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IntroductionAs mobile communications evolve, mobileend-users will be offered wideband multi-media capabilities. The associated multi-media streams require that the networksshould be more flexible than present-daynetworkswhich are based on time-

division multiplexing (TDM)at provid-ing bandwidth on demand. The networksmust thus evolve toward cell- and packet-based technologies. As the transport tech-nology evolves, the network paradigm alsochanges. The vertical networkswith a sep-arate network dedicated to each individualapplicationwill be replaced by a horizon-tally layered network architecture. Mediagateways will play a fundamental role in theevolution toward the new network archi-tecture, mediating at the crossing point be-tween existing and new transmission tech-nologies and network types.

Ericsson is working actively with rele-vant standardization bodies and other fora(such as the ITU, ETSI, MSF, 3GPP andIETF) to bring about this network archi-tecture and to ensure open and standardizedinterfaces. In the universal mobile telecom-munications system (UMTS), the horizon-tally layered architecture1 divides the net-work into an application layer; network control layer; and common connectivity layer.

In the network control layer (Figure 1), theMSC server controls circuit-mode servicesand the serving GPRS support node (SGSN)server controls packet-mode services. In theconnectivity layer, the media gateway usesopen interfaces to connect to different typesof node in the core network and external net-works. The media gateway control interface(H.248) facilitates a separation of networkcontrol and connectivity layers. The inter-face to the UMTS terrestrial radio access net-work (UTRAN) is called Iu. In Figure 1, avoice call between the UTRAN and PSTNis interconnected by two media gateways.Media gateway A switches ATM or routesIP traffic. It also provides interworkingfunctions between ATM and IP. The MSCserver and gateway MSC/transit switchingcenter (TSC) server control media gateway B,by means of H.248 control paths. Mediagateway B processes the media stream andprovides interfaces to the PSTN. In this ap-proach, the voice coder is only inserted whenneeded at the edge of the core network. Thisgives improved voice quality and facilitatesmore efficient use of bandwidth in the corenetwork.

In the connectivity layer, the Gn inter-face handles packet-mode traffic betweenthe media gateway and the gateway GPRSsupport node (GGSN).2 The SGSN servercontrols the media gateway by means ofH.248.

216 Ericsson Review No. 4, 2000

Media gateway for m obile networks

Magnus Fyr, Kai Heikkinen, Lars-Gran Petersen, and Patrik Wiss

The telecommunications community is migrating toward a new network

architecture based on horizontal layers. Call control and connectivity,

which have traditionally been bundled in telecommunication networks, are

being separated into different layers. The connectivity layer is based pri-

marily on asynchronous transfer mode (ATM) and Internet protocol (IP)

transmission. Access networks and the core network are parts of the con-

nectivity layer. The connectivity layer provides interfaces to legacy net-

works, such as the public switched telephone network (PSTN). The lay-

ered architecture is being deployed in third-generation mobile

networksthat is, the universal mobile telecommunication system

(UMTS). Ericsson has taken an active role in promoting this new architec-

ture in relevant standardization bodies.Media gateways (MGW) have been introduced to bridge between differ-

ent transmission technologies and to add service to end-user connec-

tions. Smooth step-by-step migration toward the new network architec-

ture is achieved by splitting the mobile services switching center (MSC)

and the serving GPRS support node (SGSN) into a media gateway and

servers.

The authors describe the role of the media gateway in third-generation

mobile networks, and present Ericssons new media gateway product,

which is based on the Cello packet platform.

MSCserver

HSS

MGW

MGW

GGSN

UTRAN

Core networkconnectivity layer

(QoS-enabled ATM or IP)

MAPMAP

Call controlCall control

H.248 H.248

H.248GTP-C

lu

Gn

ATM

CallcontrolQ.1901(BICC) GMSC/TSC-server

PSTN/ISDN/PLMN

Internet/intranet

SGSNserver

Figure 1Layered network architecture, open interfaces.


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Ericsson Review No. 4, 2000 217

Quality of service (QoS) plays a centralrole in the new networks: it is a means of

providing end-users with agreeable serviceand is essential for network management.The media gateway facilitates quality of ser-vice by supporting ATM and IP traffic en-gineering through a combination of ATM traffic managementfor ATM;

and multiprotocol label switching (MPLS)

and differentiated services (DiffServ)for IP.

