Xu Xianghui,Chen Yunqing

（China Telecom Corporation Ltd. Beiing Research Institute, Beijing 100035, P. R. China）

Abstract:Carrier Ethernet（CE）is g radually stepp ing away from standard ization and testing to dep loyment and app lication in live networks.Although g reat im p rovem ent has been made in the re liab ility o f various CE technologies,there is need for imp rovement in their Quality of Service（QoS）,Operation,Adm inistration and Maintenance（OAM）,especially the m ulti-vendor interoperab ility.The dep loyment o f CE should be service-oriented,and the factors,such as maturity of related technolog ies and standards,dep loyment costs,and the com p lexity of network reconstruction as well as the interoperability with other vendors,shou ld be taken into full consideration to flexib ly select app rop riate networking technology for d ifferent app lication scenarios accord ing to technical features.

A s IP technology is gradually rep lacing trad itionaltechnologies,the trend ofa full IPnetwork is irresistib le.Enjoying the innate advantage of convergence w ith IP,Ethernetg radually goes beyond the app lications w ithin LocalArea Networks（LAN）by virtue of its advantages such as betterp rice-performance ratio perMb bandw id th,sim p lemanagement,flexib le service loading,and low cost,and has become one of the lead ing networking technologies forMetropolitan Area Networks（MANs）.On the one hand,the developmentof Ethernet technologies is reflected by the rate im p rovement from 10Mb/s to 10 Gb/s;on the otherhand,it is reflec ted by enhanced performance.New technologies forQuality of Service（QoS）guarantee,reliability and manageability are constantly emerging.

When app lied into operators'networks,metro Ethernet technology has tomeet the carrier-g rade requirements.Currently,common Ethernetagg regation networks adop ted by operators in broadband access networks ofMANs cannotsatisfy the requirements ofNGN,IPTV and otherhigh-quality services.Specifically,itcannotmeet the requirements for reliability p rotection,and cannotp rovide good service managementand network operation and maintenance ow ing to its poorOperation,Adm inistration and Maintenance（OAM）capabilities;moreover,itis difficult to estab lish an end-to-end secured channel.In order to distinguish from trad itionalEthernetapp lications and reflect the carrier-g rade features,the telecom industry has p roposed the concep tofCarrier Ethernet（CE）,which refers tometro Ethernetw ith scalability,QoS,reliability,security and manageability thatcan be used in operators'networks.Today,ithas become one im portant task foroperators to adop tCE networking technology to op tim ize their IPMANs for im p lementing theirmulti-service bearing capabilities.

1 Status Quo of CE Technologies

Driven by both themarketand technologies,CE technologies and p roducts are flourishing.Although there is a w ide variety ofCE technologies availab le,the grades of those devices are atdifferent levels.Themainstream technologies inc lude trad itionalEthernet enhancement technology,VirtualPrivate LocalArea Network Service（VPLS）,Provider Backbone Transport（PBT）,and ResilientPacketRing（RPR）.

1.1 Traditional Ethernet Enhancement Technology

The trad itionalEthernetenhancement technology p rovides high reliability and scalability based on the op tim ization and im p rovementof trad itionalEthernet architecture.It is characterized by no substantialchanges in the interface and forward ing features of traditional Ethernet.The imp rovementand enhancementmainly occurs at the control layer to achieve carrier-grade p rotection.Trad itionalEthernet enhancement technologiesmainly inc lude Ethernet ring technology,link p rotec tion technology,and Multip le Spanning Tree Protocol（M-STP）.The traditionalEthernetenhancement technology can be imp lemented on the Ethernetsw itches in live networks by simp le software upgrade,w ith no need of rep lacing devices in the live networks.

EthernetAutomatic Protection Switching（EAPS）［1］is an enhanced Fast ConvergentAlgorithm p roposed for traditionalEthernetsw itches,and is mainly used in Ethernet ring topology.EAPS-based sw itches exchange Virtual LAN（VLAN）controlmessages to detect loop faults;themasternode w illconduct link switching once any faults take p lace.EAPS has Layer2 p rotectionmechanism for ring and mesh topology,and itoffers VLAN p rotection.Moreover,itcan p rovide recovery time of100m s while the faultdetection is fully automatic w ithout the intervention ofOAM.

Although ring technology enhances network reliability,itstillhas certain weaknesses,such as a big numberof times of remote node scheduling on the ring,and lim ited scalability.EAPSonly supports ring networking,which restric ts its flexibility.In add ition,this technology,w ithout fairness algorithm,is notsuitab le forb roadband Internetaccess and other burstservices w ith huge traffic,and may result in unfairoccupancy ofbandw id th among devices.

