mobility

management between RNS and CN. In addition, to establish

call routes to MSs, each MSCS needs to function as a

GMSCS.

An MGW in UMTS R4 implements bearer processing

functions between different networks. It implements UMTS

voice communication, multimedia service, CS domain data

service and interworking between PSTN and UMTS and

between 3G and 2G networks. It also supports GSM and

UMTS radio networks as well as all existing interfaces with

legacy network elements.

3 Current network structure

The flat (full meshed) structure is mostly selected in

building the legacy 2G CNs by mobile operators whose

network size is not large. However, the flat structure will no

longer fit in the new environment with growing traffic

volume and more new NEs. So, there are two options in

planning the UMTS R4 networks: flat structure or layered

structure.

Figure 2 Flat structure of switching network (see online versionfor colours)

The flat structure enables each NE to connect with every

other NE in the network either physically, for example,

through leased lines, or logically, for example, through T1

or E1. The flat structure (full meshed structure) is similar

to the point to point structure in the internet, bypassing

the tandem routes and communicating directly between

two NEs. The flat structure possesses a feature of high

redundancy and simple connectivity. But its network




topology will become more and more complicated when the

network keeps expanding. For example: if 51 2G visiting

MSCs are distributed in the legacy GSM network, there are

at least 51 * 50 = 2,550 routes to be configured to deliver

the signalling message or carry the TDM-based traffic for

the non-local calls. With a flat structure, the number of

connection for the least route is given by:

( 1) Least route number N N = × − (1)

where N is the number of network elements in the network.

The layered structure does not require direct links

between all the network elements, but provides some

tandem elements (class 2–4 switches) in the tandem layer to

connect all the local exchanges in the local access layer.

In this scheme, the traffic or signalling routing between the

NEs takes place either directly if they are connected or

indirectly through the tandem NEs (3GPP TS 25.415). The

layered structure simplifies the network topology and

reduces the link resources. In a UMTS R4 CN, tandem

mobile switching centre server (TMSCS) or call mediation

node (CMN) can be built in a tandem layer to converge and

forward the signalling messages such as bear independentcall control (BICC) message or integrated services

digital network user part (ISUP) message between two

MSCSs. Similarly, tandem MGW, if needed, may also be

provisioned to forward the traffic between any two visiting

MGWs. For example: if a pair of TMSCS or CMN are built

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

number is calculated as follows:

1

1 2

( 3) 2 3

K N K

Least route number K N K

K N K K K K

× =⎧⎪

= × + =⎨⎪ × + + × − × ≥⎩

(2)

where N is the number of network elements in the network

(tandem elements excluded). K is the number of tandem

elements.

Figure 3 Layered structure of switching network (see onlineversion for colours)

Table 1 compares the flat with layered structure. Based on

the same traffic, the layered structure, compared to flat

structure, saves the link resources for local exchanges via

traffic converging and forwarding. The flat structure has a

lower CAPEX due to no investment on the tandem network

elements. However, the reduced CAPEX does not guarantee

to offset its higher cost of OPEX.

Table 1 Comparison between flat and layered structure

Characters Flat network Layered network Preferred

Network

processing

capability

Depends on

subscriber

number or traffic

model

Depends on

subscriber

number or traffic

model

No

difference

Number of

links N * ( N – 1) K * N + K + K *

( K – 3)/2

Layered

structure

Data

configuration

and

maintenance

Heavy work

load; one

element revised,

all others revised

Easy to operate;

individual

revisal

Layered

structure

CAPEX No investment

on tandem layer, but link budget

is higher

More CAPEX

on tandemnetwork

elements, saves

link budget

Not clear

OPEX Maintenance

scale larger;

higher OPEX

Simplified

structure; lower

OPEX

Layered

structure

4 The architecture of local network

4.1 Integration mode

The integration mode in Figure 4 has been widely applied

in GSM CNs. It strictly complies with the administrative

division. All the NEs achieve localised deployment. The

MSC, HLR, short message centre (SMC) and BSC are

distributed at the same physical location. In signalling

transmission, ISUP protocol is adopted. The signalling

messages delivered between MSCs; mobile application part

(MAP) is delivered between MSC and HLR and between

HLR and SMC. In voice transmission, TDM-based E1 or T1

is selected to carry traffic between MSCs.

