Zhang Zailong1,Fang Jun2, Yu Jinghai2

（1. Institute of Inform ation and Network Technologies, Nanjing University of Posts and Telecommunications, Nanjing 210003, P. R. China;

2. ZTE Corporation, Shenzhen 518057, P. R. China）

Abstract:The Metro Ethernet Forum（MEF）put forward the concep t of Carrier Ethernet（CE）to im p rove Ethernet technology and m ake it a transm ission convergence layer solution for Next Generation Network（NGN）.Provider Backbone Transport（PBT）is the result of the enhancement and im p rovement of the early Ethernet technologies and it is the new version of CE imp lementation techno logy and standard which is p rom ising.Through studying the PBT-related technologies,PBT network structure,PBT advantages as the transm ission convergence layer solution and its trend of future deve lopm ent,it is conc luded that PBT can be used as the p re ferred technology o f the transm ission convergence layer in the NGN,though there are som e p rob lems to be solved for PBT.

Metropolitan Area Network（MAN）is notonly a bridge between Wide Area Network（WAN）and LocalArea Network（LAN）,but also a convergence area for transm ission networks,access networks,and service networks.For the telecom MAN,ithas to carry and converge various types of services now and in the future in add ition to trad itional telecom networks and data networks.As a result,fixed network operators and mobile operators are now competing intensivelyw ith each other in the field ofMAN,attemp ting to find a technology thatcan im p rove network capacity and operation and maintenance efficiency greatly,supportmultip le service types and reduce operation and maintenance costs.

The nextgeneration MAN should be ab le to transportboth packetservices and circuitservices.The operators have been looking fora transm ission convergence layer technology.There is w ide recognition in the telecom community that IP p rotocolis help ful for transformation from circuit-based services to packet-based services and it should be used as the basis ofnew services.But there are stillmany p rob lems to be add ressed before IP routing technologies are app lied into transm ission convergence layer.Currently,IP/Multi-ProtocolLabel Sw itching（MPLS）［1］has been w idely app lied into telecom networks as a service layerand convergence layer technology,especially in telecom backbone and core networks.

Meanwhile,the facts are that traffics of mostdata services ofp rovidernetworks startand end w ith Ethernet,and thatnew service flows are increasing sharp ly,whichmakesmany telecom operators consider Ethernetas a transm ission convergence layersolution for their NGNs.Sim ilarly,some p rob lems have to be solved before Ethernetcan really enter telecom markets and be accep ted.Forexam p le,itmustp rove to be ab le to p rovide services of the same quality as currentservices.That is to say,itmustbe ab le to deliver carrierg rade services.

Consequently,the concep tofCarrier Ethernet（CE）and related solutions are introduced.CEwas firstp roposed by Metro EthernetForum（MEF）at the beginning of2005.Its technologies are notspecific to one kind ofnetwork technologies but inc lude any technologies thatmeet the criteria regarding support for Ethernetservice types and service performance.The c riteria inc lude:

（1）Standard ized Ethernetservices:Point-to-pointand point-to-multipoint Ethernetconnections should be estab lished w ith certainmeans.The service types to be supported inc lude EthernetPrivate Line（EPL）,Ethernet VirtualPrivate Line（EVPL）,Ethernet Private LAN（EPLAN）,and Ethernet VirtualPrivate LAN（EVPLAN）.

（2）Scalability:Service bandw id th and scale should be flexib ly expanded to supporta w ide range ofEthernet services,at rates from 1Mb/s to 10 Gb/s.

（3）Carrier-grade reliability:The p rotection sw itching mechanism should recovera faultw ithin 50m s,leaving the users unaware of the fault.Traditional Ethernetuses link aggregation and Spanning Tree Protocol（STP）to p rovide p rotection.Thismethod consumes lots of lines and ports,and any link failure has to be recovered w ithin seconds,muchmore than 50m s thatcarrier Ethernet requires.Any CE technology should ensure service p rotection sw itching time to be shorter than 50m s.In add ition to network p rotection,node devices adop t redundancy technology to enab le active/standby sw itchover function.In case of failure,the active and standby devices can be quickly sw itched over in severalm illiseconds,w ith user services notaffected.

