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									  IP Technology in WCDMA/GSM core networks
  Heino Hameleers and Christer Johansson

Mobility and the Internet, the two most dynamic forces in communications                                    ning—the future will be even more spec-
today, meet in the design and implementation of the mobile core network.                                    tacular given that the number of people con-
Support for new end-user services and a common transport technology                                         nected to the Internet will continue to in-
are the main drivers of integration of IP technology into our systems. The                                  crease and that GPRS and WCDMA mobile
IP multimedia application plays a special role in providing these new end-                                  networks will enable connectivity virtually
                                                                                                            everywhere and at any time with any device.
user services. IP transport technology addresses the vision of multi-
                                                                                                            A new form of interactive communication
service backbone networks, based on a single network layer technology.                                      behavior is emerging from the combination
Ericsson provides complete solutions and products to support deploy-                                        of different media with applications that are
ment of new IP-based services and transport networks. Moreover,                                             randomly invoked by multiple users and
Ericsson’s flexible core network architecture allows operators to address                                   end-systems.
these drivers in an independent way.
   The main parts of this article describe how the requirements for the two                                 Drivers of IP technology
main drivers for IP technology are met in the mobile core network. Paying
special attention to support for the IP multimedia application, the authors                                 There are two main arguments that drive
describe how support for IP applications is implemented. They then                                          the integration of IP technology into mo-
describe how IP transport technology can be supported, including site                                       bile core networks:
                                                                                                            • support for (new) IP applications to gen-
configurations and specific issues like quality-of-service and network
                                                                                                               erate (new) revenues; and
redundancy.                                                                                                 • a common transport technology to reduce
                                                          Introduction                                      The entire mobile telecommunications in-
                                                                                                            dustry is funded out of the end-user’s pock-
                                                          The marriage of mobile communication and          et. Therefore, to ensure future growth in the
                                                          the Internet has the potential to produce a       industry, end-user value needs to be en-
                                                          revolution whose scope far outpaces that as-      hanced. Service and application offerings are
                                                          sociated with the advent of the personal          the prime drivers of the entire network and
                                                          computer (PC). In parallel with the fantas-       terminal evolution. End-to-end IP solutions
                                                          tic worldwide growth of mobile subscrip-          must have access to the dynamic IP appli-
                                                          tions, the fixed Internet and its service of-     cations industry. This is the main prerequi-
                                                          ferings have grown at a rate far exceeding all    site for success in the world of third-
                                                          expectations. And this is only the begin-         generation mobile networks. In this world,
                                                                                                            visualization and real-time behavior are fun-
                                                                                                            damental components for bridging distance
                                                                                                            and for giving the end-user experience of
                                                                                                            “presence” a broader spectrum of human
Figure 1                                                                                                       IP is gradually becoming a dominating
Different core network functions in the layered network architecture.                                       transport technology thanks to recent ad-
                                                                                                            vances in optics and routing technology and
                                                                                                            the impact that these have had on price/per-
                                                                                                            formance. When combined with other key
                                                                 networks                                   technologies, such as IP-based virtual pri-
                                                                                                            vate networks (VPN), IP enables a new gen-
                                                                                                            eration of advanced multiservice networks.
                                                                                                            The use of a common infrastructure based
                                            Control layer
                        MSC      GMSC                                                                       on a single technology simplifies network
                        server   server    server            CSCF           BGCF    MGCF                    implementation and operation and helps re-
          Circuit-Ł                                                                      IM subsystem and
          switched                                                    MRFC               packet-switched
                                                                                                            duce costs. Ericsson’s core network archi-
          domain                                    HSS                                  domain             tecture and product solutions allow opera-
                                                                                                            tors to address these two main drivers of IP
                                                                        MGW                                 technology individually.

                                                                                                            Core network overview
                                                               SGSN                                         The mobile core network is at the heart of
                                          Connectivity layer
                                                                                                            present-day mobile communication net-
  User equipment and                                                                 Service and external   works. It provides support for network fea-
  access networks                                                                    networks               tures and telecommunications services, in-