Migration toward the newnetworkWhen described in terms of the new net-work architecture, Ericssons AXE systemshave traditionally comprised both the net-work control and connectivity layers in thesame node. To migrate toward the new ar-chitecture, the AXE MSC is first divided in-ternally, creating an MSC server and a mediagateway. The node can then be physicallysplit (Figure 2): the MSC server is based on

3GPP Third-generation Partnership ProjectAMR Adaptive multirateAPI Application program interfaceATM Asynchronous transfer modeBICC Bearer-independent call controlCDMA Code-division multiple accessCORBA Common object request broker

architectureDSP Digital signal processorDTMF Dual-tone multifrequencyET Exchange terminalETSI European Telecommunication

Standards InstituteFTP File transfer protocolGGSN Gateway GPRS support node

GPB General-purpose boardGPRS General packet radio serviceGSM Global system for mobile

communicationGTP Gateway tunneling protocolGTP-C GTP controlGTP-U GTP user plane

GUI Graphical user interfaceHSS Home subscriber serverHTML Hypertext markup languageHTTP Hypertext transfer protocolIETF Internet Engineering Task ForceIIOP Internet inter-object request broker

protocolIP Internet protocolIPv4 IP version 4IPv6 IP version 6IRP Integrated reference pointISDN Integrated services digital networkITU International Telecommunication

UnionL2TP Layer 2 tunneling protocol

LER Label edge routerMGW Media gatewayMIB Management information baseMPC Multiparty callMPLS Multiprotocol label switchingMSB Media stream boardMSC Mobile services switching center

MSF Multiservice Switching ForumMTP Message transfer partPDP Packet data protocolQoS Quality of serviceRNC Radio network controllerRTP Real-time transport protocolSCCP Signaling connection control partSCTP Stream control transport protocolSDH Synchronous digital hierarchySGSN Serving GPRS support nodeSNMP Simple network management

protocolSPB Special-purpose boardSS7 Signaling system no. 7STP Signal transfer point

TDM Time-division multiplexingTSC Transit switching centerTTC Telecommunication Technology

CommitteeUDP User datagram protocolUMTS Universal mobile

telecommunications system

BOX A, TERMS AND ABBREVIATIONS

SGSN

MSC

MSCserver

Mediagateway

Server - control layer

Media gateway - connectivity layer

Step 1

Step 2

SGSN

MSC

SGSNserver

Figure 2Ericsson server-gateway split migrationpath.


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AXE and the media gateway is based on theCello packet platform (CPP).3

A similar migration takes place in theGPRS network. The current SGSN is splitinto an internal server and a media gateway.These can then be physically splitall con-nectivity network functionality is ported tothe media gateway. The media gateway isthus common to circuit-mode and packet-mode core networks. The media gateway willalso be introduced into GSM core networks.

Media gatewayThe media gateway comprises several func-tional entities. The physical node is divid-ed into several virtual media gateways. Aspecific server controls each virtual mediagateway, which shares resource componentsthat are visible from the resource componentdatabase. However, resource componentscan also be preconfigured, by identity andtype, for any virtual media gateway. Specialcare has been taken to provide clean and ro-bust interfaces between the virtual mediagateway, resource component database, and

resource components. This modular ap-proach facilitates smooth upgrades on differ-ent sides (Figure 3).

When an H.248 message arrives from theserver, it is broken down and delivered tothe virtual media gateway to which it be-longs. The connection handler sets up thestate logic for the connection and allocatesavailable resource components according tothe resource component database. Thesecomponents are interconnected to process astream and carry it through the media gate-way from one network to another.

Resource components are composed ofmedia framing components and mediastream components. Media framing compo-nents terminate different protocol layersfor example, IP, user datagram protocol(UDP) and real-time transport protocol(RTP). The media stream componentsprocess voice and data calls. Resource com-ponents can be seen as versatile buildingblocks in the functional architecture. Theyalso extract relevant data on performanceand fault management according to the re-quirements in H.248 and according to thecorresponding managed objects in the man-

aged object model. The resource compo-nents interconnect to the Cello packet plat-form by means of application program in-terfaces (API).

The media gateway is designed as an ap-plication on the Cello packet platform,


MGW node

VirtualMGW

MGWapplication

SGWapplication

Resourcecomponentdatabase

Resource components

Media framingcomponent

Switch fabricCPP

API API

Embedded real-timerouter

Physical interfaces

ATM/AAL2 switch

Operation and maintenance

Media streamcomponent

H.248 message handler

Connection handler

Figure 3Functional architecture of the media gateway.