▲Figure 1. Evolution of CE frame format.

（4）Quality of Service（QoS）:End-to-end service performance should be guaranteed.The performance indexes inc lude calland connection setup speed（e.g.end-to-end delay and delay jitter）,throughput that reflects availab le bandw id th and is related to bandw id th,error rate,cache capacity and p rocessor's capability.The data services carried overearly Ethernetin LAN are insensitive to delay,and at the same time,Transm ission ControlProtocol（TCP）retransm issionmechanism can tolerate loss of few data packets in the Ethernet.As a result,d ifferentiated QoS c lasses are unnecessary.But forCE technologies thatare required to carry various services,the trad itional＂best effort＂servicemodel,whichmakes no attemp t to d ifferentiate between traffic flows,is d ifficult to guarantee QoS.CE uses Differentiated Service（DiffServ）modelto achieve QoS.The im p lementation p rocesses of this service modelinc lude flow c lassification,mapp ing,congestion control,and queue scheduling.

（5）Carrier-g rade service management:Quick service connec tion,Operations,Adm inistration and Maintenance（OAM）and customer networkmanagementshould be p rovided.CE can p rovide powerfuland com p lete networkmanagement functions,as wellas the capabilities for end-to-end unified network management,c lustermanagement,stackmanagementand visualized image management.In add ition to perform ing regularoperations such as configuration,monitoring,userdata samp ling and analysis,the networkmanagement function should im p lement the follow ing operations:automatically discover network failures and repair them timely;automatically d iscovernew service nodes and configure end-to-end services for them;measure end-to-end performance and learn the network status in realtime.

There aremany CE technologies,of which the three p revailing ones are TransportMPLS（T-MPLS）,Provider Backbone Transport（PBT）［2］and Provider VirtualLAN Transport（PVT）.Among the three technologies,T-MPLS is the maturestin standard ization,while PVT and PBT com pete w ith each other.PBT excels PVT in terms ofcom patibility w ith trad itionalEthernetand interwork w ith othernetwork technologies.On the whole,PBT,a connec tion-oriented Ethernet technology,is themost p rom ising among allCE technologies.

1 PBT Overview

The concep tofCE has attracted w idesp read concern of the telecom industry since itwas p roposed by MEF.It is defined as a solution for IP,Ethernet and Time-Division Multip lexing（TDM）service transport in the MAN.The instability of the quality of traditional IP technologies,the app lication of IP-based broadband services（e.g.network TV and video communications）and the strong demand forQoS urge the operators to p lan the construction of the nextgeneration MAN thatmeets new service requirements.

Themain reason why trad itional Ethernetcannotdeliver carrierg rade services lies in its non-connectivity.Besides,its scalability is restricted by lim ited VLAN numbers and Med ia Access Control（MAC）add ress struc ture which is d ifficult to expand.To offerhigh quality services,it is required to setup point-to-pointconnection;to imp rove the scalability,the flatadd ress space mustbe changed into hierarchicalone.Such technologies as VLAN［3］,Provider Bridge（PB）［4］and Provider Backbone Bridge（PBB）［5］are designed to solve the scalability and managementp rob lems,while the p rotocols Ethernet in the First Mile（EFM）［6］and Connectivity Fault Management（CFM）［7］p rovide all-sided OAM solutions.As new technologies emerge continuously,an integ rated technology is needed to effectively combine various technologies and make them suitab le for currentnetwork operations aswellas to p rovide operab le telecom services.PBT is developed in such situation.

PBTcomes from PBB,i.e.MAC-in-MAC technology.MAC-in-MAC is a technology based on MAC stacks.With this technology,the customerMAC add ress is encapsulated into the backbone MAC add ress,enab ling the customer traffic flow to be isolated,thus im p roving Ethernet's scalability and service security.The key feature ofPBB is that itintroduces 24-bit I-TAG to identify services.Being quite suitab le for interworking w ith other technologies,such as MPLS,I-TAG is now used to identify a service rather than a virtualnetwork.The IEEE calls PBTas PBB-Traffic Engineering（PPB-TE）.Figure 1 illustrates the evolution of frame formats ofd ifferent technologies［8-9］.