14                                                                                                                              Ericsson Review No. 1, 2002
cluding essential functions such as session        It also supports the migration to new trans-
and call control, charging, mobility, and se-      port technologies, since the upper layers are
curity. Because of its central role in the over-   independent of the transport technology de-
all architecture, the mobile core network in-      ployed in the connectivity layer. The layered
terfaces with and coordinates other network        architecture also allows different, optimized
elements, including user equipment, radio          technologies to be deployed in the connec-
access networks and service networks.              tivity layer (which is payload-processing-
                                                   intensive) instead of in the control layer
Core network architecture                          (which is transaction-oriented). Figure 1
Ericsson’s core network solution is based on       shows how the different core network func-
the separation of functionality into a control     tions fit into the layered architecture.
layer, a connectivity layer, and an applica-          In the circuit-switched domain, the mo-
tion layer. The control layer hosts network        bile services center (MSC), gateway MSC
control servers that are in charge of call or      (GMSC) and transit services switching cen-
session set-up, modification, and release.         ter (TSC) servers are part of the control layer.
The control servers might also handle mo-
bility management, security, charging and
interworking functions that relate to exter-
nal networks at the control plane level.
                                                     BOX A, ABBREVIATIONS
   The connectivity layer hosts routers,
switches, signaling gateways, media gate-
ways and other user-plane functions. Its             3GPP      Third-generation Partnership Pro-      ISUP     ISDN user part
routers and switches provide transport ca-                     gram                                   LAN      Local area network
                                                     AAA       Authentication, authorization and      LER      Label edge router
pabilities for traffic on the control and user                 accounting                             LSP      Label switched path
planes. The media gateways facilitate inter-         AF        Assured forwarding                     MAP      Mobile application part
working on the user plane. This includes             AMR       Adaptive multirate                     MGCF     Media gateway control function
interworking between different transmis-             ATM       Asynchronous transfer mode             MGW      Media gateway
sion technologies and media formats.                 AUC       Authentication center                  MPLS     Multiprotocol label switching
                                                     BE        Best effort                            MRF      Media resource function
   The interface between the control layer           BGCF      Breakout gateway control function      MRFC     MRF control part
and the connectivity layer mainly consists           BGP       Border gateway protocol                MRFP     MRF processing part
of gateway control protocols. The network            BICC      Bearer independent call control        MSC      Mobile services center
control servers use these interfaces to ma-          BSC       Base station controller                O&M      Operation and maintenance
                                                     CN        Core network                           OSA      Open service architecture
nipulate media gateway resources in the              CPP       Cello packet platform                  OSPF     Open shortest path first
connectivity layer.                                  CS        Circuit-switched                       PBN      Public backbone network
   The application layer, which is imple-            CSCF      Call session control function          PCM      Pulse code modulation
mented as part of the service network, hosts         CSE       Customized applications for mobile     P-CSCF   Proxy CSCF
application and content servers. There are                     network-enhanced logic (CAMEL)         PHB      Per-hop behavior
                                                               service environment                    PLMN     Public land mobile network
two interfaces between the core network and          DNS       Domain name server                     PS       Packet-switched
the service network: a horizontal interface          DS        Differentiated services                QoS      Quality of service
and a vertical interface.                            DSCP      DS code point                          RAN      Radio access network
   The horizontal interface between the core         ECMP      Equal cost multipath routing           RFC      Request for comment
                                                     EF        Expedited forwarding                   RNC      Radio network controller
network and the service network refers to            FTP       File transfer protocol                 RTSP     Real-time streaming protocol
regular peer-to-peer or client/server mode of        GGSN      Gateway GPRS support node              SCS      Service capability server
operation for typical end-user applications,         GMSC      Gateway MSC                            S-CSCF   Serving CSCF
such as Web browsing, e-mail and                     GPRS      General packet radio service           SDH      Synchronous digital hierarchy
audio/video services. These applications are         GSM       Global system for mobile communi-      SDP      Session description protocol
                                                               cation                                 SGSN     Serving GPRS support node
normally invoked by an end-user but might            GSN       GPRS support node                      SGW      Signaling gateway
also be invoked by an application server.            GSTN      General switched telephone network     SIP      Session initiation protocol
   The vertical interface allows applications        GTP       GPRS tunneling protocol                SLF      Subscriber location function
that reside on specific application servers to       HLR       Home location register                 SSF      Service switching function
                                                     HSS       Home subscriber server                 STM      Synchronous transfer mode
complement or modify the normal proce-               HTTP      Hypertext transfer protocol            TDM      Time-division multiplexing
dures for setting up calls or sessions through       I-CSCF    Interrogating CSCF                     TSC      Transit services switching center
the core network. These applications inter-          IETF      Internet Engineering Task Force        UE       User equipment
work with the core network through a set of          IMS       IP multimedia subsystem                UMTS     Universal mobile telecommunica-
standardized application program interfaces          IP        Internet protocol                               tions system
                                                     IPSec     IP security                            VLAN     Virtual LAN
(API).                                               ISC       IP multimedia service control          VPN      Virtual private network
   The layered architecture allows each layer        ISDN      Integrated services digital network    WAN      Wide area network
to evolve independently and in pace with             ISP       Internet service provider              WAP      Wireless application protocol
the evolution of the market and technology.                                                           WDM      Wavelength division multiplexing

Ericsson Review No. 1, 2002                                                                                                                        15
                                                                                        SGSN                    GGSN

                                                                    networks                                                   External
                                                                                                Core network

                                                   Application                                                                                   Application
                                                    TCP/UDP                                                                                       TCP/UDP
                                                       IP                                                         IP      IP        IP     IP        IP
                                                     PDCP        PDCP    GTP     GTP     GTP                     GTP                               PDCP
                                                      RLC         RLC   UDP     UDP     UDP                     UDP     L2         L2      L2       RLC
Figure 2
                                                      MAC        MAC      IP      IP      IP     IP      IP       IP                                MAC
End-to-end protocol stack for an IP appli-
                                                       L1          L1   L1/L2   L1/L2   L1/L2   L1/L2   L1/L2   L1/L2   L1         L1      L1        L1
cation that runs on top of a WCDMA/GSM
packet-switched network.                               UE          UTRAN           SGSN         IP backbone        GGSN           IP backbone       UE

                                                  The corresponding media gateway belongs                   Two separate IP layers can be identified.
                                                  to the connectivity layer.                             The upper layer (drawn in blue) denotes the
                                                     In the packet-switched domain, both the             IP application layer, which runs between the
                                                  serving GPRS support node (SGSN) and the               user equipment (UE) and an external entity
                                                  gateway GPRS support node (GGSN) are                   with which the UE is communicating. This
                                                  considered to be part of the connectivity              would typically be an IP application server
                                                  layer—they contain some control function-              or another UE.
                                                  ality, but the dominant functionality lies in             The lower layer (drawn in red) denotes the
                                                  providing IP connectivity.                             IP transport layer, which has only local sig-
                                                     With 3GPP Release 5, one more                       nificance to the public land mobile network
                                                  “domain” is being added to the mobile core             (PLMN). The IP transport layer is needed to
                                                  network: the IP multimedia subsystem                   transport (control and user plane) traffic
                                                  (IMS). The principal network entities of the           within the mobile network. In this particu-
                                                  IMS are the                                            lar case, the layer is terminated at the
                                                  • call/session control function (CSCF);                GGSN, where the traffic leaves the PLMN;
                                                  • media gateway control function (MGCF);               routing is performed directly on the IP ap-
                                                  • breakout gateway control function                    plication layer. In other cases, IP
                                                     (BGCF);                                             application-layer traffic might continue to
                                                  • media resource function (MRF) control                be carried over an IP transport layer even
                                                     part, or MRFC;                                      after it leaves the GGSN.
                                                  • MRF processing part (MRFP);
                                                  • media gateway (MG); and
                                                  • signaling gateway (SG).
                                                                                                         Support for IP
                                                  The master subscriber database, called the             applications
BOX B, HISTORY OF IP MULTIMEDIA                   home subscriber server (HSS), is common to
STANDARDIZATION                                   the circuit-switched domain, the packet-               Role of the core network in providing
                                                  switched domain, and the IP multimedia                 IP applications
Support for IP multimedia applications in         subsystem.                                             With respect to the IP application layer, the
mobile core networks was first discussed in                                                              role of the mobile core network has tradi-
1999 in the 3G.IP forum. The 3G.IP forum          The two roles of IP                                    tionally been limited to providing a tunnel
was an industry consortium initially consist-
ing of eight of the main operators and ven-       The two roles that IP plays in the mobile              that allows the UE to communicate with an-
dors, including Ericsson. It had set itself the   core network are also expressed in the core            other IP host. This support is implemented
goal of defining an IP technology based           network protocol stacks. Figure 2 shows an             in the GPRS support nodes (GSN).3
architecture for the next generation of mobile    end-to-end protocol stack for an IP appli-                In general, IP applications are transpar-
networks that would support voice, data and
multimedia services.
                                                  cation running on top of a WCDMA/GSM                   ent to the core network. This is true for all
   This network architecture proposal was         packet-switched network. The traffic                   IP applications but one: the session initia-
brought into the 3GPP forum. 3GPP has             leaves the mobile core network at the                  tion protocol (SIP) application. The main
accepted the proposal and has since spent         GGSN. There are also other scenarios in                difference compared with other IP applica-
considerable effort to define a complete end-     which traffic from the GGSN remains in                 tions is the communication model. Most IP
to-end architecture, including solutions for
essential functions such as security, charging    the core network. However, for the sake of             applications target a client-server model.
and QoS. 3GPP selected SIP as the session         describing the roles of IP technology in the           The file transfer protocol (FTP) allows a
control protocol and has also mandated IPv6       mobile core network a simplified scenario              client to download files from a server, the
for IP multimedia applications.                   is used.                                               hypertext transfer protocol (HTTP) and

