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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.
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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).
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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
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96 Y. Ouyang and M.H. Fallah
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)
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Evolving core networks from GSM to UMTS R4 version 97
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
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98 Y. Ouyang and M.H. Fallah
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
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Evolving core networks from GSM to UMTS R4 version 99
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)
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100 Y. Ouyang and M.H. Fallah
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)
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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
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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