Resourcecomponentdatabase

ATM AAL2

Media framing Media framingMedia stream

Resource components

luFH Voicecoder

Connection handler(one connection chain)

Echocanceller RTP UDP IP

Logical view

Connectionchain

Figure 4Connection model.


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which is a fully redundant telecom platformthat can be used for several different prod-ucts. Its robust, real-time control systemand efficient cell transport system guaran-tee cost-effective applications that supporttime-division multiplexing, ATM and IPtransmission. The execution platform of theCello packet platform offers a scalable clus-ter of intercommunicating processors, a dis-tributed, real-time operating system, a dis-tributed real-time database, and O&M sup-port. Internally, the Cello packet platformembraces ATM (including ATM adaptationlayer 2, AAL2) switching capabilities. Acell-switching fabric allows functionality tobe distributed across boards and subracks.

A real-time router4, which is embeddedin the media gateway, provides distributedforwarding at wireline speeds to all inter-faces. The real-time router handles IPv4, IPv6, header compression,

IP security protocol (IPsec), and differen-tiated services (DiffServ);

provides MPLS edge functionality, in-cluding traffic engineering and protec-tion;

provides advanced firewall filtering andclassification; and

supports virtual private networks (VPN).Figure 4 shows a simplified view of the con-nection model. The connection handler,which keeps a logical view of the connection

chains, interconnects resource componentsby means of the switch fabric. The resourcecomponents can thus have any physical lo-cation in the node. Figure 4 shows a con-nection chain with compressed voice overATM/AAL2 that is converted to uncom-pressed voice over IP (VoIP). In this exam-ple, voice coding and echo-cancellation ser-vices are performed on the voice stream.

By inserting different kinds of mediaframing and media stream components, itis possible to set up processing of virtuallyany media stream on demand. Only a smallset of component types is needed, and it isnot necessary to prepare in advance for allforthcoming combinations of functionalityin a connection chain.

The media gateway comprises severalphysical interfaces ranging from 1.5 to 155Mbit/s. Future releases will include 622Mbit/s and 2.5 Gbit/s interfaces and Giga-bit Ethernet. The interface to the switch fab-ric is not dependent on rate or technology.Operators can upgrade the switch fabric, in-terfaces, or processor boards without dis-turbing traffic. Three kinds of processor

board are available (Figure 5): The general-purpose board (GPB) targets

processes that execute on the distributedprocessor cluster.

The special-purpose board (SPB) is aworkhorse for protocol termination. The

Main functions Virtual MGW logic Message handler Connection handler Resource component database O&M SGW routing

Media framing components GTP-U IP-termination UDP termination RTP termination L2TP IuFH AAL1,2,5

Media stream components Interactive messaging Multi party call Speech coder Echo canceller DTMF sender/receiver Tone-sender/receiver Circuit switch data Charging

1.5, 2, ...622 Mbit/s

1.5, 2, ...155 Mbit/s

1.5, 2, ..622 Mbit/s2.5... Gbit/s

Fast EthernetGigabit Ethernet

Mass storage

Micro processor

DSP

Processing boards

Network interface boards

ATM-termination GPBSwitchfabric

SPB

MSB

TDM-termination

IP-termination

Ethernet-term.

Figure 5Hardware view and functional example ofdistribution.


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board, which comprises several standard

microprocessors, is meant to handle mostof the media framing components.

The media stream board (MSB) is a work-horse for processing media streams (Fig-ure 6). It is composed of several standarddigital signal processors (DSP). Nearly allmedia stream components are processedon this board. To allow efficient and flex-ible resource handling, all media streamcomponents in the media gateway arethought of as belonging to a commonpool.

Resource components

The resource components can be allocatedto any of the processor boards. However, foroptimum performance, some board types

have been developed for use with specific

components (Figure 5). The resource com-ponents can be allocated in needed numbersand type relative to the boards total pro-cessing capacity. The concept also allows forfuture hardware accelerationthat is, someresource components can be implementedin dedicated hardware to further increase ca-pacity and reduce footprint. Hardware sup-port for the resource components that ter-minate ATM, AAL2, IP and TDM is pro-vided on associated network interfaceboards.

Media framing components

The main task of the media framing com-ponents (Box B) is to convert protocols be-tween different transmission networks andto adapt them to the media stream compo-nents (Box C).

The media framing components also sup-port SGSN functionality, handling user datatraffic between the radio network controller(RNC) and the GGSN in the GPRS networkby interconnecting two gateway tunnelingprotocol (GTP) media framing components.The GTP tunnels are thus adapted in themedia gateway between the Iu and Gn in-

terfaces (Figure 1). Using this same mecha-nism, it is easy to process the streams by in-serting a media stream component. For ex-ample, with the outlined architecture, theGPRS network can easily be enhanced withreal-time media streams.