PBT is a connection-oriented Ethernet technologyw ith telecom network characteristics,and ithas the follow ing technicalfeatures:

（1）PBT is based on MAC-in-MAC butnotequivalent to MAC-in-MAC.Its core concep tis to enab le CE services to be de facto connective bymeans of networkmanagementand control configuration,thus realizing telecom transportnetwork functions,inc luding p rotection sw itching,OAM,QoSand TE.

（2）Ituses Backbone Customer Destination MAC（B-DA）and Backbone VLAN ID（B-VID）to forward services,allow ing the operators to controlCE and isolate customer flows.Hence,the Customer VID（C-VID）in the inner layer,is unnecessarily unique in the network.DifferentB-DAs can use the same C-VID fordata frame forward ing w ithout any conflict.

（3）It turns offMAC's self-learning func tion based on VLAN,thus avoid ing the flood ing ofb roadcastpackets and d ropp ing those data packets whose destinations cannotbe found in forwarding tab le.

（4）It is com patib le w ith trad itional Ethernetb ridge hardware.As a result,the data frame forward ing does not require updating the network's intermed iate nodes ormodifying data frames.Consequently,the forward ing is highly efficient.

（5）Itsupports such connection-oriented network specific functions as bandw id thmanagement and Connection Adm ission Control（CAC）tomanage network resources.It enab les the connections setup by configuration at the networkmanagement centeror via Network Controller（NC）,thus flexib le routing and TE are easily achieved.

PBT is basically an enhanced version ofPBB;but it turns offsome functions of PBB.Forexamp le,it turns offMAC self-learning func tion,allow ing the PBT-based devices to discard the data whose destinations are unknown rather than flood them to allpotential destinations;itd isab lesmulticast function,letting PBT d rop rather than forward themulticastdata;it turns off b roadcast learning func tion because PBT paths are p redefined;and it removes the p rotocols thatare used to p revent loops in the network,which become unnecessary because the forwarding paths ofdata frames are p reset,thus im p roving network utilization.Moreover,the operators canmanage the loads on d ifferent routes to p reventunbalance.

PBTadop ts CFM mechanism defined in IEEE 802.1ag tomonitor the tunnels of the network in a continuous way.When an ac tive tunnelfails towork,PBTw ill automatically transfer its services onto a p re-created standby circuit,p roviding the required resiliency.As a result,the device can gain a failure sw itching time of15m s.The purpose of PBT is to help the operators achieve the follow ing ob jec tives:deliverguaranteed,determ inistic services in a large Ethernet architec ture;ensure reliability,manageability and scalability of the network in order to transportmultimedia services for the enterp rises;allow the operators tomake fulluse ofmetropolitan Ethernet's advantages in running and costduring theirnetworks evolved to convergence architecture.

2 PBT Network Architecture

IEEE hasmade a series ofnew standards based on old Ethernet standards.These new standards,inc lud ing 802.1Q,802.1ad,802.1ag,802.1ah and 802.3ah,have supp lemented the old ones.PBT is an Ethernet transm ission solution based on these standards.Having an independent,connection-oriented and packetsw itching-based transm ission layer,PBT can notonly transportEthernet services,butalso transparently transport other services thatare carried on it.

Figure 2 illustrates PBTnetwork architecture［10］.Adop ting MAC-in-MAC encapsulation,PBTencapsulates the Ethernetdata frames of term inaluser into CE frame header,thus two MAC add resses are generated.In the p rovider core network,the encapsulated data frames w illbe forwarded to the laterMAC add ress.The app lication of MAC-in-MAC encapsulation greatly im p roves the Ethernet's scalability and its capability as a network transm ission technology.In otherwords,w ith MAC-in-MAC used in Ethernet,network hierarchy and broadcastdomain isolation are realized,making Ethernetoperation possib le.With PBT,itis possib le to constructcarriergrade networks based on Ethernet technologies w ithoutany othersupporting network.PBT canmeet the basic requirements for carrier app lications,p rovid ing manageab le and p rotected point-to-pointconnections.Its connections are direc tly p rovided by the networkmanagementsystem rather than w ith MAC self-learningmechanism of Ethernet,enab ling amore reliab le and simp lernetwork.