16                                                                                                                               Ericsson Review No. 1, 2002
wireless application protocol (WAP) allow            This approach limits the functional and          BOX C, IP MULTIMEDIA PROTOCOLS
clients to download electronic pages with         protocol dependency between the home and
content from a server, and the real-time          visited networks, thereby                           The session initiation protocol (SIP) has been
streaming protocol (RTSP) allows clients to       • minimizing the restrictions put on the            defined in IETF RFC 2543. It describes a way
“stream” content from a server. By contrast,         services that can be deployed in the home        of supporting session-based applications
SIP primarily targets a client-to-client com-        network; and                                     over IP networks that involve one or more
                                                                                                      participants. In mobile terms this means that
munication model.                                 • increasing the rate at which services can         it allows a mobile client to set up an IP ses-
   How is SIP supported in the mobile core           be deployed.                                     sion to another mobile client. An updated
network? The main parts are defined as the        In addition to the personalized services pro-       version of SIP is expected in 2002.
IP multimedia subsystem (IMS). Together           vided by the home network, the home and                 SIP “uses” the session description proto-
with the circuit-switched domain and the          the visited networks can each provide local         col (SDP) to describe the nature of the multi-
                                                                                                      media sessions—that is, which media are
packet-switched domain, the IMS builds the        services—however, these services are not            included in the session, in which format the
3GPP mobile core network.                         tied to the user profile in the home network.       media will be transported, and so on. SDP
                                                     Although this implies that control sig-          has been defined in IETF RFC 2327.
IMS: providing IP session control                 naling must always go through the home
To understand the IMS architecture, one           domain, the actual payload must not. Pay-
must first understand the basic concepts on       load is routed independently of control sig-
which the architecture has been built:            naling and can follow the optimal (shortest)
the home-visited-interworking architecture        path for efficient transmission and optimal
and functional entities, and services.            quality of service (QoS).
                                                     If the IMS is to interconnect with legacy
Home-visited-interworking                         networks, some functionality must be pro-
In second-generation mobile systems, such         vided to enable the systems to interwork. To
as GSM, services are provided by the PLMN         allow for the optimal user-plane transport
in which the subscriber is roaming. Person-       path (for example, by keeping the session in
alized service information is transferred         its own network as long as possible), this
from the home PLMN to the visited PLMN.           functionality can be placed in an inter-
This approach requires that both the home         working network outside of the home and
PLMN and the visited PLMN support the             visited networks. Home, visited and inter-
service to be provided. That is, the services     working networks can be physically differ-
that can be provided to the end-user repre-       ent networks or they can be implemented in
sent the lowest common denominator of             one and the same network.
what the home PLMN and the visited
PLMN can support.
   In the future, the differentiation between
network operators will be made at the ap-
plication and service levels, instead of at the
access and network levels. Roaming sub-
scribers will no longer be the exception, but       Figure 3
will have become the norm. This implies             The visited network provides connectivity to the home network.
that the provision of seamless services for
roaming subscribers is increasingly impor-                                                                          Services
tant.                                                                                                               Control
   Ericsson has proposed the concepts of home                                                                       Connectivity
and visited to describe the architecture of
modern communication networks. Home de-
                                                                                           Home network
notes user data and services, whereas visited                                              Session control
denotes connectivity and mobility. This im-                                                user control and
plies that the main task for the visited net-
work is to provide a subscriber with (mo-
bile) connectivity to the home network (Fig-
ure 3). The home network hosts user data,
session control and services.
   Acceptance of these concepts implies that
subscribers are always roaming in a visited
network. However, the services are con-
trolled from the home network, regardless
of which visited network the subscriber is                     Visited networks                                Interworking networks
roaming in.                                                    Connectivity and mobility                       Interworking

Ericsson Review No. 1, 2002                                                                                                                      17

                                           Visited network                                              Home network

                                             P-CSCF                                                               MRFC

                                                   SGSN              GGSN

                                                                                       MRFP                     Service and
                                                                                                                external IMS
                          User equipment                                                                        networks
                          and access

Figure 4
Basic IMS architecture.