Examples of media framing components

Asynchronous transfer mode (ATM). ATM adaptation layer 1 (AAL1). ATM adaptation layer 2 (AAL2). ATM adaptation layer 5 (AAL5). Internet protocol (IP).

User datagram protocol (UDP). Real-time transport protocol (RTP). Layer 2 tunneling protocol (L2TP). Multiprotocol label switching-label edge

router (MPLS-LER). Gateway tunneling protocol (GTP). Time-division multiplexing (TDM).

BOX B, MEDIA FRAMING COMPONENTS

Figure 6Photograph of the media stream board.


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Signaling gatewayIn many network configurations, mediastreams and signaling share the same phys-ical lines to the media gateway. A signalinggateway function is needed to convey thesignaling messages across different trans-port domains. For example, call control mes-sages must be exchanged for a call that spansan IP-based core network and the PSTN.These messages are set up between theservers but the call is transmitted throughthe media gateway node. To reduce the foot-print, the signaling gateway applicationwhich provides signaling interworking be-tween IP, ATM, and TDM networkshasbeen co-located in the media gateway node.The signaling gateway application also con-tains signal transfer point (STP) functional-ity, in order to relay signaling system no. 7(SS7) messages over the message transferpart (MTP3 and MTP3b) in TDM and ATMnetworks as well as over the stream controltransport protocol (SCTP) in IP networks.

Operation andmaintenanceThe media gateway is managed by a Web-based thin-client application that can be runfrom a standard Web browser and Java vir-tual machine on any computer, locally or re-motely. The thin-client application displaysa graphical presentation of the managementapplication but does not store data on thenode. The client communicates with themedia gateway using the hypertext transferprotocol (HTTP), Internet inter-ORB pro-tocol (IIOP), file transfer protocol (FTP),Telnet, and the simple network manage-ment protocol (SNMP). The connection be-tween the media gateway and the thin-clientcan be localover an Ethernet cable; or remoteusing a secure intranet.Dedicated management applications areembedded in the media gatewaythat is,it contains all the software it needs to man-age the node. These applications (written in

Java) typically use a common object requestbroker architecture (CORBA) or IIOP in-terface to manipulate the management dataof the node.

The graphical user interface (GUI) has thesame look and feel as all other network ele-ment interfaces in Ericssons third-generation core network and radio accessnetwork. Online multilanguage HTMLdocumentationwhich provides Help,

Examples of media stream components

Voice coderthe adaptive multirate (AMR)voice codec is the default voicecoding/decoding algorithm for UMTS. Allmodes of the AMR voice codec are sup-ported. This allows operators to choose thesubset of voice codecs they want to use intheir networks.

Echo cancellerecho cancellers attenuate echo generated from the con-

version between four-wire and two-wiretransmission in the PSTN; and

reduce mobile crosstalk. Circuit-switched datathe circuit-switched

data application provides modem function-ality to the PSTN and ISDN.

Multiparty callsupport for conversationsbetween more than two parties.

Tone sender/receiverthe tone sender/receiver provides tones to be sent to andreceived from end-users.

DTMF sender/receiverthe DTMFsender/receiver sends DTMF tones to the farend of the connection as requested by amobile station. It also receives DTMFtonesfor example, tones that are to beused with the interactive messaging appli-cation.

Interactive messagingthe interactive mes-

saging application provides subscribers withinformative messages on special conditionsin the network or conditions that pertain tothe service in use.

Chargingthe charging function supportsthe generation of volume-based charginginformation for GPRS services.

BOX C, MEDIA STREAM COMPONENTS

The Cello packet platform (CPP) is a new plat-form from Ericsson that primarily targets nodeapplications in the second- and third-generation access and core networks. It fea-tures a modular design that allows applicationsto scale from small base stations up to verylarge network nodes, such as a radio networkcontroller or media gateway. The CPP allowsdifferent applications to be co-located in thesame network node. Current applications onthe CPP include

UTRAN radio network controller; UTRAN radio base stations; GERAN (GSM EDGE radio access network)

real-time routers; cdma2000 base station controllers; cdma2000 radio base stations; and media gateways.

BOX D, APPLICATIONS ON THE CPP


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search functions, and interactive support fortask-oriented operationsis integratedinto the media gateway. This guaranteesthat the information is always consistentwith the software release.