1.2 VPLS

Developed based on point-to-point Multip le ProtocolLabelSw itching（MPLS）,VPLS［2］is a Layer2 Virtual Private Network（VPN）technologywith multi-point interoperability;itexpands Wide Area Network（WAN）MPLS to the access layerofEthernet.From the users'viewpoint,itseem s thatall the sites are connected to one ded icated LAN.From the viewpointofservice p roviders,IP/MPLS infrastructure can be reused to p rovide various services.VPLS adop ts two-layerMPLS labelencapsulation,independentofspecific physical topology,and itcan supportvarious logic topologies,so ithas highernetworking flexibility.In add ition,the traffic engineering ofMPLS can be used to op tim ize resources configuration.VPLS mainly has the follow ing advantages:

·FastRerouting（FRR）technology,used to rep lace Spanning Tree Protocol（STP）and Rap id Spanning Tree Protocol（RSTP）p rotection of Ethernet,which imp roves the p rotection switching time up to 50ms;

·The scalab le Access ControlList（ACL）capability and ACL controlof every user thatenab le safer controland policymechanism;

·Good Layer2 Aggregation capability and ability to im p lement HierarchicalVPLS（HVPLS）,which further enhances network scalability and allows the usernumberexpanded to them illion level;

·Ab lility to d ifferentiate and ensure d ifferent traffic flows ofa user,and enab le sim p le service configuration and fastservice p rovisioning;

·A c learboundary between the operatornetwork and the user network thatmakes iteasier for management.

VPLS estab lishes signaling on Layer2 devices by adop ting com p licated Layer3 p rotocols,so ithas higherequipmentcost.Furthermore,ithasmore p rotocol stack hierarchies and more comp licated operation configurations.Therefore,itis especially expensive for large-scale MANsw ith thousands ofnodes to use VPLS.In addition,the adop tion ofdual MPLS labels increases p rotocol overhead,resulting in poorefficiency when forward ing smallpackets.However,for large-scale MANsw ith strong demands fornew advanced services like IPTV,VPLS is stilla forward-looking technology op tion,especially for the core network.

1.3 PBT

PBT［3］is the enhanced Media Access Control（MAC）in MAC technology.Defined in IEEE 802.1ah,MAC in MAC encapsulates carrier's Ethernet frame header into user's Ethernet frame,which enab les eachmessage to have two MAC add resses.The core network transfers traffic only accord ing to the operators'MAC add ress.Allthe nodes,excep t for edge nodes,are not required to learn userMAC add resses,so the capacity requirementofMAC add ress lists is reduced.Moreover,two-layerMAC add resses give a c learboundary between the operatornetwork and the usernetwork,safe and easy for management.Based on MAC in MAC,PBT turns off the MAC learning function,and elim inates the b roadcast function thatmay resultin MAC flood and restrict the network scale.Moreover,ituses VLAN Identifier（VID）+MAC（60 bit）as the only g lobaladd ress.By introducing connection-oriented backbone network tunnelp rotectionmechanism,PBT can configure backbone network tunnels through the networkmanagementand controlconsole,which enab les stric t QoS,bandw id th reservation,and 50ms p rotection.

PBT technology takes advantage of hidden MAC add resses ofMAC in MAC,while itstillhas the follow ing p rob lem s:

·Atp resent,itdoes notsupport signaling,which can only be dep loyed by networkmanagementdevices.Add itionalMAC header is required,so it has high celloverhead;

·Itcan only p rovide point-to-point services,and fails to supportcommon point-to-multipointservices in Ethernet;

·Itcan only p rovide one-to-one path p rotection while the local repair functionin MPLSTraffic Engineering（MPLS-TE）is notintroduced.It leads to the increase ofp rotec tion paths in network,and the convergence rate cannotguarantee 50m s once paths increase;·This solution requires rep lacing all the devices in a live network.Moreover,its connection w ith DigitalSubsc ribe Line Access Multip lexer（DSLAM）cannotbe p rotected,so itcannotp rovide end-to-end p rotection.

1.4 RPR

RPR［4］is a new type ofMAC technology standardized by IEEE 802.17working g roup.Itis a MAC p rotocolworking in Open System Interoperability（OSI）p rotocolstack Layer2,irrelevantof physicalenvironment.Itcan run over Synchronous Op ticalNetwork（SONET）/Synchronous DigitalHierarchy（SDH）,Ethernetand DenseWaveleng th Division Multip lexing（DWDM）networks.RPR technology combines the reliability broughtby SDH's self-healing function w ith the advantages of Ethernetinc lud ing cost-effec tiveness,broad bandw id th,flexibility,and scalability.Based on ring topology,itp rovides bandw id th management fordata op tim ization and high performancemulti-service transm ission solution.