                          Architecture and functional entities              tail records for input to the charging process.
                          The IP multimedia system is built around             The HSS, which is an evolution of the
                          the call/session control function, of which       home location register (HLR) and authenti-
                          there are three different kinds:                  cation center (AUC), holds the subscriber
                          • the interrogating CSCF (I-CSCF);                profile and keeps track of which core net-
                          • the proxy CSCF (P-CSCF); and                    work node is currently handling the sub-
                          • the serving CSCF (S-CSCF).                      scriber. It also supports subscriber authen-
                          The P-CSCF is the UE’s first point of con-        tication and authorization functions (AAA).
                          tact with the IMS. The P-CSCF forwards SIP           In networks with more than one HSS, the
                          messages received from the UE to a SIP serv-      subscriber location function (SLF) provides
                          er in the home network (and vice versa). The      information on the HSS that contains the
                          P-CSCF might also modify an outgoing re-          profile of a given subscriber.
                          quest according to a set of rules defined by         The media resource function (MRF),
                          the network operator (for example, address        which contains the functionality for manip-
                          analysis and potential modification).             ulating multimedia streams, supports
                             The I-CSCF function, which forms the en-       multiparty multimedia services, multi-
                          trance to the home network, hides the inner       media message playback and media conver-
                          topology of the home network from other           sion services. The Third-generation Part-
                          networks and provides flexibility for select-     nership Project (3GPP) has split the MRF
                          ing an S-CSCF.                                    into a control part (MRFC) and a processing
                             The S-CSCF performs the session control        part (MRFP). Figure 5 depicts a scenario in
                          services for the UE. This includes routing        which a multimedia session interworks with
                          originating sessions to external networks         a general switched telephone network
                          and routing terminating sessions to visited       (GSTN).
                          networks. The S-CSCF also decides whether            The BGCF selects the network in which
                          or not an application server is required to re-   the interworking is to be performed. If the
                          ceive information on an incoming SIP ses-         interworking is performed in the home net-
                          sion request to ensure appropriate service        work, the BGCF selects an MGCF. If the in-
                          handling. This decision is based on infor-        terworking is to be performed in another
                          mation received from the HSS (or other            network, the BGCF selects another BGCF
                          sources, such as an application server).          or an MGCF.
                             All CSCF functions can generate call de-          The MGCF provides interworking func-

18                                                                                                 Ericsson Review No. 1, 2002

                                                       Home network
                              HSS/SLF                  S-CSCF

                                          I-CSCF      MRFC                      GSTN interworking


                              SGSN          GGSN

                                                                 TRA                External GSTN
                                                                       MGW          networks        Figure 5
 User equipment                                                                                     A multimedia session interworks with a
 and radio access                                                                                   general switched telephone network
 networks                                                                                           (GSTN).

tionality between SIP session control sig-         besides providing real-time services (such as
naling from the IMS and ISUP/BICC call             video-conferencing), the IMS provides non-
control signaling from the external GSTN           real-time services (such as instant messag-
networks. It also controls the media gate-         ing).
way that provides the actual user-plane in-           Unlike second-generation mobile sys-
terworking functionality (for instance, for        tems (such as GSM), specific IMS services
converting between AMR- and PCM-coded              will not be standardized. The 3GPP is mere-
voice). The signaling gateway provides bear-       ly defining the architecture framework and
er interworking functionality for the control      service capabilities that can be used to build
signaling (ISUP/IP – ISUP/TDM).                    services. The actual services are implement-
                                                   ed on top of these capabilities by network
Services                                           vendors, operators or third parties.
The 3GPP is working to define the IMS ar-             In keeping with IETF principles, the end-
chitecture, but its concepts and protocols         points of the system contain considerable in-
were derived from the Internet Engineering         telligence for supporting services with
Task Force (IETF). The IMS enables the con-        little or no assistance from the network.
vergence of, and access to, voice, video, mes-     However, there are also scenarios in which
saging, data and Web-based technologies            the network provides value-added ser-
for the wireless user. The main tools that the     vices—for instance, to provide a presence
IMS provides to build these services are           service or to optimize the use of resources
• peer-to-peer addressing architecture for         for conferencing services.
   IP-based sessions;                                 To support these network-controlled or
• flexible integration of “any” type of media      network-assisted services, the 3GPP is
   into sessions;                                  defining an IMS service creation environ-
• integration with other IP applications,          ment. The S-CSCF and the HSS each feature
   such as RTSP, HTTP, and so on; and              one or more vertical service creation inter-
• end-to-end QoS charging and security ar-         faces, and the 3GPP is deliberating whether
   chitecture.                                     or not it will also include them in the MRFC.
Although the IMS mainly targets client-to-            Horizontal interfaces might also be con-
client services, its tools can be used to facil-   sidered. A horizontal interface would apply
itate or enhance client-server-based services,     to a scenario in which the S-CSCF routes
such as those found in gaming. Moreover,           control of the user plane to an external ap-

Ericsson Review No. 1, 2002                                                                                                                  19
                                                                                                   turned to the core network, the session is
                                                                      Core                         established to the terminating user equip-
                                                                     network                       ment. Example: a voice session with ses-
                                                                                                   sion forwarding.
                                             a)           Pure client-to-client services        d)The core network passes session control
                                                                                                   via a vertical interface to an application
                                                                                                   server. When session control has been re-
                                                         Core             Service
                                                                                                   turned to the core network, the session is
                                                        network           network                  passed to another application server.
                                                                                                   Example: a gaming session in which the
                                             b)       Pure client-to-server services               service network determines the closest
                                                                                                   gaming server.
                                                                                                e) The main session runs between user
                                                                                                   equipment. In addition, there may be one
                                                                   network                         or more sessions running between the user
                                                                                                   equipment and the service network. The
                                                                                                   user equipment coordinates these multi-
                                                                    Core                           ple sessions. Example: a voice session dur-
                                                                                                   ing which both clients view the same
                                                                                                   streaming video.
                                             c)       Assisted client-to-client services
                                                                                                Core network products
                                                                                                Given the complex functional architecture
                                                               Service                          of the IMS, careful consideration ought to
                                                                                                be given to the mapping between functions
                                                                                                and actual products. The result of imple-
                                                                                                menting each function as a separate physi-
                                                                Core                Service     cal box would be a complex system that is
                                                               network              network
                                                                                                extremely hard to operate.
                                                                                                  In all probability, the first commercial
                                             d)        Assisted client-to-server services       IMS products will be deployed on a small
                                                                                                scale. Therefore, cost-efficient, entry-level
                                                                                                solutions should exist that provide enough
                                                                     Service                    capacity to handle the initial IMS traffic. As
                                                                                                the market evolves, more flexible and pow-
                                                                                                erful configurations will also be required.
                                                                                                  The Ericsson IMS product line will fulfill
                                                                     network                    near-term and long-term requirements by
Figure 6                                                                                        implementing the IMS functions in an in-
Overview of five service execution scenar-                                                      tegrated yet modular way, by
                                             e)                   Multiservice
ios.                                                                                            • adhering to a horizontally layered archi-
                                                                                                • providing scalability from entry-level (in-
                                                                                                  tegrated) to high-end (distributed) con-
                                              plication server. In that case, the application     figurations;
                                              server would have full control over session       • using a modular software architecture (for
                                              routing and media. Figure 6 gives an                example, separate S-CSCF, I-CSCF and
                                              overview of five service execution scenarios:       P-CSCF modules) that allows for distrib-
                                              a) The main service functionality lies in the       uted solutions;
                                                 user equipment (UE). The core network          • implementing servers on TSP; and
                                                 can provide assistance for some value-         • implementing gateway and user-plane
                                                 added services. Example: a video-phone           functions on CPP.4
                                                 session between two clients.                   Figure 7 shows some example IMS product
                                              b)The core network passes the session via a       configurations. The trial site provides full
                                                 horizontal interface to an application serv-   IMS functionality, including support for
                                                 er. Example: a gaming session.                 video-conferencing and interworking with
                                              c) The core network passes session control        GSTNs. It also provides a local service exe-
                                                 via a vertical interface to an application     cution environment on the CSCF.
                                                 server. After session control has been re-       The figure also shows some long-term