Figure 4 Integration mode of local network (see online version

for colours)




In introducing R4-based NEs into the legacy local network,

the easiest way is to directly replace the current 2G NE

(MSC) with the new 3G NEs (MSCS and MGW). We do

not need to modify the legacy network topology, but only

need to replace the NEs in the network and allocate more

link resources to accommodate increased traffic in 3G

phase. However, it does not help achieve the all-IP target

in evolving the legacy networks since the transmission

medium is still based on TDM not IP or ATM in the

integrated mode.

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 privatenetwork 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

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 andMGW (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

by considering such factors as current TDM resources,

physical bearer preference, the schedule to deploy IP/ATM

private network for MSCS and MGW, CAPEX and OPEX.

With TDM option, MSCSs are moved upward to locate

in the tandem layer, while MGWs are distributed into local

networks in the end layer. Through interface Nc, the MSCSs

communicate with each other via ISUP messages carried by

TDM links. The interface Mc between MSCS and MGW

is the only portion that has achieved the IP transport via

the newly built IP private network which may extend to

interface Nc or Nb according to the respective plans of

wireless carriers. Figure 6 shows the topology of TDM

option which achieves IP transport in interface Mc.

Figure 6 TDM option of detached mode (see online version

for colours)

Figure 7 IP option of detached mode (see online versionfor colours)

With IP or ATM option, MSCSs are centralised, while

MGWs are distributed to deploy respectively. Due to the

availability of IP or ATM bearer, the MSCSs are able to

apply BICC to substitute ISUP protocol, in which a circuit

identification code (CIC) is specific to TDM, in interface

Nc via the IP private network. Defined by ITU-T Q1901




Series Q and ITU-T Q1902.1 to 5 Series Q, BICC is

developed to be interoperable with any type of bearer. It has

no knowledge of the specific bearer technology which is

referenced in the binding information (Cho and Kim, 2008).

Either IP or ATM option achieves the non-TDM (IP or

ATM) transport in interface Mc, Nc and interface Nb which

enables the MGWs from different local networks to deliver

the voice traffic via IP/ATM transport. The only exception

exists in interface E between the MGW and legacy 2G

MSC, which only allows TDM bearer for voice delivery,

but does not support the evolution to IP or ATM bearer.

Figure 7 shows the topology with IP option which achieves

IP transport in interface Mc, Nc and Nb.

Table 2 Summaries of the integration and detached mode

Integration mode Detached mode with

TDM option

Detached mode with

IP/ATM option

Networking

characters

Integrated

deployment withTDM bearer

Detached

deploymentwith TDM

bearing voice

Detached

deploymentwith IP

bearing voiceCAPEX and

OPEX

Typical SS

architecture in

local layer; high

integrability

Separation

control from

bearer;

balanced

capability inlocal level;

optimised

resource

distribution

Higher cost

in initial

investment,

but benefits

for thelong-term

R4

evolution

Change to IP

interfaces; hugemodifications

to the current

network topology

Change to IP

interfaces

One step

evolution

QoS No difference

with existing 2G

network

Not much

difference

with existing

2G network

Differentiated

service

(DiffServ);

multiprotocollabel switching

(MPLS)

5 The architecture of tandem network

The tandem network is responsible for converging

and forwarding the voice traffic and signalling messages

between two visiting MGWs, two MSCSs or two MSCs.

As mentioned in Section 3, the tandem network can be

organised into either a flat structure when the network size (represented by the number of NEs in the network) is

small or a layered structure if the network volume keeps

expanding. In addition, another factor impacting the tandem

network structure is the operation and maintenance (O&M).

It is suggested that the mobile operators estimate the

allowable tolerance of flat networking structure from both

the O&M aspect and network size aspect.

Based on flat structure in Figure 8, any two MSCSs or

visiting MGWs have direct connection. There is no longer

an actual tandem layer existing in the network. However,

the visiting MSCSs and MGWs in the end layer play the

tandem function as well.

The layered structure is preferred if either the network

size or the O&M load exceeds the threshold of flat structure.