Currently,there are two types ofPRP devices:pure RPR and MSTP devices w ith embedded RPR.Most leading op ticalcommunication equipment vendors in China choose to develop the Multi-Service TransportPlatform（MSTP）w ith embedded RPR,few ofwhich are putinto app lication.

1.5 Technology Comparison

From the abovementioned mainstream CE technologies,itis easy to conc lude that:

（1）In respec tof reliability,allthe lead ing CE technologies availab le can meet the requirementof50m s,and only the Ethernet ring p rotection technology fulfilled by software upg rade has the p rotection sw itching time ofabout 100ms.However,the currentservices unnecessarily require stric t50ms p rotection,forexam p le,users are generally impercep tib le to fault recovery ofa voice service in 150-250m s.

（2）In respec tofQoS,excep t that VPLS and RPR have strictQoS guarantee mechanism,otherCE technologies basically use p riority guarantee mechanism such as 802.1P,which has no imp rovement in nature com pared w ith trad itionalEthernet.Moreover,itis anotherQoSguarantee solution to imp lementhierarchicalQoSat the access point like Broadband Remote Access Server/Service Router（BRAS/SR）,so as to reduce the requirements forQoS guarantee in CE.

（3）In respectofOAM,mostCEs fulfill OAMmanagementbased on 802.1ag/ITU-TY.1731.Atp resent,VPLS and PBT take the lead in OAM imp lementation,butotherCE technologies can only realize certain basic OAM,such as link detection.

2 CE Standardization and Products

Many internationalstandard ization organizations have carried out the work on CE standards from d ifferent perspec tives.

ITU-Tmainly focuses on the architecture ofoperators'networks,therefore,its standards aboutEthernet technology and services emphasize regulating how to bear Ethernet frames overd ifferent transportnetworks,inc luding SDH,Op ticalTransport Network（OTN）,Asynchronous Transfer Mode（ATM）and MPLS.

Instead ofdefining Ethernetservices,IEEEmainly focuses on standard ization for Ethernet technologies.IEEE is working on Ethernetstandards,inc luding 802.1QVLAN,STP（802.1d,802.1w,802.1s）,802.3ad link convergence,802.3ahEFM,and 802.17 RPR.

IETFmainly focuses on how to p rovide Ethernetservices in packet networks.The IETFPWE3 working g roup isworking on standard ization forservice encapsulation and service simulation analog.The L2 VPN working group is responsib le for specifying im p lementation solutions for L2 VPN p rovided by operators,inc luding Virtual PrivateWire Services（VPWS）and VPLS.

The Metro EthernetForum（MEF）［5］,as the chiefstandard ization organization for CE,ismainly responsib le for the follow ing technology development tasks:

（1）Metro Ethernetarchitecture:MEF p roposed metro Ethernetarchitec ture independentofvarious technologies,and the reference pointofUserNetwork Interface（UNI）.

（2）Metro Ethernetservices:MEF defined themetro Ethernetservice framework from user's perspective and specified service types.

（3）Metro Ethernetp rotection and QoS:MEFp roposed p rotec tionmodes,mechanism and QoS function framework formetro Ethernet,thatis to say,it defined the QoS functions and characteristics required by executing and maintaining Service Level Agreement（SLA）.

（4）Metro Ethernetmanagement:MEF p roposed the networkmanagement interface ofmetro Ethernetand formulated specifications for interfaces in network hierarchy,subnetwork partition,subnetwork topology and network connection.

With regard to CE p roducts,most devices availab le support the abovementioned mainstream technologies.In add ition,several vendors have launched d istinctive CE p roducts,among which VPLS is relatively mature,and these devices have small-scale dep loyments and app lications in the networks ofsuch operators as British Telecom and Southwestern BellCorporation（SBC）.At p resent,the Ethernet ring technology is notmature;althoughmany sw itch vendors have launched IETFRFC 3619 on Extreme Networks'EAPS-based Ethernet ring devices,these devices cannot interoperate w ith each other because ofdifferentimp lementation methods used,such as d ifferentstate machines,different frame formats and software/hardware im p lementation.Currently,operators and standard ization organizations are actively p romoting EAPS interoperability standards to solve the abovementioned p rob lem.RPR technology ismature,but itgets less support from vendors.

Atp resent,there is no big breakthrough in CE p roducts in respect ofQoS and OAM compared w ith traditionalEthernetsw itches.In add ition,CE p roducts cannotsupportcircuit emulation well.Therefore,CE p roducts are stillnotmature on the whole.Although various CE technologies can basicallymeethigher requirements on reliability,there is stillneed for im p rovementin QoS and OAM.Generally speaking,CE is undergoing tests in operators'networks worldw ide,shortof large-scale dep loymentcases.