20                                                                                                                  Ericsson Review No. 1, 2002
configurations that enable handling larger        3GPP functional entities                     Platform
volumes of IMS traffic. In this particular sce-
nario, we can differentiate access, service and
                                                         Application servers
gateway sites. The access sites can host
P-CSCF and possibly also I-CSCF function-                                                            TSP
ality to handle the interface to the UE and
to forward sessions to the appropriate ser-              HSS                 SLF
vice sites.
   The service sites host the functions that
provide the actual session control and ser-
                                                         S-CSCF              MGCF
vices. The gateway site handles the interface
to external GSTNs.                                       I-CSCF              BGCF
   Figure 8 shows only a handful of many
possible configurations. Given the flexibil-             P-CSCF              MRFC                    TSP
ity of the IMS product offering, many other
configurations can be created to suit specif-
ic network deployment scenarios.                         MG                  SG

                                                         MRFP                                       CPP      Figure 7
IP connectivity                                                                                              Example IMS product configurations.

Most mobile operators have introduced (or
are in the process of introducing) packet-
based transport technologies into their net-         Ericsson’s work with these issues has re-
works. IP networks are being introduced for       sulted in the multiservice IP network pre-
the intranet when WAP servers are intro-          sented in this article. The basic assumption
duced—following the introduction of               is that an operator who is building a multi-
GPRS—and when operators become Inter-             service IP network is not willing to sacrifice
net service providers (ISP). At the same time     the characteristics associated with tradi-
some operators are introducing asynchro-          tional services that are provided in TDM and
nous transfer mode (ATM) backbones to de-         ATM networks. We must thus solve the
crease transport costs for traditional voice
services. Ordinarily, this is done at the tran-
sit level of the network.
   More and more operators are seeing a need        Figure 8
to harmonize the introduction of packet-            Example configurations.
based networks and have concluded that the
                                                                                           Home service sites
best move is to invest in a single multi-
service network. But the question remains:                                                          S-CSCF     HSS         S-CSCF
which basic technology should be used,                                                              MRFC                   I-CSCF

ATM or IP? ATM is a mature technology
for traditional voice services and for running
                                                                                                    MRFP          SIP AS
IP. But for operators to run IP over ATM
they must still invest in routers.
   IP is mature technology for best-effort          Visited access sites                                                        GSTN interworking site
traffic. An operator who wants to deploy IP         Includes GSNs
                                                                             P-CSCF                                                       MGCF
technology as the basis for a multiservice                                   I-CSCF                                                       I-CSCF
network that also supports traditional voice
services (real-time) must consider several
new aspects. At this time, there is no well-                                                                                                 MG, SG
established dominant network design for
this deployment. Emerging technologies,                                               Trial sites
such as IP-VPNs, differentiated services,                                              HSS                       CSCF
and resilience mechanisms, are now avail-                                              MGCF                      MRFC

able in router products for these kinds of
carrier-class IP network. These technologies                                                        MG
also provide wire-speed routing for accept-                                                         MRFP
able delay and jitter.

Ericsson Review No. 1, 2002                                                                                                                           21
                                                                  problem of running telephony over IP                 common backbone for WCDMA/GSM ser-
                                                                  multiservice networks while preserving the           vices, ISP services or fixed network services
                                                                  characteristics of present-day networks,             (Figure 9). The challenge is to find an IP
                                                                  such as quality of service, congestion con-          network solution which integrates security,
                                                                  trol, security, and resilience.                      resilience, QoS, dual-stack IPv4/IPv6, and
                                                                     The objective is to run all services over the     bandwidth efficiency, and which can handle
                                                                  same IP network, which makes the IP layer            connection-oriented services in a “connec-
                                                                  the converging layer.                                tionless” network. One possible network so-
                                                                     In the near future we expect to see a shift       lution is described below, but variations of
                                                                  from synchronous digital hierarchy (SDH)             this solution are possible (depending on the
                                                                  to IP on wavelength division multiplexing            operator’s specific prerequisites).
                                                                  (WDM). Today there is a lot of surplus ca-
                                                                  pacity in fiber networks. The deployment of          The structure of the IP infrastructure
                                                                  solutions based on, for example, Ethernet            The IP infrastructure is made up of two main
                                                                  technologies, instead of SDH on layer 2 can          tiers: a backbone tier, which is used to carry
                                                                  give a dramatic reduction in transmission            all traffic between sites; and a site infra-
                                                                  costs. To realize this potential, the services       structure tier, in which the site IP infra-
                                                                  must be transported in a packet-based form.          structure extends IP connectivity to the core
                                                                                                                       network elements at the site. Each site IP
                                                                  IP-based core network architecture                   infrastructure is attached to the backbone
                                                                                                                       tier through one or more edge routers, which
                                                                  General                                              serve as traffic aggregation points and de-
                                                                  In an IP-based WCDMA/GSM core net-                   marcation points between the local IP net-
                                                                  work, all core network elements use the con-         work and the backbone IP network domain.
                                                                  nectivity services of a common IP infra-                The backbone IP network is a shared net-
                                                                  structure to interconnect user traffic and in-       work, which also interconnects sites be-
                                                                  ternal signaling.                                    longing to other service networks (non-
                                                                     A key component of the IP infrastructure          WCDMA/GSM networks, such as ISPs or
                                                                  is an IP backbone network that is used as a          fixed service network providers). The IP
                                                                                                                       backbone tier provides wide-area IP con-
                                                                                                                       nectivity between sites. It is designed for
                                                                                                                       simple high-speed packet transport and is
                                                                                                                       typically built with large backbone routers
                                                                                                                       interconnected with fast links, such as
Figure 9                                                                                                               Ericsson’s AXI 520/580 routers intercon-
Service provisioning over an IP infrastructure.
                                                                                                                       nected with Gigabit links.
                                                                                                                          In this article, we assume that the back-
                                                                                       Future services                 bone IP infrastructure is run by a single op-
                                                                                                                       erator. The routers support different classes
                                                                 Other services
                                                                                                                       of service and layer-3 VPNs. They also sup-
                       Telephony services                                                                              port layer-2 encapsulation, which can be
                                                                                                                       used in parallel to IP traffic to carry Frame
                                                                                                                       Relay or ATM between sites.
       IP services
                                                                                                                          The site IP infrastructure tier extends IP
                                                                                                                       connectivity to the various WCDMA/GSM
                                                                                                                       network elements. Each site has a local IP
                                                                                 Other S                               infrastructure that is connected to the vari-
                                                          Fixed N ice            sites
                                                          multiserv                                                    ous elements at the site and serves as a liai-
                                  sites                   sites                         Site                           son to the IP backbone through edge
            ISP                                                                            tructure
                                                                   Site          IP infras                             routers. The site IP infrastructure is adapt-
            sites                                                       ucture                           IP tructure
                                            Site               infrastr                                  infras
                                                          IP                                                           ed to low-cost, high-capacity traffic capa-
                                   IP   infrastr
                           ture                                                                                        bilities within the site, typically using Fast
             IP infr                                                                                                   Ethernet, Gigabit Ethernet, and LAN
                                                                      e                                                switches that can make use of virtual LAN
                                                     IP ba                                                             (VLAN) techniques. The site IP infrastruc-
                                                                                                                       ture is duplicated to guarantee full avail-
                                                                                                                          The edge routers connect the site IP net-
                                                                                                                       work to the IP backbone. In terms of struc-