An appropriate opportunity to separate the tandem layer

from end layer is at the time of building the IP-based

SS MGW in the legacy network. The tandem NEs are

advised to be provisioned with the deployment of IP

(soft-switching) based MGW at the end layer.

The CMN in Figure 9 relays BICC protocol. From

Van Deventer et al. (2001), the CMN may be useful in a

large-scale BICC network with a large number of interface

serving nodes (ISNs), where the CMN would route the

BICC messages. In this paper, the ISN denotes MGWs.

Therefore, it is concluded that, with the independent tandem

NEs provisioned in tandem layer, the signalling links

between MSCS and CMN (or TMSCS) are available to

deliver BICC messages for the long distance (non-local) call

triggered by a soft-switching MGW in the local network.

Tandem MGWs are also built with CMNs or TMSCS to

forward the IP voice media stream between two visitingMGWs.

CMN can be co-configured with TMSCS in markets

with relatively fewer soft-switching MGWs or with lower

traffic in the local layer. CMN can also be independent from

TMSCS when the number or the traffic of MGWs keeps

growing. Based on the independent structure that CMN

separates from TMSCS, the independent CMN is only

responsible for relaying BICC message, while TMSCS is

responsible for delivering ISUP messages only. To achieve

this independent structure, extra signalling links and

routes are configured between the new CMN and visiting

MGWs in local networks. The MGWs from different

local networks, but under the same MSCS have direct

connections. The MGWs from different local networks and

under different MSCS communicate with each other via the

tandem MGW in the tandem layer.

Below is a summary for the scenarios in which

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

forwarded via the independent CMN 2 in market B. The

signalling routes follow this path: local MSCS in market A

to CMN 1 in market A to CMN 2 in market B to local

MSCS in market B. The voice traffic goes through this way:

local MGW in market A to tandem MGW to local MGW in

market B. The red and blue curve in Figure 9 denotes thesignalling and traffic path respectively for this scenario.

If the called party belongs to the TDM-based MSC in

market B while the calling party registered in the IP-based

MGW in market A, the routing will be pointed to TMSC

server 2 which handles ISUP messages. The signalling

routes follow this path: local MSCS in market A to CMN 1

in market A to TMSCS 2 in market B to local TDM MSC in

market B. The voice traffic goes through this way: local




MGW in market A to tandem MGW to local MGW in

market B.

If the calling party registered in IP-based MGW in

market B while the called party registered in IP-based

MGW in market A, the signalling routes follow this path:

local MSCS in market B to CMN 2 in market B to CMN 1

in market A to MSCS in market A. The voice traffic goes

through this way: local MGW in market B to tandem MGW

to local MGW in market A.

If the calling party registered in IP-based MGW in

market B while the called party registered in TDM-

based MSC in market A, the signalling routes follow this

path: local MSCS in market B to CMN 2 in market B to

TMSCS 1 in market A to local TDM MSC in market A. The

voice traffic goes through this way: local MGW in market B

to tandem MGW to local MSC 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)




6 The architecture of the gateway network

In 2G, 2.5G and R99 phases, the gateway structure at

CN side is not so complicated that the gateway NEs such as

gateway mobile switching centre (GMSC) stands at the

border of network to exchange MAP message with

HLR and ISUP message with visiting MSC in the CN or

exchange ISUP and telephone user part (TUP) messages

with PSTN side. Meanwhile, regarding the voicetransmission, GMSC is also the gateway to exchange the

TDM-based G.711 voice stream between GSM side and

PSTN side. Gateway NEs in UMTS R4 network, split into

GMSCS and GMGW, and are also physically distributed at

the border of the CN to achieve the functions of signalling

conversion and traffic transition between PLMNs, between

PLMN and PSTN, between PLMN and IMS or between

PLMN and NGN. How to deploy the gateway NEs which

interconnect with NGN, IMS and PSTN network, to some

extent, decides whether or not the wireless carrier is able to

achieve fixed mobile convergence (FMC).

Take the interconnection between UMTS and

NGN/PSTN as an example: in Figure 10, a pair of GMSCS

and GMGW provisioned at the border of UMTS CN

connects with a pair of SS and PSTN switch at NGN side

via a back to back format in which the medium to carry

traffic and transit signalling is still TDM-based E1 or T1.