The media gateway supports integratedreference points (IRP). These recommenda-tions from the 3GPP provide interoperabil-ity between managed network elements,such as the media gateway, and multiple-technology or multivendor management

systems. An IRP is an information modelthat defines the interface between a networkelement and the network management sys-tems.

ProductsThe media gateway can be configured tomeet the needs of different operators. To fa-cilitate installation, it comes in a limitednumber of preconfigured subracks. Re-source components can be active by defaultor they can be activated gradually by meansof software keys. Hot-swapping applies toall boards, and new types of resource com-ponent can be downloaded and put into ser-vice without disturbing traffic. Additionalsubracks can be introduced to upgrade ca-pacity as traffic increases. Before delivery,the subracks are fully tested and configuredat the factory.

Main subrack

The main subrack has internal links to theother subracks. It also terminates high-speed physical interfaces and contains cen-

tral processing functions.

Interface extension subrack

The interface extension subrack containshigh-speed interfaces to handle traffic inlarge media gateway nodes.


H.248 is a new protocol whose role is to con-trol media gateways from servers. The proto-col has been developed by the InternationalTelecommunication Union (ITU) and InternetEngineering Task Force (IETF).7, 8 H.248defines a connection model, which is a centralconcept for describing the logical entities with-in the media gateway that can be controlled bythe server. Thanks to this model, differenttransmission media can coexist, and mediastreams can be processed, in the connection.H.248 allows an authenticated server to estab-

lish, move, modify, remove, and obtain eventsthat have been reported on a connection orgroup of connections. A server can audit themedia gateway to determine the extent of itscapabilities. H.248 is a framework protocolthat is, new capabilities can be added bymeans of packages and profiles.

BOX E, H.248

Circuit-switched services subrack

(high-speed)


(high-speed)

Main subrack

Circuit-switched services subrack(high-speed)


(high-speed)

Circuit-switched services subrack(high-speed)

Interface

extensionsubrack

Connection field

Connection field Connection field Packet-switchedservices subrack

Figure 7Example of media gateway configuration.


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Circuit subrack with high-speed

interfaces

The circuit subrack contains high-speed in-terfaces and a corresponding number ofmedia stream boards to handle the traffic onthe interface boards.

Circuit subrack with low-speed

interfaces

The circuit subrack contains low-speed in-terfaces and a corresponding number ofmedia stream boards to handle the traffic onthe interface boards.

Packet subrack with forwarding engine

interfaces

The packet subrack contains wireline-speedforwarding engines and special-purposeboards for terminating GPRS IP traffic.

ConclusionMedia gateways play an important role in theevolution toward the new, horizontally lay-ered network architecture. The media gate-

ways are positioned in the connectivity layerat the crossing point between different net-works. They are fully controlled from serversvia the H.248 protocol. In the connectivitylayer, operators can select the transmissiontechnology that best suits their needsATM, IP or TDM. By introducing MSC andSGSN servers that work together with themedia gateways, operators can migrate theirnetworks toward the new architecture.

The media gateway is built on the Cellopacket platform. This platform is also usedin several other network nodes, such as basestations, the radio network controller, andreal-time routers. The media gateway hasalso been designed for use in all-IP networksthat support real-time voice over IP (VoIP).It contains an embedded real-time routerand an ATM/AAL2 switch with extensivesupport for quality of service and traffic en-gineering. The core of the media gatewayarchitecture is flexible. This means that onlythe network interface boards are specific toa given transmission technologycurrentand emerging.

1 Dahlin, S. and rnulf, E.: Network evolutionthe Ericsson way. Ericsson Review Vol.76(1999):4, pp. 174-181.

2 Ekeroth, L. and Hedstrm, P-M.: GPRS sup-port nodes. Ericsson Review Vol.77(2000):3, pp. 156-169.

3 Reinius, J.: Cello An ATM transport andcontrol platform. Ericsson Review Vol.76(1999):2, pp. 48-55.

4 Brje, J., Lund, H-. and Wirkestrand, A.:Real-time routers for wireless networks.Ericsson Review, Vol. 76(1999): 4,pp. 190-197.

5 Witzel, A.: Control servers in core network.

Ericsson Review Vol. 77(2000): 4, pp. 234-243.6 Granbohm, H. and Wiklund, J.: GPRS Gen-

eral packet radio services. Ericsson ReviewVol. 76(1999):2, pp. 82-88.

7 Recommendation H.248, ITU8 Megaco Protocol, RFC 2885

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