22                                                                                                                                         Ericsson Review No. 1, 2002
ture, they belong to the site IP network do-
main and to the IP backbone domain, and
participate in routing protocols in both do-                                                                                                   UTRAN
mains. The edge routers contain advanced
functions (for instance, MPLS LER function,
2547bis, BGP, and filtering) for defining a                                                                                                                                              networks
                                                  Management                    Primary site                                             Secondary site
service agreement between the site IP net-
work and the IP backbone. A site typically                                                                              RNC                                                   RNC
uses a pair of edge routers connected to dif-
ferent core routers in the IP backbone.
Different types of site                                                                                                                                                                      PSTN
In a practical network design the physical        Secondary site                                                            AXI
equipment is grouped together in sites. Sites                                                          AXI       IP backbone core                    AXI
can be grouped by type, depending on their
                                                                          AXI                                                AXI                                                             MGW
role in the network. For the core network,                                                                                                                                   AXI
three types of site can capture the needs of           RNC
                                                                                                       IP backbone                                                                           RNC
                                                                                                           edge                             AXI
an operator. Other types can also be defined                                                                                                                               Concentrator site
to suit specific operator conditions.
• The primary site includes a complete set of
  functions needed for a WCDMA/GSM
  network (control servers, media gateways,                                                                                 Peering IP
  GPRS support nodes, and radio access                                                                                      networks
  network controllers). A primary site                                UTRAN
  might also include a service network con-
  figuration. To distribute redundant load,
                                                 Figure 10
  a network can have several primary sites.      Mapping of different site types on the IP backbone.
• The secondary site contains media gate-
  ways, GPRS support nodes, and radio ac-
  cess network controllers. If necessary, sec-
  ondary sites can also have peering con-
  nections to other networks.
• The concentrator site, which includes media
  gateways and radio access controllers, is      Figure 11
  used for concentrating load far out in the     Example configurations of a primary site.
Peering connections to other networks can
be made from any site. Figure 10 shows the                                                                                                              ng
mapping of different site types on the IP                                                                                           O&MSS
backbone. The primary site is the most im-                                                                                          CN-

portant type. In fact, a complete network
                                                                                   fig.                                     HSS                                                     POI, fixed network
can be built exclusively from primary sites.                               rk   con                    MSC r                                                                        interface
                                                                     e two                           F serve
Figures 11 shows example configurations of                    vic en
                                                                                          Dir    CSC                                                                                 Peering to other
                                                                           ail,                                                                                                      PLMN (TDM)
a primary site.                                                        b, m g
                                                                    We eamin
                                                                     str                                                                                                              Peering to other
                                                                                                                 astruc                                                               PLMN (ATM)
Logical networks and VPNs                                                                                   infr
                                                                                                       IP                                                                                External BSCs
Logical networks                                                                                                                        SG W
                                                                                                                                        MG                           BS
Different kinds of information are ex-                                                                    S   N             RN
                                                       ther     or                                     SG
changed between sets of network elements.         To o s via IP ork                        SN
                                                   site e netw                        GG
Different types of network (STM/TDM,                cor
ATM, IP and SS7) can be used to handle dif-                                                                                           ist ribu
                                                                                                                             AT    Md
ferent kinds of information flows, each with
well-defined quality of service and little or                                                                                                                                 ork
                                                 GGSN may be                                                                                                               etw
                                                                                                                                                                 ce   ss n
no connectivity between the networks. It is      standalone or
                                                                                                                                                           io ac
                                                 hosted in                                                                                       , rad
thus possible to have complete separation of     edge router
                                                                                                                                        ub s
traffic between information flows.                                                                                                 To h                                                  TDM/STM
                                                                                                                   ra  te                                                                Ethernet
   In the context of a multiservice IP net-                                                                  o rpo
work, the IP infrastructure must be able to                                                        Gi t