▲Figure 2. PBT network architecture.

3 Benefits of PBT

PBTenab les the c reation of connec tion-oriented Ethernet tunnels thatallow service p roviders to offer dedicated Ethernet linksw ith guaranteed,determ inistic performance levels.PBT is designed tomeetor exceed the functionality ofMPLS Resource Reservation Protocol（RSVP）-TE［11］tunnels.With these capabilities,PBToffers service p roviders severalnew alternatives to dep loying nextgeneration MANs in term s ofboth the＂tunneling＂technology and the＂services＂thatitsupports（Figure 3）［12］.

▲Figure 3. MAN tunneling and services technology options.

As a traffic-engineered tunneling technology,PBT p rovides an alternative to dep loying MPLS tunnels（e.g.RSVP-TE）in the MANs and supports multip lexing ofany EthernetorMPLS service inside a PBT tunnel.Therefore,service p roviders can delivernative Ethernetin add ition to MPLS-based services over PBT tunnels.This flexibility allows service p roviders to dep loy native Ethernetservices initially,and MPLS services ifand when they need to.As both a tunneling and services infrastructure technology,PBT delivers the follow ing benefits to service p roviders:

（1）Scalability:By turning off the com p licated MAC learning,broadcast and STP features of trad itionalEthernet,PBTavoids the possib le broadcast storm s in customernetworks,and removes the undesirab le broadcast functionality thatcreates MAC flood ing and lim its the size of the network.Additionally,it takes effectivemeasures to get rid of lim itations on service scalability.For instance,itadop ts full VID+MAC（60 bit）add ress as the g lobally unique add ress and destination-based forward ing,enab ling 260 tunnels in the service p rovider network;it forwards data with MAC-in-MAC encapsulation scheme and＂B-VID+B-MAC＂mode.VID is used to identify a specific path,which is not unique in the network,so the add ress spaces of the customerand p rovider networks are expanded.

（2）Hard QoS:In PBT,the path for information forwarding is d irectly p rovided by the network management/controlp lane rather than depends on traditional flood ing and learning.By specifying the path a packet takes across the network,service p roviders can now traffic engineer their Ethernetnetworks.Without over-p rovisioning network capacity,PBT achieves hard QoS,allow ing bandw id th reservation and a p rotection sw itching time of50ms.This,in turn,allows the service p rovider tomaxim ize network utilization and hence reduce the costper bitcarried.

（3）TDM Support:With two-layer encapsulation scheme p rovid ing simp le point-to-pointpath,PBT can interwork w ith existing WAN technologies w ithouta com p lex signaling mechanism.It supports various Ethernetservices as wellas MPLS-based services,inc lud ing VPLS,virtualpseudow ire services at Layer2 and IPVPN services at Layer3.Therefore,it is quite flexib le.The very low latency ofEthernetswitches is combined w ith the determ inistic traffic flow of PBT,p rovid ing an idealp latform to emulate trad itionalTDM/circuitemulation services.

（4）Security:When using point-to-pointEthernetconnection across the network,anym isconfiguration or packet leakage becomes obvious immed iately.Thismeans the traffic is p rotected from incorrectoperations,malicious intentorunintentional leakage ofpackets to its end-points forwhich theywere not intended.PBT technology also shields the realMAC add ress of the customer.

（5）Network Management:PBT adop tsmany networkmanagement functions defined by IEEE and ITU,as wellas existing mature operation and maintenance systems.With these networkmanagement func tions being transp lanted from physical layeror overlaid network layer into data link layer,PBT p rovides carrierg rade network management func tions sim ilar to Synchronous DigitalHierarchy（SDH）.

（6）Service Management:The fact that the Operation SupportSystem（OSS）is aware of the route taken by each service enab les alarm correlation,service-faultcorrelation,and service-performance correlation.Italso enab les p rotection switching for maintenance purposes to be performed in a controlled manner thatguarantees performance against the Service Level Agreement（SLA）.