Therefore, this option does not achieve the all-IP structure

on the gateway level. Figure 10 also displays the typical

position of gateway NEs in UMTS and NGN networks.

The alternative is to build an integrated soft-switch

(ISS) gateway centre to achieve direct intercommunication

between UMTS and NGN networks. Including ISS server

and integrated MGW, the ISS gateway centre integrates

and converges the gateway functions used to play by the

individual gateway NEs such as GMSCS, GMGW and GSS

distributed at the borders of UMTS and NGN network.

This option helps the network actually achieve the IP

structure in the gateway layer. Compared to the separated

gateway structure in Figure 10, the integrated gateway

centre in Figure 11 provides the integrated signalling

process capability to support both IP and SS7 signalling,

integrated media intercommunication capability to complete

the conversion between multiple voice media streams

such as G.711, AMR, G.729 and G.723, and integrated

interconnection capability to provide multiple interfaces to

different access networks such as TDM interface for PSTN

and GSM network, ATM or IP interface for UMTS and

NGN networks.

As per the integrated structure shown in Figure 11, it

is suggested that integrated the SS server in the gateway

centre supports the multiple signalling protocol conversion

function. SIP-I/T, as an extension of SIP protocol, is

advised to apply between ISS server and SS. SIP I/T is also

the basic protocol in IMS, so it helps the NGN side to

converge with the IMS network provisioned from UMTSR5. On the other side between ISS and visiting MSCS,

BICC protocol is applied to comply with the same protocol

adopted in interface Nc between MSCSs in UMTS network.

Therefore, the most important requirement on the ISS server

is to support the protocol conversion between SIP I/T and

BICC/ISUP. The integrated gateway is required to achieve

the codec conversion between different voice formats and

between different video formats. For example: it needs to

support the conversion of G.711/G.729/G.723/AMR voice

stream and H.263 video stream.

Figure 10 Gateway NEs between UMTS and NGN network (see online version for colours)




Figure 11 Integrated gateway structure for UMTS and NGN network (see online version for colours)

7 Conclusions and future work

Mobile operators, especially those with GSM legacy

networks, need to evolve their existing 2G GSM networks

to an all-IP network. This transition is a process that needs

to be managed effectively over a period of time. The paper

first gave an overview of UMTS network including its

architecture and topology, and then described two network

structures for legacy network evolution: flat structure and

layered structure. The pros and cons of the two structures

are compared so that mobile operators can adopt an

appropriate strategy to plan the architecture of their UMTSCN. Based on the theoretical considerations, the paper

proposed a three-layer structural network for CS domain

of UMTS R4 CN. A detail description of the architecture,

topology and intercommunication of local layer, tandem

layer and gateway layer is provided.

The current literature is focused more on RAN and

overlooks the CN. A lot of design philosophy and proposed

architecture have been applied in the plan of UMTS radio

network. However, not much effort, however, has been

made on how to evolve UMTS CN. This may be explained

by two facts that CN in either logical or physical structure is

more complicated than RAN and the internal throughput or

traffic in CN may vary by different vendors’ NEs. This

paper explored both RAN and CN to provide a proposedsolution for evolving a legacy network to an all-IP-based

network with IMS and system architecture evolution (SAE)

capable.

The discussion of NGN, FMC or voice over IP (VOIP)

eventually boils down to ‘pure IP’ or ‘all-IP’, which is the

vision of every wireless or wire line operator. The evolution

from TDM to IP is a lengthy process, but never just a

simple task of replacing the circuit-based NEs in the legacy

network with new IP-based NEs. Consequently, forklift, as

a radical way, is not an optimal strategy for the mobile

operators to evolve their legacy networks. As discussed in

this paper, the layered design philosophy does not mean to

place the different NEs in a hierarchy in the network at

once, but to help the mobile operators steadily transit from

traditional circuit-based network to IP-based network step

by step and layer by layer. The proposed architecture with

its evolution path, as discussed in this paper, has been

partially deployed in some tier 1 mobile operators in

Asia and the Pacific area. We are continuing our study of

converging with IMS and SAE networks.

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