3 CE Construction and Dep loyment Strategy

Currently,the competition among telecom operators has shifted from backbone network to MAN.Ithas become a common goal to construct metro transportnetworks thatare cost-effective,highly efficient,and can supportmulti-service integ rated bearing.It is an im portant issue forg lobal operators to solve the p rob lem of app lication and dep loymentofCE accord ing to its characteristics and in lightof the ac tualneeds of the operators.

3.1 Driving Force: Service Development

Foroperators,service requirements guide theirCE dep loyment.New multimed ia services like Voice over InternetProtocol（Vo IP）and IPTV require the metro b roadband access network enhancing the follow ing capabilities:

·Highly flexib le scalability in bandwid th,capacity,and services;

·Fastp rotection recovery capability tomeet the operating requirements of suchimportantservice as Softsw itch and IPTV;·QoS guarantee foroffering differentiated services from both dimensions ofusers and services;

·Strict traffic and service perm ission controlcapabilities to ensure safe operation ofhigh-value c losed services.

3.2 Starting Point: Network Reconstruction

Existing metro b roadband access networks are shortof resiliency and reliability,because ofsing le node and sing le link faults in the star topology.Moreover,QoS and securitymechanism ofsome devices in the networks still needs to be im p roved.Tomeetservice requirements,operators should solve the follow ing p rob lem s in theirb roadband access networks:

·Provide carrier-g rade reliability and stability,troub leshootsing le node and sing le link faults in the star topology,p rovide Layer2 domain p rotection,and avoid multip le cascade asmuch as possib le;

·Supportmulti-service dep loyment,imp rove QoSmechanism forQoS guarantee high-g rade real-time services,and simultaneously offer d ifferentiated services;

·Im p rove networkmanageability,p rovide abundantOAMmeasures,and reduce Operation Expend iture（OPEX）;

·Enab le high-bandw id th access to meetusers'increasing demand for bandw id th.

3.3 Dep loyment Scope

Chinese operators generally have large-scale MANs,w ith BRAS/SR dep loyed at the lower layer.Therefore,for them,CE is positioned at the Layer2 access and aggregation layerbelow the services access point.In the future,the possibility to use CE networking for the whole MAN w ith thematurity of technology remains.

3.4 Dep loyment Strategy

CE dep loymentbasically aims tomeet demands ofusers and services.Itshould take effort to imp rove network reliability,d ifferentiated services and OAM capability,and fulfillservice isolation and multi-service integ rated bearing.Meanwhile,itshould focus on imp lementing high-levelQoSguarantee forboth operators'self-operating transformation services and high-end VIP customers'access.

Under the p rerequisite ofmeeting the requirements ofservice sustainab le development,returns on investment should be taken into fullconsideration.CE dep loymentshould be im p lemented in phases accord ing to network situation,actualoperation cond itions and the development trend ofbroadband access network technology,so as to reach the goals ofdevelop ing services,reducing network construction cost,and avoid ing investment risks.

The selection ofCE technology should take into fullconsideration the maturity of related technologies and standards,dep loymentcost,the comp lexity ofnetwork reconstruc tion,and the interoperability w ith other vendors.All the technologies have their strong and weak points,so the selection should be flexib le according to d ifferent app lication scenarios.Forexamp le,VPLS can be app lied at themetro core network to p rovide bandw id th services for VIP customers likemetro VPNs;EAPS can be app lied in ring topology to bear operators'self-operating key services w ith high requirements for reliability.

Among the CE technologies,the enhanced Ethernet ring solution based on trad itionalEthernet technology g reatly im p roves its fastp rotection sw itching and multicastdup licating,solves such p rob lem s as weak p rotection capability of trad itionaldata network and long time for fault recovery,and can be im p lemented by the reconstruction of live networksw ith low dep loymentcost.Therefore,it is currently a feasib le solution.However,there are stillmany p rob lem s to be solved,such as its unsatisfied QoS,OAM,circuitsimulation support,and especially its failure to support themulti-vendor interoperability;itis a long way to fulfillits large-scale app lication.Operators shallactively p romote the im p rovementof related technologies and interoperability w ith othervendors in lightof theiractual network and service demands.

4 Conclusion

Driven by both service and technology,CE hasmade g reatp rogress and become one of the leading technologies in MAN.Ithas the carrier-grade features,butitpaysmuch for com p lexity and cost.CEneeds technology op tim ization and standard ization,so as to lay a solid foundation for large-scale app lications in operators'networks.