PBT delivers the scalability,Traffic Engineering（TE）,QoS and manageability thatEthernetdoes not support,allow ing service p roviders to use Ethernetas an infrastruc ture for converged,nextgeneration MANs to supportbusiness and residentialvoice,video and data services.The fact that PBT is enab led bymaking a small alteration to the normalEthernetbehavior means that this technology can be easily im p lemented on existing Ethernet hardware.As a result,there is no requirement to introduce com p lex and expensive network overlay technologies（e.g.MPLS）in the MAN.PBT combines the superiority of Ethernetw ith thatof MPLS.With a sim p lerde-layered network and sim p ler devices,the initial CAPEX costs are reduced,so is the operationalburden,which results in recurring savings.

4 Development Trend of PBT

Currently,the operators around theworld are evaluating PBT/PBB technologies.Although PBThas notan effec tive automatic configuration system,which may have im pacton its scalability,itcan p rovide effective,connection-oriented and packet-based network capabilities.However,PBT is far from perfect.For exam p le,ithas not the capability of point-to-multipoint transm ission.In p rac tice,PBT is often dep loyed in the p roviderbackbone network;while in the convergence layer,PBB technology is used because it is ab le to transport point-to-multipointservices.Therefore,the combination ofPBB and PBT can meet the requirement for point-to-multipointservices.

Those in favorof PBT technology think Ethernetsw itches are usually cheaper than IP/MPLS routers and this situation w illnotchange in the future.Somemanufacturers have developed specialconfiguration and management systems for PBT,and c laimed that their systems canm inim ize the configuration.Moreover,they do not think the standard ization ofPBTw ill lead to a g reat increase in comp lexity.

The standardization of PBT technology has started quickly and is now accelerating.So far,related d rafts have been released successively.In June 2007,the interconnec tion between Provider Edges（PEs）was successfully demonstrated for the first time.This,from anotheraspec t,p roves thatPBT technology can be easily dep loyed and im p lemented w ith existing Ethernet hardware,and its standard ization related research is notcom p licated.

The attitudes and p ractices ofsome lead ing operators,aswellas the PBT-related tests and commercialization made by some operators,show thatPBT has gradually become a＂through train＂for transformation ofg lobaloperator networks ow ing to its technical features and p roven business costmodel.Surely,the operators have to consider theirown developmentstrategies and networks before they decide whether to selectand when to dep loy PBT-based Ethernet solutions.

5 Conclusions

Telecom operators are facing a series of challenges,d riving them to look fora new technology or solution to c reate theirown com petition edges.The introduction of PBToffers the operators certain opportunities.As an attrac tive network and technicalconcep t,PBThasmany advantages:based on PBB subsets and severalEthernetstandards,PBT can p rovide a sim p le,connection-oriented transm ission solution,which canmeet the challenges the telecom operators face and can be used in existing p rovider networks;combining the superiority of Ethernetw ith thatofMPLS,PBTcan p rovide a new,flat,low-cost convergence architecture forMAN,avoid ing excessive dependence on IP/MPLS core.Itcan be used to construct a network w ith technicaladvantages.In the nextgeneration networks,PBT is no doubta p referred technology for transm ission convergence layer.

However,in p raising the advantages ofPBT,telecom operators have to keep a c learhead on the challenges they face.For instance,PBT is a new technical concep t,and there is notany off-the-shelfp roduct to supportit;PBT has notbeen standard ized yet,and currentstandards are sub jec t to change,hence itis difficult to find a solution that can solve allp rob lem s;the N-square p rob lem ofhuge routing tab le in PBT results in lots ofconnection,whichmakes themanagementmore d ifficult;PBT can only be used for ring networking,which restric ts its flexibility;PBThas notany equity algorithm,notsuitab le forsuch heavy traffic and burstservices as b roadband Internetbecause the unfair occupancy ofbandw id ths among devices is likely to occur;the add itional encapsulation in PBTand MAC-in-MAC would inevitab ly lead to an increase of hardware cost.