Ericsson Review No. 1, 2002                                                                                                                                                                              23
                                            handle this exchange of information flows.            RFC 2547 bis is used to separate logical net-
                                            To facilitate traffic separation and ensure           works into layer-3 VPNs in the backbone.
                                            quality of service for the different traffic          Virtual LAN tagging gives characteristics
                                            types, the WCDMA/GSM core network is                  that are similar in nature to those of an ATM
                                            conceptually divided into a number of log-            network and gives the operator good tools
                                            ical networks.                                        for controlling the different traffic flows
                                               Each logical network encompasses a par-            through the IP network. Another simple
                                            ticular kind of information flow between a            mapping would be to avoid the use of VPNs
                                            designated set of functional entities in the          and instead use filtering and BGP commu-
                                            WCDMA/GSM network elements. Fur-                      nities in the routers and client nodes.
                                            thermore, each logical network has a set of             Information flows can be encrypted using
                                            requirements with respect to connectivity,            IPSec in the client nodes or IPSec VPNs be-
                                            QoS, network availability, and so on.                 tween edge routers.
                                               The functional entities reside in
                                            WCDMA/GSM network elements at differ-                 Network redundancy and resilience
                                            ent sites. To support the logical networks,           The network redundancy principles are
                                            the IP infrastructure is configured into sev-         based on the assumption that the network
                                            eral virtual networks. Logical networks are           should be able to withstand single-failure
                                            implemented as virtual networks. Depend-              situations and resume service to its users
                                            ing on the requirements for the supported             with very short restoration time. Further-
                                            logical network, the virtual networks are             more, it is assumed that
                                            implemented using an appropriate set of ca-           • the IP infrastructure—that is, the site IP
                                            pabilities in the site and backbone IP infra-            network and the IP backbone network—
                                            structures (Box D).                                      can be configured to support alternative
                                                                                                     paths through the IP infrastructure; and
                                            Mapping of logical networks to VPNs                   • redundant access can be provided to the
                                            Logical networks can be grouped according                infrastructure.
                                            to their specific characteristics into virtual        The main routing protocols in the IP back-
                                            networks (security, redundancy and re-                bone will be OSPF or IS-IS and BGP-4. At
                                            silience, addressing, QoS, and scaling). This         present, large networks require anywhere
                                            helps operators to decide which VPN tech-             from a few seconds to tens of seconds to con-
                                            nology to apply for each network.                     verge link state protocols, such as open
                                               Figure 12 shows one possible implemen-             shortest path first (OSPF) and IS-IS. This is
                                            tation. In this example, BGP/MPLS IETF                too long for time-critical WCDMA/GSM

Figure 12
Possible implementation of grouping logi-    Site A                                                                    Site B
cal networks into VPNs.                                                                                                   VRF           MGW
                                                                CS payload/Nb
                                                 MGW                                                                      VRF
                                                                Operation and
                                                                                           VRF                                          SGW
                                                                maintenance                VRF                            VRF          MSC-S
                                                                                           VRF                                          HLR
                                                MSC-S                                      VRF                                         SGSN
                                                 HLR                                       VRF

                                                 SGSN           GTP signaling and
                                                                payload                                                Site C
                                                 GGSN           Gi, different encapsulations
                                                                                                                          VRF           MGW
                                                           On site: traffic separation                                                  SGW
                                                                  using VLAN                                              VRF          MSC-S

                                                                                          Between sites: BGP/MPLS according
                                                                                                    to RFC 2547bis                     GGSN

24                                                                                                                    Ericsson Review No. 1, 2002

  Signaling network                                  from the internal UMTS traffic. Traffic is cur-
  Used to carry UMTS signaling traffic such as       rently of best-effort class only, but future traf-
  H.248 and BICC signaling, making use of            fic can be of any class of service.
  SCTP as transport protocol. The signaling net-     Gi toward corporate networks
  work has application level awareness of alter-     Carries user traffic between GGSN and an
  native addresses to signaling recipients. Traf-    appropriate access point to the corporate net-
  fic volume is relatively low (a few Mbit/s to a    work. Separate virtual networks, thus allowing
  site). Signaling traffic must be well protected    for overlapping addresses, will be required for
  from external traffic to avoid possibilities of    each corporate network with several access
  network intrusion.                                 points in the IP infrastructure.
  ISDN voice network                                 Gn, Gp traffic
  Carries circuit voice and data traffic between     User traffic carried in GTP tunnels between
  media gateways. The traffic is characterized       GSN nodes. QoS requirements are depending
  by short packet traffic, with high QoS require-    on the carried user traffic.
  ments.                                             O&M network
  Gi toward ISP                                      Used for operation and maintenance of UMTS
  Carries user traffic between GGSN and the          core network components. Very high availabil-
  Internet. The traffic must be well separated       ity and security requirements exist.

traffic. Therefore, to reduce fail-over times       Admission control
to 50 ms or less, an operator should not sole-      Various mechanisms and policies are used
ly rely on layer-3 redundancy in the IP back-       for controlling the amount of traffic that is
bone. One solution is to rely on underlying         injected into the IP backbone. Admission
SDH mechanisms. Another is to use MPLS              control is exercised at the edges of the net-
with redundant secondary LSPs and the               work and serves to protect the backbone
MPLS fast reroute mechanism.                        from being overloaded. An overloaded back-
   The OSPF protocol with equal cost multi-         bone results in packet loss and increased de-
path routing (ECMP) is the recommended              lays. There are three main ways of control-
method of applying the network-                     ling admission:
provided multipath principle in the site IP         • admission control in client nodes;
infrastructure. ECMP distributes the traffic        • policing of external interfaces; and
between multiple paths between routers and          • policing of internal interfaces.
allows fast fail-over.                              In principle, every node that generates a sub-
                                                    stantial amount of non-best-effort traffic
Quality of service                                  should perform admission control—for ex-
IP quality-of-service capabilities are imple-       ample, GSN, MGW, data-intensive appli-
mented using overprovisioning, admission            cations servers, such as streaming servers and
control, and differential services (DiffServ or     O&M nodes.
Within the site, overprovisioning gives sim-
ple management and is the cheapest way of
guaranteeing QoS. In the backbone, how-
ever, other means must be added due to the
cost of bandwidth.
   The extent to which overprovisioning can
                                                      BOX E, USE OF DIFFERENTIAL SERVICES CODE POINTS
be reduced depends on how sophisticated
the congestion control mechanisms are. The
amount of overprovisioning needed is de-              EF PHB will be used for traffic that has            BE is proposed for UMTS background QoS
termined by fault situations (link or router          requirements on lowest delay, the UMTS con-         traffic.
                                                      versational QoS class.                              Network control signaling is assumed to have
failure), traffic concentration in conjunction        AF4 is reserved for interactive traffic.            a separate code point for PHB.
with “abnormal” events, and the provision             AF3 PHB is proposed for UMTS streaming              Different kinds of O&M traffic may have differ-
of best-effort capacity (capacity must never          QoS class.                                          ent requirements on the DiffServ PHB. Thus
be completely starved).                               AF2 PHB is proposed for CS and PS signal-           multiple DiffServ PHBs will be used for O&M.

Ericsson Review No. 1, 2002                                                                                                                           25
                                                                                                    ferentiate between independent traffic
                                                            User IPv6
                                                                                                    flows, several DiffServ per-hop behaviors
                                                                                                    (PHB) have been proposed (Box E). When
                                                                             GGSN          App.
                                                                                           server   tunneling traffic over GTP, the DSCP in the
                                                         UTRAN                                      SGSN and GGSN is marked as follows:
                                                                                                    • The DSCP in the end-to-end IP header
                                                                                                       can be set by the UE.
Figure 13                                                Transport      Transport IPv4 or IPv6      • In the uplink this setting can be over-
Coexistence of IPv6 and IPv4.                            ATM
                                                                                                       written by the GGSN in accordance with
                                                                                                       the PDP context (APN) when the DSCP
                                                                                                       is forwarded over Gi.
                                                                                                    • The DSCP in the outer IP header is set ac-
                                                                                                       cording to the PDP context (APN) in the
                                                                                                       SGSN (uplink and downlink) and for the
                                                                                                       GGSN (downlink).
                                                Admission can be controlled in different            The different routers are configured to
                                              ways and according to different princi-               schedule and prioritize traffic packets ac-
                                              ples/algorithms. For instance, a trunk-based          cording to their DSCP. Ericsson’s AXI
                                              model can be used for voice traffic between           routers and the embedded router in the
                                              media gateways. The media gateway con-                media gateway and GSN provide rich mech-
                                              trols the amount of traffic on the routes to          anisms for this.
                                              other media gateways. Before new sessions
                                              are accepted, the originating node must               IPv6
                                              check that the required bandwidth is avail-           Ericsson’s IP solutions support IPv4 and
                                              able for the destination.                             IPv6. To have enough IP addresses for every
                                                                                                    connected terminal, operators will need to
                                              DiffServ                                              use IPv6 between end-users and applica-
                                              In the proposed QoS solution, differential            tions. IPv4 and IPv6 will coexist in the IP
                                              services constitute a cornerstone for han-            backbone for a long time. Therefore,
                                              dling quality of service at the IP layer. Var-        Ericsson products include dual-stack
                                              ious applications in the client nodes and             IPv4/IPv6 implementations.
                                              end-user applications or clients in terminals
                                              mark IP packets. Examples of applications             Ericsson products for the IP
                                              in client nodes are the ISDN application in           infrastructure
                                              the media gateway, the GTP encapsulation              The IP solution described above can be sup-
                                              function in the SGSN/GGSN, and the                    ported using Ericsson products. The
                                              SIGTRAN application in the HLR. To dif-               RXI 820 real-time router capabilities are


NetScreen is a trademark of NetScreen
Technologies, Inc.

Extreme Networks is a registered trade-
mark of Extreme Networks, Inc, Santa
Clara, California in the United States, and
may be registered in other countries.

26                                                                                                                     Ericsson Review No. 1, 2002
being embedded in the media gateway            Conclusion
(AXM 101) together with SIGTRAN capa-
bilities and signaling gateway functions.      Mobility and the Internet, the two most dy-
The media gateway will also include trans-     namic forces in communications today, con-
formation functions for payload between        verge in the design and implementation of the
ATM, TDM and IP. In addition, IP and           Ericsson mobile core network. Support for
SIGTRAN capabilities are being included        new services and a common transport tech-
in server nodes, such as AXE.                  nology are the main drivers for the integra-
   The packet platform for GSN nodes con-      tion of IP technology into the core network.
tinues to evolve to include new functional-       Ericsson is fully committed to the intro-
ity and improved performance.                  duction of
   For the site IP infrastructure, Ericsson    • IP-based technologies in its products; and
uses products from partners: NetScreen for     • the mobile core network solutions that ad-
firewalls, and Extreme for LAN switches.          dress mobility and the Internet.
These products are well suited for carrier-    Ericsson’s flexible core network architecture
class networks with high-availability archi-   allows operators to address mobility and the
tecture and hardware-supported filtering       Internet independently.
and forwarding. Ericsson’s partnerships in-       The introduction of GPRS is the first step
clude the identifying and implementing of      toward supporting IP-based applications.
specific mobile network requirements.          Support for new IP multimedia services is
   Ericsson IP-Works develops DNS and          now being implemented according to the
DHCP products adapted to mobile net-           ongoing 3GPP standardization work out-
works.                                         lined in this article.
   Ericsson’s partnership with Juniper Net-       New IP-based connectivity solutions are
works aims at providing a carrier-class        being introduced step by step with the ob-
router family with enhancement for the mo-     jective of decreasing transport costs. The
bile core network. Ordinarily, edge routers    basis of these solutions are IP-based multi-
are based on the AXI 520 series (equal to      service networks based on carrier-class
Juniper M20, M40 series). A new GGSN           routers. Other IP technologies, such as
product J20 has been developed on the Ju-      VPN, resilience, quality of service, and se-
niper router platform. Either the AXI 520      curity mechanisms, are needed to provide
or AXI 580 (equal to Juniper M20, M40,         the characteristics we associate with carrier
M160) can be used for the core routers in      networks.
the backbone. The partnership between             The new IP-based multiservice network
Ericsson and Juniper combines unique com-      also opens up the way for much cheaper
petence in mobile systems with that of         transmission techniques in the long-haul
building carrier-class routers.                network.


                                                                                               1 IP Multimedia Subsystem Stage 2, 3GPP
                                                                                                 technical specification 3G TS 23.228
                                                                                               2 Peck, M., Granström, P and Olson, S.: The
                                                                                                 future of communication using SIP, Erics-
                                                                                                 son Review Vol. 80(2002):1, pp. 30-37.
                                                                                               3 Ekeroth, L. and Hedström, P-M.:GPRS
                                                                                                 Support Nodes, Ericsson Review Vol.
                                                                                                 77(2000):3, pp. 156-169
                                                                                               4 Reinius, J.: Cello—An ATM transport and
                                                                                                 control platform, Ericsson Review Vol.
                                                                                                 76(1999):2, pp. 48-55
                                                                                               5 Session Description Protocol, RFC 2327
                                                                                               6 Session Initiation Protocol, RFC 2543

Ericsson Review No. 1, 2002                                                                                                            27

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